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

Adjacent N11L and L12N mutations in the antiparallel beta-ribbon of Arc repressor result in dramatic changes in local structure in which each P-strand is replaced by a right-handed helix. The full solution structure of this "switch" Arc mutant shows that irregular 3,, 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 alpha-helices A and B and changes in sidechain 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 similar to 1000 Angstrom(2) of additional solvent-accessible surface area, and the N termini of the 3(10) 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 wildtype Arc, showing how stepwise evolutionary transformations can arise through small changes in amino acid sequence. (C) 2003 Elsevier Science Ltd. All rights reserved.