A quantitative look at protein diffusion in cells and crowded media

生命系统是高度动态的。特别是，大多数细胞过程涉及分子在细胞区室之间或细胞区室之间的运动。因此，了解细胞如何工作将需要了解蛋白质如何在细胞环境中移动。在这种情况下，我的研究重点是蛋白质扩散。我的目标是了解蛋白质在细胞内部（拥挤和非平衡环境）的扩散与简单流体中的扩散有何不同，以及这些差异如何影响涉及蛋白质扩散的许多细胞过程。为此，我们将利用显微镜的最新发展，目前允许使用荧光技术在从细胞大小（约10微米）到细胞骨架特征网格大小（约50 nm）的长度尺度范围内表征扩散。我们将讨论的一个问题是蛋白质扩散的长度尺度依赖性。有强有力的证据表明，在细胞中，蛋白质在短尺度上的扩散速度比在大尺度上的扩散速度快。为了更好地理解这种现象的起源，我们将在拥挤的聚合物溶液和凝胶中模拟细胞环境进行可变长度尺度的扩散测量。使用仿生模型系统将允许精确和系统地控制可能影响扩散的参数，例如拥挤剂浓度或温度。这将是必不可少的，以测试近年来提出的不同模型的有效性，以解释在拥挤的环境中的扩散。* 我们要讨论的另一个问题是活细胞的有效温度。简单液体中的扩散是由与流体分子的热碰撞引起的，导致扩散系数与流体的绝对温度成比例。然而，最近的几项研究表明，在细胞中，非热随机过程在直径小至100 nm的探针颗粒的扩散中发挥作用。这导致扩散过程具有高于预期的扩散系数，即更高的有效温度。我们将测试这种效应是否扩展到小颗粒（约5 nm）的扩散，如蛋白质。如果合适，我们将努力确定非热随机噪声的来源。* 我们的工作将参与推进细胞生物学更定量的方法。我们将提供定量数据，这将有助于区分不同的模型之间的异常扩散拥挤的环境，如细胞内部。反过来，这将有助于开发涉及扩散的许多细胞过程的更好模型。我们的工作还将提供一些例子，说明为简单物理系统开发的统计力学定律如何需要修改，以便与细胞环境等生命系统相关。**

Living systems are highly dynamic. In particular, most cellular processes involve motions of molecules across or between cellular compartments. Thus understanding how cells work will require understanding how proteins move in the cellular environment. In this context, my research focuses on protein diffusion. My goal is to understand how protein diffusion in the cell interior (a crowded and non-equilibrium environment) might differ from diffusion in simple fluids, and how these differences might impact the many cellular processes involving protein diffusion. To do so, we will take advantage of recent developments in microscopy, which presently allow characterizing diffusion, using fluorescence techniques, over a range of length-scales extending from the size of a cell (~ 10 microns) to the characteristic mesh size of the cell cytoskeleton (~50 nm).*** One question we will address is that of the length-scale dependence of protein diffusion. There is strong evidence that, in cells, proteins diffuse faster over short length-scales than they do over large length-scales. In order to better understand the origin of this phenomenon, we will perform variable length-scale diffusion measurements in crowded polymer solutions and gels mimicking the cellular environment. Using biomimetic model systems will allow precisely and systematically controlling parameters such as crowding agent concentration or the temperature, that may influence the diffusion. This will be essential in order to test the validity of the different models that have been proposed in recent years to explain diffusion in crowded environments. *** Another question we will address is that of the effective temperature of a living cell. Diffusion in simple liquids is caused by thermal collisions with the fluid molecules, resulting in a diffusion coefficient that is proportional to the fluid's absolute temperature. However, several recent studies have shown that, in cells, non-thermal stochastic processes play a role in the diffusion of probe particles as small as 100 nm in diameter. This results in a diffusion process with a higher than expected diffusion coefficient, i.e. a higher effective temperature. We will test whether this effect extends to the diffusion of small (~5 nm) particles such as proteins. If appropriate, we will work to establish the source of the non-thermal stochastic noise. *** Our work will participate in advancing a more quantitative approach to cell biology. We will supply quantitative data that will help discriminate between different models of anomalous diffusion in crowded environments such as the cell interior. In turn, this will help develop better models of the many cellular processes that involve diffusion. Our work will also provide examples of how the laws of statistical mechanics developed for simple physical systems need to be modified in order become relevant in living systems such as the cellular environment. **