CHARACTERIZATION OF A PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOINOSITIDE 3-KINASE FROM MAMMALIAN-CELLS

Background: As phosphoinositides can serve as signalling molecules wit hin cells, the enzymes responsible for their synthesis and cleavage ar e likely to be involved in the transduction of signals from the cell s urface through the cytoplasm. The precise role of the phosphoinositide 3-kinase that has been cloned from mammalian cells is not known, but it has been implicated in receptor-stimulated mitogenesis, glucose upt ake and membrane ruffling. The enzyme can use phosphatidylinositol (Pt dlns), Ptdlns 4-phosphate and PtdIns (4,5)-bisphosphate as substrates in vitro, but it seems to phosphorylate PtdIns (4,5)-bisphosphate pref erentially in vivo. The VPS34 gene product of yeast, by contrast, is a phosphoinositide 3-kinase homologue implicated in vacuolar protein so rting that apparently utilizes only Ptdlns as a substrate. The signifi cance of this difference in lipid-substrate preference and its relatio nship to the functions of the two phosphoinositide kinases is unknown. Results: We have characterized a distinct PtdIns-specific phosphoinos itide 3-kinase activity in mammalian cells. Unlike the previously iden tified, broad-specificity mammalian phosphoinositide kinase, this enzy me is resistant to the drug wortmannin and uses only PtdIns as a subst rate in vitro; it therefore has the capacity to generate Ptdlns 3-phos phate specifically. The newly characterized enzyme, which was purified by chromatography from cytosol, has biochemical and pharmacological c haracteristics distinct from those of the broad-specificity enzyme. Co nclusions: The enzyme we have characterized may serve to generate PtdI ns 3-phosphate for fundamentally different roles in the cell from thos e of PtdIns (3,4)-bisphosphate and/or PtdIns (3,4,5)-trisphosphate. Fu rthermore, the functions of the VSP34 gene product, which may not be r elevant to the broad-specificity mammalian phosphoinositide 3-kinase, may be related to those of the enzyme we describe.