The Crystal Structure of the Ca2+-ATPase 1 from Listeria monocytogenes reveals a Pump Primed for Dephosphorylation
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###Reviewer #3:
In this manuscript by Hansen et al., the authors describe three low (3.0 to 4.0 Å) resolution crystal structures of Ca2+-ATPase from Listeria, a gram positive bacterium. Two are crystal structures of wild type protein with B eF3- and AlF4- in the absence of Ca2+, thus, likely to represent the E2P ground state and E2~P transition state. The third one is a structure of a G4 mutant, in which 4 Gly residues are inserted into the A-domain -M1 linker, with BeF3- and Ca2+-present in crystallisation, designed to capture the E2P[Ca2+] state. Authors state, however, the three structures are virtually the same and that the E2·BeF3- crystal structure represents a state just prior to ("primed for") dephosphorylation. They also propose that proton counter transport "mechanism" is different from that of SERCA.
As Listeria Ca2+-ATPase …
###Reviewer #3:
In this manuscript by Hansen et al., the authors describe three low (3.0 to 4.0 Å) resolution crystal structures of Ca2+-ATPase from Listeria, a gram positive bacterium. Two are crystal structures of wild type protein with B eF3- and AlF4- in the absence of Ca2+, thus, likely to represent the E2P ground state and E2~P transition state. The third one is a structure of a G4 mutant, in which 4 Gly residues are inserted into the A-domain -M1 linker, with BeF3- and Ca2+-present in crystallisation, designed to capture the E2P[Ca2+] state. Authors state, however, the three structures are virtually the same and that the E2·BeF3- crystal structure represents a state just prior to ("primed for") dephosphorylation. They also propose that proton counter transport "mechanism" is different from that of SERCA.
As Listeria Ca2+-ATPase has been studied by a single molecule FRET, its crystal structures will certainly contribute to our understanding of ion pumping. Furthermore, different from SERCA, Listeria Ca2+-ATPase transports only one Ca2+ per ATP hydrolysed. Therefore, how site I is managed is an interesting topic, although let's not forget the same 1:1 stoichiometry is observed with plasma membrane Ca2+-ATPase (PMCA), for which an EM structure appeared in 2018 (ref. 9). The authors indeed find that the Arg795 side chain extends into binding site I. This part is solid and a more elaborate (and interesting) discussion could be made than what is currently described.
Another solid finding is that the two E2·BeF3- crystal structures are similar to the E2·AlF4- crystal structure, although how similar is unclear as a structural superimposition reporting an RMSD is not provided and the presented figure makes it difficult to judge directly; the structures are viewed from almost one direction, which makes it infeasible to discern the differences in M1 and M2 and in the horizontal rotation of the A-domain. Two or three structures are superimposed, but with cylinders and again viewed from only one direction. As the authors designate that the structures represent H+ occluded states, it is important to clearly show the extracellular gate is really closed to H+ (not only to Ca2+ as well). For completeness, they should also examine the effect of crystal packing on the A-domain position.
With regard to the point that the E2·BeF3- structure is "primed for dephosphorylation", only Fig. 2 is shown, in which differences appear to be the path of the TGES loop and the orientation of the Glu167/183 side chain. Their atomic models show that there is plenty of space for the Glu167 sidechain to take an orientation similar to that of Glu183 in SERCA. The authors should, however, provide an omit annealed Fo-Fc map for the Glu167 side chain and explain why that is the preferred and only orientation. If a Glu side chain is free to move, it could adopt in less than a nanosecond a different orientation. If it does, then the difference in the orientation of the Glu side chain does not sufficiently explain "the rapid dephosphorylation observed in single-molecule studies". The authors place further emphasis on proton occlusion and countertransport. However, this part of the manuscript is more speculative and, as detailed later should, at least, be entirely moved to the Discussion section.
As mentioned, the authors place a larger emphasis on proton countertransport. Here a number of issues show up. First of all, I think they have frequently used the term "occlusion" improperly. From my understanding, occlusion of a site (or ion) means that the site (or ion) is inaccessible from either side of the membrane. This means more than closure of the gates, as the two gates have to stay closed for a substantial length of time (i.e. locked). It is experimentally well established with SERCA that Ca2+ ions are occluded in E1P species. It can be shown that the lumenal gate is closed for Ca2+ in the E2 state. However, that does not necessarily mean that the gate for H+ is also closed. As far as this reviewer knows, nobody has actually demonstrated that H+ is occluded, even in the E2 state of SERCA.
