State-specific morphological deformations of the lipid bilayer explain mechanosensitive gating of MscS ion channels
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The manuscript reports a new structure of the small conductance mechanosensitive channel MscS from E. coli in the open state, together with coarse-grained and atomistic molecular dynamics simulations of MscS and the related channel MSL1 of plant mitochondria in closed and open states. The important finding is that the surrounding lipid bilayer is severely distorted in the closed state only, with the protein inducing high curvature in the inner leaflet due to the membrane protruding into the cytoplasm. The authors argue convincingly that the role of membrane tension is to increase the energy of the protein-membrane system in this closed state compared to the relatively flat-membrane open state, in contrast to the previous proposal that tension-induced gating is driven by expansion of the in-plane area of the protein. The finding may be relevant for the understanding of ion channel mechano-sensation more generally, including of the PIEZO1 channel.
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Abstract
The force-from-lipids hypothesis of cellular mechanosensation posits that membrane channels open and close in response to changes in the physical state of the lipid bilayer, induced for example by lateral tension. Here, we investigate the molecular basis for this transduction mechanism by studying the mechanosensitive ion channel MscS from Escherichia coli and its eukaryotic homolog MSL1 from Arabidopsis thaliana. First, we use single-particle cryo-electron microscopy to determine the structure of a novel open conformation of wild-type MscS, stabilized in a thinned lipid nanodisc. Compared with the closed state, the structure shows a reconfiguration of helices TM1, TM2, and TM3a, and widening of the central pore. Based on these structures, we examined how the morphology of the membrane is altered upon gating, using molecular dynamics simulations. The simulations reveal that closed-state MscS causes drastic protrusions in the inner leaflet of the lipid bilayer, both in the absence and presence of lateral tension, and for different lipid compositions. These deformations arise to provide adequate solvation to hydrophobic crevices under the TM1-TM2 hairpin, and clearly reflect a high-energy conformation for the membrane, particularly under tension. Strikingly, these protrusions are largely eradicated upon channel opening. An analogous computational study of open and closed MSL1 recapitulates these findings. The gating equilibrium of MscS channels thus appears to be dictated by opposing conformational preferences, namely those of the lipid membrane and of the protein structure. We propose a membrane deformation model of mechanosensation, which posits that tension shifts the gating equilibrium towards the conductive state not because it alters the mode in which channel and lipids interact, but because it increases the energetic cost of the morphological perturbations in the membrane required by the closed state.
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Author Response
Reviewer #1 (Public Review):
Major
The observations on the hook lipids are critical and should be documented better. Based on previous work, it had been proposed that the hook lipids are associated with the inner leaflet and that they leave upon (partial) channel opening. In contrast, the present MD simulations indicate these lipids are associated with the outer leaflet and that their association to the channel persists on opening. These critical observations need to be documented better.
i) Do the authors observe hook lipids in the cryoEM structure of the open channel? If yes, data should be shown. If no, then the discrepancy between MD and EM should be explicitly addressed.
The resolution of the original cryo-EM density map of MscS in PC14 nanodiscs was not sufficient to reveal clear densities for the “hook” …
Author Response
Reviewer #1 (Public Review):
Major
The observations on the hook lipids are critical and should be documented better. Based on previous work, it had been proposed that the hook lipids are associated with the inner leaflet and that they leave upon (partial) channel opening. In contrast, the present MD simulations indicate these lipids are associated with the outer leaflet and that their association to the channel persists on opening. These critical observations need to be documented better.
i) Do the authors observe hook lipids in the cryoEM structure of the open channel? If yes, data should be shown. If no, then the discrepancy between MD and EM should be explicitly addressed.
The resolution of the original cryo-EM density map of MscS in PC14 nanodiscs was not sufficient to reveal clear densities for the “hook” lipids. However, through further analysis we have now obtained an improved map to 3.1-Å resolution that offers new insights into this question – see Figure 2 – Figure Supplement 1. The new map confirms all the characteristics previously determined for the open conformation: same helical movements resulting in a similar opening of the pore, and the absence of lipids blocking it, all indicating a conducting conformation. In addition, the new map reveals a series of densities consistent with the dimensions of a phospholipid headgroup near the C-terminus of TM2 (facing the outside), filling a small cavity in-between adjacent TM1 helices. This position is precisely that occupied by the hook lipids in the close MscS structure obtained in PC18 nanodiscs. A headgroup residing in this density would also be well positioned to interact directly with Arg88, a key element in the hook-lipid interaction site, whose mutation leads to a strong loss-of-function phenotype (Reddy et al, 2019). These consistencies notwithstanding, we want to be cautious in this interpretation; these densities are of the same intensity as and blend with that of the nanodisc lipid, and so it is not possible to discern the acyl chains, which were more clearly resolved in the closed state. Therefore, while the new densities are consistent with a model in which the hook lipids are a structural feature of both closed and open states, as indicated by the simulation data, additional experimental data (or further improvements in the map) will be needed for an unequivocal assignment.
