The Interaction of Vinculin with Actin

Vinculin can interact with F-actin both in recruitment of actin filaments to the growing focal adhesions and also in capping of actin filaments to regulate actin dynamics. Using molecular dynamics, both interactions are simulated using different vinculin conformations. Vinculin is simulated either with only its vinculin tail domain (Vt), with all residues in its closed conformation, with all residues in an open I conformation, and with all residues in an open II conformation. The open I conformation results from movement of domain 1 away from Vt; the open II conformation results from complete dissociation of Vt from the vinculin head domains. Simulation of vinculin binding along the actin filament showed that Vt alone can bind along the actin filaments, that vinculin in its closed conformation cannot bind along the actin filaments, and that vinculin in its open I conformation can bind along the actin filaments. The simulations confirm that movement of domain 1 away from Vt in formation of vinculin 1 is sufficient for allowing Vt to bind along the actin filament. Simulation of Vt capping actin filaments probe six possible bound structures and suggest that vinculin would cap actin filaments by interacting with both S1 and S3 of the barbed-end, using the surface of Vt normally occluded by D4 and nearby vinculin head domain residues. Simulation of D4 separation from Vt after D1 separation formed the open II conformation. Binding of open II vinculin to the barbed-end suggests this conformation allows for vinculin capping. Three binding sites on F-actin are suggested as regions that could link to vinculin. Vinculin is suggested to function as a variable switch at the focal adhesions. The conformation of vinculin and the precise F-actin binding conformation is dependent on the level of mechanical load on the focal adhesion.

纽蛋白（Vinculin）可通过两种途径与纤维状肌动蛋白（F-actin）发生相互作用：一是将肌动蛋白丝招募至正在形成的黏着斑，二是对肌动蛋白丝进行加帽以调控肌动蛋白动力学。本研究采用分子动力学（molecular dynamics）模拟了这两类相互作用，共使用了四种不同的纽蛋白构象：仅包含纽蛋白尾结构域（vinculin tail domain, Vt）的构象、处于闭合构象的完整纽蛋白、处于开放I构象的完整纽蛋白，以及处于开放II构象的完整纽蛋白。开放I构象由结构域1远离Vt的运动所产生；开放II构象则由Vt与纽蛋白头部结构域完全解离形成。对肌动蛋白丝上纽蛋白结合过程的模拟结果显示：仅Vt即可沿肌动蛋白丝结合；处于闭合构象的纽蛋白无法沿肌动蛋白丝结合；而处于开放I构象的纽蛋白则可沿肌动蛋白丝结合。该模拟证实，在开放I构象形成过程中，结构域1远离Vt的运动足以使Vt能够结合至肌动蛋白丝表面。针对Vt加帽肌动蛋白丝的模拟共探索了六种潜在结合结构，结果表明纽蛋白可通过Vt上原本被D4及邻近纽蛋白头部结构域残基遮蔽的表面，与肌动蛋白丝倒刺端的S1和S3位点结合，从而实现对肌动蛋白丝的加帽。在结构域1发生解离后，D4进一步与Vt分离，由此形成开放II构象。对开放II型纽蛋白与肌动蛋白丝倒刺端结合的模拟显示，该构象可支持纽蛋白发挥加帽功能。本研究提出，纤维状肌动蛋白上存在三个结合位点，可与纽蛋白相互作用。纽蛋白被认为是黏着斑处的可变调控开关。纽蛋白的构象及其与F-actin的精确结合模式，取决于黏着斑所受机械载荷的大小。