A novel bifunctional phospholipase C that is regulated by G alpha(12) 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 phosphatidylin ositol 4,5-bisphosphate (PIP2) to generate the second messengers inositol 1 ,4,5 trisphosphate and diacylglycerol, leading to a cascade of intracellula r responses that result in cell growth, cell differentiation, and gene expr ession. Here we describe the founding member of a novel, structurally disti nct fourth family of PI-PLC, PLC epsilon not only contains conserved cataly tic (X and Y) and regulatory domains (C2) common to other eukaryotic PLCs, but also contains two Res-associating (RA) domains and a Ras guanine nucleo tide exchange factor (RasGEF) motif. PLC epsilon hydrolyzes PIP2, and this activity is stimulated selectively by a constitutively active form of the h eterotrimeric G protein Ga alpha (12). PLC epsilon and a mutant (H1144L) in capable of hydrolyzing phosphoinositides promote formation of GTP-Ras, Thus PLC epsilon is a RasGEF, PLCe, the mutant H1144L, and the isolated GEF dom ain activate the mitogen-activated protein kinase pathway in a manner depen dent on Res but independent of PIP, hydrolysis, Our findings demonstrate th at PLC epsilon is a novel bifunctional enzyme that is regulated by the hete rotrimeric G protein Ga alpha (12), and activates the small G protein Ras/m itogen-activated protein kinase signaling pathway.