ii) Please show the comparison of the position and coordination of the hook lipids in MD simulations and in the closed (and/or open) structures.
See new Figure 2 – Figure Supplement 1 in comparison with Figure 5 and new Figure 4 – Figure Supplement 1.
iii) The authors acknowledge that the volume of the cavity where the hook lipids are located decreases on channel opening. How does this not affect the association of the hook lipids with the protein?
There appears to be a misunderstanding. The hydrophobic cavities that explain the membrane protrusions discussed in the manuscript are not where the “hook” lipids are observed – we hope to have fully clarified this in the new Figure 4 – Figure Supplement 1. These hydrophobic cavities are underneath each of the TM1-TM2 hairpins, on the cytoplasmic side of the transmembrane domain of the channel; accordingly the protrusions are formed in and exchange lipids with the inner leaflet of the bilayer. Upon reorientation of the TM1-TM2 hairpin, i.e. in the open state, these cavities indeed become smaller but more importantly, they become embedded in the membrane – and hence the protrusions are largely eliminated – see Figure 8 – Figure Supplement 1. The sites where the “hook” lipids observed are elsewhere in the structure, towards the outer entrance of the pore; these lipids originate in the outer leaflet. As discussed in the manuscript, the geometry of these sites in the experimentally determined structures of closed and open states is largely invariant; consistent with that observation, the occupancy of the “hook” lipid sites is also similar when simulations of closed and open states are compared. At this point, therefore, it is unclear whether the “hook” lipids are involved in tension sensing; it is plausible that their primary role is structural (for both open and closed states).
iv) Past work revealed several lipids in MscS structures near these cavities besides the hook lipids, and their ordered dissociation from the channel was proposed to be important for gating. Do the simulations show lipids in these cavities?
Yes. Previous structural studies identified individual lipid densities under the TM2-TM3 hairpins. Our data show these lipids are not isolated sites but integrated into a larger morphological feature.
v) Does the occupancy of the hook lipids in MD simulations change between the open and closed conformations? This should be analyzed.
Please see our answer to point (iii).
vi) Is the occupancy of other lipids in the nearby cavity altered upon channel opening?
Please see our answer to point (iii).
vii) Is the exchange of lipids near Ile150 affected by the conformational change?
Please see our answer to point (iii).
I am a bit confused by the claim that "The comparison clearly highlights the reduction in the width of the transmembrane span of the channel upon opening, and how this changed is well matched by the thickness of the corresponding lipid nanodiscs (approximately from 38 to 23 Å)."
This statement has been clarified. Our intention was to state is that in the open conformation stabilized by PC14, the increased tilt of the TM1-TM2 hairpins towards the midplane of the bilayer leads to a reduction in the hydrophobic width of the protein parallel to the membrane normal. (This reduction is clearly illustrated by our simulation data – see Figure 8 – Figure Supplement 1.) This change correlates with the reduction in thickness from the PC18 to the PC14 nanodiscs, explaining why the latter stabilizes the open state while the former stabilizes the closed state.
i. How was the nanodisc membrane thickness determined? This should be described.
ii. I do not see a ~15A change in the vertical length of the channel protein or of the nanodisc. While the panels in Fig.2 clearly show a vertical compression of the membrane, it appears that the ~15 A claim might be overstated. Adding a panel with measurements would be helpful to quantify this claim. If this is difficult on the membrane, maybe measurements could be performed on the protein.
The reviewer is correct. The original estimate, based on a cursory measurement of distances between two sets of protein atoms seemingly aligned with the water-lipid interface, turned out to be less accurate than expected. A better and more reproducible estimate has now been derived from the OPM database (https://opm.phar.umich.edu/). Using V3 of the database the closed-state is 32.6 Å and the open is 25.8 Å. The change is 6.8 Å. This is the value we now report.
iii. What happens to the N-terminal cap structure in the open state? What are the rearrangements that allow the extracellular ends of the TM1 to disassemble the cap.
