In lymphocytes, integration of Ca2+ and other signaling pathways results in productive activation, while unopposed Ca2+ signaling leads to tolerance or anergy. We show that the Ca2+-regulated transcription factor NFAT has an integral role in both aspects of lymphocyte function. Ca2+/calcineurin signaling induces a limited set of anergy-associated genes, distinct from genes induced in the productive immune response; these genes are upregulated in vivo in tolerant T cells and are largely NFAT dependent. T cells lacking NFAT1 are resistant to anergy induction; conversely, NFAT1 induces T cell anergy if prevented from interacting with its transcriptional partner AP-1 (Fos/Jun). Thus, in the absence of AP-1, NFAT imposes a genetic program of lymphocyte anergy that counters the program of productive activation mediated by the cooperative NFAT:AP-1 complex.

NFAT transcription factors play a key role in the immune response. The activation of NFAT proteins is controlled by calcineurin, the calmodulin-dependent phosphatase that is inhibited by the immunosuppressive drugs cyclosporin A and FK506. Here we identify a short conserved sequence in NFAT proteins that targets calcineurin to NFAT. Mutation of a single residue in this sequence impairs the calcineurin-mediated dephosphorylation and nuclear translocation of NFAT1. Peptides spanning the region inhibit the ability of calcineurin to bind to and dephosphorylate NFAT proteins, without affecting the phosphatase activity of calcineurin against other substrates. When expressed intracellularly, a corresponding peptide inhibits NFAT dephosphorylation, nuclear translocation, and NFAT-mediated expression in response to stimulation. Thus, disruption of the enzyme-substrate docking interaction that directs calcineurin to NFAT can effectively block NFAT-dependent functions.

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