ISOZYME-SELECTIVE STIMULATION OF PHOSPHOLIPASE C-BETA-2 BY G-PROTEIN BETA-GAMMA-SUBUNITS

HYDROLYSIS by phospholipase C (PLC) of phosphatidylinositol 4,5-bisphosphate is a key mechanism by which many extracellular signalling molecules regulate functions of their target cells1,2. At least eight distinct isozymes of PLC are recognized in mammalian cells3,4. Receptor-controlled PLC is often regulated by G proteins, which can be modified by pertussis toxin in some cells but not in others5,6. In the latter cells, PLC-beta1, but not PLC-gamma1 or PLC-delta1, may be activated by members of the alpha(q)-subfamily of the G protein alpha-subunits7-10. An unidentified PLC in soluble fractions of cultured human HL-60 granulocytes is specifically stimulated by G protein betagamma subunits purified from retina and brain11. Identification of a second PLC-beta complementary DNA (PLC-beta2) in an HL-60 cell cDNA library9 prompted us to investigate the effect of purified G protein betagamma subunits on the activities of PLC-beta1 and PLC-beta2 transiently expressed in cultured mammalian cells. We report here that PLC-beta1 and PLC-beta2 were stimulated by free betagamma subunits and that PLC-beta2 was the most sensitive to betagamma stimulation. Thus stimulation of PLC by betagamma subunits is isozyme-selective and PLC-beta2 is a prime target of betagamma stimulation. Activation of PLC-beta2 by betagamma subunits may be an important mechanism by which pertussis toxin-sensitive G proteins stimulate PLC.