Remarkably slow folding of a small protein

Equilibrium denaturation of the 72 amino acid alpha/beta-protein MerP, by acid, guanidine hydrochloride, or temperature, is fully reversible and follows a two-state model in which only the native and unfolded states are populated, A cis-rr arts equilibrium around a proline peptide bond causes a heterogeneity of the unfolded state and gives rise to a slow- and a fast folding population, With a rate constant of 1.2 s(-1) for the major fast folding population, which has none of the common intrinsically slow steps, MerP is the slowest folding protein of this small size yet reported. (C) 1997 Federation of European Biochemical Societies.