The phosphoinositide cascade plays a central role in the transduction of many extracellular stimuli. Activation of plasma membrane-associated phosphoinositide-specific phospholipase C stimulates the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdInsP2 ) and generates inositol 1,4,5-trisphosphate (Ins(1,4,5)P3 ), which in turn releases calcium from intracellular stores (1 ).