Agonist-dependent formation of the second messengers inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG).
The inositol lipids that function in signalling are embedded in the inner leaflet of the plasma membrane. The precursor lipid is phosphatidylinositol (PtdIns), which is successively phosphorylated, first on the 4-position to form PtdIns4P and then on the 5-position to form PtdIns4,5P2. Activated cell-surface receptors are coupled through the G protein Gq/11 to phospholipase C (PLC) that hydrolyses PtdIns4,5P2 to generate inositol 1,4,5-trisphosphate (InsP3). InsP3 activates Ca2+ signalling, and diacylglycerol (DAG) stimulates protein kinase C (PKC) to initiate protein phosphorylation (Module 2: Figure PKC structure and activation).