Thermal unfolding of the archaeal DNA and RNA binding protein Ssh10.

The reversible thermal unfolding of the archaeal histone-like protein Ssh10b from the extremophile Sulfolobus shibatae was studied using differential scanning calorimetry and circular dichroism spectroscopy. Analytical ultracentrifugation and gel filtration showed that Ssh10b is a stable dimer in the pH range 2.5–7.0. Thermal denaturation data fit into a two-state unfolding model, suggesting that the Ssh10 dimer unfolds as a single cooperative unit with a maximal melting temperature of 99.9°C and an enthalpy change of 134kcal/mol at pH 7.0. The heat capacity change upon unfolding determined from linear fits of the temperature dependence of ΔHcal is 2.55kcal/(molK). The low specific heat capacity change of 13cal/(molK residue) leads to a considerable flattening of the protein stability curve (ΔG (T)) and results in a maximal ΔG of only 9.5kcal/mol at 320K and a ΔG of only 6.0kcal/mol at the optimal growth temperature of Sulfolobus.