NER: Dynamics of Flagellar Polymorphism

这是纳米级探索性研究奖。总的想法是了解细菌鞭毛在顺时针和逆时针旋转之间“切换”的机制，长期目标是利用这些信息开发可用于实际或商业应用的纳米级开关。细菌鞭毛是一种非凡的结构，直径为纳米级，在宏观尺度上具有与任何已知材料不同的力学和动力学特性。鞭毛蛋白由11根自组装的细丝组成，它们可以在多种左旋和右旋螺旋形态之间相互转换，这是单体构象转变的结果，改变了亚纳米尺度上的包装特性。这些微小的分子重排对更大范围内的细胞动力学，特别是细胞运动具有重要的影响。在趋化过程中，多个为细胞提供推力的旋转鞭毛作为它们嵌入细胞膜的旋转马达，不时地改变旋转方向。当捆绑在一起时，细菌会线性移动；在运动逆转时，束的解体会产生一个翻转事件，使细胞的方向随机化。这些“奔跑”和“翻滚”的运动产生了构成趋化性基础的随机行走。马达反转也会引发手性的传播反转，将逆时针旋转的左手螺旋变成顺时针旋转的右手螺旋。这些相互转换不仅是由旋转马达的扭矩引起的，也可能是由流过鞭毛的流体引起的。鞭毛的纳米力学性质，特别是动态多态的实验研究很少。最近，戈尔茨坦博士和他的合作者开发了第一个理论模型，解释了早期的、主要是定性的、关于外部流动产生的手性转变的实验。该连续体模型将单体填料的多稳定性纳入弹性理论的非线性扩展，并通过细长体流体力学与流体流动耦合。通过这种方式，它将纳米尺度上的构象转变与细胞尺度上的动力学联系起来，并捕捉到这些实验的许多关键特征。它还对手性变换的起始、频率和速度做出了许多尖锐的预测，但迄今尚未得到验证。该项目将利用最近在鞭毛荧光成像方法方面取得的重要进展，结合光学捕获方法，在流体应力或外力的明确可控条件下对鞭毛进行首次实验，为鞭毛动力学创建“低雷诺数风洞”。与此同时,Drs。Goldstein和Kessler将利用他们为研究描述弹性细丝的过阻尼弯曲和扭曲的耦合偏微分方程而开发的最新技术，扩展理论方法，与这些实验进行定量接触。这些研究将提供关于纳米尺度下多态的应力诱导构象转变的重要信息，并可能为这些分子开关在微流体或MEMS应用中的应用开辟道路。

This is a Nanoscale Exploratory Research award. The general idea is to understand the mechanism by which bacterial flagella "switch" between rotating clockwise and rotating counterclockwise, with the longer-term goal of using this information to develop nanoscale switches that can be used in practical or commercial applications. Bacterial flagella are remarkable structures, nanometers in diameter, that possess mechanical and dynamical properties unlike any known material on the macroscale. Composed of eleven filaments self-assembled from the protein flagellin, they may interconvert between multiple helical morphologies of both left-and right-handedness as a consequence of monomer conformational transitions that change packing properties on the subnanometer scale. These minute molecular rearrangements have important consequences for cellular dynamics on a range of larger scales, especially so in cellular motility. During chemotaxis, the multiple rotating flagella that provide thrust to the cells bundle and unbundle as their rotary motors, embedded in the membrane, episodically change rotational direction. When bundled, the bacterium moves linearly; disintegration of the bundle upon motor reversal creates a tumbling event that randomizes the cell's orientation. These motions of "run "and "tumble "produce the random walks that underlie chemotaxis. The motor reversal also initiates a propagating chirality reversal, turning a left-handed helix that had been rotated counterclockwise into a right-handed helix rotated clockwise. These interconversions occur not only from the torques of rotary motors, but may also be triggered by fluid flow past flagella.Little is known experimentally about the nanomechanical properties of flagella and especially of dynamic polymorphism. Very recently, Dr. Goldstein and his collaborators developed the first theoretical model that explains the early, mostly qualitative, experiments on chirality transitions produced by external flows. This continuum model incorporates monomer packing multistability into a novel nonlinear extension of elasticity theory, coupled via slender-body hydrodynamics to fluid flow. In this way, it connects conformational transitions at the nanoscale to dynamics at the cellular scale, and captures many of the key features of those experiments. It also makes a number of sharp predictions regarding the initiation, frequency, and velocity of chirality transformations, to date untested. This project will use recent, important developments in fluorescent imaging methods for flagella, coupled with optical trapping methods, to conduct the first experiments on flagella under well-defined and controllable conditions of fluid stress or external forces, creating a "low Reynolds number wind tunnel" for flagellar dynamics. Concurrently, Drs. Goldstein and Kessler will extend the theoretical approach to make quantitative contact with these experiments, using the latest techniques they have developed for the study of the coupled partial differential equations that describe overdamped bending and twisting of elastic filaments. These studies will yield important information about the stress-induced conformational transitions underlying polymorphism at the nanoscale, and perhaps open the way toward the use of these molecular switches in microfluidics or MEMS applications.