The CD4 associated tyrosine protein kinase p56lck is positively regulated through its site of autophosphorylation

Antibody-mediated aggregation of the CD4 T-cell surface antigen activates bound p56lck molecules and can result in increased intracellular tyrosine protein phosphorylation. To evaluate the basis of the CD4 induced tyrosine phosphorylation signal, we have studied the ability of CD4 to regulate the function of p56lck when these two molecules are co-expressed in non-lymphoid cells. Our studies indicate that cross-linking of CD4 is capable of activation of p56lck in fibroblasts in a manner analogous to that previously reported for T-lymphocytes. They also demonstrate that replacement of the major site of autophosphorylation of p56lck (tyrosine 394) by a phenylalanine residue abolishes the ability to activate p56lck by CD4 cross-linking, implying that this residue is critical for the positive regulation of the Lck tyrosine kinase activity by CD4. Contrary to what we have previously reported for an antigen-dependent murine T-cell clone, as well as murine thymocytes, the CD4 induced activation of p56lck observed in fibroblasts does not result in marked changes in Lck tyrosine phosphorylation, suggesting that other lymphoid specific components may be required for these tyrosine phosphorylation changes.