{"id":"8b2e2c83-1e5c-4798-9e5a-e44949c68e68","arxiv_id":"2604.00470","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"A stochastic model with contact-dependent gating of the proximal MotB ion channel reproduces the concave CCW and linear CW torque-speed curves of the bacterial flagellar motor.","lead":"The paper proposes that the bacterial flagellar motor's asymmetric torque-speed behavior—concave in one direction, linear in the other—arises from a contact-dependent gate controlling when ions are released from one stator subunit. Generalists may read it as a case where molecular structure and stochastic modeling jointly explain a long-standing motor puzzle and name testable mutations.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"MD shows a structural asymmetry, but the paper never computes or measures the ion-release rates; the central kg,CCW≫kg,CW asymmetry is an assumed parameter that the model is then fit to reproduce.","rationale":"Reader's weakest assumption is exactly the unresolved translation from structural asymmetry to kinetic rates. I read the paper's central argument as: strong coupling makes Vr irrelevant; therefore only k(θs) can differ; contact-dependent gating provides that difference; and MD shows a contact difference. Each step is defended except the final rate step. The strong-coupling analysis is independently supported by analytic expressions and the stall-torque/IMF linearity argument; the algebraic derivation of Eq. 10 is transparent. The conditional verdict is appropriate. I do not see a deeper internal inconsistency that would warrant rejection. The concern is empirical/computational support for kg, not a logical flaw. Hence no change to the reader's verdict. Credit is given for the model's explicit predictions (mutation effects on hr and kg) and the self-identified limitation, which the paper honestly states.","tokens_in":22147,"tokens_out":6498,"duration_ms":63592,"concrete_test":"Re-run the MD systems used here (PDB 8UCS/8UMD/8UMX) with enhanced-sampling methods (e.g., umbrella sampling or metadynamics along the ion-release coordinate of the proximal MotB channel) to compute the potential of mean force and mean first-passage release times for CCW and CW conformations. If the computed kg,CCW/kg,CW ratio is not significantly greater than 1 (or, say, <5), the gating asymmetry is unsupported and the torque–speed difference would need another mechanism. A complementary check: mutate the identified contact residues and measure the torque–speed knee shift predicted by the model.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires that the MotA–FliG contact difference in CCW versus CW changes the proximal MotB ion-release rate by a large amount (kg,CCW ≫ kg,CW). This quantity is never computed or measured. kg is introduced as a free parameter in the gating model and fit to the torque–speed curves it is meant to explain. The MD support is structural: cavity volume drops from 915.97 to 508.42 ų and H-bond count from 185 to 123 at the MotA–FliG interface. These metrics do not determine an ion-release rate: a smaller cavity can raise or lower a barrier depending on where the transition state sits, and interfacial H-bond count is not a rate. Moreover, the measured cavity is in the MotA–FliG interface, not in the MotA–MotB release channel, so the chain from cryo-EM contact difference to kg asymmetry is doubly indirect. The authors explicitly concede in the Discussion: 'Our current MD simulations reveal structural asymmetry at the MotA–FliG interface but do not yet resolve the detailed ion translocation process or the corresponding kinetic rates.' If the actual release-rate ratio is close to 1, the proposed mechanism cannot generate the observed CW–CCW asymmetry. This is a gap in support, not an internal contradiction; it is the reason the result should remain conditional.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a stochastic mechanochemical model of the bacterial flagellar motor with explicit rotor–stator coupling and two-ion stator kinetics. It first argues, from Eqs. (1)–(7) and SI analysis, that under physiological conditions the motor operates in a tight-engagement regime, so the stall torque is essentially independent of the rotor–stator interaction potential Vr. The no-gating version of the model yields a linear torque–speed curve, consistent with CW rotation. To explain the concave CCW curve, the authors introduce a contact-dependent gating mechanism: interaction between MotA and FliG modulates the ion release rate kg of the proximal MotB channel. MD simulations of the CCW and CW cryo-EM structures show a larger interfacial cavity and