Myxococcus xanthus spore coat protein S, a stress-induced member of the betagamma-crystallin superfamily, gains stability from binding of calcium ions.

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.