Force-Dependent Platelet Binding of the Von Willebrand Factor A1-A2 Complex from Atomistic Simulations

Abstract

Von Willebrand factor (VWF) is a huge multimeric extracellular protein playing a crucial adhesive role in hemostasis. In response to shear stress, VWF binds to the GP1bα receptor of platelets via its A1 domain, to build a plug that stops the bleeding of an injured blood vessel. Recent experimental studies revealed that the binding of GP1bα is altered by the presence of individual A2 subunits, suggesting that A2, adjacent to A1, interacts with A1, thereby shielding the binding site for GP1bα. In addition, electron microscopy studies measured the distance between the center of masses of A1 and A2, varying from ∼4.6 nm, in which the two subunits may be in contact, to ∼11 nm, in which the two subunits are not in contact (Zhou YF et al. EMBO J. (2011) 30: 4098-4111). Apart from these distance estimates, little is known on how A1 and A2 interact with each other and thereby modify GP1bα binding, and how this inhibition is sensitive to mechanical forces in flowing blood. We address these issues by using molecular docking, and equilibrium and force probe molecular dynamics simulations. Our study predicts a stable structure of the A1-A2 complex, in which A2 binds to the same site in A1 as GP1bα does, over a time scale of hundreds of nanoseconds. This inhibition is relieved under a stretching force, explaining how A2 can inhibit Gp1bα binding to A1 in a shear-flow dependent manner. Overall, our simulations suggest a new and intriguing mechanism on an intermolecular auto-inhibition that allows a shear-sensitive growth of blood coagulates, which reconciles previous and can be directly tested by future experiments.