Shielding the front-strand beta 3 of the von Willebrand factor A1 domain inhibits its binding to platelet glycoprotein Ibalpha.

Platelet adhesion to damaged vessel wall and shear-induced platelet aggregation necessitate binding of the von Willebrand factor (VWF) A1 domain to platelet GPIbalpha. Blocking this interaction represents a promising approach to the treatment of arterial thrombosis. Comparison of amino acid sequences of the VWF A1 domain in several species, expressing VWF recognized by the blocking monoclonal antibody AJvW-2, suggested 9 residues (His563, Ile566, Asp570, Ala581, Val584, Ala587, Arg616, Ala618, and Met622) to contribute to the epitope for AJvW-2 or to be part of the GPIbalpha-binding site. Glutathione-S-transferase (GST)-human VWF A1 fusion proteins, in which these amino acids were mutated to their murine counterparts, were tested for their capacity to bind AJvW-2 or heparin, to interfere with botrocetin- or ristocetin-mediated VWF binding to GPIb, or to induce flow-dependent platelet tethering in a perfusion chamber. Thus, mutations His563Arg, Ile566Leu, Asp570Ala, and Ala587Thr, clustered on the outer surface of the A1 domain, dramatically impaired binding of AJvW-2 to A1. The His563Arg, Ile566Leu, and Asp570Ala mutations also impaired the binding of heparin, which competes with AJvW-2 for binding to A1. Perfusion studies revealed that His563, Ile566, Asp570, Arg616, and Ala618 take part in GPIbalpha binding, their mutation-impairing platelet recruitment. In agreement with the surface distribution of VWF type 2M mutations, this study demonstrates overlapping of the epitope for AJvW-2 and the GPIbalpha-binding site, located around the front pocket of the A1 domain and defined by strands beta3, beta4, and helix alpha3, and it provides a mechanistic basis for VWF neutralization by this antibody.