Enhanced T cell signaling in cells bearing LAT molecules resistant to ubiquitylation

Linker for activation of T cells (LAT) plays a central role in T cell activation by nucleating signaling complexes that are critical for the propagation of T cell signals from the plasma membrane to the cellular interior. The roles of phosphorylation and palmitoylation in LAT function have been well studied, but not much is known about other strategies by which the cell modulates LAT activity. We have focused on LAT ubiquitylation and have mapped the sites on which LAT is ubiquitylated. To elucidate the biological function of this process, we substituted LAT lysines with arginines. This resulted in a dramatic decrease in overall LAT ubiquitylation. Ubiquitylation‐resistant mutants of LAT were internalized at rates comparable to wild‐type LAT in a mechanism that required Cbl family proteins. However, these mutants displayed a defect in protein turnover. Cells bearing mutant LAT had a hyperresponsive phenotype and displayed elevated T cell signaling. Though phosphorylation of LAT was not affected, elevated phosphorylation of downstream signaling proteins was observed. These results support LAT ubiquitylation as a molecular checkpoint for attenuation of T cell signaling.