more hydrogen bonds in CCW, which the authors interpret as supporting faster ion release in CCW. Setting kg,CW = 0 and choosing kg,CCW values, the model reproduces the experimentally observed concave CCW torque–speed curve and its IMF dependence. The paper concludes that differential gating strength, not rotor–stator mechanics, is the origin of CW–CCW asymmetry, and proposes specific mutations to test this.","tokens_in":22508,"tokens_out":4429,"duration_ms":46098,"significance":"If the proposed mechanism is correct, this would be a notable advance: it connects recently resolved stator–rotor structures to the long-standing torque–speed asymmetry puzzle and offers quantitative, mutatable predictions. The paper's analytical work on the strong-coupling limit is a genuine strength: the Fokker–Planck treatment in SI Eqs. (8)–(21) gives a clean argument that the torque–speed curve is independent of Vr in the tight-engagement regime, and the derived linear relation (Eq. (10)) and gated-curve expression (SI Eq. (26)) are checkable. The mutational predictions for MotA Asp86/Glu94 and FliG Asp241/Asp284/Asp289 are concrete and falsifiable. The main weakness is that the central kinetic asymmetry kg,CCW ≫ kg,CW is assumed and fitted, not computed or measured. The MD data are structural and do not by themselves determine an ion-release rate. Thus the paper currently establishes a plausible mechanism and an internally consistent model, but not the quantitative link between molecular asymmetry and motor function claimed in the abstract.","major_comments":[{"comment":"The central claim that kg,CCW ≫ kg,CW is the origin of the asymmetric torque–speed curves is supported only by parameter choice, not by measurement or calculation. In Fig. 5C, kg,CW = 0 and kg,CCW is varied from 1×10^5/s to 2.5×10^5/s to generate the concave shape; in Fig. 5D and the SI, the IMF dependence is imposed through kg = 18000[(IMF/kBT)−3.65] s^−1, chosen to match the Lo et al. data. The discussion explicitly concedes: 'Our current MD simulations reveal structural asymmetry at the MotA–FliG interface but do not yet resolve the detailed ion translocation process or the corresponding kinetic rates.' The MD observables (cavity volume 915.97→508.42 Å^3; H-bonds 185→123) do not determine a rate: a smaller cavity can raise or lower a release barrier depending on where the transition state lies, and the measured cavity is at the MotA–FliG interface, not in the MotA–MotB release channel","section":"Differential contact-dependent gating explains the CW-CCW asymmetry (Fig. 5C; SI 'Torque-speed curve with gating')"},{"comment":"The model's 'prediction' of the CCW curve is partly a fit. The normalized gated torque–speed relation, SI Eq. (26), contains a single shape parameter σ = (γ/β)(αα0)/(α+α0)^2 that depends on kg and the gating position θg through α0 and γ. The authors state that increasing kg or decreasing θg increases concavity, and the simulation parameters in Fig. 5D are selected to match the same experimental data the model is meant to explain. Consequently, the agreement in Fig. 5D demonstrates consistency, not independent confirmation. The paper should provide a sensitivity analysis of the predicted curves to kg and θg, and ideally independent constraints on these parameters from single-stator measurements or from the MD, so that the concave shape is not merely encoded by free parameters.","section":"Eqs. (10)–(11) and SI Eq. (23)–(26)"},{"comment":"The quantitative MD support for the gating asymmetry is based on a cavity-volume metric whose definition is not physically appropriate as presented. Fig. S7's caption says the cavity volume is estimated by the convex hull of interfacial Cα atoms within 5.0 Å of the opposing protein. A Cα convex hull is not a solvent-excluded or molecular-surface volume; it depends on the arbitrary 5.0 Å cutoff and residue selection, and it does not measure the hydration or steric occlusion of the ion-release pathway. In addition, the reported values 915.97 and 508.42 Å^3 and the H-bond counts 185 and 123 are given without error bars or a definition of the relevant region. Since these numbers are the only quantitative MD evidence for kg,CCW > kg,CW, the analysis should be redone with a standard cavity-definition method (e.g., solvent-excluded surface or channel-geometry analysis) and with replica-to-repli","section":"SI 'Molecule Dynamics Simulation for MotA–FliG Contact Interface' and Fig. S7"}],"minor_comments":[{"comment":"Typo: 'with slopes ng' should be 'with slope s_ng'.","section":"Eq. (10)"},{"comment":"The legend lists several potential shapes (Quadratic, Linear, Cosine, U-shape), but the figure