In the open conformation part of the N-terminal cap appears to re-folds into TM1 extending its length as this helix tilts to embed itself at the membrane/water interface. The detailed side-chain structure of this domain is not clearly resolved but the C trace can be approximately inferred.
The data shown in Fig. 6 is cryptic and should be explained better in the main text. As it stands there is a cursory mention in pg. 12 and not much else.
i. It would be helpful if the authors showed the position of Ile150 in the structure.
Please see the revised version of Figure 6 and the corresponding caption.
ii. Does the total number of lipids in proximity of Ile150 change over time? Or the fold change represents ~1:1 exchange of lipids in the pocket?
Please see the revised version of Figure 6. The total number of lipids in proximity of Ile150 in closed MscS, i.e. the number of lipids filling the cavities under the TM1-TM2 hairpins, is approximately constant at any given timepoint; in both the CG and AA representations, we find about 4 lipids for each of the 7 subunits. However, these are not always same lipid molecules. For example, in a period of 20 s of CG simulation, 40 different lipid molecules were observed to transiently reside in each of protrusions. We trust that this new format of the figure will be more intuitive than the original version.
iii. I am confused by the difference in the maximum possible fold-change in unique lipids, does this reflect the difference in total number of lipids in each leaflet in each system? If so, I am a bit confused as to why there is a ~30% difference in the AA simulations whereas the values are nearly identical for the CG one.
Please see the revised version of Figure 6. For clarity we have eliminated the concept of fold-change (and maximum fold-change, relative to the total number of lipids in each leaflet), and now simply quantify the number of lipids in proximity to each site.
iv. Is it possible to quantify the residence time of the lipids in the pocket of each subunit?
Please see the revised version of Figure 6. From the data presented in panels C and D, it can be deduced that a full turnover takes 2-4 microseconds in the CG representation of the system; in the AA representation, we observe a turnover of about 75% in 10 microseconds, on average over all subunits.
The authors state on Pg. 21 "Nevertheless, we question the prevailing view that density signals of this kind are evidence of regulatory lipid binding sites; that is, we do not concur with the assumption that lipids regulate the gating equilibrium of MscS just like an agonist or antagonist would for a ligand-gated receptor-channel." I am a bit confused by this statement. In principle, binding and unbinding of modulatory ligands can happen on relatively fast time scales, so the observation that in MD simulations lipids exchange on a faster time scale than that of channel gating is not sufficient to make this inference. Indeed, there is ample evidence from other channels (i.e. Trp channels, HCN channels etc) where visualization of similar signals led to the identification of modulatory lipid binding sites. Thus, while I do not necessarily disagree with the authors, I would encourage them to tone down the general portion of the statement.
The statement has been rephrased as “Nevertheless, our data puts into question the prevailing view that density signals of this kind necessarily reflect long-lasting lipid immobilization, as one might expect for an agonist or antagonist of a ligand-gated receptor-channel.”
Reviewer #2 (Public Review):
- Are the structures stable in the membrane also without the weak restraints on the dihedral angles? Continuing at least one of the atomistic simulations without restraints for about 1 microsecond in a tension-free membrane would address a possible concern that the severe membrane distortion could go away by a more extensive relaxation of the channel structure.
Please see our responses to the Editor.
- Does the observed effect occur also in membranes with physiologically relevant PE lipids? Performing a simulation with a lipid mix closer to that in E. coli (and thus high in PE) would address a possible concern that the observed effect is not physiologically relevant.
Please see our responses to the Editor.
- Please include a figure showing that the lipid positions in the MD simulations match the lipid densities in the cryo-EM maps.
Rather than re-rendering images already published, or generating new images that might not adequately represent the authors’ interpretation of their own data, we have to opted to specify the specific figures in previous studies where lipid densities under the TM1-TM2 hairpin have been clearly highlighted, for both MscS and MSL1. Specifically, for MscS, see Figure 4 in Zhang et al. [Ref. 16] and Figures 3-5 and Supplementary Figure 11 in Flegler et al [Ref. 15]; for MSL1, see Supplementary Figure 8 in Deng et al [Ref. 18].
- Is the reported mobility of helices TM2-TM3 of MSL1, as deduced from a comparison of different cryo-EM structures (ref 18), sufficient to impact the lipid organisation?
