HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Gain of von Willebrand factor–binding function by mutagenesis of a species-conserved residue within the leucine-rich repeat region of platelet glycoprotein Ib

Glycoprotein (GP) Ib , a member of the leucine-rich repeat (LRR) protein family, mediates platelet adhesion to immobilized von Willebrand factor (VWF). We investigated the role in VWF binding of charged residues in the LRR region of GP Ib that are conserved in human, canine, and murine proteins. Substitution of His86 with either Ala or Glu resulted in a gain of VWF-binding function as judged by increased VWF binding in the presence of the modulators ristocetin and botrocetin and by enhanced adhesion of Chinese hamster ovary (CHO) cells expressing the mutant GP Ib to immobilized VWF under conditions of flow. This is the first report of a gain-of-function phenotype resulting from mutations in the LRR region of GP Ib . Because His86 is 2 nm away from the region of GP Ib with the largest surface of contact with VWF, the data suggest that the LRRs regulate GP Ib affinity for VWF allosterically. (Blood. 2005;106:1982-1987)