Stimulation by G protein betagamma subunits of phospholipase C beta isoforms in human platelets

Different phospholipase C (PLC) isoforms were located in human platelet cytosol and membranes. PLCgamma2 and PLCbeta3b were mainly located in the cytosol and PLCbeta2 and PLCbeta3a 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 betagamma subunits were purified from human platelet cytosol and membrane fractions. Two PLC fractions from membranes were identified as PLCbeta2 and PLCbeta3a, and one from cytosol was PLCbeta3b. These PLCbeta isoforms were activated by the purified betagamma subunits of brain G proteins in the order PLCbeta3b > PLCbeta3a > PLCbeta2. Western blot analysis of gamma subunits of the purified platelet G proteins with antibodies against various standard gamma subunits revealed that the major component of the gamma subunit of Gi2 and Gq was gamma5, and that gamma7 was a minor component. Studies using various subtypes of betagamma subunits, betagamma2, betagamma3, and betagamma7 purified from bovine brain, betagamma5 from bovine lung, or betagamma12 from bovine spleen, failed to show differences in their ability to stimulate the isolated platelet PLCbeta isoforms. These results suggest that the betagamma subunits of Gi2 and Gq have similar efficacy in regulation of effectors in human platelets.