Activation-modulated Association of 14–3–3 Proteins with Cbl in T Cells

14-3-3 proteins have recently been implicated in the regulation of intracellular signaling pathways via their interaction with several oncogene and protooncogene products. We found recently that 14–3–3 associates with several tyrosine-phosphorylated proteins and phosphatidylinositol 3-kinase (PI3-K) in T cells. We report here the identification of the 120-kDa 14–3–3τ-binding phosphoprotein present in activated T cell lysates as Cbl, a protooncogene product of unknown function which was found recently to be a major protein-tyrosine kinase (PTK) substrate, and to interact with several signaling molecules including PI3-K, in T lymphocytes. The association between 14–3–3τ and Cbl was detected both in vitro and in intact T cells and, in contrast to Raf-1, was markedly increased following T cell activation. The use of truncated 14–3–3τ fusion proteins demonstrated that the 15 C-terminal residues are required for the association between 14–3–3 and three of its target proteins, namely, Cbl, Raf-1, and PI3-K. The findings that 14–3–3τ binds both PI3-K and Cbl, together with recent reports of an association between Cbl and PI3-K, suggest that 14–3–3 dimers play a critical role in signal transduction processes by promoting and coordinating protein-protein interactions of signaling proteins. 14-3-3 proteins have recently been implicated in the regulation of intracellular signaling pathways via their interaction with several oncogene and protooncogene products. We found recently that 14–3–3 associates with several tyrosine-phosphorylated proteins and phosphatidylinositol 3-kinase (PI3-K) in T cells. We report here the identification of the 120-kDa 14–3–3τ-binding phosphoprotein present in activated T cell lysates as Cbl, a protooncogene product of unknown function which was found recently to be a major protein-tyrosine kinase (PTK) substrate, and to interact with several signaling molecules including PI3-K, in T lymphocytes. The association between 14–3–3τ and Cbl was detected both in vitro and in intact T cells and, in contrast to Raf-1, was markedly increased following T cell activation. The use of truncated 14–3–3τ fusion proteins demonstrated that the 15 C-terminal residues are required for the association between 14–3–3 and three of its target proteins, namely, Cbl, Raf-1, and PI3-K. The findings that 14–3–3τ binds both PI3-K and Cbl, together with recent reports of an association between Cbl and PI3-K, suggest that 14–3–3 dimers play a critical role in signal transduction processes by promoting and coordinating protein-protein interactions of signaling proteins.