CD4 serves as a receptor for major histocompatibility complex class II antigens and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gpl20. It is coupled to the protein-tyrosine kinase p56kk, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56kk possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56kk generates intracellular signals is unclear, although it has the potential to interact with various downstream molecules. One such downstream target is the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase), which has been found to bind to activated pp64W" and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56"k complex as judged by the presence of PI 3-phosphate generated from anti-CD4 immunoprecipitates and detected by high-pressure liquid chromatographic analysis. However, surprisingly, CD4-p56kk was also found to associate with another lipid kinase, phosphatidylinositol 4-kinase (PI 4-kinase). The level of associated PI 4-kinase was generally higher than PI 3-kinase activity. HIV-1 gpl20 and antibody-mediated cross-linking induced a 5-to 10-fold increase in the level of CD4-associated PI 4-and PI 3-kinases. The use of glutathione S-transferase fusion proteins carrying Lck-SH2, Lck-SH3, and Lck-SH2/SH3 domains showed PI 3-kinase binding to the SH3 domain of p56kk, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56kk. CD4-p56kk contributes PI 3-and PI 4-kinases to the activation process of T cells and may play a role in HIV-1-induced immune defects.CD4 is a 55-kDa transmembrane glycoprotein present in T helper cells and serves as a receptor for major histocompatibility complex class II antigens and also for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gpl20 (6,18,41,55,60 bisphosphate (PI-4,5-P2) (10, 12). It is a heterodimeric protein, comprising a regulatory subunit (p85) and a catalytic subunit (p110) (13,22,31,51,64). p85 includes two Src homology 2 (SH2) domains which mediate binding to phosphotyrosine residues within the cytoplasmic tail of receptor tyrosine kinases such as the platelet-derived growth factor receptor (PDGF-R) (17,24,39). p85 also binds to activated pp6Q' (26, 67, 78; see