A negatively charged domain of LAT mediates its interaction with the active form of Lck.

We have shown previously that in T cells, LAT co-immunoprecipitates with the active but not the inactive-'closed' form of Lck, and that this interaction impacts negatively on Lck activity. Here we confirm that activation of T cells induced a transient LAT/Lck association within 4 min after stimulation, returning to basal levels by 30 min. Interestingly, autoimmune T cells isolated from patients with systemic lupus erythematosus, which contain a larger pool of active Lck and LAT, exhibited increased LAT/Lck association compared to healthy controls. To identify the domain of LAT responsible for its interaction with active Lck, a series of LAT truncation mutants were constructed and tested in co-immunoprecipitation experiments. We found that the segment comprising residues 112-126 of human LAT is required for its interaction with Lck. This domain is rich in negatively charged amino acids and is conserved among different species. Therefore, in addition to the conserved tyrosines, the 112-126 domain identified here could be important for certain functions of LAT in T cells.