STRUCTURAL SIMILARITY OF A DEVELOPMENTALLY-REGULATED BACTERIAL SPORE COAT PROTEIN TO BETA-GAMMA-CRYSTALLINS OF THE VERTEBRATE EYE LENS

The solution structure of Ca2+-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 ''Creek keys'': the second and fourth moths form standard Greek keys, whereas the first and third moths 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 moth organization and domain packing, indicating a different mode of oligomerization and a divergent evolutionary pathway from the beta gamma-crystallins.