Furthermore, the authors presume that protons enter the binding sites through a different pathway from that used for Ca2+ release, citing ref 26. However, if it does, can closure of the gate for Ca2+ really mean closure for the gate for H+? This seems a contradictorily statement as the authors designate that the E2·BeF3- state in Listeria Ca2+-ATPase as a proton occluded state (p.12). Apparent closure of the gate for Ca2+ on the extracellular side in a crystal structure seems insufficient for such a statement. One must keep in mind that a crystal structure merely provides a possible conformation in that particular state. It may not, however, represent the most populated conformation for that state. It is equally plausible that the E2·BeF3- complex takes a closed conformation for only a small fraction of the time. At this resolution it is simply not possible to determine if H+ occupies the binding site in the crystal structure. Furthermore, although it may be possible to show the gate is closed for Ca2+, it would be very difficult to show the gate is closed for H+. Thus, more experimental evidence is required to support that the structure represents a H+ occluded state.
The authors write in the Abstract "Structures with BeF3- mimicking a phosphoenzyme state reveal a closed state, which is intermediate of the outward-open E2P and the proton-occluded E2-P* conformations known for SERCA". In essence this statement is fine, although what "closed" means is still unclear to me. In Figure 1, the authors state that "LMCA1 structures adopt proton-occluded E2 states". This statement is a bit misleading, because, in E2·BeF3-, the lumenal (extracellular) gate can in fact be opened and closed, at least with SERCA. As the authors recognize (p.14), the BeF3- complex of SERCA can be crystallised in two conformations, one with the lumenal gate is closed (with thapsigargin) and the other with the gate open; yet, they write "In SERCA, the calcium-free BeF3 -complex adopts an outward-open E2P state,..." p.8). This is for lumenal (extracellular) Ca2+, not for H+. Further evidence is required to establish that the extracellular gate of LMCA1 is fixed in a closed position for H+ in E2·BeF3-. Again more experimental evidence is required to support that E2·BeF3- is a H+ occluded state.
The authors write that "SERCA has two proposed proton pathways: a luminal entry pathway [26] and a C-terminal cytosolic release pathway [27] (p. 9). One has to be careful here, as the luminal entry pathway has not been experimentally confirmed in SERCA. The authors write that "The luminal proton pathway has been mapped to a narrow water channel …” [26]. But since the pathway is not confirmed in SERCA I don't think it can be used to justify that the corresponding part of LMCA1 is mainly hydrophobic and that protons cannot enter through this pathway.
The description on the exit pathway for H+ also needs clarification. They describe (p. 10; first line) "In SERCA it consists of a hydrated cavity...[27]. ... M7 in LMCA1 further blocks the pathway ... and LMCA1 therefore does not appear to have a C-terminal cytosolic pathway either" and rationalize that "This may explain why no distinct proton pathways are required in LMCA1". I think it should be made clearer that this is a proposal rather than an established fact.
As H+ release takes place in the E2 to E1 transition the authors state that the E2·BeF3- structure of LMCA1 is different from that of SERCA. However, I don't think they can confidently make such statements without E1 and E2 structures of LMCA1. Furthermore, these descriptions (discussion) should not be in the "Results" section. As they conclude that LMCA1 use the Ca2+ release pathway, which is assumed to be the same as that in SERCA (even though no Ca2+ release pathway is visualised in their crystal structures), for H+ entry, why does SERCA not use the same pathway? I think experimental evidence is required for a proposal that H+ binds to E309 from the cytoplasmic side.
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###Reviewer #2:
The manuscript by Hansen et al. presents three new structures of LMCA1, Ca2+-ATPase 1 from Listeria monocytogenes. They determined structures with BeF and AlF, and a Gly4 linker form of LMCA1 in complex with BeF. This latter structure is at 3 Å resolution and was very challenging. The other two structures are at low 4 Å resolution. These structures are a follow up to an excellent single-molecule fluorescence study of the same enzyme. The structures support the main conclusion of that work that LMCA1 more rapidly progresses through the dephosphorylation step of the reaction cycle. The manuscript is well written, the structures and findings are interesting and make a significant contribution, and the work seems ideally suited for this journal. There are no substantive concerns with the manuscript. Overall the R factors are …
###Reviewer #2:
The manuscript by Hansen et al. presents three new structures of LMCA1, Ca2+-ATPase 1 from Listeria monocytogenes. They determined structures with BeF and AlF, and a Gly4 linker form of LMCA1 in complex with BeF. This latter structure is at 3 Å resolution and was very challenging. The other two structures are at low 4 Å resolution. These structures are a follow up to an excellent single-molecule fluorescence study of the same enzyme. The structures support the main conclusion of that work that LMCA1 more rapidly progresses through the dephosphorylation step of the reaction cycle. The manuscript is well written, the structures and findings are interesting and make a significant contribution, and the work seems ideally suited for this journal. There are no substantive concerns with the manuscript. Overall the R factors are high for the structures, particularly the 3 Å resolution structure for which they should be lower. However, the authors offer a reasonable explanation for this in the supplemental information provided.
