MTCL2 promotes asymmetric microtubule organization by crosslinking microtubules on the Golgi membrane

  1. Risa Matsuoka
  2. Masateru Miki
  3. Sonoko Mizuno
  4. Yurina Ito
  5. Chihiro Yamada
  6. Atsushi Suzuki

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Abstract

The Golgi complex plays an active role in organizing asymmetric microtubule arrays, which are essential for polarized vesicle transport. The coiled-coil protein MTCL1 stabilizes microtubules nucleated from the Golgi membrane. Here, we report an MTCL1 paralog, MTCL2, which preferentially acts on the perinuclear microtubules accumulated around the Golgi. MTCL2 associates with the Golgi membrane through the N-terminal coiled-coil region and directly binds microtubules through the conserved C-terminal domain without promoting microtubule stabilization. Knockdown of MTCL2 significantly impaired microtubule accumulation around the Golgi, as well as the compactness of the Golgi ribbon assembly structure. Given that MTCL2 forms parallel oligomers through homo-interaction of the central coiled-coil motifs, our results indicate that MTCL2 promotes asymmetric microtubule organization by crosslinking microtubules on the Golgi membrane. Results of in vitro wound healing assays further suggest that this function of MTCL2 enables integration of the centrosomal and Golgi-associated microtubules on the Golgi membrane, supporting directional migration. Additionally, the results demonstrated the involvement of CLASPs and giantin in mediating the Golgi association of MTCL2.

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  1. Version published to 10.1242/jcs.259374
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    Reply to the reviewers

    First of all, I sincerely appreciate the critical reading of our manuscript by the reviewers.

    Point-by-point responses to the reviewer #1’s comments

    Most of the key conclusions are valid but the main one should be either reinforced or tuned down.

    Through our study, we want to indicate that MTCL2 preferentially associates with perinuclear MTs accumulated around the Golgi complex, and its target is not necessarily restricted to “Golgi-associated (nucleated) MTs.” In this sense, the sentences in the previous manuscript, such as “MTCL2 preferentially associates with Golgi-associated MTs” and “MTCL1 and 2 …. are specifically condensed on Golgi-associated MTs,” were overstatements and completely misleading.

    According to reviewer#1’s comment, we carefully revised these sentences throughout the manuscript and eliminated ambiguity on this point as far as possible.

    The corresponding revisions are as follows.

    In particular, the authors tend to give central role to MTCL2 in regulating the formation and organization of Golgi-associated MT network, and conversely in organizing Golgi elements, without considering the other factors identified (the authors cite relevant papers though but do not discuss this). They should analyze the function of MTCL2 in relation to the role of CLASP2, AKAP450, Golgi-g-Tubulin, or even EB proteins (like EB3).

    I agree with the above comment since it is important to analyze how MTCL2 preferentially associates with the perinuclear MTs accumulated around the Golgi complex.

    In the revised manuscript, we included new data analyzing knockdown effects of CLASP1/2 and AKAP450 on the subcellular localization of MTCL2 (Fig. 7A). These data indicate that CLASPs but not AKAP450 are required for the preferential localization of MTCL2 to the perinuclear MTs around the Golgi. We also demonstrate that the minimum Golgi-localizing region of MTCL2 (the N-terminal coiled-coil region) physically associates with CLASP2 (Fig. 7B), further supporting the idea that CLASPs mediate the Golgi association of MTCL2. Additional involvement of another Golgi element, giantin, is also suggested through Fig. 7C and Appendix Fig. S6. We believe that these revisions significantly improved the weakness previously pointed out by the reviewer.

    I also do not think that carrying out super resolution microscopy is enough to "reveal the possibility that MTCL2 mediates the association of the Golgi membrane with stabilized MTs". More generally, the authors cannot conclude that MTCL2 preferentially associated to Golgi-MT only from their immunofluorescence and KD experiments. The centrosome (the main MTOC) is indeed also localized in the perinuclear area. Easy to do additional experiments may help to confirm these conclusions (see below). Also, the authors could strengthen the way the study how MTCL1 and MTCL2 binds to microtubules and Golgi (see below). The localization or interaction of MTCL2 with Golgi-associated MT is not directly shown.

    Previously, we demonstrated that the N-terminal region of MTCL2 shows clear Golgi-localization activity, whereas the C-terminal region directly binds to MTs. These data support our conclusion that MTCL2 mediates the association of the Golgi membrane with general MTs (although not with Golgi-associated or stabilized MTs).

    In the revised manuscript, we reinforced these data by newly revealing that four-point mutations (4LA) in the first coiled-coil motif disrupt the Golgi localization of the N-terminal region of MTCL2 (Fig. 4D). Thereafter, we found that introduction of the same mutations in full-length MTCL2 abolished its preferential association to the perinuclear MTs accumulating around the Golgi without affecting its localization to MTs (Fig. 4E and F). In addition, we provide data on candidate molecules mediating the Golgi association of MTCL2, as stated above (Fig. 7). These results reinforce our immunofluorescence analysis results (Fig. 2) and indicate that the preferential association of MTCL2 to perinuclear MTs accumulating around the Golgi is facilitated by physical interactions between the N-terminal region of MTCL2 and the Golgi-resident proteins, such as CLASPs and giantin.

    The title should be changed also. I am not sure I understand what an asymmetric microtubule network means in this context. I guess that the authors mean non-centrosomal microtubule network.

    We acknowledge the confusion caused by our previous manuscript. By “an asymmetric MT network” we meant not equivalent to “non-centrosomal MT network.”

    In many cases, microtubules do not elongate radially (symmetrically) from the centrosome but intensely accumulate around the Golgi area and show asymmetric organization (see Meiring et al. Curr. Opin. Cell Biol. 62: 86-95, 2020). “An asymmetric MT network” in the title corresponds to this asymmetric array of general MTs accumulating around the GA.

    The present findings that MTCL2 depletion severely disrupted MT accumulation around the Golgi and induced random and rather symmetric arrays of MTs (Fig. 5A) are very impressive. We believe that the knockdown/rescue experiments in this study strongly support the title by demonstrating that MTCL2 facilitates MT accumulation around the Golgi through its dual binding activity to MTs and the Golgi membrane.

    We changed the title in the revised manuscript but still used the term “asymmetric microtubule organization” based on these rationalities.

    The authors also state that tubulin acetylation is induced by MTCL1 C-MTBD but it may simply be stabilized. They should also clarify if MTCL2 regulates Golgi-dependant nucleation microtubules.

    Yes, we think that MTCL1 C-MTBD enhances tubulin acetylation by simply stabilizing the polymerization state of MTs (see Kader et al. PLos One 12: e0182641, 2017). As for the second point, please see our response below to the comment (7).

    I was not convinced by the use of the quantification of "skewness", in particular in figure 5B. Whether a Wilcoxon test is adequate is unclear to me.

    I understand that utilization of skewness, a measure of the asymmetry of distribution, might not be popular in previous studies. In fact, the skewness of tubulin signal distribution in pixels does not indicate in which way MTs distribute asymmetrically by themselves. However, quantification of this statistical parameter does not require any arbitrary factors and thus eliminates the chance of using discretion as far as possible. Therefore, we are confident that this is the best way to estimate the asymmetric organization of microtubules, which are severely affected by various conditions, without any preconception.

    The two biological phenomena we attempted to elucidate here (microtubule arrays and Golgi ribbon expansion) are thought to be context-dependent in each cell (for example, cell cycle, cell densities, etc.). Therefore, we do not have any substantial reason to assume a normal distribution for variation of the two values (skewness of tubulin signal distribution and Golgi ribbon expansion angle) in our cell population. Therefore, we considered that the Wilcoxon test, being a non-parametric rank test, was the most appropriate and safest test to use.

