The cytoplasmic domain of CD4 is required for stable association with the lymphocyte-specific tyrosine protein kinase p56lck.

The CD4 T cell surface molecule binds MHC class II determinants expressed on antigen-presenting cells. CD4 is thought to enhance T cell activation by serving as an adhesion molecule as well as possibly by transducing an independent intracellular signal during the process of antigen stimulation. The recent observation that CD4 is physically associated with the Src-related tyrosine protein kinase p56lck suggests that tyrosine phosphorylation might be involved in these CD4 "signaling" events. The results presented in this report demonstrate that deletion of the cytoplasmic domain of CD4 significantly diminishes its ability to stably associate with p56lck. This observation provides a biochemical basis for the decreased ability of this mutant CD4 molecule to enhance T cell activation during suboptimal antigen stimulation.