In the naming convention used in Ref. 18, TM3 in MSL1 corresponds to TM1 in MscS. Different channels in this family feature different N-terminal domains preceding TM1. MscS features a short helix that has been referred as the N-cap, which lies on the membrane surface. MSL1 from Arabidopsis however features two additional TM helices – which confusingly Ref. 18 refers to as TM1 and TM2, while the key hairpin adjacent to the pore domain is referred to as TM3-TM4. Neither TM1 or TM2 in MSL1 are clearly resolved, presumably because they are indeed mobile, but they are in any case peripheral and therefore not likely to be critically influential for the morphological changes in the membrane that we discuss in the manuscript. Indeed, our simulations of MSL1 do not, by design, include those two N-terminal helices – in part because, as mentioned, they are poorly resolved, but also so that the results can be directly contrasted with MscS. Nevertheless, both channels show very similar deformations in the membrane for the closed state, and an elimination of these deformations in the open state.
- Did the initial lipid configuration in atomistic MD simulations already contain the deformations of the inner leaflet, or did these form spontaneously both in coarse-grained and atomistic simulations?
Please see our responses to the Editor.
- Did the earlier MD simulations of the closed-state structure 6PWN of MscL give any indications on the membrane deformation?
The simulation reported in Reddy et al alongside the structure of closed MscS in PC18 [Ref. 17] did not reveal the kind of deformations observed in this study, most probably due to insufficient equilibration time. However, that simulation did reveal a translational displacement of the channel relative to what had been previously assumed to be the transmembrane span. In retrospect, it seems clear that the observed translation was driven by the strong hydrophobic mismatch between the protein surface and the flat lipid bilayer; the membrane deformations we now observe represent the adaptation that ultimately minimizes that mismatch.
- Are there distinct interactions between the headgroups of distorted inner-leaflet lipids with charged amino acids? If so, are these amino acids conserved?
Please see the new Figure 4 – Figure Supplement 1. As discussed in the manuscript, the interior of the cavities formed under the TM1-TM2 hairpins, and flanked by TM3a and TM3b, are lined almost entirely by hydrophobic residues. Charged and polar amino-acids are only observed on the outer face of the TM1-TM2 hairpin and are primarily in contact water.
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eLife assessment
The manuscript reports a new structure of the small conductance mechanosensitive channel MscS from E. coli in the open state, together with coarse-grained and atomistic molecular dynamics simulations of MscS and the related channel MSL1 of plant mitochondria in closed and open states. The important finding is that the surrounding lipid bilayer is severely distorted in the closed state only, with the protein inducing high curvature in the inner leaflet due to the membrane protruding into the cytoplasm. The authors argue convincingly that the role of membrane tension is to increase the energy of the protein-membrane system in this closed state compared to the relatively flat-membrane open state, in contrast to the previous proposal that tension-induced gating is driven by expansion of the in-plane area of the protein. The …
eLife assessment
The manuscript reports a new structure of the small conductance mechanosensitive channel MscS from E. coli in the open state, together with coarse-grained and atomistic molecular dynamics simulations of MscS and the related channel MSL1 of plant mitochondria in closed and open states. The important finding is that the surrounding lipid bilayer is severely distorted in the closed state only, with the protein inducing high curvature in the inner leaflet due to the membrane protruding into the cytoplasm. The authors argue convincingly that the role of membrane tension is to increase the energy of the protein-membrane system in this closed state compared to the relatively flat-membrane open state, in contrast to the previous proposal that tension-induced gating is driven by expansion of the in-plane area of the protein. The finding may be relevant for the understanding of ion channel mechano-sensation more generally, including of the PIEZO1 channel.
