Type I phosphatidylinositol-4-phosphate 5-kinases - Cloning of the third isoform and deletion/substitution analysis of members of this novel lipid kinase family

Type I phosphatidylinositol 4-phosphate (PtdIns(4)P) 5-kinases (PIP5K) catalyze the synthesis of phosphatidylinositol 4,5-bisphosphate, an essential lipid molecule in various cellular processes. Here, we report the cloning of the third member (PIP5K gamma) and the characterization of members of the type I PIP5K family. Type I PIP5K gamma has two alternative splicing forms, migrating at 87 and 90 kDa on SDS-polyacrylamide gel electrophoresis. The amino acid sequence of the central portion of this isoform shows approximately 80% identity with those of the alpha and beta isoforms, Northern blot analysis revealed that the gamma isoform is highly expressed in the brain, lung, and kidneys. Among three isoforms, the beta isoform has the greatest V-max value for the PtdIns(4)P kinase activity and the gamma isoform is most markedly stimulated by phosphatidic acid. By analyzing deletion mutants of the three isoforms, the minimal kinase core sequence of these isoforms were determined as an approximately 380-amino acid region, In addition, carboxyl-terminal regions of the beta and gamma isoforms were found to confer the greatest V-max value and the highest phosphatidic acid sensitivity, respectively, It was also discovered that lysine 138 in the putative ATP binding motif of the alpha isoform is essential for the PtdIns(4)P kinase activity, As was the case with the alpha isoform reported previously (Shibasaki, Y., Ishihara, H., Kizuki, N., Asano, T., Oka, Y., Yazaki; Y. (1997) J. Biol. Chem. 272, 7578-7581), overexpression of either the beta or the gamma isoform induced an increase in short actin fibers and a decrease in actin stress fibers in COS7 cells, Surprisingly, a kinase-deficient substitution mutant also induced an abnormal actin polymerization, suggesting a role of PIP5Ks via structural interactions with other molecules.