J. Aramburu

The flow of information from calcium-mobilizing receptors to nuclear factor of activated T cells (NFAT)-dependent genes is critically dependent on interaction between the phosphatase calcineurin and the transcription factor NFAT. A high-affinity calcineurin-binding peptide was selected from combinatorial peptide libraries based on the calcineurin docking motif of NFAT. This peptide potently inhibited NFAT activation and NFAT-dependent expression of endogenous cytokine genes in T cells, without affecting the expression of other cytokines that require calcineurin but not NFAT. Substitution of the optimized peptide sequence into the natural calcineurin docking site increased the calcineurin responsiveness of NFAT. Compounds that interfere selectively with the calcineurin-NFAT interaction without affecting calcineurin phosphatase activity may be useful as therapeutic agents that are less toxic than current drugs.

NFAT transcription factors are related to NF-kappaB/Rel proteins and form cooperative complexes with Fos and Jun on DNA. We have identified an NFAT-related protein, NFAT5, which differs from the conventional NFAT proteins NFAT1-4 in its structure, DNA binding, and regulation. NFAT5 contains a NFAT-like Rel homology domain, conserves the DNA contact residues of NFAT1-4, and binds DNA sequences similar to those found in the regulatory regions of well-characterized NFAT-dependent genes. However, it lacks the majority of Fos/Jun contact residues and does not bind cooperatively with Fos and Jun to DNA. Unlike NFAT1-4, whose nuclear import is tightly regulated by calcineurin-mediated dephosphorylation, NFAT5 is a constitutively nuclear phosphoprotein regardless of calcineurin activation. These features suggest that unlike the conventional NFAT proteins, NFAT1-4, which activate gene transcription by integrating inputs from calcium/calcineurin and protein kinase C/mitogen-activated protein kinase signaling pathways, NFAT5 participates in as-yet-unidentified signaling pathways in diverse immune and nonimmune cells.