    To demonstrate that MTCL2 associated to Golgi-MT, microtubule regrowth experiments following nocodazole treatment have to be conducted (time course). Another efficient way to analyze such events, as shown by the Kaverina and the Akhmanova labs for example, is to use fluorescent EB proteins (e.g. EB3) to image microtubule plus ends and back-track them to identify nucleation points. Carrying out such an experiment (nocodazole way-out and EB tracking) in the presence or absence of MTCL2 would allow to confirm, or not, the functional hypothesis of the authors.

    We did not want to demonstrate that MTCL2 preferentially associates with “Golgi-MTs.” From this point of view, we do not think the experiments suggested by reviewer#1 were necessarily required for our study.

    However, there is no doubt that one of the main components of the “perinuclear MTs accumulating around the Golgi” is “Golgi-associated (nucleated) MTs.” In this sense, we still agree with reviewer#1’s comment that it is better to examine whether MTCL2 is involved in MT nucleation from the Golgi membrane. The results of these experiments will be informative for readers particularly because we previously reported that MTCL1 stabilizes Golgi-associated (nucleated) MTs.

    In keeping with the above consideration, we have performed both experiments (nocodazole way-out and EB tracking) according to the previous studies (for example, Sanders et. al. M.B.C. vol. 28; 3181-3192, 2017). However, we ultimately decided against the inclusion of the data as we could not overcome large cell-to-cell deviations.

    Nevertheless, we believe that our current dataset adequately answers and supports the specific questions we explored. Briefly, if these experiments succeed to demonstrate the functional importance of MTCL2 for the development of Golgi-nucleated microtubules, they will not necessarily indicate the physical interaction of MTCL2 with Golgi-associated microtubules. In this respect, as described above, we have significantly supplemented data on the molecular mechanisms by which MTCL2 mediates MT–Golgi interactions. This improvement must sufficiently compensate lack of data from the experiments suggested by reviewer#1.

    Several circumferential data suggest that MTCL2 is not involved in the development of Golgi-associated (nucleated) MTs in contrast to MTCL1. We discussed this issue in the “Discussion” of the revised manuscript.

    *Additionally, carrying electron microscopy analysis would be important to qualify better the effects observed on Golgi complexes upon depletion. The authors mention the effects on the "morphology of Golgi ribbon" but it is rather unclear. *

    We did not perform electron microscopy analysis, because we are not implicating a change in the ultrastructure of the Golgi apparatus in MTCL2-knockdown cells. We specifically want to demonstrate that MTCL2 knockdown changes the assembly structures of the Golgi ribbons, and we believe that it is feasible to do so by light microscopy. We realize that using the term “Golgi morphology” may be misleading in this context. In the revised manuscript, we replaced this term with appropriate ones, such as “assembly structures of the Golgi stacks” or “compactness of the Golgi ribbon.”

    Last, because the authors compare the way MTCL1 and MTCL2 bind microtubules, and suggest intriguing differences, domain swapping experiments between these two isoforms would be important to carry out.

    We conducted the suggested experiments and obtained interesting results. However, we ultimately decided against their inclusion given that the functional difference between MTCL1 and 2 is not the main point of discussion in our study.

    Some studies are referred but the published data not actually used (with the exception of the final scheme). The authors should comment on the fact that other Golgi-associated MT binding proteins have been shown to be involved in the mechanisms highlighted here. Why they would not take over in the absence of MTCL2 should be properly discussed.

    In the revised manuscript, we included data regarding the involvement of CLASPs and AKAP450 in the Golgi association of MTCL2. Accordingly, we introduced their roles in the development of Golgi-associated MTs as far as possible in the “Introduction” (see lines 29-36 and 38-42), “Results” (see lines 306-309 and 344-347), and “Discussion” (see lines 398-402 and 442-444).

    *Similarly, in the discussion, the authors indicate that SOGA has been found as an interacting partner of CLASP2. As CLASP2 is a microtubule binding protein also localized at the Golgi complex and binding to acetylated microtubules, the authors should at least comment on the putative role of the interaction between MTCL2 and CLASP2 in the phenotypes they described. The role of the interaction between CLASP2 and MTCL2 should be discussed and ideally tested. *

    As described above, we provided the data indicating the role of the interaction between MTCL2 and CLASP2 in the revised manuscript.

    In the introduction, page 3 line 74-77, the authors wrote « The resultant N-terminal fragment is released into the cytoplasm to suppress autophagy by interacting with the Atg12/Atg5 complex, whereas the C-terminal fragment is secreted after further cleavage (see Fig. 1A, boxed illustration). » while on the Fig1 the boxed area indicates that SOGA bears Atg16 and Rab5 binding domains. Please double check the interacting partners of SOGA1.

    Thank you for pointing this out. The sentence in the “Introduction” was revised to “… interacting with the Atg12/Atg5/Atg16 complex” (Rev. Endocr. Metab. Disord. 15, 137–147, 2014).

    Figure 1 B and C are not cited in the main text.

    These figures were cited in the “Introduction” section (line 65 in the previous manuscript). In the revised manuscript, these figures were replaced with Fig. EV1 A and C and cited in the “Introduction” section (line 59) as well as in the legend to Fig. 1 (line 757).

    Figure 1E: a loading control is needed to evaluate the expression level of SOGA/MTCL2 in the mouse tissues.

    Sample loading in each lane shown in previous Fig. 1E (Fig. 1D in the revised manuscript) was normalized by total protein amount (25 mg), as indicated in the figure legends. However, we have decided to add the data for a-tubulin expression in each lane as a reference, although they are not equal for each lane.

    In the liver, the size of the bands is different than in other tissues (smaller size). The authors might comment if these smaller bands correspond to the cleaved version of SOGA that was previously described in mouse hepatocyt

    In Fig. 1D of the revised manuscript, we added arrowheads indicating the bands of smaller sizes observed in some tissues such as the liver. In addition, we commented on them in the corresponding part of the “Results” section by describing that “we cannot exclude a possibility that MTCL2 is subjected to the reported cleavage and works as SOGA in these tissues.”

    *Figure 2A: single color picture for the anti-tubulin immunolabeling would help to see the distribution of microtubules in the perinuclear area. The perinuclear region is a crowded area with many intracellular compartments accumulating there as well as cytoskeleton elements. *

    We completely revised Fig. 2 following the reviewers’ suggestion. To provide single-color pictures for the anti-MTCL2 and anti-tubulin immunolabeling, we added new pictures examining colocalization of MTCL2 with MTs at the peripheral regions where densities of both signals are rather low. In Fig. 2B, the colocalization was further examined via a line scan analysis across MTs. Finally, we have included new data demonstrating that exogenously expressed MTCL2 similarly colocalized with MTs even at the peripheral regions when its expression was suppressed to the endogenous level (Fig. 2C).

    *Figure 2C: same comment as above, a single-color picture for the anti-MTCL2 and anti-GM130 immunolabeling are required. *

    Owing to the space limitation, we could not include a single-color picture for the anti-GM130 immunolabeling in Fig. 2, although we enlarged their merged figure so that readers easily agree with our statement: “some overlapped with the Golgi marker signals” (lines 146-147).

    Alternatively, we included a new Appendix Fig. S8, in which immunofluorescence signals of MTCL2 and CLASP1/2 (A) or giantin (B) are compared at a super-resolution microscopic level. In these figures, we included single-color pictures together with merged data.

    page 7, line 132-134: the authors state: « Close inspection using super-resolution microscopy further revealed the possibility that MTCL2 mediates the association of the Golgi membrane with stabilized MTs (Fig. 2D, arrows). » To my opinion, the data are over-interpreted. The signals partially co-localize but this does not indicate a function of MTCL2 in mediating the interaction.

