Immunochemical and amino-terminal sequence comparison of two cytoadhesins indicates they contain similar or identical β subunits and distinct α subunits

Platelet membrane glycoprotein (GP) IIb-IIIa is functionally and antigenically related to proteins present on many cell types, suggesting that it is a member of the proposed cytoadhesin family of membrane proteins. We have compared the purified tissue vitronectin receptor (VnR) with GP IIb-IIIa. Anti-VnR immunoprecipitated GP IIb-IIIa and a related endothelial cell protein. In immunoblots, GP IIIa reacted with anti-VnR and the β subunit of the VnR reacted with poly and monoclonal anti-GP IIIa. In contrast, the α subunit of the VnR failed to react either with a polyclonal anti-GP IIb or with monoclonal anti-GP IIb. Furthermore, the amino-terminal sequence of GP IIIa and the β subunit of VnR were identical at determined residues while the α subunit and the GP IIb were different, but showed 33% identity. These data indicate the identity or close homology of GP IIIa and the β subunit of the VnR. In contrast, the α subunit and GP IIb are distinct polypeptides which may be homologous. Since GP IIb-IIIa and the VnR differ in ligand recognition specificity, the data also suggest that this specificity may be governed by the α subunit of cytoadhesins.