Reviewer #2 (Public review):
Major concern 1: The manuscript does not clearly establish a bona fide GS filament state.
The authors repeatedly refer to GS "filaments," but the data presented appear to support primarily a di-decameric assembly rather than a well-defined filamentous polymer.
A two-decamer reconstruction can define a putative inter-decamer interface, but it cannot by itself demonstrate propagation of a repeating filament geometry. To establish a bona fide filament, the authors should provide evidence for a reproducible one-dimensional assembly, such as at least three consecutive repeating units or equivalent quantitative evidence that the same inter-decamer transform propagates along an assembly axis.
In the current manuscript, many of the supporting 2D classifications appear to contain at most two adjacent GS decamers. This is particularly evident in the time-resolved cryo-EM datasets shown in Supplementary Figures 9-10, where I do not see convincing 2D classes corresponding to filaments. The same concern applies to other datasets, including Supplementary Figures 2, 6, and 11, where the apparent assemblies are primarily two-decamer particles.
Moreover, many of the selected "filament" classes show only one well-resolved GS decamer, while the neighboring decamer density is blurred. This suggests substantial variability in the relative position and/or orientation of adjacent decamers. Such heterogeneity is difficult to reconcile with a stable repeating filament geometry.
Therefore, the authors should explicitly define what they mean by "filament." If their evidence supports only a di-decameric or short oligomeric assembly, the terminology should be changed accordingly throughout the manuscript.
Symmetry concern
Given the low quality and heterogeneity of the 2D classifications for the putative "filament" classes, the use of D5 symmetry requires stronger justification. The current reconstructions primarily show the result after applying D5 symmetry to a two-decamer assembly. The authors should show reconstructions of the same particle sets processed under C1, C5, and D5 symmetry, and explain why D5 symmetry is justified.
This is particularly important because the claimed interface density and ligand interpretation are sensitive to symmetry averaging. Without showing how the reconstruction behaves under less restrictive symmetry assumptions, it is difficult to determine whether the final D5 map reflects a true biological assembly or a symmetry-imposed interpretation.
Filament abundance and physiological relevance
Even under the authors' broad classification criteria, the filament-like population appears to be a minor species. In some datasets, especially Supplementary Figure 10, the apparent filament fraction is very low, approximately 2-10%. This raises a major concern: if GS filaments are rare even under high-concentration cryo-EM conditions, are they expected to form to a meaningful extent under physiological conditions?
The authors propose a concentration-dependent assembly mechanism. If so, the relevance of GS filamentation in the lower-concentration cellular environment becomes even less clear. The authors should quantify filament abundance as a function of GS concentration and glutamine concentration, ideally under conditions closer to physiological ranges.
K52/C53 interface mutations
The authors use K52 and C53 as filament-interface residues, but the mechanistic contribution of these residues to filament assembly remains insufficiently explained. Why should K52A or C53A disrupt filament formation? Is the effect due to loss of a specific side-chain contact, altered local electrostatics, reduced crosslinker accessibility/reactivity, local structural destabilization, or nonspecific disruption of the interface?
The manuscript states that the interface is "concentration dependent and driven primarily by electrostatic interactions," but the data presented before that statement do not clearly establish this. The authors should explicitly identify the interacting electrostatic partners and provide structural or biochemical evidence supporting this interpretation.
Functional linkage between filamentation and kinetics is weak.
The authors should establish the oligomeric state of GS under the actual assay conditions. In particular, what is the filament fraction during the Figure 2F / Supplementary Figure 15 kinetic assays? Is the change in KM ammonia quantitatively correlated with filament abundance?
This is currently unclear. The direct comparison between decamer and 2-decamer fractions does not robustly show a functional difference, and the later glutamine-addition assays are interpreted as filament-mediated without directly demonstrating the filament fraction under the same assay conditions.
In Figure 2F, WT, K52A, and C53A already show different ammonia-dependent kinetic parameters in the absence of added glutamine. K52A and C53A appear to have lower basal kcat/KM ammonia and higher KM ammonia than WT even without glutamine. The authors should explain why these interface mutants already alter basal ammonia kinetics. Without such an explanation, K52A and C53A cannot be treated as clean controls that selectively disrupt glutamine-stabilized filamentation.
Major concern 2: The interface density is not convincingly assigned to glutamine.
The second foundational issue is the assignment of the interface density to glutamine. At present, the evidence is not sufficient to support the conclusion that glutamine is the ligand at this interface.
The local density at the interface appears weak and likely has lower local resolution than the reported global resolution. The current density could represent a low-occupancy or symmetry-averaged amino-acid-like density rather than a confidently assigned glutamine molecule.
Ligand pose and hydrogen bonding.
The proposed glutamine pose also requires more rigorous validation. The authors state that glutamine forms hydrogen bonds with interface residues, including K52, C53, and E55. These hydrogen bonds should be shown explicitly in a figure, with distances listed.
