Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA.
Proc Natl Acad Sci U S A. 2011 Sep 20;108(38):15822-7. doi: 10.1073/pnas.1108999108. Epub 2011 Sep 8.
Bacterial cells possess multiple cytoskeletal proteins involved in a wide range of cellular processes. These cytoskeletal proteins are dynamic, but the driving forces and cellular functions of these dynamics remain poorly understood. Eukaryotic cytoskeletal dynamics are often driven by motor proteins, but in bacteria no motors that drive cytoskeletal motion have been identified to date. Here, we quantitatively study the dynamics of the Escherichia coli actin homolog MreB, which is essential for the maintenance of rod-like cell shape in bacteria. We find that MreB rotates around the long axis of the cell in a persistent manner. Whereas previous studies have suggested that MreB dynamics are driven by its own polymerization, we show that MreB rotation does not depend on its own polymerization but rather requires the assembly of the peptidoglycan cell wall. The cell-wall synthesis machinery thus either constitutes a novel type of extracellular motor that exerts force on cytoplasmic MreB, or is indirectly required for an as-yet-unidentified motor. Biophysical simulations suggest that one function of MreB rotation is to ensure a uniform distribution of new peptidoglycan insertion sites, a necessary condition to maintain rod shape during growth. These findings both broaden the view of cytoskeletal motors and deepen our understanding of the physical basis of bacterial morphogenesis.
细菌细胞拥有多种参与广泛细胞过程的细胞骨架蛋白。这些细胞骨架蛋白是动态的，但这些动态的驱动力和细胞功能仍知之甚少。真核细胞骨架的动力学通常是由马达蛋白驱动的，但到目前为止，还没有发现驱动细胞骨架运动的细菌马达蛋白。在这里，我们定量研究了大肠杆菌肌动蛋白同源物 MreB 的动力学，该蛋白对于细菌棒状细胞形状的维持是必不可少的。我们发现 MreB 以持续的方式围绕细胞的长轴旋转。虽然先前的研究表明 MreB 动力学是由其自身的聚合驱动的，但我们表明 MreB 的旋转不依赖于其自身的聚合，而是需要肽聚糖细胞壁的组装。细胞壁合成机制因此要么构成了一种新型的胞外马达，对细胞质中的 MreB 施加力，要么是间接需要一个尚未确定的马达。生物物理模拟表明，MreB 旋转的一个功能是确保新的肽聚糖插入位点的均匀分布，这是在生长过程中保持棒状形状的必要条件。这些发现拓宽了对细胞骨架马达的看法，并加深了我们对细菌形态发生的物理基础的理解。