    We deleted the previous Fig. 2D and the corresponding sentence. By doing so, we ceased to suggest that MTCL2 functions to mediate MT–Golgi interactions only based on immunofluorescence data.

    *Figure 3: Another way of merging the anti MTCL2 and GS28 pictures have to be provided. The pictures are difficult to interpret with the current display. *

    We deleted the previous Fig. 4 and ceased to discuss colocalization of MTCL2 with Golgi proteins only based on immunolabeling data as mentioned above.

    Figure 4C: please indicate the meaning of « ppt »

    We included the explanation of “ppt” in the legends to the corresponding figure (Fig. 3C in the revised manuscript) as follows (lines 801-802):

    “ppt represents the MT precipitate obtained after centrifugation (200,000 × g) for 20 min at 25°C.”

    *Figure 5B and C: for easier reading of the figure, it would be useful to annotate with MTCL2 construct is overexpressed following doxycycline treatment (MTCL2 WT (A) and MTCL2 delta C-MTBD (C)). *

    We followed the suggestion. Please see new Fig. 5 and Fig. EV4 and 5.

    Figure 6 A and C: the labels are wrong. Bottom pictures correspond to anti-GM130 immunostaining not anti-tubulin. If I am not mistaken, it is MTCL2 delta C which is studied in panel C.

    Thank you for pointing this out. We corrected this error in Fig. EV5 (previous Fig. 6) in the revised manuscript.

    Page 11, line 212: Supplementary Figure 2 (knockdown in RPE1 cells) is intended to be cited not Supplementary Figure 3.

    Thank you for pointing this out. We corrected the error in the revised manuscript appropriately.

    Figure 8A: single color pictures are needed to appreciate the distribution of the signals

    One of the major comments of three reviewers have been provided on Fig. 8, which reports that MTCL1 and 2 differentially regulate microtubules. We agree that the previous data in Fig. 8 A–C are rather preliminary. Although we could improve these figures according to the reviewers’ comments, we decided to omit these data and cease the discussion that MTCL1 and 2 localize with microtubules in a mutually exclusive manner, as this was not the main focus of the study.

    Point-by-point responses to the reviewer #2’s comments

    *In figure 1D, a loading control should be included for the Western Blot probing for V5-mMTCL2 in HEK293T cells. *

    We did include loading controls for the indicated lanes. However, because the HEK293T cell extract in lanes 1–3 was diluted, the signals were too weak to be visualized in this figure (Fig. 2C in the revised manuscript).

    The authors use the anti-SOGA antibody to detect MTCL2. However, in Figure 1A they do not show the sequence similarity between this region in MTCL1 and MTCL2. The authors should include this, as well as show that the anti-SOGA antibody is specific for MTCL2 and does not detect MTCL1.

    In new Fig. EV1, we included amino acid sequence alignment data for the region corresponding to the used anti-SOGA1 antibody epitope. The data indicate significant divergence of the sequence from MTCL1 (6% homology, 23% similarity).

    We also included new western blot data (Fig. 1B in the revised manuscript) demonstrating that anti-SOGA1 antibody does not react with MTCL1 exogenously expressed in HEK293T cells.

    Line 132-134. The authors conclude that MTCL2 possible mediates association between Golgi membrane and stabilized MTs based on localization microscopy only. This is an overstatement and should be corrected. Not only is the microscopy technique used able to produce resolution of 140nm, which is not enough to show direct association; the staining techniques used (double antibody staining) ensures the fluorophores are approximately 20-30nm away from the intended target (MTs, MTCL2, or Golgi). Thus, the conclusion drawn is overstated and should be refined at this point in the manuscript.

    I agree with reviewer#2’s comment that the previous data in Fig. 2D are insufficient to draw the conclusion that MTCL2 mediates the association between the Golgi membrane and stabilized MTs. We deleted the figure and the corresponding sentence reviewer #2 indicated.

    We want to demonstrate that “MTCL2 mediates the association between the Golgi membrane and MTs (not restricted to the stabilized MTs).” In this sense, we have already obtained supportive data in the previous manuscript that the N-terminal region of MTCL2 has clear Golgi-localization activity, whereas the C-terminal region directly binds to MTs.

    In the revised manuscript, we reinforced these data by revealing that four-point mutations (4LA) in the first coiled-coil motif disrupt the Golgi localization of the N-terminal region of MTCL2 (Fig. 4D). Thereafter, we found that introduction of the same mutations in full-length MTCL2 abolished its preferential association to the perinuclear MTs accumulating around the GA without affecting its colocalization to MTs (Fig. 4E and F). We also provide data on candidate molecules mediating the Golgi association of MTCL2 (Fig. 7). These results reinforce our immunofluorescence analysis results (Fig. 2) and indicate that the preferential association of MTCL2 to perinuclear MTs accumulating around the Golgi is facilitated by physical interactions between the N-terminal region of MTCL2 and the Golgi-resident proteins, such as CLASPs and giantin.

    *The authors should include some quantification of MTCL2 signals along stabilized microtubules near the Golgi and in peripheral regions of the cell in Figure 2. This will show that MTCL2 preferentially localizes to MTs in the Golgi region but not the periphery, as the authors claim (lines 124-130). This quantification could be in the form of linescans along or across MT signals. *

    We included a line scan data across peripheral MTs to confirm MTCL2 colocalization with MTs (Fig. 2C). However, it is difficult to perform a line scan for the perinuclear regions where both signals of MTCL2 and MTs are too dense. Therefore, we demonstrate the preferential colocalization of MTCL2 to the perinuclear MTs by comparing peripheral signals of MTCL2 with that of MAP4 (Fig. 2D).

    The authors show that ectopic expression of the C-terminus of MTCL2 can rescue MTCL2 siRNA phenotypes. Since the N-terminus localizes strongly to the Golgi membrane, the authors should do corresponding experiments with this fragment, to determine if membrane binding of MTCL2 can have a similar rescue effect or if MT binding is essential. This is especially important for the Golgi-ribbon organization (Figure 6).

    We did not include data indicating rescue activity of the C-terminal fragment of MTCL2. In the previous Fig. 5 and 6, we demonstrated that MTCL2 lacking the C-terminal microtubule-binding region does not show rescue activities. Therefore, we did not follow reviewer#2’s suggestion directly.

    However, we included new data indicating that an MTCL2 mutant (4LA) that associates with MTs but not with the Golgi membrane also lacks rescue activities for asymmetric MT organization and Golgi ribbon compactness (new Fig. 5 and Fig. EV4). I hope these revisions are satisfactory.

    *Line 261-2. The authors claim that MTCL1 and MTCL2 function in a mutually exclusive manner. As with point 3, this is an overstatement based solely on localization microscopy. The authors cannot draw this conclusion from the data associated with this statement (Figure 8A) and it should be refined to reflect that they only comment on the respective localization patterns of MTCL1 and MTCL2. Additionally, to show that MTCL1 and MTCL2 do not overlap on MTs, the authors should include linescans along MTs showing the anti-V5 and anti-MTCL1 intensities. *

    One of the major comments of three reviewers have been provided on Fig. 8, which reports that MTCL1 and 2 differentially regulate microtubules. We agree that the previous data in Fig. 8 A–C are rather preliminary. Although we could improve these figures according to the reviewers’ comments, we decided to omit these data and cease the discussion that MTCL1 and 2 localize with microtubules in a mutually exclusive manner, as this was not the main focus of the study.

