Abstract
von Willebrand factor (vWf) is well known to interact with platelet membrane glycoprotein Ib (GPIb) in the presence of botrocetin as well as ristocetin. We studied the role of botrocetin on vWf binding to GPIb. The amount of vWf binding to GPIb was dependent on the concentration of botrocetin. Labeled botrocetin showed the specific binding to vWf and its tryptic fragments of 116kDa and 52/48kDa (residues 449-728). 116kDa fragment was a homodimer of 52/48kDa fragment. III-T2 fragment, also dimer, was composed of two pairs of disulfide-linked subunits, two 35-kDa heavy chains (residues 273-511) and two 10-kDa light chains (residues 674-728) and failed to bind to botrocetin. These results strongly suggest that the missing portion of III-T2 fragment corresponding to the amino acid residue from 512 to 673, is crucial for the binding domain of botrocetin. The synthetic peptides of vWf and anti-vWf monoclonal antibodies that inhibit the ristocetin-induced vWf binding to GPIb, failed to inhibit the binding in the presence of botrocetin, whereas glycocalicin and anti-GPIb antibody inhibited the binding in the presence of both botrocetin and ristocetin. These data strongly suggest that vWf binds to GPIb after the conformational change of vWf by the binding to botrocetin.