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###Reviewer #1:
Structural comparison is an important tool to understanding how proteins function at the molecular level. The mechanistic premise of obtaining LMCA1 structures from the gram-positive bacteria Listeria monocytogenes was to understand how Ca2+ pumps have different Ca2+ stoichometies to the mammalian SERCA and how they are proton coupled differently. Per molecule of ATP hydrolyzed, SERCA exports two Ca2+ ions in exchange for 2 or 3 protons, whereas LMCA1 exports a single Ca2+ and perhaps 1 proton in return.
The paper describes two intermediate states of LMCA1 and from my understanding a mechanism is proposed based on structural differences in ionisable groups at the Ca2+ binding site, in particular the positioning of Arginine 795 that in SERCA is a glutamate. Since a previous crystal structure of LMCA1 was determined the …
###Reviewer #1:
Structural comparison is an important tool to understanding how proteins function at the molecular level. The mechanistic premise of obtaining LMCA1 structures from the gram-positive bacteria Listeria monocytogenes was to understand how Ca2+ pumps have different Ca2+ stoichometies to the mammalian SERCA and how they are proton coupled differently. Per molecule of ATP hydrolyzed, SERCA exports two Ca2+ ions in exchange for 2 or 3 protons, whereas LMCA1 exports a single Ca2+ and perhaps 1 proton in return.
The paper describes two intermediate states of LMCA1 and from my understanding a mechanism is proposed based on structural differences in ionisable groups at the Ca2+ binding site, in particular the positioning of Arginine 795 that in SERCA is a glutamate. Since a previous crystal structure of LMCA1 was determined the new mechanistic insights rely heavily on the details achieved by the improved resolution. While this is technically an important achievement, just the assignment of side-chains in the current structures is not sufficient to reach the mechanistic conclusions reached and, as such, the current paper is unfortunately too preliminary. Proton-coupling pathways are mechanistically difficult to detangle and require extensive experimentation, such as ITC, mutagenesis and transport measurements as well as computational approaches. Indeed, ion or proton coupling pathways that alter energetics are rarely just the result from differences in a few residues. For example, glucose (GLUT) transporters are passive sugar transporters, whilst the bacterial counterparts are proton coupled. The proton coupling in the bacterial proteins is due to single aspartic acid residue in TM1. Whilst one can convert the bacterial sugar transporters to be no longer proton coupled by the mutagenesis of this TM1 residue to asparagine, you cannot make GLUT transporters proton coupled by mutating the corresponding asparagine residue to aspartic acid.
One would have liked the authors to biochemically demonstrate how they could evolve LMCA1 to function similar to SERCA. This would have broader implications in our understanding of how biological systems can evolve substrate coupling and energetics.
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##Preprint Review
This preprint was reviewed using eLife’s Preprint Review service, which provides public peer reviews of manuscripts posted on bioRxiv for the benefit of the authors, readers, potential readers, and others interested in our assessment of the work. This review applies only to version 1 of the manuscript.
###Summary:
We all agreed that the LMCA1 complex structures are an important step forward for providing a structural framework for piecing together an ion pumping model to follow on from the previous smFRET studies. Nonetheless, two of the reviewers think that the mechanistic conclusions reached - based solely on crystal structures - require further validation. In particular, further experimental work (and likely computational) is required to i) confirm the hitherto designated crystallographic "states" and ii) to begin …
##Preprint Review
This preprint was reviewed using eLife’s Preprint Review service, which provides public peer reviews of manuscripts posted on bioRxiv for the benefit of the authors, readers, potential readers, and others interested in our assessment of the work. This review applies only to version 1 of the manuscript.
###Summary:
We all agreed that the LMCA1 complex structures are an important step forward for providing a structural framework for piecing together an ion pumping model to follow on from the previous smFRET studies. Nonetheless, two of the reviewers think that the mechanistic conclusions reached - based solely on crystal structures - require further validation. In particular, further experimental work (and likely computational) is required to i) confirm the hitherto designated crystallographic "states" and ii) to begin clarify how LMCA1 and SERCA have different Ca2+:H+ stoichiometries as there are other, plausible models.
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