SH2 DOMAIN FUNCTION IS ESSENTIAL FOR THE ROLE OF THE LCK TYROSINE KINASE IN T-CELL RECEPTOR SIGNAL-TRANSDUCTION

Tyrosine kinase activity is required for signal transduction through t he T cell antigen receptor (TCR), The Src family tyrosine kinase Lck a ppears to play a key role in the initiation of TCR signaling events. W e have investigated the role of the phosphotyrosine-binding Src homolo gy-2 (SH2), domain of Lck in TCR signaling. Lck containing a mutation in the phosphotyrosine binding pocket of the SH2 domain was expressed in an Lck-deficient cell line, We found that, in contrast to wild-type Lck, the SH2 domain mutant was unable to restore even the earliest TC R-mediated signaling events, To investigate the role of the Lck SH2 do main, we examined the association of tyrosine phosphoproteins with Lck , The predominant associated phosphoprotein was the ZAP-70 tyrosine ki nase, which has also been implicated in the initiation of TCR signalin g, In addition, the zeta subunit of the T cell receptor was found to w eakly associate with Lck. Further analysis indicated that the SH2 doma in of Lck can directly recognize both ZAP-70 and zeta in immunoprecipi tates from TCR-stimulated cells. Our findings demonstrate that the SH2 domain of Lck is essential for the initiation of signaling events fol lowing TCR stimulation probably as a result of-its ability to mediate an interaction between Lck and the ZAP-70 tyrosine kinase and/or the z eta subunit of the T cell receptor.