Multidomain structure of a recombinant streptokinase. A differential scanning calorimetry study

The temperature dependence of the heat capacity function of a recombinant s treptokinase (rSK) has been studied by high-sensitivity differential scanni ng microcalorimetry and circular dichroism as a function of pH in low- and high-ionic strength buffers. At low ionic strength it is found that this pr otein, between pH 7 and 10, undergoes four reversible and independent two-s tate transitions during its unfolding, suggesting the existence of four dom ains in the native structure of the protein. This result reconciles previou s conflicting reports about the number of domains of this protein obtained by differential scanning calorimetry and small-angle X-ray scattering. The number of two-state transitions decreases when the pH of the medium is decr eased, without noticeable changes in its circular dichroism spectrum. A pla usible localization of the four domains in the streptokinase sequences is p roposed and their thermodynamic parameters are given. Increase of ionic str ength to 200 mM NaCl affects positively the protein stability and confirms the existence of four reversible two-state transitions. Above 200 mM NaCl t he protein stability decreases, resulting in low percentage of reversibilit y, and even irreversible transitions.