    *In Figure 8C the authors show acetylated tubulin staining in cells depleted of MTCL2. Based on this localization pattern, it seems the MT network is not grossly altered, as was shown in Figure 5 where perinuclear accumulation of MTs was lost. The authors should comment on whether acetylated tubulin presence and localization is altered in MTCL2-depleted cells. This is also mentioned in the discussion where the authors conclude that the major function of MTCL2 is to crosslink and accumulate MTs in the Golgi region. However, based on acetylated tubulin staining patterns, stable MTs seem to still accumulate in the Golgi region. The authors need to show this accumulated population of stable MTs is no longer crosslinked in the absence of MTCL2 to support their claim. *

    Acetylated microtubules represent a minor fraction of the perinuclearly accumulated microtubules. From the point of this view, it could be possible that the accumulation of perinuclear microtubules is severely affected, whereas that of acetylated microtubules is not. MTCL1 might crosslink these acetylated microtubules.

    In any case, we have decided to delete the previous Fig. 8 A–C, as stated above.

    *To investigate potential functional overlap between MTCL1 and MTCL2, the authors should include a double depletion experiment where MT organization and Golgi organization are investigated. The currently shown experiments do not test a functional relationship between the two paralogs. Additionally, the authors should show Western Blot analysis of MTCL1 levels in MTCL2-depleted cells, and vice versa. While there does not seem to be an overlap in localization patterns of the two proteins, that does not mean there is no functional relationship. *

    We did not follow reviewer#2’s comment because of the reason stated above.

    *Lines 120-30 and 297-9. The authors state that based on the localization pattern of MTCL2 it mostly localizes along MTs in the perinuclear region (shown in Figure (2). Then, in the discussion they state MTCL2 preferentially localizes to Golgi membranes. Please clarify which of the two sites MTCL2 localizes to preferentially. *

    We agree that we should be more careful while describing the subcellular localization of MTCL2. We revised the information in the manuscript to indicate that MTCL2 preferentially localizes to perinuclearly accumulated microtubules showing partial colocalization to the Golgi membrane.

    *Loss of Golgi organization as described in Figures 6 does not appear in polarized cells in Figure 7. The authors should comment on the loss of the phenotype in polarized cells. *

    Since RPE1 cells cultured at high density show abnormally elongated shapes, as described in the original text (line 238; in the revised text, line 326), Golgi ribbons in these cells do not appear to be as expanded. However, their loss of compactness in MTCL2-knockdown cells can be easily recognized in the previous Fig. 7C (corresponding to Fig. 6C in the revised manuscript).

    The authors should consider using colorblind friendly palettes in figures. For example, magenta/green instead of red/green and magenta/cyan/yellow instead of red/blue/green. Additionally, for tri-color images the combination red/green/white (Figure 4B, 7C) should be avoided, as overlapping red/green signals will show up as yellow which is difficult to distinguish from the white signals. Finally, human eyes detect shades of red much poorer than for example green. Therefore, the main point of a figure should not be in red. For example, MTCL2 is frequently shown as red signal in a merged image and should be replaced with a different color.

    We incorporated the reviewer’s suggestion.

    *The authors claim the mouse MTCL2 protein lacks 203 N-terminal amino acids. Authors should clarify in the text that this is relative to mouse MTCL1. The authors should also include the human comparisons, as they work on human cell lines in the majority of the manuscript. *

    I am afraid that this comment is based on a misunderstanding by reviewer #2, because we did not claim that mouse MTCL2 lacks 203 N-terminal amino acids. Instead, we described that SOGA, a mouse MTCL2 isoform, lacks 203 N-terminal amino acids compared to the full-length mouse MTCL2, the cDNA of which was used in this work.

    *In Figure 1D the authors show Western Blots where various amounts of HEK293T extracts were probed for exogenously expressed MTCL2. As a control, authors should include a non-transfected control. From Figure 1E, it would be expected that HEK293 (kidney cells) would not express endogenous MTCL2, but the control should be included anyway. *

    In the revised Fig. 2B, we included a lane in which a non-transfected HEK293T cell extract was loaded, according to reviewer #2’s comment (see lanes indicated as mock).

    *In Figure 3, the color scheme in the final column of images should be changed. Red/white contrast is very poor and no conclusions can be drawn from these images. Additionally, the authors should include a box to show where the inset is located in the overview images. *

    In the revised manuscript, we deleted the “final column of images using red/white contrast” from Fig. 2D (previous Fig. 3), to avoid drawing a conclusion on the interaction between MTCL2 and the Golgi membrane only from immunofluorescence data.

    In addition, we included boxes in the overview images to show where the inset is located, wherever it is required in the revised manuscript.

    Authors claim that MTCL2 is not detected near more dynamic MTs in the periphery of the cell and references Figures 2A and 3. They should include annotation in the figures to highlight this. This can be done with arrowheads or other markings, or with additional insets enlarging a peripheral region of the cell.

    To respond to the comment, we separately provided enlarged views of perinuclear and peripheral regions in the revised Fig. 2.

    *The authors should clarify in the main text and figure legend which superresolution microscopy technique was used in Figure 2D. *

    As mentioned above, we deleted the previous Fig. 2D.

    *The authors use methanol fixation to examine localization of MTCL2, MTs, and Golgi. Methanol extracts lipids and thus affects intracellular membrane compartments, and can affect the localization pattern of GM130, a Golgi matrix protein. The authors should include samples fixed with a crosslinking fixative to ensure their conclusions drawn from methanol-fixed samples are not affected by the choice of fixative. *

    According to the reviewer’s suggestion, we included additional data obtained using PFA fixations (Fig. EV2). PFA fixation also revealed a similar localization pattern of MTCL2 to that obtained by methanol fixation.

    *In Supplementary Figure 1B a third, relatively high expressing cell can be seen in the top panel. The GM130 signal for this cell seems to be comparable to non-transfected cells in the same image. Can the authors address this? Alternatively, to show differences in expression levels between these three cells in that panel and others, authors could use a heatmap LUT of the V5 signal to differentiate expression levels more clearly in different cells. *

    I am unsure whether the reviewer is referring to the cell located at the bottom-left corner of the panel in the previous Supplementary Fig. 1B (Appendix Fig. S1B in the revised manuscript). The cell shows a rather normal distribution pattern of exogenous MTCL2 similar to the endogenous one. We think this is the reason why it maintains a rather normal assembly structure of the Golgi ribbon. We included the word “frequently” in the sentence (line 153 in the revised text) to indicate that high levels of exogenous MTCL2 do not disrupt the normal Golgi ribbon structure. We do not think it is necessary to differentiate the expression levels of exogenous MTCL2 more clearly by using a heatmap, since this issue is not critical for the conclusions of this paper.

    *Line 139. How was the ectopic expression 'suppressed to endogenous levels'? The panels in Suppl Fig. 1 of 'low expression' clearly show increased MTCL2 signal when compared to non-transfected cells in the same panel still. This would suggest ectopic expression is still above endogenous levels. *

    We did not suppress the expression actively. We identified the cells expressing exogenous MTCL2 at low levels comparable to those of endogenous MTCL2. The information provided in line 139 of the previous text is not accurate. Thank you for pointing out this issue; we revised the sentence as follows: “However, when the expression levels were similar to the endogenous levels, … (lines 154-155 in the revised text)”

    *Figure 5C. The label for MTCL2 construct should read mMTCL2 ΔC-MTBD to clarify the expression construct used. *

    Since the labeling in previous Fig. 5 and 6 was confusing, we revised them all by adding the name of the expressed MTCL2 mutant under the label “+dox” (see Fig. 5, Fig. EV4, and Fig. EV5 in the revised manuscript).

