Folding of Circular and Permuted Chymotrypsin Inhibitor 2: Retention of the Folding Nucleus

Abstract

The 64-residue chymotrypsin inhibitor 2 (CI2) folds by a two-state nucleation−condensation mechanism, whereby secondary and tertiary structure coalesce concomitantly in the transition state around Ala 16 in the helical N-cap. Permutation of the SH3-domain of α-spectrin apparently shifts its folding nucleus to another region of the protein, suggesting that a protein's transition state may be altered by altering the protein's connectivity. We have characterized the structure of the transition state of a circular and a permuted version of CI2 by a protein engineering study encompassing 11 mutations. Circular CI2 was obtained by the introduction of cysteines at residues 3 and 63 and linking them by disulfide bond formation. Subsequent cyanogen−bromide cleavage of the scissile bond, Met 40−Glu 41, yielded permuted CI2. Circular and permuted CI2 also fold according to a two-state mechanism. Permutation does not affect the folding rate constant, but circularization increases it 7-fold. The transition states of circular and permuted CI2 are essentially unchanged from that of wild-type CI2. Importantly, the folding nucleus around Ala16 is retained. These results complement a previous observation that the transition state for association of two CI2 fragments (residues 1−40 and 41−64, generated by CNBr cleavage) is very similar to the folding transition state of intact CI2. The similarity of rate constants for folding of wild-type and permuted CI2, and their value relative to that for the association of fragments, allows us to estimate the gain in entropy of activation on having the separate fragments linked: 18.3 cal M^-1 K^-1; i.e. an effective molarity of 10⁴ M. The contrast between the retention of the folding nucleus on permutation of CI2 and its change for the SH3-domain of α-spectrin probably arises because the latter was cleaved in its folding nucleus whereas cleavage at sites other than 40−41 in CI2 is very destabilizing. Whether or not a folding nucleus can be changed probably depends on the specific protein and its permissivity to permutation.