An evolutionary bridge to a new protein fold

Are repressor bearing the N11L substitution (Arc-N11L) is an evolutionary i ntermediate between the wild type protein, in which the region surrounding position 11 forms a beta -sheet, and a double mutant 'switch Arc,' in which this region is helical. Here, Arc-N11L is shown to be able to adopt either the wild type or mutant conformations. Exchange between these structures o ccurs on the millisecond time scale in a dynamic equilibrium in which the r elative populations of each fold depend on temperature, solvent conditions and ligand binding. The N11L mutation serves as an evolutionary bridge from the beta -sheet to the helical fold because in the mutant, Leu is an integ ral part of the hydrophobic core of the new structure but can also occupy a surface position in the wild type structure. Conversely, the polar Asn 11 side chain serves as a negative design element in wild type Arc because it cannot be incorporated into the core of the mutant fold.