Role of electrostatic interactions for the stability and folding behavior of cold shock protein

The impacts of three charged-residue-involved mutations, E46A, R3E, and R3E/L66E, on the thermostability and folding behavior of the cold shock protein from the themophile Bacillus caldolyticus (Bc-Csp) were investigated by using a modified Gō-like model, in which the nonspecific electrostatic interactions of charged residues were taken into account. Our simulation results show that the wild-type Bc-Csp and its three mutants are all two-sate folders, which is consistent with the experimental observations. It is found that these three mutations all lead to a decrease of protein thermodynamical stability, and the effect of R3E mutation is the strongest. The lower stability of these three mutants is due to the increase of the enthalpy of the folded state and the entropy of the unfolded state. Using this model, we also studied the folding kinetics and the folding/unfolding pathway of the wild-type Bc-Csp as well as its three mutants and then discussed the effects of electrostatic interactions on the folding kinetics. The results indicate that the substitutions at positions 3 and 46 largely decrease the folding kinetics, whereas the mutation of residue 66 only slightly decreases the folding rate. This result agrees well with the experimental observations. It is also found that these mutations have little effects on the folding transition state and the folding pathway, in which the N-terminal beta sheet folds earlier than the C-terminal region. We also investigated the detailed unfolding pathway and found that it is really the reverse of the folding pathway, providing the validity of our simulation results.