Tyr Regulates Lymphocyte-Specific Tyrosine Kinase Activity in T Cells.

TCR signaling critically depends on the tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase). Two phosphotyrosines, the activating pTyr and the inhibitory pTyr, control Lck activity. Recently, pTyr in the Lck SH2 domain emerged as a third regulator. How pTyr may affect Lck function remains unclear. In this study, we explored the role of Lck Tyr using CRISPR/Cas9-targeted knock-in mutations in the human Jurkat T cell line. Our data reveal that both Lck pTyr and pTyr are controlled by Lck Tyr Lck with a nonphosphorylated SH2 domain (Lck Phe) displayed hyperactivity, possibly by promoting Lck Tyr transphosphorylation. Lck Glu mimicking stable Lck pTyr was inhibited by Tyr hyperphosphorylation. To overcome this effect, we further mutated Tyr The resulting Lck Glu/Phe displayed strongly increased amounts of pTyr both in resting and activated T cells. Our results suggest that a fundamental role of Lck pTyr may be to protect Lck pTyr and/or pTyr to maintain a pool of already active Lck in resting T cells. This provides an additional mechanism for fine-tuning of Lck as well as T cell activity.