TYPE-I PHOSPHATIDYLINOSITOL-4-PHOSPHATE 5-KINASES SYNTHESIZE THE NOVEL LIPIDS PHOSPHATIDYLINOSITOL 3,5-BISPHOSPHATE AND PHOSPHATIDYLINOSITOL 5-PHOSPHATE

Inositol phospholipids regulate a variety of cellular processes includ ing proliferation, survival, vesicular trafficking, and cytoskeletal o rganization. Recently, two novel phosphoinositides, phosphatidylinosit ol-3,5-bisphosphate (PtdIns-3,5-P-2) and phosphatidylinositol-5-phosph ate (PtdIns-5-P), have been shown to exist in cells. PtdIns-3,5-P-2, w hich is regulated by osmotic stress, appears to be synthesized by phos phorylation of PtdIns-3-P at the D-5 position. No evidence yet exists for how PtdIns-5-P is produced in cells. Understanding the regulation of synthesis of these molecules will be important for identifying thei r function in cellular signaling. To determine the pathway by which Pt dins-3,5-P-2 and PtdIns-5-P might be synthesized, we tested the abilit y of the recently cloned type I PtdIns-4-P 5-kinases (PIP5Ks) alpha an d beta to phosphorylate PtdIns-3-P and PtdIns at the D-5 position of t he inositol ring.We found that the type I PIP5Ks phosphorylate PtdIns- 3-P to form Ptdins-3,B-P-2. The identity of the PtdIns-3,B-P-2 product was determined by anion exchange high performance liquid chromatograp hy analysis and periodate treatment. PtdIns-3,4-P-2 and PtdIns-3,4,5-P -3 were also produced from PtdIns-3-P phosphorylation by both isoforms . When expressed in mammalian cells, PIP5K I alpha and PIP5K I beta di ffered in their ability to synthesize PtdIns-3,5-P-2 relative to PtdIn s-3,4-P-2. me also found that the type I PIP5Ks phosphorylate PtdIns t o produce PtdIns-5-P and phosphorylate PtdIns-3,4-P-2 to produce PtdIn s-3,4,5-P-3. Our findings suggest that type I PIP5Ks synthesize the no vel phospholipids PtdIns-3,B-P-2 and PtdIns-5-P. The ability of PIP5Ks to produce multiple signaling molecules indicates that they may parti cipate in a variety of cellular processes.