CALCINEURIN BINDS THE TRANSCRIPTION FACTOR NFAT1 AND REVERSIBLY REGULATES ITS ACTIVITY

NFAT1 (previously termed NFATp) is a cytoplasmic transcription factor involved in the induction of cytokine genes. We have previously shown that the dephosphorylation of NFAT1, accompanied by its nuclear transl ocation and increased DNA binding activity, is regulated by calcium- a nd calcineurin-dependent mechanisms, as each of these hallmarks of NFA T1 activation is elicited by ionomycin and blocked by the immunosuppre ssive drugs cyclosporin A and FK506 (Shaw, K. T.-Y., Ho, A. M., Raghav an, A., Kim, J., Jain, J., Park, J., Sharma, S., Rao, A., and Hogan, P . G. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 11205-11209). Here we show that the activation state of NFAT1 in T cells is remarkably sensi tive to the level of calcineurin activity. Addition of cyclosporin A, even in the presence of ongoing ionomycin stimulation, results in reph osphorylation of NFAT1, its reappearance in the cytoplasm, and a retur n of its DNA binding activity to low levels. Similar effects are obser ved upon removal of ionomycin or addition of EGTA. We also demonstrate a direct interaction between calcineurin and NFAT1 that is consistent with a direct enzyme-substrate relation between these two proteins an d that may underlie the sensitivity of NFAT1 activation to the level o f calcineurin activity. The NFAT1-calcineurin interaction, which invol ves an N-terminal region of NFAT1 conserved in other NFAT family prote ins, may provide a target for the design of novel immunosuppressive dr ugs.