panel does not distinguish the curves. Please make the color/shape legend explicit.","section":"Fig. 3A"},{"comment":"The IMF values and how kg scales with IMF are not stated in the caption. The SI gives kg = 18000[(IMF/kBT)−3.65] s^−1, but this should be stated in the main text or the caption so the reader can evaluate the fit.","section":"Fig. 5D"},{"comment":"The probability densities Pg and Pd are introduced without a clear definition. Define them explicitly in terms of the gating and default pathways before using them in the boundary conditions.","section":"SI Eq. (22)"},{"comment":"The mapping of PDB entries 8UCS, 8UMD, and 8UMX to the CCW and CW states is not stated. Please specify which structures correspond to which rotational state and whether the two CW structures were merged or treated separately.","section":"SI 'Molecule Dynamics Simulation...'"},{"comment":"The predicted effects of charge-neutralizing substitutions on the knee speed are plausible but qualitative. If the authors intend these as quantitative predictions, the model parameters that translate a changed contact probability into a changed kg should be specified.","section":"Discussion, testable predictions"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is mechanically well built and the strong-coupling derivation is careful, but the central mechanistic claim currently rests on an unmeasured kinetic parameter. The stress-test concern about kg is real and lands directly on the paper's main conclusion. I would not recommend rejection, because the gap is in principle closable with additional simulations or experiments, and the modeling framework is valuable. However, the paper's title and abstract overstate what is demonstrated: the MD shows structural asymmetry, not a rate asymmetry. A major revision that either computes release rates, provides a sensitivity analysis over kg/θg, or reframes the claim from 'explains' to 'proposes a mechanism consistent with' would be appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper is worth reading for the strong-coupling analysis alone. The derivation that, under tight rotor–stator engagement, the torque-speed relation is linear, independent of the rotor–stator potential, and depends only on the ion-translocation kinetics is careful and convincing. The analytical form of the gated torque-speed curve (Eq. 26) is a nice compact result, and the structural argument that the MotA–FliG interface differs between CW and CCW is supported by MD with clear geometric metrics: cavity volume drops from ~916 to ~508 ų and hydrogen bonds from 185 to 123. That the same stall torque arises in both directions because slippage is negligible in both cases is a genuine insight.\n\nThe soft spot, as the stress-test note says, is the load-bearing step. The entire mechanism rests on the claim that the tighter CW interface reduces the ion-release rate of the proximal MotB subunit enough to make kg,CCW ≫ kg,CW. That rate asymmetry is never computed or measured. kg is a free parameter in the gating model, and the values used (kg = 0 for CW, various large values for CCW) are chosen to reproduce the experimental torque-speed curves. The MD measures cavity geometry and hydrogen bonds at the MotA–FliG interface, not the release channel, and a smaller cavity or fewer H-bonds does not directly give you a release barrier. The paper's own Discussion concedes exactly this: the simulations \"do not yet resolve the detailed ion translocation process or the corresponding kinetic rates.\" So the central explanatory asymmetry is an assumption, and the model is then fit to the data using that assumption. That is a gap in support, not an internal contradiction—the mechanism is plausible and the predictions are concrete, including specific mutagenesis targets. But it means the paper is a conditional explanation, not a demonstrated one.\n\nThe SI is thorough on the strong-coupling limit and the Fokker-Planck treatment, and the IMF dependence is handled transparently, though several parameters (τ+, k, γ1, c, γ2) are tuned to match the Lo et al. data. No code or data are shipped, which is a minor reproducibility hit. The MD cavity volumes are reported as single numbers without error bars on the key comparison, though the SI shows ensemble-averaged trajectories with standard deviations.