Identification and Isolation of a Platelet GPlb-like Protein in Human Umbilical Vein Endothelial Cells and Bovine Aortic Smooth Muscle Cells

Glycoprotein lb (GPIb) is an intrinsic platelet membrane pro-tein that plays a major role in platelet adherence and mediates ristocetin-dependent platelet von Willebrand factor binding. Recent reports that the platelet membrane glycoprotein com-plex Ilb/IlIa is expressed in several cell types prompted us to look for GPIb expression in other vascular cells. Immunoper-oxidase staining of human stomach and skin histologic sections with polyclonal as well as monoclonal anti-GPIb antibody re-vealed the presence of GPIb in the endothelial cell and smooth muscle cell layers. Western blotting using monospecific poly-clonal anti-GPIb antibodies confirmed the presence of immu-noreactive GPIb in human umbilical vein endothelial and bo-vine aortic smooth muscle cell cultures. Fab fragments of a monoclonal anti-GPIb antibody were used to immunoprecipi-tate 13Hileucine labeled GPIb from metabolically labeled cells. The GPIb in these cells was functional as measured by risto-cetin-dependent cell agglutination and by vWF binding. Endo-thelial cells as well as smooth muscle cells bound "MI-labeled vWF in a ristocetin-dependent manner, with a Kd of 7.9 nM