The photosystem I supercomplex from a primordial green alga Ostreococcus tauri harbors three light-harvesting complex trimers
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eLife assessment
The fundamental work represents an important contribution to our understanding of the diversity of photosynthetic mechanisms across the branches of phototrophic life, with the first high-resolution structure (2.9 Å) of a photosynthetic complex from a primitive green alga. This is a valuable resource for understanding function and evolution of light-harvesting antennas. The evidence is convincing in suggesting that the mechanism found here is distinct from the classical antenna state transitions seen in other organisms studied thus far.
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Abstract
As a ubiquitous picophytoplankton in the ocean and an early-branching green alga, Ostreococcus tauri is a model prasinophyte species for studying the functional evolution of the light-harvesting systems in photosynthesis. Here, we report the structure and function of the O. tauri photosystem I (PSI) supercomplex in low light conditions, where it expands its photon-absorbing capacity by assembling with the light-harvesting complexes I (LHCI) and a prasinophyte-specific light-harvesting complex (Lhcp). The architecture of the supercomplex exhibits hybrid features of the plant-type and the green algal-type PSI supercomplexes, consisting of a PSI core, an Lhca1-Lhca4-Lhca2-Lhca3 belt attached on one side and an Lhca5-Lhca6 heterodimer associated on the other side between PsaG and PsaH. Interestingly, nine Lhcp subunits, including one Lhcp1 monomer with a phosphorylated amino-terminal threonine and eight Lhcp2 monomers, oligomerize into three trimers and associate with PSI on the third side between Lhca6 and PsaK. The Lhcp1 phosphorylation and the light-harvesting capacity of PSI were subjected to reversible photoacclimation, suggesting that the formation of Ot PSI-LHCI-Lhcp supercomplex is likely due to a phosphorylation-dependent mechanism induced by changes in light intensity. Notably, this supercomplex did not exhibit far-red peaks in the 77 K fluorescence spectra, which is possibly due to the weak coupling of the chlorophyll a 603- a 609 pair in Ot Lhca1-4.
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Author Response
Reviewer #1 (Public Review):
This is a carefully written manuscript describing the structure of a low-light inducible PSI complex from Ostreococcus tauri. The work expands our knowledge of how photosynthetic systems react to changes in light conditions and shows how this ecologically important green alga utilizes its unique antenna, Lhcp.
In general, I find that the work described in the manuscript is of high quality. The cryoEM maps obtained by the authors clearly show the addition of lhcp trimers to PSI under low light conditions and the distinction between lhcp1 and lhcp2 appears sound together with the identification of the phosphorylation site and its binding in the PSI complex.
We thank the reviewer for taking the time and carefully studying the manuscript.
Reviewer #2 (Public Review):
When O. tauri cells …
Author Response
Reviewer #1 (Public Review):
This is a carefully written manuscript describing the structure of a low-light inducible PSI complex from Ostreococcus tauri. The work expands our knowledge of how photosynthetic systems react to changes in light conditions and shows how this ecologically important green alga utilizes its unique antenna, Lhcp.
In general, I find that the work described in the manuscript is of high quality. The cryoEM maps obtained by the authors clearly show the addition of lhcp trimers to PSI under low light conditions and the distinction between lhcp1 and lhcp2 appears sound together with the identification of the phosphorylation site and its binding in the PSI complex.
We thank the reviewer for taking the time and carefully studying the manuscript.
Reviewer #2 (Public Review):
When O. tauri cells are grown under low light, PSI has six classical LHCIs (Lhcas), four on one side of the PSI core and two on another, and three trimers of the "Lhcp" antenna proteins on a third side, thus surrounding the PSI core. Lhcp Trimer 2 consists of 1 Lhcp1 and 2 Lhcp2; Trimers 1 and 3 are solely Lhcp2. Careful examination of carotenoid positions suggested that certain serve as "molecular staples" in holding the three monomers of a trimer together.
The resolution of the structure is high enough to determine the positions of all the chlorophylls and carotenoids and to establish the correct chemical composition. All the proteins determined by LCMS/MS were located and modeled. Of particular interest were the minor polypeptides PsaO, PsaL, PsaH, and PsaK, which are in between the PSI core and the trimers, and are involved in binding the trimers to the core.
There is a very detailed comparison of Lhcp trimers with LHC trimers of plants and Chlamydomonas. One of the conclusions is that Chl b requires a Gln rather than a Glu at a certain position, which may otherwise be occupied by a carotenoid. Another is that the increased distance between Lhca5 and 6 may be responsible for the lack of "red" Chls.
This led to a detailed analysis of potential energy transfer pathways in the holocomplex based on distances between pigments and how the trimers interact with the small PSI subunits PsaO, PsaL, PsaH, and PsaK. This section is unfortunately rather tedious to read because the individual monomers in each different trimer are suddenly designated by capital letters. This is not explained properly in the text or in the legend in Fig. 10.
Thank you for pointing this out. We now provide the detailed explanation of the labels of the individual monomers in the text: Among the three trimers associated with PSI, Trimer 1, formed by monomers S, T, and U, may have a crucial role in mediating the excitation energy transfer between Trimer 2, formed by monomers P, Q, and R, and Trimer 3, formed by monomers V, W, and X.
