Genetic and biochemical studies have shown that the phosphatidylinositol (PtdIns) 3-kinase encoded by the yeast VPS34 gene is required for the efficient sorting and delivery of proteins to the vacuole. A human homologue of the yeast VPS34 gene product has recently been characterized as part of a complex with a cellular protein of 150 kDa (Volinia, S., Dhand, R., Vanhaesebroeck, B., MacDougall, L. K., Stein, R., Zvelebil, M. J., Domin, J., Panaretou, C., and Waterfield, M. D. (1995) EMBO J. 14, 3339-3348). Here, cDNA cloning is used to show that the amino acid sequence of this protein, termed p150, is 29.6% identical and 53% similar to the yeast Vps15p protein, an established regulator of Vps34p. Northern blot analysis showed a ubiquitous tissue distribution for p150 similar to that previously observed with PtdIns 3-kinase. Recombinant p150 associated with PtdIns 3-kinase in vitro in a stable manner, resulting in a 2-fold increase in lipid kinase activity. Addition of phosphatidylinositol transfer protein (PI-TP) further stimulated the lipid kinase activity of the p150.PtdIns 3-kinase complex 3-fold. A PtdIns 3-kinase activity could also be co-immunoprecipitated from human cell lysates using anti-PI-TP antisera. This observation demonstrates that an interaction between a PtdIns 3-kinase and PI-TP occurs in vivo, which further implicates lipid transfer proteins in the regulation of PtdIns 3-kinase activity. These results suggest that the Vps15p.Vps34p complex has been conserved from yeast to man and in both species is involved in protein trafficking.