The LxVP and PxIxIT NFAT motifs bind jointly to overlapping epitopes on calcineurin's catalytic domain distant to the regulatory domain.

The serine/threonine phosphatase calcineurin (Cn) targets the nuclear factors of activated T cells (NFATs) that activate cytokine genes. Calcium influx activates Cn to dephosphorylate multiple serine residues within the ∼200 residue NFAT regulatory domain, which triggers joint nuclear translocation of NFAT and Cn. The dephosphorylation process relies on the interaction between Cn and the conserved motifs PxIxIT and LxVP, which are located N- and C-terminal to the phosphorylation sites in NFAT's regulatory domain. Here, we show that an NFATc1-derived 15-residue peptide segment containing the conserved LxVP motif binds to an epitope on Cn's catalytic domain (CnA), which overlaps with the previously established PxIxIT binding site on CnA and is distant to the regulatory domain (CnB). Both NFAT motifs partially compete for binding but do not fully displace each other on the CnA epitope, revealing that both segments bind simultaneously to the same epitope on the catalytic domain.