Novel Gain-of-function Mutations of Platelet Glycoprotein Ibα by Valine Mutagenesis in the Cys209–Cys248 Disulfide Loop: FUNCTIONAL ANALYSIS UNDER STATIC AND DYNAMIC CONDITIONS

Abstract

Platelet-type von Willebrand disease is a bleeding disorder resulting from gain-of-function mutations of glycoprotein (GP) Ibalpha that increase its affinity for von Willebrand factor (vWf). The two known naturally occurring mutations, G233V and M239V, both enrich the valine content of an already valine-rich region within the Cys(209)-Cys(248) disulfide loop. We tested the effect of converting other non-valine residues in this region to valine. Of 10 mutants expressed in CHO cells as components of GP Ib-IX complexes, four displayed a gain-of-function phenotype (G233V, D235V, K237V, and M239V) based on (125)I-vWf binding and adhesion to immobilized vWf. The remainder displayed loss-of-function phenotypes. The gain-of-function mutants bound vWf spontaneously and had a heightened response to low concentrations of ristocetin or botrocetin, whereas the loss-of-function mutants bound vWf more poorly than wild-type GP Ibalpha. No distinct gain- or loss-of-function conformations were identified with conformation-sensitive antibodies. Compared with cells expressing wild-type GP Ibalpha, cells expressing the gain-of-function mutants rolled significantly more slowly over immobilized vWf under flow than wild-type cells and were able to adhere to vWf coated at lower densities. In aggregate, these data indicate that the region of GP Ibalpha bounded by Asn(226) and Ala(244) regulates the affinity for vWf.