p56lck association with CD4 is required for the interaction between CD4 and the TCR/CD3 complex and for optimal antigen stimulation.

By fluorescence resonance energy transfer, we have previously demonstrated that upon anti-CD3 mAb-mediated activation of a murine T cell hybridoma expressing human CD4, CD4 moves into close association with the TCR/CD3 complex. It was shown that this association between CD4 and the TCR/CD3 complex was dependent upon the presence of an intact CD4 cytoplasmic domain. We have now expressed, in a murine T cell hybridoma, mutated forms of CD4 containing cysteine to serine point mutations at positions 420, 422, or 430. The mutations at positions 420 and 422, but not 430, abolish association with p56lck. By using fluorescence resonance energy transfer, we demonstrate that mutations of CD4 which fail to interact with p56lck are unable to associate with the TCR/CD3 complex under conditions in which wild-type CD4 and the 430 mutant CD4 do associate with the TCR/CD3 complex. In addition, these mutants have a diminished response to CD4-dependent stimuli. We conclude that the association between CD4 and the TCR/CD3 complex during T cell activation plays an important role in CD4-dependent responsiveness and this association requires the interaction of CD4 with p56lck. These results also suggest that a substrate for p56lck may be expressed in the TCR/CD3 complex.