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

Cold shock proteins (Csps) from mesophiles and thermophiles differ widely i n their stabilities, but show close structural similarity. Sequence variati ons involve mainly charged groups, supporting the view that ion pairs contr ibute significantly to the free energy of stabilization. Based on the known 3D structure of mesophilic Bacillus subtilis CspB and the modeled structur e of hyperthermophilic Csp from Thermotoga maritima (TmCsp), D9 and H61 of TmCsp have been substituted by asparagine to find out whether the eliminati on of predicted ion pairs has a destabilizing effect. Thermal unfolding exp eriments show that the D9N mutant is destabilized by 9 degrees C, whereas H 61N exhibits unaltered wild type behavior. The results are in agreement wit h preliminary NMR data which confirm the predicted structure only for the N -terminal ion pair. (C) 1999 Federation of European Biochemical Societies.