Cultured Human Endothelial Cells Synthesize a Plasma Membrane Protein Complex Immunologically Related to the Platelet Glycoprotein IIb/IIIa Complex

We have previously demonstrated that endothelial cells synthesize a plasma membrane protein indistinguishable from platelet glycoprotein (GP) lla. The present study provides evidence for a further analogy between the platelet and the endothelial cell membrane by showing that cultured endothelial cells also synthesize a membrane protein complex immunologically related to the platelet GP llb/GP IIIa complex. This evidence is based on the following observations: (1) Cl7, a murine monoclonal antiplatelet GP IIIa antibody, consistently precipitates two proteins, apparent molecular weights, respectively, 115,000 and 125,000 reduced and 95,000 and 135,000 nonreduced, from metabolically (36S-methionine) as well as surface 125l-labeled cultured human endothelial cells; (2) upon crossed Immunoelectrophoresis of solubilized endothelial cells against a polyclonal rabbit antiplatelet antiserum and 125l-labeled C17 IgG, a single precipitate of the protein(s) recognized by Cl7 is observed. As judged by their mobility in 9% polyacrylamide gels, both endothelial proteins appear to have a somewhat larger molecular weight than their platelet counterparts. Patterns obtained by crossed immunoelectrophoresis are also indicative of a difference in electrophoretic behavior of the platelet GP IIb/IIIa complex and the endothelial cell protein complex.