Adhesive Properties of the Isolated Amino-terminal Domain of Platelet Glycoprotein Ibalpha in a Flow Field

We have examined the interaction between the amino-terminal domain of platelet glycoprotein (GP) Ibalpha and immobilized von Willebrand Factor (vWF) under flow conditions in the absence of other components of the GP Ib-IX-V complex. Latex beads were coated with a recombinant fragment containing GP Ibalpha residues 1-302, either with normal sequence or with the single G233V substitution that causes enhanced affinity for plasma vWF in platelet-type pseudo-von-Willebrand disease. Beads coated with native fragment adhered to vWF in a manner comparable to platelets, showing surface translocation that reflected the transient nature of the bonds formed. Thus, the GP Ibalpha extracellular domain is necessary and sufficient for interacting with vWF under high shear stress. Beads coated with the mutated fragment became tethered to vWF in greater number and had lower velocity of translocation than beads coated with the normal counterpart, suggesting that the G233V mutation lowers the rate of bond dissociation. Our findings define an approach for studying the biomechanical properties of the GP Ibalpha-vWF bond and suggest that this interaction is tightly regulated to allow rapid binding at sites of vascular injury, while permitting the concurrent presence of receptor and ligand in the circulation.