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.