Solution structure of switch Arc, a mutant with 3₁₀ helices replacing a wild-type β-ribbon

Adjacent N11L and L12N mutations in the antiparallel β-ribbon of Arc repressor result in dramatic changes in local structure in which each β-strand is replaced by a right-handed helix. The full solution structure of this "switch" Arc mutant shows that irregular 3₁₀ helices compose the new secondary structure. This structural metamorphosis conserves the number of main-chain and side-chain to main-chain hydrogen bonds and the number of fully buried core residues. Apart from a slight widening of the interhelical angle between α-helices A and B and changes in side-chain conformation of a few core residues in Arc, no large-scale structural adjustments in the remainder of the protein are necessary to accommodate the ribbon-to-helix change. Nevertheless, some changes in hydrogen-exchange rates are observed, even in regions that have very similar structures in the two proteins. The surface of switch Arc is packed poorly compared to wild-type, leading to ∼1000Ų of additional solvent-accessible surface area, and the N termini of the 3₁₀ helices make unfavorable head-to-head electrostatic interactions. These structural features account for the positive m value and salt dependence of the ribbon-to-helix transition in Arc-N11L, a variant that can adopt either the mutant or wild-type structures. The tertiary fold is capped in different ways in switch and wild-type Arc, showing how stepwise evolutionary transformations can arise through small changes in amino acid sequence.