    __ *In Figures 6A and 6C the label shows a-tubulin, but the staining is of a Golgi marker. *__

    Thank you for pointing this out. We corrected this error in the corresponding figure (Fig. EV5) in the revised manuscript.

    *In Figures 6B and 6D the different conditions should be separated more in the graph, the datapoints overlap. *

    In the revised manuscript, we significantly improved the presentation of the statistical data shown in the previous Figs. 5 and 6 (Fig. 5 and Figs. EV4 and 5 in the revised manuscript). In these improvements, we determined to only include data of biological replicates in a single typical experiment in the main figures. Automatically, data points in the previous Fig. 6B and D were decreased in number and do not overlap anymore (see Figs. EV4 and EV5D). Instead, we have included new figures (Appendix Fig. S4) in which the results of technical replicates (three independent experiments) are presented.

    *Lines 246-7. The authors claim the Golgi-associated and centrosomal MTs can be easily distinguished in MTCL2 knockdown cells. They should include annotation in the corresponding figures to highlight these different populations. *

    We followed the reviewer’s suggestion by adding arrows in Fig. 6C of the revised manuscript.

    *Figure 8A. A horizontal line is missing in the panel showing MTCL/a-tub merge. *

    Thank you for pointing this out. As mentioned above, we deleted the previous Fig. 8A from the manuscript.

    *Figures 8C and 8D. The acetylated tubulin staining in control cells (control RNAi and GFP) in these panels vary greatly. Can the authors comment on this? *

    Expression of MTCL1 C-MTBD induces tubulin acetylation intensely. Therefore, to obtain appropriate pictures under non-saturated conditions, we had to decrease the gain of photomultiplier of the confocal microscopy system for the previous Fig. 8D. This is why acetylated tubulin signals in control cells appear to be too weak in the previous Fig. 8D than those in Fig. 8C.

    In any case, we deleted the previous Fig. 8C in the revised manuscript as stated above. The previous Fig. 8D is solely included in Fig. EV3.

    Additionally, there appears to be an increase in acetylated tubulin on the Western Blot (8E) shown in cells expressing GFP-MTCL2 CMTB that is not reflected in the image in Figure 8D. Since a significant population of GFP-MTCL2 CMBT localizes to the nucleus, it is possible that the functional population of GFP-MTCL2 CMBT that can stabilize MTs is much lower than GFP-MTCL1 CMBT despite showing equal levels in the Western Blot. The author should compare signal intensity in the cytosol of GFP-expressing cells and base their analysis of acetylated tubulin levels on cells where cytosolic levels are comparable.

    We agree with this reviewer’s comment and did not include WB data in Fig. EV3B corresponding to the previous Fig. 8D.

    As for quantification of the fluorescence data in Fig. 8D, we provided a typical result on the acetylate-tubulin signals normalized by GFP and a-tubulin signals in the boxed regions where cytosolic GFP signals are comparable.

    Point-by-point responses to the reviewer #____3’s comments

    *While the standard fluorescence images are of good quality, the quality of the super-resolution microscopic images is quite low and insufficient. Fig. 8A looks like an enlarged standard laser scanning microscope image, but does not achieve the resolution of a super-resolution image by far, which should be well below the µm range. However, such a resolution would be required to support the claim that MTCL1 and 2 locate on MTs in a mutually exclusive manner. (Negative) data from immunoprecipitation experiments also provide only weak evidence for the absence of a heterocomplex. I also fear that the fixation process creates artifacts. Experiments to image living cells would definitely bolster the data and also provide information about the dynamics of the interactions. *

    One of the major comments of three reviewers have been provided on Fig. 8, which reports that MTCL1 and 2 differentially regulate microtubules. We agree that the previous data in Fig. 8 A–C are rather preliminary. In the revised manuscript, we deleted these data and ceased to discuss that MTCL1 and 2 localize with microtubules in a mutually exclusive manner, as this was not the main focus of the study.

    We also deleted the previous Fig. 2D (showing another super-resolution image) and the corresponding sentence. By doing so, we ceased to suggest that MTCL2 functions in mediating MT–Golgi interactions only based on immunofluorescence data.

    It would also be relevant to confirm that the results are not a cell line artifact in HeLa cells.

    In the previous manuscript, we included data indicating that the knockdown effects observed in HeLa-K cells (reduced accumulation of MTs around the Golgi as well as lateral expansion of the Golgi ribbon) are also induced in RPE1 cells by MTCL2 knockdown (Supplementary Fig. 2 in the previous manuscript). We included the same figure in the revised manuscript as Appendix Fig. S4.

    *A standard method for detecting microtubule association in cultured cells would be to use an extraction protocol. This has to be done to show that MTCL2 actually behaves like a microtubule-associated protein (MAP). *

    In the revised manuscript, we included new immunofluorescence data obtained using PFA fixation with or without pre-extraction, which revealed a similar localization pattern of MTCL2 to that obtained by methanol fixation (Fig. EV2). Pre-extraction was performed using BRB80 buffer supplemented with 0.5% TX-100 and 4 mM EGTA for 30 s, according to a protocol provided by Dr. Mitchison Laboratory.

    *I don't see that the study proves that MTCL2 is essential for the organization of an asymmetric microtubule network as the title claims. The experiments shown in Fig. 5 demonstrate a change in the skewness of the pixel intensity distribution dependent on the presence of MTCL2, which may indicate a contribution of MTCL2 (provided that the fixation and staining do not produce an artifact). However, they do not prove that MTLC2 is essential. *

    We cannot understand how an artifact due to the fixation and staining may be responsible for the results shown in the previous Fig. 5 (Fig. 5 and Figs. EV4 and 5 in the revised manuscript).

    In many cases, microtubules do not elongate radially (symmetrically) from the centrosome but intensely accumulate around the Golgi area and show asymmetric organization (see Meiring et al. Curr. Opin. Cell Biol. 62: 86-95, 2020). “An asymmetric MT network” in the title corresponds to this asymmetric array of general MTs accumulating around the Golgi complex.

    In this respect, our findings that MTCL2 depletion severely disrupted MT accumulation around the Golgi and induced random and rather symmetric arrays of MTs (Fig. 5A) are very impressive. We believe that the knockdown/rescue experiments in this study strongly support the title by demonstrating that MTCL2 facilitates MT accumulation around the Golgi through its dual binding activity to MTs and the Golgi membrane.

    We are unable to comprehend the reviewer’s standpoint in not allowing us to conclude the essential role of MTCL2 in the organization of an asymmetric microtubule. However, the title in the revised manuscript was changed as follows.

    “MTCL2 promotes asymmetric microtubule organization by crosslinking microtubules on the Golgi membrane”

    *There is also a large oversampling of the data by plotting each individual cell from only two separate experiments. It would be better and more reliable to present the data as the mean of the experiments (then of course more than 2 would be required). The same applies to the experiments in which the "Golgi ribbon expanding angle" was determined (Fig. 6). *

    In my opinion, statistical theories based on an ideal assumption cannot simply be applied to the quantitative analysis of biological phenomena. In our case, the MT distributions, as well as the Golgi ribbon expansion angles significantly deviate in a context-dependent manner in each cell (for example, cell cycle, cell densities, etc.). The deviation of these values between each cell (in biological replicates) is much larger than the experimental deviation, which is mainly dependent on the stochastic element (in technological replicates). I understand that this is the reason why many journals in cell biology do not necessarily require “three” independent experiments for statistical analysis.

    In the revised manuscript, however, we included data from three independent experiments for all rescue experiments (Fig. 5, Figs. EV4 and 5, and Appendix Fig. S4) to further demonstrate the reliability of our data.

