Transmembrane Helix Dynamics of Bacterial Chemoreceptors Supports a Piston Model of Signalling

Transmembrane α-helices play a key role in many receptors, transmitting a signal from one side to the other of the lipid bilayer membrane. Bacterial chemoreceptors are one of the best studied such systems, with a wealth of biophysical and mutational data indicating a key role for the TM2 helix in signalling. In particular, aromatic (Trp and Tyr) and basic (Arg) residues help to lock α-helices into a membrane. Mutants in TM2 of E. coli Tar and related chemoreceptors involving these residues implicate changes in helix location and/or orientation in signalling. We have investigated the detailed structural basis of this via high throughput coarse-grained molecular dynamics (CG-MD) of Tar TM2 and its mutants in lipid bilayers. We focus on the position (shift) and orientation (tilt, rotation) of TM2 relative to the bilayer and how these are perturbed in mutants relative to the wildtype. The simulations reveal a clear correlation between small (ca. 1.5 Å) shift in position of TM2 along the bilayer normal and downstream changes in signalling activity. Weaker correlations are seen with helix tilt, and little/none between signalling and helix twist. This analysis of relatively subtle changes was only possible because the high throughput simulation method allowed us to run large (n = 100) ensembles for substantial numbers of different helix sequences, amounting to ca. 2000 simulations in total. Overall, this analysis supports a swinging-piston model of transmembrane signalling by Tar and related chemoreceptors.跨膜α螺旋（transmembrane α-helices）在众多受体中发挥关键作用，可将信号从脂质双层膜的一侧传递至另一侧。细菌趋化受体（bacterial chemoreceptors）是这类系统中研究最为透彻的一类，大量生物物理与突变数据表明，TM2螺旋在信号转导中发挥核心作用。具体而言，芳香族氨基酸残基（色氨酸（Trp）与酪氨酸（Tyr））以及碱性氨基酸残基（精氨酸（Arg））可帮助α螺旋锚定至膜上。大肠杆菌（E. coli）Tar蛋白及相关趋化受体的TM2区域中携带此类残基突变的突变体，提示螺旋的位置与/或取向变化参与了信号转导过程。本研究通过对脂质双层环境中Tar蛋白TM2螺旋及其突变体开展高通量粗粒度分子动力学（coarse-grained molecular dynamics, CG-MD）模拟，探究了这一现象的详细结构基础。本研究聚焦于TM2螺旋相对于脂质双层的位置（位移）与取向（倾斜、旋转），以及突变体相较于野生型时这些参数发生的扰动情况。模拟结果显示，TM2螺旋沿脂质双层法线方向的微小位移（约1.5埃（Å））与下游信号转导活性的变化之间存在显著相关性。螺旋倾斜与信号转导活性的相关性较弱，而螺旋扭转与信号转导之间几乎不存在相关性。此次针对相对细微变化的分析之所以能够实现，得益于高通量模拟方法可针对大量不同的螺旋序列开展大规模（n=100）的集合模拟，总模拟次数总计约2000次。综上，本分析结果支持Tar蛋白及相关趋化受体通过“摆动活塞模型”实现跨膜信号转导的假说。