Cloning of a human type II phosphatidylinositol 4-kinase reveals a novel lipid kinase family

Phosphoinositide lipids regulate numerous cellular processes in all eukaryo tes, The versatility of this phospholipid is provided by combinations of ph osphorylation on the 3 ', 4 ', and 5 ' positions of the inositol head group . Two distinct structural families of phosphoinositide (PI) kinases have so far been identified and named after their prototypic members, the pi 3-kin ase and phosphatidylinositol (PtdIns) phosphate kinase families, both of wh ich have been found to contain structural homologues possessing PI 4-kinase activity. Nevertheless, the prevalent PtdIns 4-kinase activity in many mam malian cell types is conferred by the widespread type II PtdIns 4-kinase, w hich has so far resisted molecular characterization. We have partially puri fied the human type II isoform from plasma membrane rafts of human A431 epi dermoid carcinoma cells and obtained peptide mass and sequence data. The re sults allowed the cDNA containing the full open reading frame to, be cloned . The predicted amino acid sequence revealed that the type II enzyme is the prototypic member of a novel, third family of PI kinases, We have named th e purified protein type II alpha and a second human isoform, type II beta. The type II alpha mRNA appears to be expressed ubiquitously in human tissue s, and homologues appear to be expressed in all eukaryotes.