Fig. 1.

A: Phosphatidylinositol (PI). B: Naturally occurring PIs. Phosphatidylinositol contain a D-myo-inositol head group, a glycerol backbone, and two fatty acids at the C1 and C2 acyl positions of glycerol (A). Phosphorylation of multiple free hydroxyls in the inositol head group generates several phosphorylated PI derivatives. Differential phosphorylation at the 3, 4, and 5 positions allows for the generation of seven distinct phosphoinositides (B). PI-4-P, phosphatidylinositol 4-phosphate; PI3-P, phosphatidylinositol 3-phosphate; PI-5-P, phosphatidylinositol 5-phosphate; PI-3,4-P₂, phosphatidylinositol 3,4-bisphosphate; PI-4-5-P₂, phosphatidylinositol 4,5-bisphosphate; PI-3-5-P₂, phosphatidylinositol 3,5-bisphosphate; PI-3,4,5-P₃, phosphatidylinositol 3,4,5-trisphosphate.