Nanomechanical properties of MscL α helices: A steered molecular dynamics study

Gating of mechanosensitive (MS) channels is driven by a hierarchical cascade of movements and deformations of transmembrane helices in response to bilayer tension. Determining the intrinsic mechanical properties of the individual transmembrane helices is therefore central to understanding the intricacies of the gating mechanism of MS channels. We used a constant-force steered molecular dynamics (SMD) approach to perform unidirectional pulling tests on all the helices of MscL in M. tuberculosis and E. coli homologs. Using this method, we could overcome the issues encountered with the commonly used constant-velocity SMD simulations, such as low mechanical stability of the helix during stretching and high dependency of the elastic properties on the pulling rate. We estimated Young's moduli of the α-helices of MscL to vary between 0.2 and 12.5 GPa with TM2 helix being the stiffest. We also studied the effect of water on the properties of the pore-lining TM1 helix. In the absence of water, this helix exhibited a much stiffer response. By monitoring the number of hydrogen bonds, it appears that water acts like a ‘lubricant’ (softener) during TM1 helix elongation. These data shed light on another physical aspect underlying hydrophobic gating of MS channels, in particular MscL.

机械敏感离子通道（mechanosensitive channels, MS channels）的门控过程，由跨膜螺旋响应双层膜张力时产生的层级化运动与形变级联所驱动。因此，解析单个跨膜螺旋的本征力学特性，是阐明MS通道门控机制复杂细节的核心要点。本研究采用恒力牵引分子动力学（steered molecular dynamics, SMD）方法，对结核分枝杆菌与大肠杆菌的MscL同源蛋白的全部跨膜螺旋开展单向牵引测试。相较于常规使用的恒速SMD模拟，该方法可有效规避其存在的诸多缺陷：例如拉伸过程中螺旋的力学稳定性不足，以及弹性特性对牵引速率的高度依赖。本研究估算得到MscL的α螺旋杨氏模量介于0.2 GPa至12.5 GPa之间，其中TM2螺旋（transmembrane helix 2, TM2）的刚度最高。此外，本研究还探究了水分子对孔衬TM1螺旋（transmembrane helix 1, TM1）力学特性的影响。在无水环境下，该螺旋展现出更强的刚度响应。通过监测氢键数量变化，可发现水分子在TM1螺旋拉伸过程中起到了类似“润滑剂（软化剂）”的作用。上述研究结果为解析MS通道（尤其是MscL）的疏水门控机制背后的另一物理维度提供了关键线索。