SummaryRecently, the synthesis by cultured human endothelial cells of a membrane protein complex immunologically related to platelet glycoprotein (GP) IIb/IIIa complex was demonstrated. Since platelet GP IIIa is known to carry the platelet alloantigen Zwaor PaA1, studies were performed to establish whether this antigen is also expressed on endothelial cells. The present report describes the results of these studies, which provide evidence for the presence of the Zwaor PlA1antigen on the surface of cultured human endothelial cells. This evidence is based on the following observations: (1) cultured endothelial cells react with anti‐Zwa(PlA1) antibodies as shown by indirect immunofluorescence: (2) two proteins are precipitated by anti‐Zwa(PlA1) antibodies from lysates of125I‐labelled endothelial cells with an electrophore‐tic mobility corresponding with that of GP IIb and IIIa; (3) anti‐Zwa(PlA1) reacts specifically, as shown by immunoblotting of sodium‐dodecylsulphate polyacrylamide gels of solubilized endothelial cells, with a protein with a mobility similar to that of plate
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