Biophysics Graduate Group, Chemistry Department, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2012 Feb 21;109(8):2913-8. doi: 10.1073/pnas.1114292109. Epub 2012 Jan 30.
Mechanical cues affect many important biological processes in metazoan cells, such as migration, proliferation, and differentiation. Such cues are thought to be detected by specialized mechanosensing molecules linked to the cytoskeleton, an intracellular network of protein filaments that provide mechanical rigidity to the cell and drive cellular shape change. The most abundant such filament, actin, forms branched networks nucleated by the actin-related protein (Arp) 2/3 complex that support or induce membrane protrusions and display adaptive behavior in response to compressive forces. Here we show that filamentous actin serves in a mechanosensitive capacity itself, by biasing the location of actin branch nucleation in response to filament bending. Using an in vitro assay to measure branching from curved sections of immobilized actin filaments, we observed preferential branch formation by the Arp2/3 complex on the convex face of the curved filament. To explain this behavior, we propose a fluctuation gating model in which filament binding or branch nucleation by Arp2/3 occur only when a sufficiently large, transient, local curvature fluctuation causes a favorable conformational change in the filament, and we show with Monte Carlo simulations that this model can quantitatively account for our experimental data. We also show how the branching bias can reinforce actin networks in response to compressive forces. These results demonstrate how filament curvature can alter the interaction of cytoskeletal filaments with regulatory proteins, suggesting that direct mechanotransduction by actin may serve as a general mechanism for organizing the cytoskeleton in response to force.
力学线索影响多细胞生物细胞中的许多重要生物学过程，例如迁移、增殖和分化。这些线索被认为是由与细胞骨架相连的专门的机械感觉分子检测到的，细胞骨架是一种细胞内的蛋白质丝网络，为细胞提供机械刚性并驱动细胞形状变化。其中最丰富的丝是肌动蛋白，它形成由肌动蛋白相关蛋白 (Arp) 2/3 复合物引发的分支网络，该复合物支持或诱导膜突起，并对压缩力表现出适应性行为。在这里，我们表明丝状肌动蛋白本身具有机械敏感性，通过响应丝弯曲使肌动蛋白分支成核的位置发生偏差。使用一种体外测定法来测量固定化肌动蛋白丝的弯曲部分的分支形成，我们观察到 Arp2/3 复合物在弯曲丝的凸面优先形成分支。为了解释这种行为，我们提出了一个波动门控模型，其中 Arp2/3 的丝结合或分支成核仅在足够大的瞬态局部曲率波动导致丝的有利构象变化时发生，并且我们通过蒙特卡罗模拟表明，该模型可以定量解释我们的实验数据。我们还展示了分支偏差如何响应压缩力增强肌动蛋白网络。这些结果表明，丝曲率如何改变细胞骨架丝与调节蛋白的相互作用，这表明肌动蛋白的直接力转导可能是一种组织细胞骨架以响应力的通用机制。