That being said, my overall judgment of the manuscript up to this point is very favorable - I'm impressed with the high quality of the data and the thoroughness of its analysis. It has long been known that when O. tauri cells are grown under high light, the PSI complex does not have the Lhcp trimers, but just has the Lhca antenna. Returning cells to low light induces the synthesis of the Lhcp trimers and the formation of the holocomplex. This could be looked at as a "low-light acclimation"; in nature, the prasinophytes are found in shallow water and hence high light exposure may be their "normal".
Thank you for the nice and insightful comments. We agree that the high light exposure is normal for prasinophytes as their habitat is surface of ocean, especially this particular ecotype of O.tauri, OTH95, which was isolated in Thau lagoon, France (Derelle et al. PNAS 2006).
The authors asked if this is related to the situation in higher plants and Chlamydomonas where HL induces phosphorylation of certain LHCII trimers which migrate from the appressed membrane regions and associate with PSI. The common factor of these two phenomena is phosphorylation, but the process referred to as a"State transition" operates in the opposite direction to the situation in O. tauri. The authors did a little experiment to see if the disappearance of the complex was reversible in the same time scale as the "state transitions" of Chlamy and plants, by exposing their normal low light cells to 1 hr of HL, then putting them back in LL. They did show that the amount of phosphorylated Lhcp1 decreased significantly in this time frame and then recovered a significant amount when returned to LL. However, using P700 oxidation to assay Lhcp trimers is not very convincing to my eyes.
In my opinion, this does not provide any evidence for a similar mechanism to "state transitions". A real understanding will have to involve studying PSII and its interaction (if any) with Lhcps. There is no indication of where the Lhcps went in 1 hour of HL--maybe they're just at the top of the gradient, minus any phosphate. I would strongly recommend deleting this section altogether.
It is important to be transparent and cautious when presenting data that are not conclusive. While it may be tempting to eliminate this section completely, we have opted to keep the data in its current form while explicitly acknowledging the limitations of the study. By doing so, we aim to prevent any definitive assertions that may potentially mislead readers. In this case, it is appropriate to acknowledge that the current data suggest that phosphorylation/dephosphorylation of Lhcp1 takes place in response to changes in light intensity, but it is not possible to conclude whether O.tauri undergoes state transitions or not. It is, however, reasonable to suggest that this direction of research could be pursued in the future. It is also important to clarify the role of the Lhcp trimers in relation to PSI whether they are constitutive antennas for PSI under LL conditions (as observed in moss) or conditional antennas during state transitions (as observed in land plants and Chlamydomonas), and to acknowledge that there may be different mechanisms at play in different organisms. This could be an interesting avenue for future research especially in the perspective of green plants evolution, but again it should be emphasized that more data is needed before any definitive conclusions can be drawn as you kindly advised.
My conclusion is that a detailed comparison with plant and Chlamydomonas PSIs shows that there are many different ways in which a photosynthetic eukaryote can evolve an effective antenna system. It gives me great pleasure to see a carefully revealed model of another solution to the light-harvesting problem.
Thank you for the insightful comment! We fully agree.
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eLife assessment
The fundamental work represents an important contribution to our understanding of the diversity of photosynthetic mechanisms across the branches of phototrophic life, with the first high-resolution structure (2.9 Å) of a photosynthetic complex from a primitive green alga. This is a valuable resource for understanding function and evolution of light-harvesting antennas. The evidence is convincing in suggesting that the mechanism found here is distinct from the classical antenna state transitions seen in other organisms studied thus far.
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Reviewer #1 (Public Review):
This is a carefully written manuscript describing the structure of a low-light inducible PSI complex from Ostreococcus tauri. The work expands our knowledge of how photosynthetic systems react to changes in light conditions and shows how this ecologically important green alga utilizes its unique antenna, Lhcp.
In general, I find that the work described in the manuscript is of high quality. The cryoEM maps obtained by the authors clearly show the addition of lhcp trimers to PSI under low light conditions and the distinction between lhcp1 and lhcp2 appears sound together with the identification of the phosphorylation site and its binding in the PSI complex.
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Reviewer #2 (Public Review):
When O. tauri cells are grown under low light, PSI has six classical LHCIs (Lhcas), four on one side of the PSI core and two on another, and three trimers of the "Lhcp" antenna proteins on a third side, thus surrounding the PSI core. Lhcp Trimer 2 consists of 1 Lhcp1 and 2 Lhcp2; Trimers 1 and 3 are solely Lhcp2. Careful examination of carotenoid positions suggested that certain serve as "molecular staples" in holding the three monomers of a trimer together.
