Phosphatidylinositol (PI) 3-Kinase and PI 4-Kinase Binding to the CD4-p56^lck Complex: the p56^lck SH3 Domain Binds to PI 3-Kinase but Not PI 4-Kinase

Abstract

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 gp120. It is coupled to the protein-tyrosine kinase p56^lck, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56^lck possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56^lck 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 pp60^src and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56^lck 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-p56^lck 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 gp120 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 p56^lck, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56^lck. CD4-p56^lck contributes PI 3- and PI 4-kinase to the activation process of T cells and may play a role in HIV-1-induced immune defects.