The CD4 and CD8 antigens are coupled to a protein-tyrosine kinase (p56lck) that phosphorylates the CD3 complex.

Many mammalian receptors have been found to regulate cell growth by virtue of a protein-tyrosine kinase domain in their cytoplasmic tail. We recently described an association of the CD4 antigen with a T-cell-specific protein-tyrosine kinase (p56lck; formerly termed pp58lck; EC 2.7.1.112). This interaction represents a potential mechanism by which T-cell growth may be regulated and offers a model by which other members of the src family (products of c-src, c-yes, c-fgr, etc.) may interact with mammalian growth factor receptors. As in the case of the CD4 antigen, the CD8 antigen appears to serve as a receptor for nonpolymorphic regions of products of the major histocompatibility complex and has been implicated in the regulation of T-cell growth. In this study, we reveal that the human CD8 antigen is also associated with the T-cell-specific protein-tyrosine kinase (p56lck). The associated p56lck kinase was detected by use of both in vitro and in vivo labeling regimes using an antiserum to the C terminus of p56lck. Two-dimensional nonequilibrium pH-gradient gel electrophoresis and sodium dodecyl sulfate/polyacrylamide gel electrophoresis demonstrated the similarity of p56lck to the protein-tyrosine kinase associated with the CD4 antigen. The catalytic activity of p56lck was revealed by the autophosphorylation of the 55- to 60-kDa kinase and the occasional labeling of a 35-kDa protein. Last, we demonstrate directly that members of the CD3 complex, including the gamma, delta, and epsilon chains, as well as a putative zeta subunit, can be phosphorylated at tyrosine residues by the CD4/CD8.p56lck complex.