鞭毛钩的动态硬化
Dynamic stiffening of the flagellar hook
摘要：
许多细菌通过一个或多个旋转的刚性螺旋鞭毛运动，使它们成为唯一已知的使用旋转作为推进手段的生物体。这种旋转是由细菌鞭毛马达提供的，这是一种特别强大的旋转分子机器。在每个鞭毛的基部是钩，这是一种软螺旋聚合物，其充当万向接头，将刚性跨膜转子的旋转耦合到刚性细胞外鞭毛的旋转。在多鞭毛细菌物种中，其中推力由流体动力学协调的鞭毛束提供，钩的柔性是特别关键的，因为束内的许多鞭毛从它们的马达显著地离轴旋转。但是，因此，由单个旋转鞭毛产生的推力对钩施加了显著的弯曲力矩。因此，钩需要同时提供离轴束形成所需的柔度和承受游泳的大流体动力所需的刚度。为了阐明钩子如何实现这一双重功能，需要测量动态条件下单个钩子的机械行为。在这里，通过新的高分辨率测量和一种新的分析钩波动在体内电机旋转珠测定，我们解决了单钩的弹性响应下增加扭转应力，揭示了一个明确的动态增加，其弯曲刚度。因此，钩的持续长度随着所施加的扭矩增加超过一个数量级。这种应变硬化允许系统在需要时是柔性的，但在高负荷下减少变形，从而允许高速的细胞运动。
英文摘要：
Many bacteria are motile by means of one or more rotating rigid helical flagella, making them the only known organism to use rotation as a means of propulsion. The rotation is supplied by the bacterial flagellar motor, a particularly powerful rotary molecular machine. At the base of each flagellum is the hook, a soft helical polymer that acts as a universal joint, coupling rotation of the rigid membrane-spanning rotor to rotation of the rigid extra-cellular flagellum. In multi-flagellated bacterial species, where thrust is provided by a hydrodynamically coordinated bundle of flagella, the flexibility of the hook is particularly crucial, as many of the flagella within the bundle rotate significantly off-axis from their motor. But, consequently, the thrust produced by a single rotating flagellum applies a significant bending moment to the hook. So, the hook needs to simultaneously provide the compliance necessary for off-axis bundle formation and the rigidity necessary to withstand the large hydrodynamical forces of swimming. To elucidate how the hook can fulfill this double functionality, measurements of the mechanical behavior of individual hooks under dynamical conditions are needed. Here, via new high-resolution measurements and a novel analysis of hook fluctuations during in vivo motor rotation in bead assays, we resolve the elastic response of single hooks under increasing torsional stress, revealing a clear dynamic increase in their bending stiffness. Accordingly, the persistence length of the hook increases by more than one order of magnitude with applied torque. Such strain-stiffening allows the system to be flexible when needed yet reduce deformation under high loads, allowing cellular motility at high speed.
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