A Raf-1-related p110 polypeptide associates with the CD4-p56lck complex in T cells.

The CD4 and CD8 antigens on T cells have been shown to associate with the Src family member p56lck and a GTP-binding protein, p32. The identification of receptor interactions with intracellular mediators is essential in the elucidation of downstream signals mediated by engagement of these receptor complexes. In this study, we report the detection of an additional 110-kDa polypeptide (p110) associated with the CD4-p56lck complex in human peripheral blood T lymphocytes and leukemic T-cell lines. p110 bound preferentially to CD4-p56lck as an assembled complex and poorly, if at all, to the individual components. p110 was recognized directly by an antiserum to the C-terminal region of the serine/threonine kinase Raf-1 and is related to a p110 polypeptide detected in anti-Raf-1 immunoprecipitates. Despite its association with the CD4-p56lck complex, p110 was found to be phosphorylated predominantly on serine residues. Furthermore, phorbol ester treatment of cells resulted in a transient increase in the detection of p110 associated with CD4-p56lck, concomitant with the modulation of CD4-p56lck from the cell surface. This Raf-1-related p110 is therefore likely to play a role in signals generated from the CD4-p56lck complex. p110 may serve as a bridge between the CD4-p56lck complex and the serine/threonine kinase pathways of T-cell activation.