Mechanisms of phospholipase C activation: a comparison with the adenylate cyclase system

Many hormones, neurotransmitters or other signaling molecules exert their biological activities through the stimulation of a specific phospholipase C. Once activated, this enzyme hydrolyzes polyphosphoinositide into inositol trisphosphate and diacylglycerol, two products known to regulate the cytosolic calcium concentration and the activity of protein kinase C, respectively. The molecular mechanisms leading to the activation of phospholipase C after the binding of the signal molecule to its specific receptor remain unclear. Yet, recent studies demonstrated that at least three molecules were implicated: the receptor, the phospholipase C and a GTP binding protein. In this review, we have summarized the properties of such systems and, more particularly, those of the vasopressin-sensitive phospholipase C present in WRK1 cells. The existence of many functional and structural analogies for the receptors which regulate adenylate cyclase activity is discussed.