Activation of A1 Domain Adhesiveness in von Willebrand Factor by Elongational Force

Abstract SCI-16

Multiple mechanisms may contribute to activation of von Willebrand factor (vWF) adhesiveness by elongational flow at sites of hemostasis, including enhancement of A1 domain exposure within vWF concatamers and conformational change within the A1 domain or its complex with GPIbα. A receptor and ligand in a single molecule (ReaLiSM) containing the A1 domain, a flexible linker, and GPIbα fused in a single polypeptide and suspended between beads using DNA handles was interrogated with a laser trap. Two pathways for unbinding representing flexed and extended states were previously reported, with the flexed, more stable state predominantly at forces above 10 pN. vWD type 2B mutations in the A1 domain selectively stabilize the extended, high affinity state, whereas platelet-type vWD mutations in GPIbα stabilize both states. With ReaLiSM, we can also measure the kinetics and force-dependence of receptor-ligand binding. Remarkably, we also see two on-rates for receptor-ligand binding, with the faster on-rate predominating above 10 pN and the slower on-rate predominating below 10 pN. vWD type 2B mutations in the A1 domain selectively increased the fast on-rate, whereas platelet-type mutations in GPIbα increased both on-rates. Our results support force-dependent conformational change as one of the mechanisms that activates vWF in hemostasis.