ARNT PAS-B has a fragile native state structure with an alternative β-sheet register nearby in sequence space

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic helix-loop-helix Period/ARNT/Single-minded (bHLH-PAS) protein that controls various biological pathways as part of dimeric transcriptional regulator complexes with other bHLH-PAS proteins. The two PAS domains within ARNT, PAS-A and PAS-B, are essential for the formation of these complexes because they mediate protein-protein interactions via residues located on their β-sheet surfaces. While investigating the importance of residues in ARNT PAS-B involved in these interactions, we uncovered a point mutation (Y456T) on the solvent-exposed β-sheet surface that allowed this domain to inter- convert with a second, stable conformation. Although both conformations are present in equivalent quantities in the Y456T mutant, this can be shifted almost completely to either end point by additional mutations. A high-resolution solution structure of a mutant ARNT PAS-B domain stabilized in the new conformation revealed a 3-resi- due slip in register and accompanying inversion of the central 1β- strand. We have demonstrated that the new conformation has > 100-fold lower in vitro affinity for its heterodimerization partner, hypoxia-inducible factor 2α PAS-B. We speculate that the pliability in β-strand register is related to the flexibility required of ARNT to bind to several partners and, more broadly, to the abilities of some PAS domains to regulate their activities in response to small-molecule cofactors.