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Platelet membrane glycoprotein (GP) IIIa forms a Ca2+-dependent heterodimer complex with GP IIb. The GP IIb-IIIa complex constitutes the fibrinogen and fibronectin receptor on stimulated platelets. A biochemically and immunologically similar membrane glycoprotein complex is present on endothelial cells. A human umbilical vein endothelial cell cDNA library was screened using oligonucleotide probes designed from peptide sequences obtained from platelet GP IIIa. A cDNA clone was sequenced and found to encode a protein of 84.5 kDa. The translated endothelial cDNA contained five sequences that corresponded to peptide sequences in platelet GP IIIa, including the amino-terminal 19 residues. Thus, the endothelial and platelet forms of GP IIIa are apparently identical. Glycoprotein IIIa consists of a long amino-terminal extracellular domain with several potential N-linked glycosylation sites and four cysteine-rich tandem repeats, a 29-residue hydrophobic transmembrane segment, and a short carboxyl-terminal cytoplasmic domain. Glycoprotein IIIa has a 47% amino acid sequence homology to “integrin,” a fibronectin receptor from chicken embryo fibroblasts. This homology suggests that GP IIIa is a member of a family of cell-surface adhesion receptors.