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 including proliferation, survival, vesicular trafficking, and cytoskeletal organization. Recently, two novel phosphoinositides, phosphatidylinositol-3,5-bisphosphate (PtdIns-3,5-P-2) and phosphatidylinositol-5-phosphate (PtdIns-5-P), have been shown to exist in cells. PtdIns-3,5-P-2, which is regulated by osmotic stress, appears to be synthesized by phosphorylation 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 their function in cellular signaling. To determine the pathway by which Ptdins-3,5-P-2 and PtdIns-5-P might be synthesized, we tested the ability of the recently cloned type I PtdIns-4-P 5-kinases (PIP5Ks) alpha and beta to phosphorylate PtdIns-3-P and PtdIns at the D-5 position of the 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 chromatography 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 differed in their ability to synthesize PtdIns-3,5-P-2 relative to PtdIns-3,4-P-2. me also found that the type I PIP5Ks phosphorylate PtdIns to produce PtdIns-5-P and phosphorylate PtdIns-3,4-P-2 to produce PtdIns-3,4,5-P-3. Our findings suggest that type I PIP5Ks synthesize the novel phospholipids PtdIns-3,B-P-2 and PtdIns-5-P. The ability of PIP5Ks to produce multiple signaling molecules indicates that they may participate in a variety of cellular processes.