The lymphocyte-specific Src family protein tyrosine kinase p56(Lck) (Lck) is essential for T cell development and activation and, hence, for adaptive immune responses. The mechanism by which Lck activity is directed toward specific substrates in response to T cell receptor (TCR) activation remains elusive. We used fluorescence lifetime imaging microscopy to assess the activation-dependent spatiotemporal changes in the conformation of Lck in live human T cells. Kinetic analysis of the fluorescence lifetime of Lck biosensors enabled the direct visualization of the dynamic local opening of 20% of the total amount of Lck proteins after activation of T cells with antibody against CD3 or by superantigen-loaded antigen-presenting cells. Parallel biochemical analysis of TCR complexes revealed that the conformational changes in Lck correlated with the induction of Lck enzymatic activity. These data show the dynamic, local activation through conformational change of Lck at sites of TCR engagement.

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