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Reviewer #1 (Public Review):
This interesting manuscript from the Perozo and Faraldo-Gomez labs investigates the molecular mechanisms underlying the activation of the mechanosensitive ion channel MscS. The authors use a clever combination of cryoEM, coarse-grained (CG) and all-atom (AA) molecular dynamics simulations to determine the first (putatively) open conformation of the WT MscS channel and to show that this channel induces profound deformations of the membrane in the closed but not in the open state. Strikingly, MD simulations reveal that, contrary to what was previously assumed, lipids occupying cavities near the closed pore (hook lipids) come from the outer rather than inner leaflets. On pore opening, the membrane adopts a more relaxed conformation where the lipids contacting the protein are in less strained and tilted …
Reviewer #1 (Public Review):
This interesting manuscript from the Perozo and Faraldo-Gomez labs investigates the molecular mechanisms underlying the activation of the mechanosensitive ion channel MscS. The authors use a clever combination of cryoEM, coarse-grained (CG) and all-atom (AA) molecular dynamics simulations to determine the first (putatively) open conformation of the WT MscS channel and to show that this channel induces profound deformations of the membrane in the closed but not in the open state. Strikingly, MD simulations reveal that, contrary to what was previously assumed, lipids occupying cavities near the closed pore (hook lipids) come from the outer rather than inner leaflets. On pore opening, the membrane adopts a more relaxed conformation where the lipids contacting the protein are in less strained and tilted conformations. The authors thus propose a mechanism for sensing tension where the equilibrium between the open and closed conformations of the channel is dictated by differences in the membrane morphology in the two states rather than by the association and dissociation of individual lipids with the protein.
Major
The observations on the hook lipids are critical and should be documented better. Based on previous work, it had been proposed that the hook lipids are associated with the inner leaflet and that they leave upon (partial) channel opening. In contrast, the present MD simulations indicate these lipids are associated with the outer leaflet and that their association to the channel persists on opening. These critical observations need to be documented better.
i. Do the authors observe hook lipids in the cryoEM structure of the open channel? If yes, data should be shown. If no, then the discrepancy between MD and EM should be explicitly addressed.
ii. Please show the comparison of the position and coordination of the hook lipids in MD simulations and in the closed (and/or open) structures.
iii. The authors acknowledge that the volume of the cavity where the hook lipids are located decreases on channel opening. How does this not affect the association of the hook lipids with the protein?
iv. Past work revealed several lipids in MscS structures near these cavities besides the hook lipids, and their ordered dissociation from the channel was proposed to be important for gating. Do the simulations show lipids in these cavities?
v. Does the occupancy of the hook lipids in MD simulations change between the open and closed conformations? This should be analyzed.
vi. Is the occupancy of other lipids in the nearby cavity altered upon channel opening?
vii. Is the exchange of lipids near Ile150 affected by the conformational change?I am a bit confused by the claim that "The comparison clearly highlights the reduction in the width of the transmembrane span of the channel upon opening, and how this changed is well matched by the thickness of the corresponding lipid nanodiscs (approximately from 38 to 23 Å)."
i. How was the nanodisc membrane thickness determined? This should be described.
ii. I do not see a ~15A change in the vertical length of the channel protein or of the nanodisc. While the panels in Fig.2 clearly show a vertical compression of the membrane, it appears that the ~15 A claim might be overstated. Adding a panel with measurements would be helpful to quantify this claim. If this is difficult on the membrane, maybe measurements could be performed on the protein.
iii. What happens to the N-terminal cap structure in the open state? What are the rearrangements that allow the extracellular ends of the TM1 to disassemble the cap.The data shown in Fig. 6 is cryptic and should be explained better in the main text. As it stands there is a cursory mention in pg. 12 and not much else.
i. It would be helpful if the authors showed the position of Ile150 in the structure.
ii. Does the total number of lipids in proximity of Ile150 change over time? Or the fold change represents ~1:1 exchange of lipids in the pocket?
iii. I am confused by the difference in the maximum possible fold-change in unique lipids, does this reflect the difference in total number of lipids in each leaflet in each system? If so, I am a bit confused as to why there is a ~30% difference in the AA simulations whereas the values are nearly identical for the CG one.
iv. Is it possible to quantify the residence time of the lipids in the pocket of each subunit?The authors state on Pg. 21 "Nevertheless, we question the prevailing view that density signals of this kind are evidence of regulatory lipid binding sites; that is, we do not concur with the assumption that lipids regulate the gating equilibrium of MscS just like an agonist or antagonist would for a ligand-gated receptor-channel." I am a bit confused by this statement. In principle, binding and unbinding of modulatory ligands can happen on relatively fast time scales, so the observation that in MD simulations lipids exchange on a faster time scale than that of channel gating is not sufficient to make this inference. Indeed, there is ample evidence from other channels (i.e. Trp channels, HCN channels etc) where visualization of similar signals led to the identification of modulatory lipid binding sites. Thus, while I do not necessarily disagree with the authors, I would encourage them to tone down the general portion of the statement.
