Interleukin-1 beta folding between pH 5 and 7: experimental evidence for three-state folding behavior and robust transition state positions late in folding.

The thermodynamic stability and folding kinetics of the all beta-sheet protein interleukin-1beta were measured between 0 and 4 M GdmCl concentrations and pH 5-7. Native interleukin-1beta undergoes a 3.5 kcal/mol decrease in thermodynamic stability, Delta, as pH is increased from 5 to 7. The native state parameter m(NU), measuring protein destabilization/[GdmCl], remains constant between pH 5 and 7, indicating that the solvent-exposed surface area difference between the native state and unfolded ensemble is unchanged across this pH range. Similarly, pH changes between 5 and 7 decrease only the thermodynamic stability, DeltaG(H)2(O), and not the m-values, of the kinetic intermediate and transition states. This finding is shown to be consistent with transition state configurations which continue to be the high-energy configurations of the transition state in the face of changing stability conditions. A three-state folding mechanism U right arrow over left arrow I right arrow over left arrow N is shown to be sufficient in characterizing IL-1beta folding under all conditions studied. The m-values of refolding transitions are much larger than the m-values of unfolding transitions, indicating that that the fast, T(2) (U right arrow over left arrow I), and slow, T(1) (I right arrow over left arrow N), transition states are highly similar to the intermediate I and native state N, respectively. Many of the folding properties of interleukin-1beta are shared among other members of the beta-trefoil protein family, although clear differences can exist.