Novel Mechanism of Bispecificity in Forkhead Transcription Factors

The forkhead transcription factor (TF) FoxN3 recognizes both the canonical forkhead DNA motif (RYAAAYA), as well as a novel alternate motif (GACGC). These two motifs are more distinct than the typically observed primary and secondary motifs of other TFs, as the two motifs cannot be aligned and are not clearly related to each other. Amino acids at the canonical base-contacting positions in the forkhead DNA binding domain are largely conserved throughout the forkhead family, so the ability of FoxN3 and other bispecific forkhead factors to recognize the alternate site cannot be explained by amino acid substitutions at these positions. Neither the mechanism of this bispecific DNA recognition nor its regulatory function are understood. Here we show that human FoxN3 does recognize both motifs in cells. We also report the crystal structures of the FoxN3 DNA binding domain bound to the canonical forkhead binding site, and separately to the alternate DNA binding site. FoxN3 adopts a remarkably similar conformation to recognize both motifs, making contacts with different DNA bases using the same amino acids. However, the DNA structure is significantly altered between the two motifs, allowing the same domain to recognize two motifs of different lengths. Changes between the two structures in the conformation of segments of the forkhead domain outside of the direct DNA-contacting positions contribute to the ability to bind the alternate site. Swapping these segments of the domain between different forkhead proteins changes the ability of proteins to recognize the alternate site, highlighting the importance of non-base-contacting positions in determining TF-DNA binding specificity. The structures present a new mechanism by which a single DBD can recognize two divergent sequences. Overall design: ChIP-seq of FoxN3 in HepG2 cells.