Fujiwara Satoru, Plazanet Marie, Matsumoto Fumiko, Oda Toshiro
Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan.
Biophys J. 2008 Jun;94(12):4880-9. doi: 10.1529/biophysj.107.125302. Epub 2008 Mar 7.
F-actin, a helical polymer formed by polymerization of the monomers (G-actin), plays crucial roles in various aspects of cell motility. Flexibility of F-actin has been suggested to be important for such a variety of functions. Understanding the flexibility of F-actin requires characterization of a hierarchy of dynamical properties, from internal dynamics of the actin monomers through domain motions within the monomers and relative motions between the monomers within F-actin to large-scale motions of F-actin as a whole. As a first step toward this ultimate purpose, we carried out elastic incoherent neutron scattering experiments on powders of F-actin and G-actin hydrated with D(2)O and characterized the internal dynamics of F-actin and G-actin. Well established techniques and analysis enabled the extraction of mean-square displacements and their temperature dependence in F-actin and in G-actin. An effective force constant analysis with a model consisting of three energy states showed that two dynamical transitions occur at approximately 150 K and approximately 245 K, the former of which corresponds to the onset of anharmonic motions and the latter of which couples with the transition of hydration water. It is shown that behavior of the mean-square displacements is different between G-actin and F-actin, such that G-actin is "softer" than F-actin. The differences in the internal dynamics are detected for the first time between the different structural states (the monomeric state and the polymerized state). The different behavior observed is ascribed to the differences in dynamical heterogeneity between F-actin and G-actin. Based on structural data, the assignment of the differences observed in the two samples to dynamics of specific loop regions involved in the polymerization of G-actin into F-actin is proposed.
F-肌动蛋白是由单体（G-肌动蛋白）聚合形成的螺旋状聚合物，在细胞运动的各个方面发挥着关键作用。F-肌动蛋白的柔韧性被认为对其多种功能很重要。了解F-肌动蛋白的柔韧性需要对一系列动力学特性进行表征，从肌动蛋白单体的内部动力学，到单体内部的结构域运动、F-肌动蛋白内单体之间的相对运动，再到F-肌动蛋白整体的大规模运动。作为朝着这个最终目标迈出的第一步，我们对用重水（D₂O）水合的F-肌动蛋白和G-肌动蛋白粉末进行了弹性非相干中子散射实验，并表征了F-肌动蛋白和G-肌动蛋白的内部动力学。成熟的技术和分析方法能够提取F-肌动蛋白和G-肌动蛋白中的均方位移及其温度依赖性。用一个由三个能量状态组成的模型进行的有效力常数分析表明，在大约150K和大约245K处发生了两个动力学转变，前者对应于非谐运动的开始，后者与水合水的转变相关。结果表明，G-肌动蛋白和F-肌动蛋白的均方位移行为不同，G-肌动蛋白比F-肌动蛋白“更柔软”。首次在不同的结构状态（单体状态和聚合状态）之间检测到内部动力学的差异。观察到的不同行为归因于F-肌动蛋白和G-肌动蛋白之间动力学异质性的差异。基于结构数据，提出将两个样品中观察到的差异归因于参与G-肌动蛋白聚合成F-肌动蛋白的特定环区域的动力学。
