Calorimetric analysis of the Ca(2+)-binding betagamma-crystallin homolog protein S from Myxococcus xanthus: intrinsic stability and mutual stabilization of domains.
The betagamma-crystallin superfamily consists of a class of homologous two-domain proteins with Greek-key fold. Protein S, a Ca(2+)-binding spore-coat protein from the soil bacterium Myxococcus xanthus exhibits a high degree of sequential and structural homology with gammaB-crystallin from the vertebrate eye lens. In contrast to gammaB-crystallin, which undergoes irreversible aggregation upon thermal unfolding, protein S folds reversibly and may therefore serve as a model in the investigation of the thermodynamic stability of the eye-lens crystallins. The thermal denaturation of recombinant protein S (PS) and its isolated domains was studied by differential scanning calorimetry in the absence and in the presence of Ca(2+) at varying pH. Ca(2+)-binding leads to a stabilization of PS and its domains and increases the cooperativity of their equilibrium unfolding transitions. The isolated N-terminal and C-terminal domains (NPS and CPS) obey the two-state model, independent of the pH and Ca(2+)-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 domain interactions, whereas the second one coincides with the denaturation of the isolated N-terminal domain. At pH 7.0, in the presence of Ca(2+), where PS exhibits maximal stability, the domain interactions at 20 degrees C contribute 20 kJ/mol to the overall stability of the intact protein.
Wenk
M
M
Institut für Biophysik und Physikalische Biochemie, Universität Regensburg, Regensburg, D-93040, Germany.
Jaenicke
R
R
D001426:Bacterial Proteins / Q000737:chemistry; D002135:Calcium-Binding Proteins / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D003459:Crystallins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D016941:Myxococcus xanthus / Q000737:chemistry*; D010446:Peptide Fragments / Q000737:chemistry; D011489:Protein Denaturation; D017510:Protein Folding; D017293:Protein S / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics; D038222:gamma-Crystallins
D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D003459:Crystallins; D010446:Peptide Fragments; D017293:Protein S; C107738:crystallin, gammaB; D038222:gamma-Crystallins