FOXC2 represses NFAT1-dependent transcription through a DNA-facilitated protein-protein interaction.

Transcription factor nuclear factor of activated T cells (NFAT) plays a central role in immune gene regulation through cooperative interactions with diverse transcriptional partners. While FOXP family members have been identified as co-regulators of NFAT1, the involvement of other FOX family proteins has remained mechanistically obscure. Here, we solved three crystal structures of NFAT1-RHR/FOXC2-DBD/ARRE DNA ternary complexes and uncovered an unexpected mode of transcriptional repression mediated by FOXC2 through direct, DNA-facilitated binding to the V-shaped groove of NFAT1's Rel-homology region (RHR). Biochemical assays revealed that DNA enhanced FOXC2-NFAT1 interaction by more than five-fold, supporting a model in which DNA acts as a structural co-factor that promotes complex formation. Mutational disruption of the FOXC2-NFAT1 interface impaired complex assembly and abrogated transcriptional repression. Functional assays further confirmed that FOXC2 suppressed NFAT1-driven transcription of multiple cytokines and chemokines, including IL2, TNF, CXCL5, and CCL2. Notably, this repressive mechanism was found to extend to other FOX proteins (FOXI1, FOXO1, and FOXK1), suggesting a broader paradigm of FOX-NFAT1 interaction. Our study defined a previously unrecognized FOX-mediated transcriptional repression mechanism and provides a structural framework for NFAT inhibition by FOX proteins, offering novel insights into the transcriptional regulation of immune-related genes.