Determinants of oligomerization of the bifunctional protein DCoHα and the effect on its enzymatic and transcriptional coactivator activities

DCoH and DCoHα are bifunctional proteins that function as 4a-hydroxytetrahydrobiopterin dehydratases and as coactivators of HNF1α-dependent transcription. Although these isoforms share sequence and structural similarity and equivalent enzyme activities, DCoH is a hyperstable tetramer whereas DCoHα readily forms dimers. Differences in quaternary structure affect the formation of the DCoH(α):HNF1α complex. Because the interface used to bind HNF1α is masked in tetrameric DCoH, the DCoH:HNF1α complex is only formed in vivo, presumably by co-translational folding. Conversely, the DCoHα:HNF1α complex readily forms in vitro. We identified residues in DCoHα that differed from those in the dimer–dimer interface of tetrameric DCoH. Mutating these residues altered the quaternary state and concomitantly the ability of the mutated proteins to affect HNF1α-dependent DNA binding. Our results indicate that three residues, Asn61, Gln45, and Lys98 in DCoHα play a role in oligomeric flexibility, which enables DCoHα to more readily interact with HNF1α and increase DNA binding.