Direct interaction of the CD38 cytoplasmic tail and the Lck SH2 domain. CD38 transduces T cell activation signals through associated Lck

CD38 ligation has been shown to induce activation of intracellular signaling cascade in T lymphocytes through a Lck-dependent pathway. However, it is not clear how Lck initiates the CD38-mediated signaling process. In the present study, we showed that CD38 and Lck were physically associated through the cytoplasmic tail and the Src homology 2 domain, respectively. This was evidenced by coimmunoprecipitation of Lck with CD38 and Lck with isolated CD38 cytoplasmic domain from T cell lysate, cell lysate of COS-7 cells cotransfected with cDNAs of Lck and CD38, or a mixture of in vitro translated CD38 and Lck. Because the CD38 cytoplasmic domain does not contain any tyrosine residue, the interaction should be independent of phosphotyrosine. The interaction was further confirmed by in vitro interaction between a purified Lck Src homology 2 domain and a nonphosphosynthetic peptide corresponding to the membrane proximal region of the CD38 cytoplasmic domain. In addition, CD38 ligation resulted in an elevated tyrosine kinase activity of the CD38-associated Lck and ultimate activation of interleukin-2 gene transcription. Furthermore, expression of a kinase-deficient Lck mutant suppressed interleukin-2 gene activation in a dose-dependent manner. These results strongly suggested that CD38 ligation indeed tranduced signals for T cell activation using its associated Lck.