A phosphatidylinositol (PtdIns) 4-kinase cDNA cloned from a rat brain cDNA library encoded a protein of 816 amino acids with a calculated molecular mass of 91654 Da. This molecule contained a lipid-kinase-unique domain and a presumed lipid/protein kinase homology domain that are found in other PtdIns 4-kinases and PtdIns 3-kinases. Furthermore, this kinase molecule had 43.3% shared identity with the presumed catalytic domain of yeast PtdIns 4-kinase, PtdInsK1, and the two molecules had a region of similarity that is not conserved in other lipid kinases. By examining PtdIns kinase activity in transfected COS-7 cells using epitope tag immunoprecipitation as well as conventional methods, the product PtdIns phosphate was identified as phosphatidylinositol 4-phosphate (PtdIns4P), but not phosphatidylinositol 3-phosphate (PtdIns3P). The PtdIns 4-kinase activity was recovered predominantly from the soluble fraction and the activity was markedly enhanced in the presence of Triton X-100 and was relatively insensitive to inhibition by adenosine. In addition, the PtdIns 4-kinase activity was completely inhibited in the presence of 10 µM wortmannin. When examined by epitope tag immunocytochemistry, the immunoreactivity for the PtdIns 4-kinase molecule was dominantly aggregated in a cytoplasmic region juxtaposed to the nuclei and was faintly but widely dispersed in the cytoplasm. By in situ hybridization analysis, the mRNA for PtdIns 4-kinase was expressed ubiquitously and was detected in most neurons throughout the grey matter of the brain, with higher expression intensity found in fetal than in adult brain.
The EMBL, GenBank, DDBJ accession number for the phosphatidylinositol 4-kinase sequence reported in this paper is D84667.