Force-induced on-rate switching and modulation by mutations in gain-of-function von Willebrand diseases

Mutations in the ultralong vascular protein von Willebrand factor (VWF) cause the common human bleeding disorder, von Willebrand disease (VWD). The A1 domain in VWF binds to glycoprotein Ib alpha (GPIb alpha) on platelets, in a reaction triggered, in part, by alterations in flow during bleeding. Gain-of-function mutations in A1 and GPIb alpha in VWD suggest conformational regulation. We report that force application switches A1 and/or GPIb alpha to a second state with faster on-rate, providing a mechanism for activating VWF binding to platelets. Switching occurs near 10 pN, a force that also induces a state of the receptor-ligand complex with slower off-rate. Force greatly increases the effects of VWD mutations, explaining pathophysiology. Conversion of single molecule k(on) (s(-1)) to bulk phase k(on) (s(-1)M(-1)) and the k(on) and k(off) values extrapolated to zero force for the low-force pathways show remarkably good agreement with bulk-phase measurements.