    In the main figures (Fig. 5, Figs. EV4 and 5), we included statistical data of a single typical experiment to demonstrate reproducibility in biological replicates in each condition. To compensate for these figures, we listed statistical data for each biological replicate of all experiments in Appendix Fig. S4 A. In Appendix Fig. S4 B and C, we further provided statistical data of technical replicates (three independent experiments) by comparing the average of each biological replicate. We concluded that this is the best way to statistically demonstrate the reliability of the biological analysis.

    We believe that the data collectively presented by these figures strongly support the reliability of our conclusions.

    *It would be good to support the claim that MTCL2 affects the Golgi ribbon structure through ultrastructural analysis (EM). *

    We did not perform electron microscopy analysis, because we are not implicating a change in the ultrastructure of the Golgi apparatus in MTCL2-knockdown cells. We specifically want to demonstrate that MTCL2 knockdown changes the assembly structures of the Golgi ribbons, and we believe that it is feasible to do so by light microscopy. We realize that using the term “Golgi morphology” may be misleading in this context. In the revised manuscript, we replaced this term with appropriate ones, such as “assembly structures of the Golgi stacks” or “compactness of the Golgi ribbon.”

    *The critical mechanistic question is which molecule on the Golgi side interacts with MTCL2, since the experiments with the deletion constructs would suggest that it is not the microstructure of the microtubules. As shown, the study is mainly descriptive in relation to this aspect. *

    We significantly improved this weakness by including new data indicating the possible involvement of CLASPs and giantin in mediating the Golgi association of MTCL2 (see Fig. 7 and Appendix Figs. S5–7).

    We also revealed that four-point mutations (4LA) in the first coiled-coil motif disrupt the Golgi localization of the N-terminal region of MTCL2 (Fig. 4D). Thereafter, we found that introduction of the same mutations in full-length MTCL2 abolished its preferential association to the perinuclear MTs accumulating around the GA without affecting its colocalization to MTs (Fig. 4E and F).

    These results reinforce our immunofluorescence results (Fig. 2) and indicate that the preferential association of MTCL2 to perinuclear MTs accumulating around the Golgi is facilitated by physical interactions between the N-terminal region of MTCL2 and the Golgi-resident proteins, such as CLASPs and giantin.

  3. Review Commons

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    Referee #3

    Evidence, reproducibility and clarity

    Matsuoka et al. describe an MTCL1 paralogue (MTCL2) that is present in vertebrates and binds to the Golgi membrane and interacts with microtubules. In contrast to MTCL1, MTCL2 contains only one microtubule binding region and does not stabilize any microtubules. The authors provide evidence that MTCL2 may be involved in accumulating microtubules on the Golgi and promote directed migration. The study is based on experiments with cell lines, predominantly HeLa cells, and relies heavily on the immunofluorescence staining of methanol-fixed cells. While the concept of a functional Golgi-microtubule interaction is interesting and may be relevant for directed migration, I am not convinced of the experimental support and interpretation provided by the authors.

    1. The study relies entirely on the examination of cell lines, mainly HeLa cells, and the immunofluorescence of fixed cells. While the standard fluorescence images are of good quality, the quality of the super-resolution microscopic images is quite low and insufficient. Fig. 8A looks like an enlarged standard laser scanning microscope image, but does not achieve the resolution of a super-resolution image by far, which should be well below the µm range. However, such a resolution would be required to support the claim that MTCL1 and 2 locate on MTs in a mutually exclusive manner. (Negative) data from immunoprecipitation experiments also provide only weak evidence for the absence of a heterocomplex. I also fear that the fixation process creates artifacts. Experiments to image living cells would definitely bolster the data and also provide information about the dynamics of the interactions.
    2. It would also be relevant to confirm that the results are not a cell line artifact in HeLa cells.
    3. A standard method for detecting microtubule association in cultured cells would be to use an extraction protocol. This has to be done to show that MTCL2 actually behaves like a microtubule-associated protein (MAP).
    4. I don't see that the study proves that MTCL2 is essential for the organization of an asymmetric microtubule network as the title claims. The experiments shown in Fig. 5 demonstrate a change in the skewness of the pixel intensity distribution dependent on the presence of MTCL2, which may indicate a contribution of MTCL2 (provided that the fixation and staining do not produce an artifact). However, they do not prove that MTLC2 is essential. There is also a large oversampling of the data by plotting each individual cell from only two separate experiments. It would be better and more reliable to present the data as the mean of the experiments (then of course more than 2 would be required). The same applies to the experiments in which the "Golgi ribbon expanding angle" was determined (Fig. 6).
    5. It would be good to support the claim that MTCL2 affects the Golgi ribbon structure through ultrastructural analysis (EM).
    6. The critical mechanistic question is which molecule on the Golgi side interacts with MTCL2, since the experiments with the deletion constructs would suggest that it is not the microstructure of the microtubules. As shown, the study is mainly descriptive in relation to this aspect.

    Significance

    The study is based on experiments with cell lines, predominantly HeLa cells, and relies heavily on the immunofluorescence staining of methanol-fixed cells. While the concept of a functional Golgi-microtubule interaction is interesting and may be relevant for directed migration, I am not convinced of the experimental support and interpretation provided by the authors.

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    Referee #2

    Evidence, reproducibility and clarity

    Summary:

    In this work, Matsuoka et al. describe a novel microtubule (MT) crosslinking factor, MTCL2. They use Western Blot analysis to show the presence of MTCL2 in various tissues and use a previously developed antibody to show its localization in cultured cells. The authors show that MTCL2 localizes along MTs in the Golgi region and that upon depletion of MTCL2, these MTs do not accumulate in the Golgi and Golgi organization is affected, leading to defects in migration. Through deletion mutant analysis, they show that MTCL2 C-terminus binds to MTs and that the N-terminus binds to Golgi membranes, though this may be lost or reduced in the full length protein. Expression of the C-terminal fragment rescues the phenotypes observed in MTCL2-depleted cells. Finally, the authors show that MTCL1 and MTCL2 show non-overlapping localization patterns and conclude they may have different functions in crosslinking and stabilizing MTs and Golgi organization.

    Major comments:

