Lou Jizhong, Zhu Cheng
Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Proc Natl Acad Sci U S A. 2008 Sep 16;105(37):13847-52. doi: 10.1073/pnas.0801965105. Epub 2008 Sep 4.
Interaction of glycoprotein Ib alpha (GPIb alpha) with von Willebrand factor (VWF) initiates platelet adhesion to injured vascular wall to stop bleeding. A major contact between GPIb alpha and VWF involves the beta-switch region, which is a loop in the unliganded GPIb alpha but switches to a beta-hairpin in the complex structure. Paradoxically, flow enhances rather than impedes GPIb alpha-VWF binding. Gain-of-function mutations (e.g., M239V) in the beta-switch reduce the flow requirement for VWF binding, whereas loss-of-function mutations (e.g., A238V) increase the flow requirement. These phenomena cannot be explained by crystal structures or energy calculations. Herein we demonstrate that the beta-hairpin is unstable without contacting VWF, in that it switches to a loop in free molecular dynamics simulations. Simulations with a novel flow molecular dynamics algorithm show that the loop conformation is unstable in the presence of flow, as it switches to beta-hairpin even without contacting VWF. Compared with the wild-type, it is easier for the M239V mutant but harder for the A238V mutant to switch to beta-hairpin in the presence of flow. These results elucidate the structural basis for the two mutants and suggest a regulatory mechanism by which flow activates GPIb alpha via inducing a loop-to-beta-hairpin conformational transition on the beta-switch, thereby promoting VWF binding.
糖蛋白 Ibα（GPIbα）与血管性血友病因子（VWF）的相互作用启动血小板对受损血管壁的黏附以止血。GPIbα 与 VWF 之间的主要接触涉及 β 转换区，该区在未结合配体的 GPIbα 中是一个环，但在复合物结构中转变为 β 发夹结构。矛盾的是，血流增强而非阻碍 GPIbα-VWF 的结合。β 转换区的功能获得性突变（例如 M239V）降低了 VWF 结合对血流的需求，而功能丧失性突变（例如 A238V）则增加了对血流的需求。这些现象无法用晶体结构或能量计算来解释。在此我们证明，β 发夹在不与 VWF 接触时是不稳定的，因为它在自由分子动力学模拟中转变为环。使用新型流分子动力学算法的模拟表明，环构象在有血流时是不稳定的，因为即使不与 VWF 接触它也会转变为 β 发夹。与野生型相比，在有血流时 M239V 突变体更容易转变为 β 发夹，而 A238V 突变体则更难。这些结果阐明了这两个突变体的结构基础，并提出了一种调节机制，即血流通过诱导 β 转换区上的环到 β 发夹的构象转变来激活 GPIbα，从而促进 VWF 结合。
