Regulation of Fyn through translocation of activated Lck into lipid rafts.

Whether or how the activation of Lck and Fyn during T cell receptor (TCR) signaling is coordinated, and their delivery of function integrated, is unknown. Here we show that lipid rafts function to segregate Lck and Fyn in T cells before activation. Coaggregation of TCR and CD4 leads to Lck activation within seconds outside lipid rafts, followed by its translocation into lipid rafts and the activation of colocalized Fyn. Genetic evidence demonstrates that Fyn activation is strictly dependent on receptor-induced translocation of Lck. These results characterize the interdependence of Lck and Fyn function and establish the spatial and temporal distinctions of their roles in the cellular activation process.