Adhesive properties of the isolated amino-terminal domain of platelet glycoprotein Ib alpha in a flow field

We have examined the interaction between the amino-terminal domain of plate let glycoprotein (GP) Iba and immobilized von Willebrand Factor (VWF) under flow conditions in the absence of other components of the GP Ib-IX-V compl ex. Latex beads were coated with a recombinant fragment containing GP Iba! residues 1-302, either with normal sequence or with the single G233V substi tution that causes enhanced affinity for plasma VWF in platelet-type pseudo -von-Willebrand disease. Beads coated with native fragment adhered to VWF i n a manner comparable to platelets, showing surface translocation that refl ected the transient nature of the bonds formed. Thus, the GP Ib alpha extra cellular domain is necessary and sufficient for interacting with VWF under high shear stress. Beads coated with the mutated fragment became tethered t o vWF in greater number and had lower velocity of translocation than beads coated with the normal counterpart, suggesting that the G233V mutation lowe rs the rate of bond dissociation. Our findings define an approach for study ing the biomechanical properties of the GP Ib alpha-vWF bond and suggest th at 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.