Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan.
Nature. 2010 Oct 7;467(7316):724-8. doi: 10.1038/nature09372. Epub 2010 Sep 15.
F-actin is a helical assembly of actin, which is a component of muscle fibres essential for contraction and has a crucial role in numerous cellular processes, such as the formation of lamellipodia and filopodia, as the most abundant component and regulator of cytoskeletons by dynamic assembly and disassembly (from G-actin to F-actin and vice versa). Actin is a ubiquitous protein and is involved in important biological functions, but the definitive high-resolution structure of F-actin remains unknown. Although a recent atomic model well reproduced X-ray fibre diffraction intensity data from a highly oriented liquid-crystalline sol specimen, its refinement without experimental phase information has certain limitations. Direct visualization of the structure by electron cryomicroscopy, however, has been difficult because it is relatively thin and flexible. Here we report the F-actin structure at 6.6 Å resolution, made obtainable by recent advances in electron cryomicroscopy. The density map clearly resolves all the secondary structures of G-actin, such as α-helices, β-structures and loops, and makes unambiguous modelling and refinement possible. Complex domain motions that open the nucleotide-binding pocket on F-actin formation, specific D-loop and terminal conformations, and relatively tight axial but markedly loose interprotofilament interactions hydrophilic in nature are revealed in the F-actin model, and all seem to be important for dynamic functions of actin.
F-肌动蛋白是肌动蛋白的螺旋组装体，是肌纤维收缩所必需的成分，在许多细胞过程中起着至关重要的作用，例如形成片状伪足和丝状伪足，作为细胞骨架的最丰富的成分和调节剂，通过动态组装和拆卸（从 G-肌动蛋白到 F-肌动蛋白，反之亦然）。肌动蛋白是一种普遍存在的蛋白质，参与重要的生物学功能，但 F-肌动蛋白的明确高分辨率结构仍然未知。尽管最近的原子模型很好地再现了来自高度取向液晶溶胶标本的 X 射线纤维衍射强度数据，但在没有实验相信息的情况下进行细化具有一定的局限性。然而，由于其相对较薄且灵活，通过电子 cryomicroscopy 直接可视化结构一直很困难。在这里，我们报告了 F-肌动蛋白结构在 6.6 Å分辨率下的结构，这是通过电子 cryomicroscopy 的最新进展获得的。密度图清楚地解析了 G-肌动蛋白的所有二级结构，如α-螺旋、β-结构和环，并使明确的建模和细化成为可能。在 F-肌动蛋白模型中揭示了复杂的结构域运动，这些运动打开了核苷酸结合口袋，形成 F-肌动蛋白，特定的 D 环和末端构象，以及相对紧密的轴向但明显松散的原纤维间相互作用亲水性质，这些似乎对肌动蛋白的动态功能都很重要。
