TYPE-I PHOSPHATIDYLINOSITOL-4-PHOSPHATE 5-KINASES ARE DISTINCT MEMBERS OF THIS NOVEL LIPID KINASE FAMILY

Phosphatidylinositol-4-phosphate 5-kinases (PIP5K) synthesize phosphat idylinositol-4,5-bisphosphate, a key precursor in phosphoinositide sig naling that also regulates some proteins and cellular processes direct ly. Two distinct PIP5Ks have been characterized in erythrocytes, the 6 8 kDa type I (PIP5KI) and 53-kDa type II (PIP5KII) isoforms. Using pep tide sequences from the erythroid 68-kDa PIP5KI, we have isolated cDNA s encoding PIP5KI alpha from human brain. Partial cDNAs obtained for a second isoform, PIP5KI beta, established that the human STM7 gene enc oded a previously unrecognized PIP5KI. However, the peptide sequences demonstrated that erythroid PIP5KI corresponded to PIP5KI alpha. Recom binant, bacterially expressed PIP5KI alpha possessed PIP5K activity an d was immunoreactive with erythroid PIP5KI antibodies. By Northern ana lysis, PIP5KI alpha and PIP5KI beta had wide tissue distributions, but their expression levels differed greatly. PIP5KIs had homology to the kinase domains of PIP5KII alpha, yeast Mss4p and Fab1p, and a new Cae norhabditis elegans Fab1-like protein identified in the data base. The se new isoforms have refined the sequence requirements for PIP5K activ ity and, potentially, regulation of these enzymes. Furthermore, the li mited homology between PLP5KIs and PIP5KII alpha, which was almost exc lusively within the kinase domain core, provided a molecular basis for distinction between type I and II PIP5Ks.