Structure of a Domain-Swapped FOXP3 Dimer on DNA and Its Function in Regulatory T Cells

SummaryThe transcription factor FOXP3 is essential for the suppressive function of regulatory T cells that are required for maintaining self-tolerance. We have solved the crystal structure of the FOXP3 forkhead domain as a ternary complex with the DNA-binding domain of the transcription factor NFAT1 and a DNA oligonucleotide from the interleukin-2 promoter. A striking feature of this structure is that FOXP3 forms a domain-swapped dimer that bridges two molecules of DNA. Structure-guided or autoimmune disease (IPEX)-associated mutations in the domain-swap interface diminished dimer formation by the FOXP3 forkhead domain without compromising FOXP3 DNA binding. These mutations also eliminated T cell-suppressive activity conferred by FOXP3, both in vitro and in a murine model of autoimmune diabetes in vivo. We conclude that FOXP3-mediated suppressor function requires dimerization through the forkhead domain and that mutations in the dimer interface can lead to the systemic autoimmunity observed in IPEX patients.

► The forkhead domain of FOXP3 exists predominantly as a domain-swapped dimer
► The domain-swapped dimer of FOXP3 simultaneously binds two distinct pieces of DNA
► Disruption of domain swapping inhibits dimerization without affecting DNA binding
► Disruption of domain swapping abrogates FOXP3-mediated suppressor functions