title: Myxococcus xanthus spore coat protein S, a stress-induced member of the betagamma-crystallin superfamily, gains stability from binding of calcium ions.
abstract: Protein S, a calcium-binding spore coat protein from the soil bacterium Myxococcus xanthus, belongs to a group of structurally related proteins, the betagamma-crystallin superfamily. Common features of this protein family are the Greek-key structural motif or crystallin fold, and the fact that all members are extremely stable long term. To investigate the correlation between the stability and Greek-key topology, protein S was cloned, expressed in Escherichia coli and purified to homogeneity. Ca2+ binding influences the native tertiary structure of protein S, whereas the secondary structure remains unaffected as shown by spectroscopic methods. Ca2+ ions enhance the conformational stability of protein S significantly. The midpoints of urea and guanidinium chloride-induced transitions show a difference of 1.4 M and 0.5 M denaturant, respectively, in the absence and in the presence of calcium. An equilibrium intermediate indicating independent domain folding can be detected at pH 2. In addition, thermal denaturation shows a clear deviation from the two-state model of folding, again with a strong stabilisation by Ca2+ ions. Temperature and denaturant-induced equilibrium transitions are fully reversible. Our data implicate a different strategy for achieving the high stability required for the biological function compared with the structurally related lens crystallins.
authors.affiliation: Institut für Biophysik und Physikalische Biochemie, Universität Regensburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Wenk
authors.forename: E M
authors.initials: EM
authors.lastname: Mayr
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002135:Calcium-Binding Proteins / Q000737:chemistry*; D002942:Circular Dichroism; D003459:Crystallins / Q000737:chemistry* / Q000378:metabolism; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008970:Molecular Weight; D016941:Myxococcus xanthus / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D003459:Crystallins