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Thermal unfolding of Sso7d from the hyperthermophile Sulfolobus Solfataricus. Stefan knapp Karolinska institutet CSB NOVUM 14157 Huddinge Sweden Sso7d is a small basic histone like protein which is highly expressed in the Archaeon Sulfolobus solfataricus. The protein has a SH3 (src-homology region 3) domain like fold and its secondary structure consists almost entirely of beta sheets. The thermal unfolding of Sso7d was studied by circular dichroism spectroscopy and differential scanning calorimetry. The unfolding transition of Sso7d can be described by a reversible two state process. Maximum stability was observed in the region between pH 4.5 and 7.0 where Sso7d unfolds with a melting temperature between 370 and 371.9 K an unfolding enthalpy between 62.9 and 65.4 kcal/mol and a heat capacity change of 620 cal/(mol K). The thermodynamic reason for the high melting temperature is a shallow stability curve with a broad stability maximum, corresponding to the very small heat capacity change which was obtained. The calculated stability curve of Sso7d has, despite of its high melting temperature, an only moderate intrinsic stability, which reaches its maximum of about 7 kcal/mol at 282 K. Sso7d is particularly poorly stabilized (1 kcal/mol) at the maximal growth temperature of Sulfolobus. A comparison of Sso7d with mesophilic SH3 domains from the eucaryotic kinases Tec, Btk and Itk revealed that SH3 domains of these proteins have at their physiological temperatures similar stabilization free energies as Sso7d at the optimal growth temperature of Sulfolobus. All SH3 domains are only weakly stabilized in terms of free energy of unfolding. The reason for the high melting temperature of Sso7d is a broadening and lifting of its stability curve. ## Back to the list of abstracts