Searching for multiple folding pathways of a nearly symmetrical protein: temperature dependent phi-value analysis of the B domain of protein A.

The B domain of protein A (BdpA) is a popular paradigm for simulating protein folding pathways. The discrepancies between so many simulations and subsequent experimental testing may be attributable to the protein being highly symmetrical: changing experimental conditions could perturb the subtle interplay between the effects of symmetry in the native structure and the effects of asymmetry from specific interactions in a given sequence. If the protein folds via multiple pathways, perturbations, such as temperature, denaturant concentration, and mutation, should change the flux of micro pathways, leading to changes in the bulk properties of the transition state. We tested this hypothesis by conducting a Phi-analysis of BdpA as a function of temperature from 25.0 degrees C to 60.0 degrees C. The Phi-values had no significant dependence on temperature and the values at 55.0 degrees C (denaturing conditions) are very similar to those at 25.0 degrees C (folding conditions), indicating the structure of the transition state does not significantly change although the experimental conditions are considerably altered. The results suggest that BdpA folds via a single dominant folding pathway.