A monoclonal antibody to human platelet glycoprotein Ilia detects a related protein in cultured human endothelial cells

We have previously described a series of monoclonal antibodies against platelet membrane glycoproteins. Two of the antibodies, B59.2 and B2.12, recognize the glycoprotein UIb-II1a complex. These two antibodies react specifically with glycoprotein (GP) IIla, as shown by immiunoblotting of sodium dodecyl sulfatepolyacrylamide gels of solubilized platelet membranes. Monoclonal B2.12, but not B59.2, binds to cultured human endothelial cells obtained from umbilical vein, internal iliac artery, and inferior vena cava. At saturation '100,000 binding sites were detected per human umbilical vein endothelial cell. When solubilized radioiodinated cells were chromatographed on a column of agarose-bound 82.12, a single radiolabeled protein was obtained whose apparent molecular weight is slightly larger than that of platelet GP IIIa. This protein incorporated I35Slmethionine when endothelial cells were labeled metabolically. These results demonstrate that human endothelial cell membranes synthesize a protein immunologically related to platelet GP IIIa