Figure 1.

Pathways of PtdIns(3,4,5)P₃ synthesis. (A) Phosphatidylinositol (PtdIns) is the basic building block of phosphorylated phosphoinositides (PIs). The inositol head group can be phosphorylated in the 3-, 4- and 5-positions to generate derivatives of PtdIns, which are listed on the right. (B) Schematic for receptor-driven PtdIns (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃) production. Agonist stimulation (tyrosine kinase or G-protein-coupled receptors) activates class I PI 3-kinase to induce phosphorylation of PtdIns(4,5)P₂ on the 3-position of the inositol group. (C) The proposed new pathway for PtdIns(3,4,5)P₃ production in Schizosaccharomyces pombe. Fission yeast enzymes are shown in blue. Their mammalian counterparts are depicted in turquoise. PIP 5-K, phosphatidylinositol-4-phosphate 5-kinase; PTEN, phosphatase and tensin homologue.