STIMULATION BY G-PROTEIN BETA-GAMMA-SUBUNITS OF PHOSPHOLIPASE-C BETA-ISOFORMS IN HUMAN PLATELETS

Different phospholipase C (PLC) isoforms were located in human platele t cytosol and membranes. PLC gamma 2 and PLC beta 3b were mainly locat ed in the cytosol and PLC beta 2 and PLC beta 3a were in both cytosol and membranes by using specific antibodies against PLC isozymes (Banno Y, Nakashima S, Ohzawa M, Nozawa Y. J Biol Chem 1996; 271: 14989-94). Three PLC fractions activated by G protein beta gamma subunits were p urified from human platelet cytosol and membrane fractions. Two PLC fr actions from membranes were identified as PLC beta 2 and PLC beta 3a, and one from cytosol was PLC beta 3b. These PLC beta isoforms were act ivated by the purified beta gamma subunits of brain G proteins in the order PLC beta 3b > PLC beta 3a > PLC beta 2. Western blot analysis of gamma subunits of the purified platelet G proteins with antibodies ag ainst various standard gamma subunits revealed that the major componen t of the gamma subunit of Gi2 and Gq was gamma 5, and that gamma 7 was a minor component. Studies using various subtypes of beta gamma subun its, beta gamma 2, beta gamma 3, and beta gamma 7 purified from bovine brain, beta gamma 5 from bovine lung, or beta gamma 12 from bovine sp leen, failed to show differences in their ability to stimulate the iso lated platelet PLC beta isoforms. These results suggest that the beta gamma subunits of Gi2 and Gq have similar efficacy in relation of effe ctors in human platelets.