Thermal unfolding of Sso7d from the hyperthermophile Sulfolobus

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

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