The folding of staphylococcal nuclease in the presence of methanol or guanidine thiocyanate.

Abstract

The effect of methanol on the folding of staphylococcal nuclease has been investigated. Equilibrium thermal unfolding transitions were monitored by fluorescence emission. The transition was very sensitive to the presence of methanol (at pH 7.0), the Tm decreased from above 50 degrees C for aqueous solution to below 0 degree C for 70% methanol. The transitions were fully reversible and conformed to two-state behavior. A linear relationship was observed between the hydrophobicity of the solvent and both the Tm and the change in delta G for unfolding. The effect of pH on the transition in 50% methanol at 0 degree C was essentially the same as for aqueous solution, with a cooperative transition in the vicinity of apparent pH (pH*) 4. The unfolding transition was determined as a function of guanidine thiocyanate in aqueous and 50% methanol solvents. The midpoints of the transitions were 0.30 and 0.20 M, respectively, at 2.1 degrees C. The kinetics of folding at 0 degree C were compared in aqueous, 50% methanol and 0.30 M guanidine thiocyanate solvents, by monitoring changes in the tryptophan fluorescence intensity. Triphasic kinetics for refolding in both aqueous and 50% methanol solutions were observed in stopped-flow experiments. In both solvent systems the slowest phase is ascribed to proline isomerization. The kinetics of refolding were monitored at subzero temperatures in 50% methanol at pH* 7.0 in manual mixing experiments. Biphasic kinetics were observed at temperatures between 0 and -35 degrees C. A third, faster phase, was inferred from the missing amplitude. The energies of activation were 20.0 and 17.2 kcal mol-1, respectively, for the two slower phases. At -33.8 degrees C, the observed pseudo first-order rate constants were 1.2 x 10(-3) and 2.1 x 10(-5) s-1. At temperatures above -35 degrees C, the sum of the observed amplitudes was essentially constant at 70-75% of the expected total amplitude. At lower temperatures the amplitude of the refolding reaction decreased, and the native state was not formed (unless the temperature was increased), due to the formation of a trapped intermediate state. This intermediate has circular dichroism and fluorescence properties consistent with a compact state with some molten globule characteristics.