Figure 9.

The phosphoinositide cycle. In the phototransduction cascade, light triggers the activation of phospholipase Cβ (PLCβ). This catalyzes hydrolysis of the membrane phospholipid PIP₂ into InsP₃ and DAG. DAG is transported by endocytosis to the endoplasmic reticulum and inactivated by phosphorylation converting it into phosphatidic acid (PA) via DAG kinase (DGK) and to CDP-DAG via CDP-DAG syntase. Subsequently, CDP-DAG is converted into phosphatidylinositol (PI), which is transferred back to the microvillar membrane, by the PI transfer protein. PIP and PIP₂ are produced at the microvillar membrane by PI kinase and PIP kinase, respectively. There are probably two PIP kinases (PIPK I, PIPK II, which are unified in the scheme). PA can also be converted back to DAG by lipid phosphate phosphohydrolase. PA is also produced from phosphatidylcholine (PC) by phospholipase D (PLD). DAG is also hydrolyzes by DAG lipase into poly unsaturated fatty acids (PUFA).