Activation of phospholipase C isozymes by G protein beta gamma subunits

The beta gamma subunits of guanine nucleotide-binding proteins (G proteins) have been shown to activate unidentified phospholipase C (PLC) isozymes (Camps, M., Hou, C., Sidiropoulos, D., Stock, J. B., Jakobs, K. H., and Gierschik, P. (1992) Eur. J. Biochem. 206, 821-831; Blank, J. L., Brattain, K. A., and Exton, J. H. (1992) J. Biol. Chem. 267, 23069-23075). To identify these target PLC isozymes, we measured the effect of bovine brain G protein beta gamma subunits on PLC-beta 1, PLC-beta 2, PLC-beta 3, PLC-gamma 1, and PLC-delta 1 activity by reconstituting purified protein components with lipid vesicles containing [3H]phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2). A nearly saturating concentration of beta gamma produced 2.5-, 4-, 8.5-, and 2-fold increases in PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-delta 1 activity, respectively, and no activation of PLC-gamma 1, in the presence of 0.2 microM free Ca2+. The beta gamma-dependent activation of the PLC-beta isozymes does not appear to be the result of increased affinity of the enzymes for Ca2+. The beta gamma-dependent PLC activation could be reversed by addition of the GDP-bound form of the alpha subunit of G(o). The alpha subunits of Gq class G proteins have been shown to activate PLC-beta isozymes in the order of PLC-beta 1 > or = PLC-beta 3 >> PLC-beta 2, which differs from the order of PLC-beta 3 > PLC-beta 2 > PLC-beta 1 for beta gamma-dependent activation. Furthermore, the half-maximal concentration of beta gamma (25 nM) required to activate PLC-beta 3 is much higher than that of Gq alpha subunits (0.6 nM) required to activate PLC-beta 1. These results suggest that the extracellular signals that induce the dissociation of G(o) or Gi, the heterotrimeric G proteins abundant in brain, should enhance the hydrolysis of PtdIns 4,5-P2 in brain primarily through activation of PLC-beta 3 (PLC-beta 2 is not detectable in brain). However, signals that activate the less abundant Gq class heterotrimers should result in the activation primarily of PLC-beta 1 and PLC-beta 3 by the corresponding alpha subunits.