Evidence for partial secondary structure formation in the transition state for arc repressor refolding and dimerization

Structure formation and dimerization are concerted processes in the refolding of Arc repressor. The integrity of secondary structure in the transition state of Arc refolding has been investigated here by determining the changes in equilibrium stability and refolding/unfolding kinetics for a set of Ala --> Gly mutations at residues that are solvent-exposed in the native Arc dimer. At some sites, reduced stability was caused primarily by faster unfolding, indicating that secondary structure at these positions is largely absent in the transition state. However, most of the Ala --> Gly substitutions in the alpha-helices of Arc and a triple mutant in the beta-sheet also resulted in decreased refolding rates, in some cases, accounting for the major fraction of thermodynamic destabilization. Overall, these results suggest that some regions of native secondary structure are present but incompletely formed in the transition state of Arc refolding and dimerization. Consolidation of this secondary structure, like close packing of the hydrophobic core, seems to occur later in the folding process. On average, Phi(F) values for the Ala --> Gly mutations were significantly larger than Phi(F) values previously determined for alanine-substitution mutants, suggesting that backbone interactions in the transition state may be stronger than side chain interactions. Mutations causing significant reductions in the Arc refolding rate were found to cluster in the central turn of alpha-helix A and in the first two turns of alpha-helix B. In the Arc dimer, these elements pack together in a compact structure, which might serve as nucleus for further folding.