Structural similarity of a developmentally regulated bacterial spore coat protein to beta gamma-crystallins of the vertebrate eye lens

The solution structure of Ca(2+)-loaded protein S (M(r) 18,792) from the Gram-negative soil bacterium Myxococcus xanthus has been determined by multidimensional heteronuclear NMR spectroscopy. Protein S consists of four internally homologous motifs, arranged to produce two domains with a pseudo-twofold symmetry axis, overall resembling a triangular prism. Each domain consists of two topologically inequivalent "Greek keys": the second and fourth motifs form standard Greek keys, whereas the first and third motifs each contain a regular alpha-helix in addition to the usual four beta-strands. The structure of protein S is similar to those of the vertebrate eye lens beta gamma-crystallins, which are thought to be evolutionarily related to protein S. Both protein S and the beta gamma-crystallins function by forming stable multimolecular assemblies. However, protein S possesses distinctive motif organization and domain packing, indicating a different mode of oligomerization and a divergent evolutionary pathway from the beta gamma-crystallins.