The Folding Pathway of Spectrin R17 from Experiment and Simulation: Using Experimentally Validated MD Simulations to Characterize States Hinted at by Experiment

We present an experimental and computational analysis of the folding pathway of the 17th domain of chicken brain α-spectrin, R17. Wild-type R17 folds in a two-state manner and the chevron plot (plot of the logarithm of the observed rate constant against concentration of urea) shows essentially linear folding and unfolding arms. A number of mutant proteins, however, show a change in slope of the unfolding arm at high concentration of denaturant, hinting at complexity in the folding landscape. Through a combination of mutational studies and high temperature molecular dynamics simulations we show that the folding of R17 can be described by a model with two sequential transition states separated by an intermediate species. The rate limiting transition state for folding in water has been characterized both through experimental Φ-value analysis and by simulation. In contrast, a detailed analysis of the transition state predicted to dominate under highly denaturing conditions is only possible by simulation.