Platelet Glycoproteins la, Ic, and Ila Are Physicochemically Indistinguishable from the Very Late Activation Antigens Adhesion-related Proteins of Lymphocytes and Other Cell Types

The very late activation antigens (VLA) are a subset of the superfamily of cell surface glycoproteins that serve as recep-tors from extracellular matrix proteins. One or more of the VLA heterodimers are present on T lymphocytes and most other cell types, including platelets. We have used VLA-spe-cific monoclonal antibodies to isolate the reactive platelet membrane molecules. We have identified them as previously characterized platelet surface glycoproteins and have com-pared them with VLA molecules isolated from lymphocytes and other cells. Utilizing one-dimensional SDS-PAGE, two-dimensional O'Farrell gel electrophoresis, and nonreduced-re-duced two-dimensional gel electrophoresis, we show that re-duced VLA molecules of platelets are composed of three chains of molecular weights 165,000, 145,000, and 140,000 that possess the physicochemical properties of platelet glyco-proteins GPIa, GPIca, and GPIIa. GPIa corresponds to the VLA 165,000 a2-chain, GPIIa corresponds to a 145,000 Mr VLA,8-chain, and GPIca corresponds to a 140,000 Mr VLA a-chain. The polypeptide structure of VLA molecules on platelets and lymphocytes are very similar or identical. Platelet proteins GPIa and GPIIa exist as a mixed heterodimer in detergent lysates and correspond with the VLA-2 heterodimer found on activated T lymphocytes and other cell types. The platelet gly-coproteins GPIIa and GPIc form a second mixed heterodimer. The mAb A-1A5, which binds to the VLA /3 chain, binds to platelet GPIIa and precipitates both the GPIIa-GPIa and GPIIa-GPIc heterodimers, and binds to 4,926±740 sites per platelet. A VLA-2-specific mAb, 12F1, which binds to the VLA a2-chain reacts with GPIa and immunoprecipitates only the GPIIa-GPIa heterodimer, and binds to 1,842±449 sites per platelet. The similarity ofVLA chains and platelet GPIIa, GPIa, and GPIc molecules suggests that these molecules may have similar functions on various cell types