Spatial Regulation of Lck Activation at the Immune Synapse Revealed by a FRET-Based Biosensor

T cell receptor (TCR) signaling is critically dependent on the Src-family kinase Lck, whose activation is tightly regulated both spatially and conformationally during antigen recognition. Here, we employ a third-generation FRET-based biosensor (TqLck-V2.3) to visualize Lck conformational dynamics in live T cells with high spatial resolution. Upon TCR engagement, we observe a paradoxical increase in whole-cell FRET signal, which immunolabeling reveals to be due to selective internalization of inactive Lck (pY505), while active Lck (pY394) remains membrane-associated and enriched at the immune synapse (IS). Using CD8 mutants that disrupt Lck binding, we demonstrate that free Lck undergoes more pronounced conformational activation than coreceptor-bound Lck. Furthermore, we show that C-terminal Src kinase (Csk) preferentially phosphorylates free Lck at Y505, while CD45 suppresses its activation via dephosphorylation of Y394, suggesting a dual regulatory mechanism that maintains free Lck in an inactive state under resting conditions. High-resolution imaging confirms sustained activation of Lck at the IS and transient inactivation at the periphery, revealing a spatially confined signaling architecture. These findings uncover a novel regulatory mechanism involving selective internalization and spatial segregation of Lck conformations, and establish TqLck-V2.3 as a powerful tool for dissecting TCR signaling dynamics.