\n\nWho is this for: biophysicists working on molecular motors, especially the flagellar motor, and theorists interested in mechanochemical coupling. It deserves a serious referee: the framework is coherent, the strong-coupling result is solid, and the gating hypothesis is eminently testable. My recommendation: send it to peer review, but the referees should push for the rate calculation—enhanced-sampling MD or QM/MM—or at minimum a clear statement that the kinetic asymmetry is an assumption to be tested, not a derived consequence.","headline":"A plausible and well-derived gating mechanism for the flagellar motor's CW–CCW asymmetry, but the central kinetic asymmetry is assumed rather than computed, so it should be treated as a testable hypothesis, not a closed explanation.","tokens_in":776,"tokens_out":941,"would_cite":false,"duration_ms":19836,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The bacterial flagellar motor's directional torque asymmetry is caused by differential ion-release gating at the rotor–stator contact, not by mechanical coupling differences.","keywords":["bacterial flagellar motor","torque-speed relation","ion gating","MotA-FliG contact","mechanochemical model","molecular dynamics","directional asymmetry","rotor-stator coupling"],"falsifier":"A direct measurement of the ion release rates of the proximal MotB channel in the CW and CCW conformational states, either by molecular dynamics free-energy calculations of the release barrier or by single-channel electrophysiology on stator complexes with the interface held in each conformation, would settle the claim: if the release rates are similar, the proposed gating asymmetry cannot generate the observed torque-speed difference. Alternatively, a mutation that equalizes the MotA–FliG contact without changing the stator's mechanical interaction should, according to the model, make the CW","tokens_in":21987,"feed_emoji":"🦠","tokens_out":2599,"duration_ms":29097,"temperature":0.7,"pith_summary":"This paper argues that the two distinct torque–speed curves of the bacterial flagellar motor—concave in counterclockwise rotation and linear in clockwise rotation—arise from different ion-release rates at the rotor-stator interface, not from differences in rotor-stator mechanics. Using a stochastic mechanochemical model constrained by experimental torque-speed data and stator-rotation measurements, the authors show the motor operates under tight rotor-stator engagement, making the torque-speed relation insensitive to the interaction potential's shape. They then propose a contact-dependent gating mechanism: the MotA–FliG contact modulates the ion release rate of the proximal MotB subunit, with a tighter contact in the clockwise state slowing release and producing the linear curve, while the more open counterclockwise contact allows faster release, suppressing torque-free waits and producing the concave curve. Molecular dynamics simulations support the structural asymmetry, showing a smaller interfacial cavity and fewer hydrogen bonds in the clockwise state. If correct, this mechanism explains why both directions share the same stall torque but differ in curve shape, and it pinpoints specific interfaces for mutational tests.","feed_headline":"Ion gating, not mechanics, sets flagellar motor direction","feed_subtitle":"A single rotor-stator contact difference explains why the motor's torque-speed curve is concave one way and linear the other.","key_machinery":"The central object is the contact-dependent gating rate kg for the proximal MotB subunit's ion release, embedded in a two-ion alternating stator cycle. The stator contains a MotB dimer with two channels that operate out of phase; after a power stroke, the proximal channel can release its ion early through a gating pathway when the MotA–FliG contact is open, allowing the next cycle to start immediately. When the contact is tight (CW state), this early release is suppressed, forcing the stator into a torque-free waiting phase until a slower baseline release. The tight-engagement regime is characterized by an engagement fraction γ = ⟨ωr⟩/(α⟨ωs⟩) near unity, with rotor-stator coupling depth hr i","core_discovery":"The central claim is that directional asymmetry in the flagellar motor's torque-speed relation is governed by contact-dependent gating of ion release: the MotA–FliG interaction modulates the ion release rate of the proximal MotB subunit, with stronger gating in the counterclockwise (CCW) state shortening torque-free waiting phases and enhancing torque, whereas weaker gating in the clockwise (CW) state yields lower torque and a linear relation. The authors establish that under physiological conditions the motor sits in a tight-engagement regime (interaction depth hr ≈ 15.4 kBT), where the torque-speed curve is independent of the rotor-stator potential's form, ruling out mechanical interaction","pith_inferences":["The proposed mechanism suggests a general design principle: in rotary molecular