A novel bifunctional phospholipase C that is regulated by Gα12 and stimulates the Ras/mitogen-activated protein kinase pathway

Three families of phospholipase C (PI-PLC beta, gamma, and delta) are known to catalyze the hydrolysis of polyphosphoinositides such as phosphatidylinositol 4,5-bisphosphate (PIP2) to generate the second messengers inositol 1,4,5 trisphosphate and diacylglycerol, leading to a cascade of intracellular responses that result in cell growth, cell differentiation, and gene expression. Here we describe the founding member of a novel, structurally distinct fourth family of PI-PLC, PLC epsilon not only contains conserved catalytic (X and Y) and regulatory domains (C2) common to other eukaryotic PLCs, but also contains two Res-associating (RA) domains and a Ras guanine nucleotide exchange factor (RasGEF) motif. PLC epsilon hydrolyzes PIP2, and this activity is stimulated selectively by a constitutively active form of the heterotrimeric G protein Ga alpha (12). PLC epsilon and a mutant (H1144L) incapable of hydrolyzing phosphoinositides promote formation of GTP-Ras, Thus PLC epsilon is a RasGEF, PLCe, the mutant H1144L, and the isolated GEF domain activate the mitogen-activated protein kinase pathway in a manner dependent on Res but independent of PIP, hydrolysis, Our findings demonstrate that PLC epsilon is a novel bifunctional enzyme that is regulated by the heterotrimeric G protein Ga alpha (12), and activates the small G protein Ras/mitogen-activated protein kinase signaling pathway.