The Src Family kinase Lck sets a critical threshold for T cell activation because it phosphorylates the TCR complex and the Zap70 kinase. How a T cell controls the abundance of active Lck molecules remains poorly understood. We have identified an unappreciated role for a phosphosite, Y192, within the Lck SH2 domain that profoundly affects the amount of active Lck in cells. Notably, mutation of Y192 blocks critical TCR-proximal signaling events and impairs thymocyte development in retrogenic mice. We determined that these defects are caused by hyperphosphorylation of the inhibitory C-terminal tail of Lck. Our findings reveal that modification of Y192 inhibits the ability of CD45 to associate with Lck in cells and dephosphorylate the C-terminal tail of Lck, which prevents its adoption of an active open conformation. These results suggest a negative feedback loop that responds to signaling events that tune active Lck amounts and TCR sensitivity.

Src家族激酶Lck为T细胞活化设定了关键阈值，因为它使TCR复合物和Zap70激酶磷酸化。T细胞如何控制活性Lck分子的丰度仍然知之甚少。我们已经确定了Lck SH2结构域中的磷酸位点Y192的一个未知的作用，该作用深刻影响细胞中活性Lck的量。值得注意的是，Y192的突变会阻断关键的TCR近端信号转导事件，并损害逆转录小鼠的胸腺细胞发育。我们确定这些缺陷是由Lck的抑制性C末端尾部的过度磷酸化引起的。我们的发现表明，Y192的修饰会抑制CD45与细胞中Lck缔合并使Lck的C末端尾部磷酸化的能力，这会阻止其采用主动的开放构象。