Regulation of phospholipase C-delta1 through direct interactions with the small GTPase Ral and calmodulin

Second messengers generated from membrane lipids play a critical role in signaling and control diverse cellular processes. Despite being one of the most evolutionarily conserved of all the phosphoinositide-specific phospholipase C (PLC) isoforms, a family of enzymes responsible for hydrolysis of the membrane lipid phosphatidylinositol bisphosphate, the mechanism of PLC-delta1 activation is still poorly understood. Here we report a novel regulatory mechanism for PLC-delta1 activation that involves direct interaction of the small GTPase Ral and the universal calcium-signaling molecule calmodulin (CaM) with PLC-delta1. In addition, we have identified a novel IQ type CaM binding motif within the catalytic region of PLC-delta1 that is not found in other PLC isoforms. Binding of CaM at the IQ motif inhibits PLC-delta1 activity, while addition of Ral reverses the inhibition. The overexpression of various Ral mutants in cells potentiates PLC-delta1 activity. Thus, the Ral-CaM complex defines a multifaceted regulatory mechanism for PLC-delta1 activation.