Spatial regulation of Lck activation at the CD8 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 an improved second-generation FRET-based biosensor (TqLckV2.3) 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 internalizat