The PI(4,5)P<sub>2</sub> cycle or simply PIP<sub>2</sub> cycle (also known as PI cycle in past) is one of the important signalling cascades underlying many cellular functions including GPCR signaling, cytokinesis, endocytosis, and apoptosis.
PIP<sub>2</sub> cycle involves steps of chemical reactions characterized by various enzymes. First hydrolysis of PIP<sub>2</sub> molecules by activated phospholipase C leads to formation of diacylglycerol and inositol trisphosphate. These two are themselves important second messengers. Diacylglycerol further converted into phosphatidic acid (PA) by DAGK enzyme. Furthermore, CDP-diacylglycerol synthetase uses CTP to create cytidine diphosphate diacylglycerol from PA. Generated CDP-DAG then condensed to PI by phosphatidylinositol synthase. This PI is then phosphorylated to PI4P and back to PIP<sub>2</sub> via action of PI4K and PIP5K