A FAMILY OF PHOSPHOINOSITIDE 3-KINASES IN DROSOPHILA IDENTIFIES A NEWMEDIATOR OF SIGNAL-TRANSDUCTION

Background: Mammalian phosphoinositide 3-kinases (PI 3-kinases) are in volved in receptor-mediated signal transduction and have been implicat ed in processes such as transformation and mitogenesis through their r ole in elevating cellular phosphatidylinositol (3,4,5)-trisphosphate. Additionally, a PI 3-kinase activity which generates phosphatidylinosi tol 3-phosphate has been shown to be required for protein trafficking in yeast. Results: We have identified a family of three distinct PI 3- kinases in Drosophila, using an approach based on the polymerase chain reaction to amplify a region corresponding to the conserved catalytic domain of PI 3-kinases. One of these family members, PI3K_92D, is clo sely related to the prototypical PI 3-kinase, p110 alpha; PI3K_59F is homologous to Vps34p, whereas the third, PI3K_68D, is a novel PI 3-kin ase which is widely expressed throughout the Drosophila life cycle. Th r PI3K_68D cDNA encodes a protein of 210 kDa, which lacks sequences im plicated in linking p110 PI 3-kinases to p85 adaptor proteins, but con tains an amino-terminal proline-rich sequence, which could bind to SH3 domains, and a carboxy-terminal C2 domain. Biochemical analyses demon strate that PI3K_68D has a novel substrate specificity in vitro, restr icted to phosphatidylinositol and phosphatidylinositol 4-phosphate, an d is unable to phosphorylate phosphatidylinositol (4,5)-bisphosphate, the implied in vivo substrate for p110. Conclusions: A family of PI 3- kinases in Drosophila, including a novel class represented by PI3K_68D , is described. PI3K_68D has the potential to bind to signalling molec ules containing SH3 domains, lacks p85-adaptor-binding sequences, has a Ca2+-independent phospholipid-binding domain and displays a restrict ed in vitro substrate specificity, so it could define a novel signal t ransduction pathway.