The resolution of the structure is high enough to determine the positions of all the chlorophylls and carotenoids and to establish the correct chemical composition. All the proteins determined by LCMS/MS were located and modeled. Of particular interest were the minor polypeptides PsaO, PsaL, PsaH, and PsaK, which are in between the PSI core and the …
Reviewer #2 (Public Review):
When O. tauri cells are grown under low light, PSI has six classical LHCIs (Lhcas), four on one side of the PSI core and two on another, and three trimers of the "Lhcp" antenna proteins on a third side, thus surrounding the PSI core. Lhcp Trimer 2 consists of 1 Lhcp1 and 2 Lhcp2; Trimers 1 and 3 are solely Lhcp2. Careful examination of carotenoid positions suggested that certain serve as "molecular staples" in holding the three monomers of a trimer together.
The resolution of the structure is high enough to determine the positions of all the chlorophylls and carotenoids and to establish the correct chemical composition. All the proteins determined by LCMS/MS were located and modeled. Of particular interest were the minor polypeptides PsaO, PsaL, PsaH, and PsaK, which are in between the PSI core and the trimers, and are involved in binding the trimers to the core.
There is a very detailed comparison of Lhcp trimers with LHC trimers of plants and Chlamydomonas. One of the conclusions is that Chl b requires a Gln rather than a Glu at a certain position, which may otherwise be occupied by a carotenoid. Another is that the increased distance between Lhca5 and 6 may be responsible for the lack of "red" Chls.
This led to a detailed analysis of potential energy transfer pathways in the holocomplex based on distances between pigments and how the trimers interact with the small PSI subunits PsaO, PsaL, PsaH, and PsaK. This section is unfortunately rather tedious to read because the individual monomers in each different trimer are suddenly designated by capital letters. This is not explained properly in the text or in the legend in Fig. 10.
That being said, my overall judgment of the manuscript up to this point is very favorable - I'm impressed with the high quality of the data and the thoroughness of its analysis. It has long been known that when O. tauri cells are grown under high light, the PSI complex does not have the Lhcp trimers, but just has the Lhca antenna. Returning cells to low light induces the synthesis of the Lhcp trimers and the formation of the holocomplex. This could be looked at as a "low-light acclimation"; in nature, the prasinophytes are found in shallow water and hence high light exposure may be their "normal".
The authors asked if this is related to the situation in higher plants and Chlamydomonas where HL induces phosphorylation of certain LHCII trimers which migrate from the appressed membrane regions and associate with PSI. The common factor of these two phenomena is phosphorylation, but the process referred to as a"State transition" operates in the opposite direction to the situation in O. tauri. The authors did a little experiment to see if the disappearance of the complex was reversible in the same time scale as the "state transitions" of Chlamy and plants, by exposing their normal low light cells to 1 hr of HL, then putting them back in LL. They did show that the amount of phosphorylated Lhcp1 decreased significantly in this time frame and then recovered a significant amount when returned to LL. However, using P700 oxidation to assay Lhcp trimers is not very convincing to my eyes.
In my opinion, this does not provide any evidence for a similar mechanism to "state transitions". A real understanding will have to involve studying PSII and its interaction (if any) with Lhcps. There is no indication of where the Lhcps went in 1 hour of HL--maybe they're just at the top of the gradient, minus any phosphate. I would strongly recommend deleting this section altogether.
My conclusion is that a detailed comparison with plant and Chlamydomonas PSIs shows that there are many different ways in which a photosynthetic eukaryote can evolve an effective antenna system. It gives me great pleasure to see a carefully revealed model of another solution to the light-harvesting problem.
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Reviewer #3 (Public Review):
The manuscript by Ishii et al describes the structural characteristics of the Ostreococcus tauri photosystem I (PSI) light-harvesting complexes, mostly under low light conditions. The bulk of the work comes from cryo-EM studies that show changes in the supercomplex structure at low light, and suggest a model where additional light harvesting complexes are recruited to the supercomplex to increase light capturing. Interestingly, the evidence presented suggests that this mechanism is distinct from the classical antenna state transitions seen in other organisms studied thus far.
The structural studies are quite interesting and overall suggest an interesting mechanism for adjusting light harvesting by PSI in this heretofore understudied species. These are exciting findings and a great example of how new …
Reviewer #3 (Public Review):
The manuscript by Ishii et al describes the structural characteristics of the Ostreococcus tauri photosystem I (PSI) light-harvesting complexes, mostly under low light conditions. The bulk of the work comes from cryo-EM studies that show changes in the supercomplex structure at low light, and suggest a model where additional light harvesting complexes are recruited to the supercomplex to increase light capturing. Interestingly, the evidence presented suggests that this mechanism is distinct from the classical antenna state transitions seen in other organisms studied thus far.
The structural studies are quite interesting and overall suggest an interesting mechanism for adjusting light harvesting by PSI in this heretofore understudied species. These are exciting findings and a great example of how new structural approaches can lead to new functional discoveries.
The manuscript is weaker when it comes to connecting these new structures to functions, and definitive cause-effect relationships are not yet provided, nor are any extensive studies on the effects of redox regulation, physiological state, etc. of the putative state transition reported, preventing a more definitive assessment of the mechanisms and physiological importance of the observed changes.
Nevertheless, the results indicate that a different (previously unknown) mode of regulation, or at least alteration, of light capture, is likely to occur in this species, adding substantially to our knowledge of the diversity of photosynthetic responses, and setting up the field to investigate the underlying mechanisms.
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