Stimulation by G Protein βγ Subunits of Phospholipase C β Isoforms in Human Platelets

Abstract

SummaryDifferent phospholipase C (PLC) isoforms were located in human platelet cytosol and membranes. PLCγ2 and PLCβ3b were mainly located in the cytosol and PLCβ2 and PLCβ3a 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 βγ subunits were purified from human platelet cytosol and membrane fractions. Two PLC fractions from membranes were identified as PLCβ2 and PLCβ3a, and one from cytosol was PLCβ3b. These PLCβ isoforms were activated by the purified βγ subunits of brain G proteins in the order PLCβ3b > PLCβ3a > PLCβ2. Western blot analysis of γ subunits of the purified platelet G proteins with antibodies against various standard γ subunits revealed that the major component of the γ subunit of Gi2 and Gq was γ5, and that γ7 was a minor component. Studies using various subtypes of βγ subunits, βγ2, βγ3, and βγ7 purified from bovine brain, βγ5 from bovine lung, or βγ12 from bovine spleen, failed to show differences in their ability to stimulate the isolated platelet PLCβ isoforms. These results suggest that the βγ subunits of Gi2 and Gq have similar efficacy in regulation of effectors in human platelets.