Fate of phosphatidylinositol 4,5-biphosphate (PIP2) in platelets and second messenger production

This figure shows the outcome of agonist activation of platelet phospholipase C beta (PLC) via the G alpha-q subunit and activation of phosphoinositol 3-kinase gamma (PI3K) via the G beta-gamma subunit. In both cases, the substrate is membrane-associated phosphatidylinositol 4,5-biphosphate (PIP2), shown in the top center of the figure. The green arrows point to the number 1 and 3 carbons in the inositol ring. Hydrolysis of PIP2 by PLC yields 1,4,5-inositol triphosphate (IP3), which raises the cytosolic Ca++ concentration by releasing sequestered Ca++ from the dense tubular system and diacylglycerol (DAG), which activates protein kinase C. Phosphorylation of the number 3 inositol carbon of PIP2 by PI3K generates the second messenger phosphoinositol 3,4,5-triphosphate (PIP3), important for stable platelet aggregation.