The glycoprotein Ib-IX-V complex is a platelet counterreceptor for P-selectin

We have identified platelet glycoprotein (GP) Ib alpha as a counterreceptor for P-selectin. GP Ib alpha is a component of the GP Ib-IX-V complex, which mediates platelet adhesion to subendothelium at sites of injury. Cells expressing P-selectin adhered to immobilized GP Ib alpha, and GP Ib alpha-expressing cells adhered to and rolled on P-selectin and on histamine-stimulated endothelium in a P-selectin-dependent manner. In like manner, platelets rolled on activated endothelium, a phenomenon inhibited by antibodies to both P-selectin and GP Ib alpha. Unlike the P-selectin interaction with its leukocyte ligand, PSGL-1 (P-selectin glycoprotein ligand 1), the interaction with GP Ib alpha required neither calcium nor carbohydrate core-2 branching or alpha(1,3)-fucosylation. The interaction was inhibited by sulfated proteoglycans and by antibodies against GP Ib alpha, including one directed at a tyrosine-sulfated region of the polypeptide. Thus, the GP Ib-IX-V complex mediates platelet attachment to both subendothelium and activated endothelium.