    1. In figure 1D, a loading control should be included for the Western Blot probing for V5-mMTCL2 in HEK293T cells.
    2. The authors use the anti-SOGA antibody to detect MTCL2. However, in Figure 1A they do not show the sequence similarity between this region in MTCL1 and MTCL2. The authors should include this, as well as show that the anti-SOGA antibody is specific for MTCL2 and does not detect MTCL1.
    3. Line 132-134. The authors conclude that MTCL2 possible mediates association between Golgi membrane and stabilized MTs based on localization microscopy only. This is an overstatement and should be corrected. Not only is the microscopy technique used able to produce resolution of 140nm, which is not enough to show direct association; the staining techniques used (double antibody staining) ensures the fluorophores are approximately 20-30nm away from the intended target (MTs, MTCL2, or Golgi). Thus, the conclusion drawn is overstated and should be refined at this point in the manuscript.
    4. The authors should include some quantification of MTCL2 signals along stabilized microtubules near the Golgi and in peripheral regions of the cell in Figure 2. This will show that MTCL2 preferentially localizes to MTs in the Golgi region but not the periphery, as the authors claim (lines 124-130). This quantification could be in the form of linescans along or across MT signals.
    5. The authors show that ectopic expression of the C-terminus of MTCL2 can rescue MTCL2 siRNA phenotypes. Since the N-terminus localizes strongly to the Golgi membrane, the authors should do corresponding experiments with this fragment, to determine if membrane binding of MTCL2 can have a similar rescue effect or if MT binding is essential. This is especially important for the Golgi-ribbon organization (Figure 6).
    6. Line 261-2. The authors claim that MTCL1 and MTCL2 function in a mutually exclusive manner. As with point 3, this is an overstatement based solely on localization microscopy. The authors cannot draw this conclusion from the data associated with this statement (Figure 8A) and it should be refined to reflect that they only comment on the respective localization patterns of MTCL1 and MTCL2. Additionally, to show that MTCL1 and MTCL2 do not overlap on MTs, the authors should include linescans along MTs showing the anti-V5 and anti-MTCL1 intensities.
    7. In Figure 8C the authors show acetylated tubulin staining in cells depleted of MTCL2. Based on this localization pattern, it seems the MT network is not grossly altered, as was shown in Figure 5 where perinuclear accumulation of MTs was lost. The authors should comment on whether acetylated tubulin presence and localization is altered in MTCL2-depleted cells. This is also mentioned in the discussion where the authors conclude that the major function of MTCL2 is to crosslink and accumulate MTs in the Golgi region. However, based on acetylated tubulin staining patterns, stable MTs seem to still accumulate in the Golgi region. The authors need to show this accumulated population of stable MTs is no longer crosslinked in the absence of MTCL2 to support their claim.
    8. To investigate potential functional overlap between MTCL1 and MTCL2, the authors should include a double depletion experiment where MT organization and Golgi organization are investigated. The currently shown experiments do not test a functional relationship between the two paralogs. Additionally, the authors should show Western Blot analysis of MTCL1 levels in MTCL2-depleted cells, and vice versa. While there does not seem to be an overlap in localization patterns of the two proteins, that does not mean there is no functional relationship.
    9. Lines 120-30 and 297-9. The authors state that based on the localization pattern of MTCL2 it mostly localizes along MTs in the perinuclear region (shown in Figure 2). Then, in the discussion they state MTCL2 preferentially localizes to Golgi membranes. Please clarify which of the two sites MTCL2 localizes to preferentially.
    10. Loss of Golgi organization as described in Figures 6 does not appear in polarized cells in Figure 7. The authors should comment on the loss of the phenotype in polarized cells.

    Minor comments:

    1. The authors should consider using colorblind friendly palettes in figures. For example, magenta/green instead of red/green and magenta/cyan/yellow instead of red/blue/green. Additionally, for tri-color images the combination red/green/white (Figure 4B, 7C) should be avoided, as overlapping red/green signals will show up as yellow which is difficult to distinguish from the white signals. Finally, human eyes detect shades of red much poorer than for example green. Therefore, the main point of a figure should not be in red. For example, MTCL2 is frequently shown as red signal in a merged image and should be replaced with a different color.
    2. The authors claim the mouse MTCL2 protein lacks 203 N-terminal amino acids. Authors should clarify in the text that this is relative to mouse MTCL1. The authors should also include the human comparisons, as they work on human cell lines in the majority of the manuscript.
    3. In Figure 1D the authors show Western Blots where various amounts of HEK293T extracts were probed for exogenously expressed MTCL2. As a control, authors should include a non-transfected control. From Figure 1E, it would be expected that HEK293 (kidney cells) would not express endogenous MTCL2, but the control should be included anyway.
    4. In Figure 3, the color scheme in the final column of images should be changed. Red/white contrast is very poor and no conclusions can be drawn from these images. Additionally, the authors should include a box to show where the inset is located in the overview images.
    5. Authors claim that MTCL2 is not detected near more dynamic MTs in the periphery of the cell and references Figures 2A and 3. They should include annotation in the figures to highlight this. This can be done with arrowheads or other markings, or with additional insets enlarging a peripheral region of the cell.
    6. The authors should clarify in the main text and figure legend which superresolution microscopy technique was used in Figure 2D.
    7. The authors use methanol fixation to examine localization of MTCL2, MTs, and Golgi. Methanol extracts lipids and thus affects intracellular membrane compartments, and can affect the localization pattern of GM130, a Golgi matrix protein. The authors should include samples fixed with a crosslinking fixative to ensure their conclusions drawn from methanol-fixed samples are not affected by the choice of fixative.
    8. In Supplementary Figure 1B a third, relatively high expressing cell can be seen in the top panel. The GM130 signal for this cell seems to be comparable to non-transfected cells in the same image. Can the authors address this? Alternatively, to show differences in expression levels between these three cells in that panel and others, authors could use a heatmap LUT of the V5 signal to differentiate expression levels more clearly in different cells.
    9. Line 139. How was the ectopic expression 'suppressed to endogenous levels'? The panels in Suppl Fig 1 of 'low expression' clearly show increased MTCL2 signal when compared to non-transfected cells in the same panel still. This would suggest ectopic expression is still above endogenous levels.
    10. Figure 5C. The label for MTCL2 construct should read mMTCL2 ΔC-MTBD to clarify the expression construct used.
    11. In Figures 6A and 6C the label shows a-tubulin, but the staining is of a Golgi marker.
    12. In Figures 6B and 6D the different conditions should be separated more in the graph, the datapoints overlap.
    13. Lines 246-7. The authors claim the Golgi-associated and centrosomal MTs can be easily distinguished in MTCL2 knockdown cells. They should include annotation in the corresponding figures to highlight these different populations.
    14. Figure 8A. A horizontal line is missing in the panel showing MTCL/a-tub merge.
    15. Figures 8C and 8D. The acetylated tubulin staining in control cells (control RNAi and GFP) in these panels vary greatly. Can the authors comment on this? Additionally, there appears to be an increase in acetylated tubulin on the Western Blot (8E) shown in cells expressing GFP-MTCL2 CMTB that is not reflected in the image in Figure 8D. Since a significant population of GFP-MTCL2 CMBT localizes to the nucleus, it is possible that the functional population of GFP-MTCL2 CMBT that can stabilize MTs is much lower than GFP-MTCL1 CMBT despite showing equal levels in the Western Blot. The author should compare signal intensity in the cytosol of GFP-expressing cells and base their analysis of acetylated tubulin levels on cells where cytosolic levels are comparable.

    Significance

    This work describes a novel MT crosslinking protein, MTCL2. The authors show that MTCL2 may function predominantly on non-centrosomal MTs associated with the Golgi and suggest a function in linking the centrosome and Golgi in polarized, migrating cells. However, the manuscript is highly descriptive as the authors do not uncover a mechanism for how MTCL2 stabilizes and crosslinks MTs and do not address potential functional interactions between MTCL1 and MTCL2. Additionally, there are some contradictory findings that are not addressed in the current manuscript.

    This work adds a new factor to an expanding list of proteins that regulate non-centrosomal MTs (reviewed in Meiring et al., 2019, Current Opinion in Cell Biology, and Sanders and Kaverina, 2015, Frontiers in Neuroscience), and would be of interest to those interested in cell biology of MT organization and function.

  5. Review Commons

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    Referee #1

    Evidence, reproducibility and clarity

    Summary:

    Provide a short summary of the findings and key conclusions (including methodology and model system(s) where appropriate).

    In this manuscript, the authors identify MTCL2, a paralog of the MTCL1 protein and study its interaction with the Golgi complex and with microtubules. A shorter version of this protein was identified before and named SOGA (suppressor of glucose from autophagy). A role of MTCL2 in regulating the polymerization of Golgi associated microtubules is reported as well as an implication in cell polarity and migration.

    Major comments:

    - Are the key conclusions convincing?

