Viscosity dependence of the folding kinetics of a dimeric and monomeric coiled coil

We measured whether solvent viscosity, and hence chain diffusion, plays a r ole in the rate-limiting step of the folding reactions of GCN4-p2', a simpl e alpha-helical coiled coil derived from the leucine zipper region of bZIP transcriptional activator GCN4. To deconvolute the dual effects of viscosog enic solvents on both viscosity, eta, and stability, earlier attempts assum ed that the cosolvent and denaturant interact to the same degree in the tra nsition state. Applying this analysis to GCN4-p2' yielded a nearly 1/eta de pendence between folding rates and viscosity for both the dimeric and the c ross-linked, monomeric versions of the coiled coil, but it revealed no such coherent relationship for cytochrome c. We also developed a method to dete rmine the relative viscosity dependence of the dimeric and monomeric forms of the coiled coil independent of the assumption concerning the transition state's relative interaction with cosolvents and denaturants. Application o f this method indicated that the effect of viscosity on both the folding an d the unfolding rates was the same for the dimeric and monomeric versions, further supporting the view that the folding of the dimeric version is fold ing-limited rather than encounter-limited. The finding that GCN4-p2' foldin g appears to exhibit a 1/eta viscosity dependence implies that the rate-lim iting step in folding is opposed predominantly by solvent-derived rather th an internal frictional forces. These results are interpreted in relation to Various models for protein folding.