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Reviewer #2 (Public Review):
The manuscript by Park et al. reports a new structure of the mechanosensitive channel MscS of E. coli in the open state and the results of extensive coarse grained and atomistic molecular dynamics (MD) simulations of MscS and the related channel MSL1 of plant mitochondria in presumed closed and open states. The major new finding is that in the closed state, the lipid bilayer contacting the channel is severely distorted. In the open state, this distortion is not present. The MD simulations forming the basis of this finding have been carefully executed and the finding is interesting and relevant for the understanding of channel mechanosensation. The MD simulations are ideally suited to probe the lipid interactions of the channel in a state-dependent manner and to identify possible membrane distortions. However …
Reviewer #2 (Public Review):
The manuscript by Park et al. reports a new structure of the mechanosensitive channel MscS of E. coli in the open state and the results of extensive coarse grained and atomistic molecular dynamics (MD) simulations of MscS and the related channel MSL1 of plant mitochondria in presumed closed and open states. The major new finding is that in the closed state, the lipid bilayer contacting the channel is severely distorted. In the open state, this distortion is not present. The MD simulations forming the basis of this finding have been carefully executed and the finding is interesting and relevant for the understanding of channel mechanosensation. The MD simulations are ideally suited to probe the lipid interactions of the channel in a state-dependent manner and to identify possible membrane distortions. However there are some issues that should be addressed.
Are the structures stable in the membrane also without the weak restraints on the dihedral angles? Continuing at least one of the atomistic simulations without restraints for about 1 microsecond in a tension-free membrane would address a possible concern that the severe membrane distortion could go away by a more extensive relaxation of the channel structure.
Does the observed effect occur also in membranes with physiologically relevant PE lipids? Performing a simulation with a lipid mix closer to that in E. coli (and thus high in PE) would address a possible concern that the observed effect is not physiologically relevant.
Please include a figure showing that the lipid positions in the MD simulations match the lipid densities in the cryo-EM maps.
Is the reported mobility of helices TM2-TM3 of MSL1, as deduced from a comparison of different cryo-EM structures (ref 18), sufficient to impact the lipid organisation?
Did the initial lipid configuration in atomistic MD simulations already contain the deformations of the inner leaflet, or did these form spontaneously both in coarse-grained and atomistic simulations?
Did the earlier MD simulations of the closed-state structure 6PWN of MscL give any indications on the membrane deformation?
Are there distinct interactions between the headgroups of distorted inner-leaflet lipids with charged amino acids? If so, are these amino acids conserved?
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Reviewer #3 (Public Review):
This paper combines experimental structures with careful molecular dynamics to address a crucially important topic in cellular biology - how are mechanosensitive ion channels gated by the membrane? There are many flavors of mechanosensitive proteins, and here the authors study MscS from e. coli and the eukaryotic homolog MSL1 from Arabidopsis. The key finding is that the closed states of both channels induce high curvature in the inner leaflet due to the membrane protruding into the cytoplasm to lipidate exposed hydrophobic patches on the protein. The open state structures exhibit far less membrane deformation. Moreover, comparing the open and closed state structures reveals that the membrane-protein surface area is not significantly different in the two states - hence all of the mathematical models to date …
Reviewer #3 (Public Review):
This paper combines experimental structures with careful molecular dynamics to address a crucially important topic in cellular biology - how are mechanosensitive ion channels gated by the membrane? There are many flavors of mechanosensitive proteins, and here the authors study MscS from e. coli and the eukaryotic homolog MSL1 from Arabidopsis. The key finding is that the closed states of both channels induce high curvature in the inner leaflet due to the membrane protruding into the cytoplasm to lipidate exposed hydrophobic patches on the protein. The open state structures exhibit far less membrane deformation. Moreover, comparing the open and closed state structures reveals that the membrane-protein surface area is not significantly different in the two states - hence all of the mathematical models to date (and many experimental models too) that posit that tension-induced gating is driven by expansion of the in-plane area of the protein must be revised. Instead, the authors convincingly argue that the role of tension is to increase the energy of the protein-membrane system in the closed state (with its large membrane deformations) compared to the flat-membrane open state. Forgive me for not going on more about the structures that have been solved here, and how they are likely more representative of the native open state than previously solved structures - I agree with the authors' assertions, and they represent a major step forward in elucidating the full gating transition in both bacterial and eukaryotic systems. This is an important discovery, and it would have been impossible without the structure and simulation coming together. Future work attempting to quantify the energy of the membrane deformations, protein free energy difference between the channels in open and closed states, and the role of tension will be essential but outside the scope of what the authors were trying to do here.
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