    Most of the key conclusions are valid but the main one should be either reinforced or tuned down. In particular, the authors tend to give central role to MTCL2 in regulating the formation and organization of Golgi-associated MT network, and conversely in organizing Golgi elements, without considering the other factors identified (the authors cite relevant papers though but do not discuss this). They should analyze the function of MTCL2 in relation to the role of CLASP2, AKAP450, Golgi-g-Tubulin, or even EB proteins (like EB3). I also do not think that carrying out super resolution microscopy is enough to "reveal the possibility that MTCL2 mediates the association of the Golgi membrane with stabilized MTs". More generally, the authors cannot conclude that MTCL2 preferentially associated to Golgi-MT only from their immunofluorescence and KD experiments. The centrosome (the main MTOC) is indeed also localized in the perinuclear area. Easy to do additional experiments may help to confirm these conclusions (see below). Also, the authors could strengthen the way the study how MTCL1 and MTCL2 binds to microtubules and Golgi (see below). The localization or interaction of MTCL2 with Golgi-associated MT is not directly shown. The title should be changed also. I am not sure I understand what an asymmetric microtubule network means in this context. I guess that the authors mean non-centrosomal microtubule network.

    - Should the authors qualify some of their claims as preliminary or speculative, or remove them altogether?

    The authors also state that tubulin acetylation is induced by MTCL1 C-MTBD but it may simply be stabilized. They should also clarify if MTCL2 regulates Golgi-dependant nucleation microtubules

    - Would additional experiments be essential to support the claims of the paper?

    Request additional experiments only where necessary for the paper as it is, and do not ask authors to open new lines of experimentation. To demonstrate that MTCL2 associated to Golgi-MT, microtubule regrowth experiments following nocodazole treatment have to be conducted (time course). Another efficient way to analyze such events, as shown by the Kaverina and the Akhmanova labs for example, is to use fluorescent EB proteins (e.g. EB3) to image microtubule plus ends and back-track them to identify nucleation points. Carrying out such an experiment (nocodazole way-out and EB tracking) in the presence or absence of MTCL2 would allow to confirm, or not, the functional hypothesis of the authors. Additionally, carrying electron microscopy analysis would be important to qualify better the effects observed on Golgi complexes upon depletion. The authors mention the effects on the "morphology of Golgi ribbon" but it is rather unclear. Last, because the authors compare the way MTCL1 and MTCL2 bind microtubules, and suggest intriguing differences, domain swapping experiments between these two isoforms would be important to carry out.

    - Are the suggested experiments realistic in terms of time and resources?

    It would help if you could add an estimated cost and time investment for substantial experiments. The proposed experiments to study Golgi-based nucleation are easy and inexpensive, as are domain swapping experiments. Electron microscopy on the other hand is quite expert and requires either internal knowledge, access to a facility or setting-up a collaboration. A few months, 3-4, would be needed.

    - Are the data and the methods presented in such a way that they can be reproduced?

    yes

    - Are the experiments adequately replicated and statistical analysis adequate?

    I am not a statistician but I was not convinced by the use of the quantification of "skewness", in particular in figure 5B. Whether a Wilcoxon test is adequate is unclear to me.

    Minor comments:

    -Specific experimental issues that are easily addressable.

    Yes

    -Are prior studies referenced appropriately?

    Some studies are referred but the published data not actually used (with the exception of the final scheme). The authors should comment on the fact that other Golgi-associated MT binding proteins have been shown to be involved in the mechanisms highlighted here. Why they would not take over in the absence of MTCL2 should be properly discussed. Similarly, in the discussion, the authors indicate that SOGA has been found as an interacting partner of CLASP2. As CLASP2 is a microtubule binding protein also localized at the Golgi complex and binding to acetylated microtubules, the authors should at least comment on the putative role of the interaction between MTCL2 and CLASP2 in the phenotypes they described. The role of the interaction between CLASP2 and MTCL2 should be discussed and ideally tested.

    -Are the text and figures clear and accurate? In general, yes. There are however quite a few problems:

    • In the introduction, page 3 line 74-77, the authors wrote « The resultant N-terminal fragment is released into the cytoplasm to suppress autophagy by interacting with the Atg12/Atg5 complex, whereas the C-terminal fragment is secreted after further cleavage (see Fig. 1A, boxed illustration). » while on the Fig1 the boxed area indicates that SOGA bears Atg16 and Rab5 binding domains. Please double check the interacting partners of SOGA1.

    • Figure 1 B and C are not cited in the main text. • Figure 1E: a loading control is needed to evaluate the expression level of SOGA/MTCL2 in the mouse tissues. In the liver, the size of the bands is different than in other tissues (smaller size). The authors might comment if these smaller bands correspond to the cleaved version of SOGA that was previously described in mouse hepatocyte.

    • Figure 2A: single color picture for the anti-tubulin immunolabeling would help to see the distribution of microtubules in the perinuclear area. The perinuclear region is a crowded area with many intracellular compartments accumulating there as well as cytoskeleton elements. • Figure 2C: same comment as above, a single-color picture for the anti-MTCL2 and anti-GM130 immunolabeling are required.

    • page 7, line132-134: the authors state: « Close inspection using super-resolution microscopy further revealed the possibility that MTCL2 mediates the association of the Golgi membrane with stabilized MTs (Fig. 2D, arrows). » To my opinion, the data are over-interpreted. The signals partially co-localize but this does not indicate a function of MTCL2 in mediating the interaction.

    • Figure 3: Another way of merging the anti MTCL2 and GS28 pictures have to be provided. The pictures are difficult to interpret with the current display.

    • Figure 4C: please indicate the meaning of « ppt »

    • Figure 5B and C: for easier reading of the figure, it would be useful to annotate with MTCL2 construct is overexpressed following doxycycline treatment (MTCL2 WT (A) and MTCL2 delta C-MTBD (C)).

    • Figure 6 A and C: the labels are wrong. Bottom pictures correspond to anti-GM130 immunostaining not anti-tubulin. If I am not mistaken, it is MTCL2 delta C which is studied in panel C.

    • Page 11, line 212: Supplementary Figure 2 (knockdown in RPE1 cells) is intended to be cited not Supplementary Figure 3.

    • Figure 8A: single color pictures are needed to appreciate the distribution of the signals

    -Do you have suggestions that would help the authors improve the presentation of their data and conclusions?

    Yes, see above

    Significance

    - Describe the nature and significance of the advance (e.g. conceptual, technical, clinical) for the field.

    It adds a new player in the machinery involved in the interplay between the Golgi complex and microtubules for their mutual organization. To me a key observation is the unlinking between the Golgi complexes and the centrosome but this observation is not really used and studied (here again, may be a nocodazole wash-out experiment and real-time analysis may help)

    - Place the work in the context of the existing literature (provide references, where appropriate).

    A large number of studies, cited by the authors, have identified proteins involved in mutual organization of Golgi membranes and microtubules. Identification and study of MTCL1 and 2 are important in this context. It also questions the role and function of the initially identified SOGA.

    - State what audience might be interested in and influenced by the reported findings.

    This is a pure cell biology study that will primarily interest people studying the Golgi complex and micrutubules. People interested by the internal organization of the cell, the interaction between the centrosome and the Golgi and intracellular polarity would also be interested, as well as people studying migration.

    - Define your field of expertise with a few keywords to help the authors contextualize your point of view. Indicate if there are any parts of the paper that you do not have sufficient expertise to evaluate.

    I studied Golgi dynamics and function as well as microtubule dynamics. I have no expertise in statistical analysis.

  6. Version published to 10.1101/2020.08.19.251967v3 on bioRxiv
  7. Version published to 10.1101/2020.08.19.251967v2 on bioRxiv
  8. Version published to 10.1101/2020.08.19.251967v1 on bioRxiv