Activation of phospholipase C-ε by heterotrimeric G protein βγ-subunits

PLC-epsilon was identified recently as a phosphoinositide-hydrolyzing phospholipase C (PLC) containing catalytic domains (X, Y, and C2) common to all PLC isozymes as well as unique CDC25- and Ras-associating domains. Novel regulation of this PLC isozyme by the Ras oncoprotein and a-subunits (G alpha (12)) of heterotrimeric G proteins was illustrated. Sequence analyses of PLC-epsilon revealed previously unrecognized PH and EF-hand domains in the amino terminus. The known interaction of G beta gamma subunits with the PH domains of other proteins led us to examine the capacity of G beta gamma to activate PLC-epsilon. Co-expression of G beta (1)gamma (2) with PLC-epsilon in COS-7 cells resulted in marked stimulation of phospholipase C activity. G beta (2) and G beta4 in combination with G gamma (1), G gamma (2), G gamma (3), or G gamma (13) also activated PLC-epsilon to levels similar to those observed with G beta (1)-containing dimers of these Gy-subunits. G beta3 in combination with the same Gy-subunits was less active, and G beta (5)-containing dimers were essentially inactive. G beta gamma -promoted activation of PLC-epsilon was blocked by cotransfection with either of two G beta gamma -interacting proteins, G alpha (i1) or the carboxyl terminus of G protein receptor kinase 2. Pharmacological inhibition of PI3-kinase-gamma had no effect on G beta (1)gamma (2)-promoted activation of PLC-epsilon. Similarly, activation of Ras in the action of G beta gamma is unlikely, because a mutation in the second RA domain of PLC-epsilon that blocks Ras activation of PLC failed to alter the stimulatory activity of G beta (1)gamma (2). Taken together, these results reveal the presence of additional functional domains in PLC-epsilon and add a new level of complexity in the regulation of this novel enzyme by heterotrimeric G proteins.