Does the elimination of ion pairs affect the thermal stability of cold shock protein from the hyperthermophilic bacterium Thermotoga maritima?

Cold shock proteins (Csps) from mesophiles and thermophiles differ widely in their stabilities, but show close structural similarity. Sequence variations involve mainly charged groups, supporting the view that ion pairs contribute significantly to the free energy of stabilization. Based on the known 3D structure of mesophilic Bacillus subtilis CspB and the modeled structure of hyperthermophilic Csp from Thermotoga maritima (TmCsp), D9 and H61 of TmCsp have been substituted by asparagine to find out whether the elimination of predicted ion pairs has a destabilizing effect. Thermal unfolding experiments show that the D9N mutant is destabilized by 9 degrees C, whereas H61N exhibits unaltered wild type behavior. The results are in agreement with preliminary NMR data which confirm the predicted structure only for the N-terminal ion pair.