Characterization of a protein that regulates the DNA-binding activity of NF-AT, the nuclear factor of activated T cells.

NF-AT is a T-cell-restricted activity that regulates interleukin 2 expression by binding to positions -285 to -254 of the interleukin 2 gene. Prior studies have shown that NF-AT is found only in T cells that are appropriately stimulated with "two signals" and is sensitive to the immunosuppressive drugs cyclosporin A and FK506. However, NF-AT has yet to be characterized biochemically. Here we show that the activity of NF-AT in electrophoretic gel mobility-shift assays is greatly enhanced by a distinct protein with an apparent molecular mass of 28 kDa. This modulator has been enriched from the flow-through of NF-AT affinity columns and enhances by severalfold the activity of the factor that elutes from these columns. The modulator has a predominantly nuclear location, but unlike NF-AT, the modulator is found in nuclear extracts of many cell types, even unstimulated T cells and T cells immunosuppressed with cyclosporin A or FK506. This modulator protein may regulate NF-AT binding activity posttranscriptionally, and it should prove useful in the isolation and biochemical characterization of NF-AT.