Calorimetric analysis of the Ca2+-binding beta gamma-crystallin homolog protein S from Myxococcus xanthus: Intrinsic stability and mutual stabilization of domains

The beta gamma-crystallin superfamily consists of a class of homologous two -domain proteins with Greek-key fold. Protein S, a Ca2+-binding spore-coat protein from the soil bacterium Myxococcus xanthus exhibits a high degree o f sequential and structural homology with gamma B-crystallin from the verte brate eye lens. In contrast to gamma B-crystallin, which undergoes irrevers ible aggregation upon thermal unfolding, protein S folds reversibly and may therefore serve as a model in the investigation of the thermodynamic stabi lity of the eye-lens crystallins. The thermal denaturation of recombinant p rotein S (PS) and its isolated domains was studied by differential scanning calorimetry in the absence and in the presence of Ca2+ at varying pH. Ca2-binding leads to a stabilization of PS and its domains and increases the c ooperativity of their equilibrium unfolding transitions. The isolated N-ter minal and C-terminal domains (NPS and CPS) obey the two-state model, indepe ndent of the pH and Ca2+-binding; in the case of PS, under all conditions, an equilibrium intermediate is populated. The first transition of PS may be assigned to the denaturation of the C-terminal domain and the loss of doma in interactions, whereas the second one coincides with the denaturation of the isolated N-terminal domain. At pH 7.0, in the presence of Ca2+, where P S exhibits maximal stability, the domain interactions at 20 degrees C contr ibute 20 kJ/mol to the overall stability of the intact protein. (C) 1999 Ac ademic Press.