Expression of the phenotypic abnormality of platelet-type von Willebrand disease in a recombinant glycoprotein Ibα fragment

The platelet GP Ib-IX receptor supports platelet adhesion and activation by binding to vWf in the exposed subendothelial matrix. An abnormal GP Ib-IX complex exists in platelet-type or pseudo-von Willebrand disease and has a characteristic increased affinity for soluble vWf resulting in impaired hemostatic function due to the removal of larger vWf multimers from the circulation. Genetic studies within an afflicted family have demonstrated that the disease is linked to a Gly²³³ → Val amino acid substitution within the α-subunit of the oligomeric GP Ib-IX complex (Miller, J. L., D. Cunningham, V. A. Lyle, and C. L. Finch. 1991. Proc. Natl. Acad. Sci. USA. 88:4761-4765). To evaluate the functional consequences of this mutation, we constructed a recombinant analogue of the α-subunit of GP Ib containing Val²³³. Experiments comparing molecules with either Gly²³³ or Val²³³ revealed that the Val substitution generates a molecule with increased affinity for vWf. The recombinant fragment reproduces the functional abnormality of the GP Ib-IX complex in platelet-type von Willebrand disease, thus establishing the molecular basis of the bleeding disorder within this family. Moreover, it becomes apparent that structural elements responsible for the regulation of hemostasis through modulation of vWf affinity for platelets reside within the α-subunit of the GP Ib-IX complex.