The proposed interaction involving C53 appears unusual and should be justified chemically and geometrically.
Glutamate has not been excluded.
The largest problem is that the authors do not adequately consider glutamate as an alternative ligand. They compare the density with phosphate and ATP/ADP, but this is not sufficient. Glutamate is present at high concentration during turnover, and it is chemically and structurally very similar to glutamine. Given the limited local density and possible orientational averaging, distinguishing glutamine from glutamate from the current cryo-EM density alone is not justified.
The authors should report or estimate the concentrations of glutamate and glutamine at the vitrification time point used for the high-resolution turnover-filament reconstruction. If glutamate is present at a much higher concentration than glutamine, the authors must explain why the interface density should be assigned to glutamine rather than glutamate.
The authors should fit both glutamine and glutamate into the interface density using the same validation criteria and compare the results. Stronger support would come from direct structural experiments, such as cryo-EM structures of GS incubated separately with glutamate and glutamine under controlled conditions.
Unless stronger evidence is provided, the claim that "glutamine binds to the filament interface" cannot be made.
Specific comments
Interface assembly statement:<br />
"These data suggest that the formation of the interface is concentration dependent and driven primarily by electrostatic interactions."
What specific data support "concentration dependent" at this point in the manuscript? Which residues or chemical groups are proposed to form the electrostatic interactions? The authors should provide a more explicit explanation.
Line 149-150:<br />
"In both scenarios, any signal is likely to be averaged out and experiments with symmetry expansion and focused classification did not yield any convincing density."
Please show these analyses. Negative results are important here because they bear directly on the reliability of the interface interpretation.
"Glutamine stabilizes larger GS filaments":<br />
What does "larger" mean? Longer filaments, more decamers per filament, or larger diameter? The authors should define this quantitatively, preferably by reporting filament-length distributions or the number of decamers per assembly.
Filament classification:<br />
The criteria used to classify particles or 2D classes as "filament" are not sufficiently clear. The authors should provide the full 2D classification results for each time-resolved dataset, including selected and discarded classes, particle numbers, and objective selection criteria. Some selected and discarded classes appear visually similar, especially in Supplementary Figures 9-10.
R298A decamer:<br />
The R298A mutant is presented as a turnover-decamer structure, not a filament structure. The authors should clarify whether R298A forms filament-like particles under comparable turnover conditions. If R298A does not form filaments, this should be reported and explained. If filament-like particles were present but excluded during processing, the authors should provide their abundance and justify why only the decameric form was analyzed. This point matters because R298A is used to connect E305-loop disorder with the proposed filament-associated mechanism, although R298A is a loop-stabilization mutant rather than a filament-interface mutant.
Line 231-233:<br />
"a reaction time that should yield a high concentration of product due to the higher enzyme concentration than previous experiments"
What is the estimated product concentration at vitrification? What concentration range qualifies as "high"? The authors should provide a quantitative estimate.
Glutamine hydrogen bonds:<br />
The proposed hydrogen bonds linking glutamine to K52, C53, and E55 should be shown explicitly with atom identities and distances.
Glutamate comparison:
What is the glutamate concentration in the same sample? Given that glutamate is chemically similar to glutamine and likely present at high concentration, why is the interface density not glutamate? The authors should compare glutamine and glutamate fitting using the same validation criteria.
Line 248-253:<br />
The speculation that apo filaments may arise from high GS concentration or residual glutamine should be moved to the Discussion. In the Results, this reads as an ad hoc explanation rather than a result directly supported by data.
Actual assay-state oligomeric distribution:<br />
What is the filament fraction under the actual kinetic assay conditions? Is the KM ammonia change quantitatively correlated with filament abundance?
Figure 2F:<br />
Why do WT, K52A, and C53A differ in basal ammonia-dependent activity even without added glutamine? The authors should explain whether these mutations alter intrinsic ammonia kinetics independent of filamentation.
Supplementary Figure 15 / Figure 2F:<br />
Please clarify the relationship between Figure 2F and Supplementary Figure 15. The kinetic constants in Figure 2F appear to depend on global fitting of progress curves shown in Supplementary Figure 15. The authors should provide replicate-level raw progress curves, between-replicate variability, fitting residuals, and individual fitted parameters.
Supplementary Figure 19:<br />
Supplementary Figure 19 should be presented consistently with Supplementary Figure 18, including the corresponding 2D classification results.
In summary, although the revised manuscript improves the presentation of cryo-EM map processing, the two foundational claims remain unresolved. The current data establish, at most, a di-decameric or filament-like GS assembly, but not a rigorously defined filamentous polymer. In addition, the interface density is not convincingly assigned to glutamine, particularly because glutamate has not been excluded as the most relevant alternative ligand. Since the proposed negative-feedback mechanism depends directly on these two points, the current evidence does not support the strength of the title, abstract, or mechanistic conclusions.