motors, the gating of the chemical transition (ion release) can be mechanically regulated by the rotor, allowing directional control of the force-speed relationship without altering the power stroke itself; this might apply to other ion-driven rotary motors such as ATP synthase's Fo domain.","A testable extension beyond the paper's scope is to measure ion release rates directly using single-molecule fluorescence or electrophysiology in the two rotational states; if the rates differ by less than the model's assumed factor, the mechanism would need revision.","The MD-observed cavity and hydrogen-bond differences between states could be used to predict mutational effects on kg before experiments, enabling a quantitative structure-kinetics map; such predictions would clarify whether steric occlusion or electrostatic/hydration effects dominate the gating difference.","The model's claim that rotor-stator mechanics are irrelevant for the torque-speed shape in the tight-coupling regime could be stress-tested by engineering stators with drastically different interaction potential shapes while keeping the gating interface fixed—if the curves remain unchanged, the paper's central dichotomy is confirmed."],"forward_implications":["Mutations that disrupt the MotA–FliG interface, such as charge-neutralizing changes at MotA Asp86/Glu94 or FliG Asp241/Asp284/Asp289, should reduce the coupling depth hr and increase stator–rotor slippage, measurably lowering stall torque under high load or high ion motive force.","Perturbations that alter the ion-release pathway or the MotA–FliG contact, including steric mutations at the interface, should shift the knee speed or concavity of the CCW torque-speed curve, providing a direct test of the gating mechanism.","The model predicts that the torque-speed curves for CCW and CW rotation remain identical at stall (same stall torque) but diverge at intermediate loads, consistent with existing measurements and offering a quantitative target for future single-stator experiments.","The dependence of the torque-speed curve on ion motive force is captured by scaling the stator potential and gating rate linearly with IMF, matching experimental data across a range of driving forces.","If the gating hypothesis holds, the same structural interface that controls directional switching also tunes the motor's load-dependent torque output, linking chemotaxis behavior to a single molecular contact."],"fun_headline_variants":["Rotor-stator gating, not mechanics, explains motor asymmetry","One interfacial contact sets flagellar motor torque asymmetry","Ion release gating yields flagellar motor directional torque shapes","Contact-dependent ion gating gives motor CCW vs CW torque curves","Flagellar motor asymmetry stems from MotA-FliG gating strength"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The assumption that the tighter MotA–FliG contact in the clockwise state translates into a sufficiently large reduction in the ion release rate (kg,CCW ≫ kg,CW) to reproduce the observed torque-speed curvature, since the MD simulations quantify structural asymmetry but do not directly compute ion translocation kinetics or rates.","fun_headline_variants_meta":{"raw":{"variants":["Rotor-stator gating, not mechanics, explains motor asymmetry","One interfacial contact sets flagellar motor torque asymmetry","Ion release gating yields flagellar motor directional torque shapes","Contact-dependent ion gating gives motor CCW vs CW torque curves","Flagellar motor asymmetry stems from MotA-FliG gating strength"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000188,"raw_usage":{"total_tokens":1207,"prompt_tokens":817,"completion_tokens":390,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":561,"completion_tokens_details":{"reasoning_tokens":313}},"tokens_in":561,"tokens_out":390,"duration_ms":4522,"temperature":1.0,"reasoning_tokens":313,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T16:58:22.685610+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A direct measurement of the ion release rates of the proximal MotB channel in the CW and CCW conformational states, either by molecular dynamics free-energy calculations of the release barrier or by single-channel electrophysiology on stator complexes with the interface held in each conformation, would settle the claim: if the release rates are similar, the proposed gating asymmetry cannot generate the observed torque-speed difference. Alternatively, a mutation that equalizes the MotA–FliG contact without changing the stator's mechanical interaction should, according to the model, make the CW","supporting_citations":[],"review_version":1}