Thermodynamic stability of proteins adapted to extreme temperatures.

Small cold shock proteins from bacteria are model proteins for many biophysical studies as a result of their small size and compact folding. Here we report a stability study of these proteins from the Antarctic psychrophile Pseudoalteromonas haloplanktis, the mesophile Escherichia coli and the hyperthermophile Thermotoga maritima. When analyzed by differential scanning calorimetry, the three proteins unfolded fully reversibly according to a perfect two-state model, which are strict prerequisites to apply equilibrium thermodynamic equations. The parameters obtained by differential scanning calorimetry allowed to calculate the thermodynamic stability of the three proteins and to draw their bell-shaped stability curves, i.e. the free energy of unfolding as a function of temperature. These curves provided fundamentals of stability for proteins adapted to extreme temperatures. For instance, the maximal stability of the three proteins was reached around room temperature and is not shifted toward high temperature for the hyperthermophilic protein, nor to low temperature for the psychrophilic one. The latter is heat-labile but is also cold-labile. Therefore, cold unfolding appears as a new but unsuspected factor limiting life at low temperatures. The distinct temperature-induced dynamics of these proteins are also discussed.