title: Myxococcus xanthus spore coat protein S may have a similar structure to vertebrate lens beta gamma-crystallins.
abstract: The Gram-negative bacterium Myxococcus xanthus has a complex life cycle during which large amounts of a protein of relative molecular mass (Mr) 19,000, known as protein S, are assembled into a spore surface coat by a process that specifically requires calcium ions. The gene for protein S has been cloned and the DNA sequence shows that the gene product is composed of four internally repeated homologous sequences, each 40 amino acids long. Although protein S resembles calmodulin both in its internally duplicated structure and its ability to bind calcium, it apparently has a beta-sheet secondary structure rather than the helix-loop-helix motifs that characterize the calmodulin family. We now show that protein S has a striking homology with the beta- and gamma-crystallins of the vertebrate eye lens which are beta-sheet proteins with internally duplicated structures. This implies that the beta- and gamma-crystallins evolved from already existing proteins, whose ancestors occurred in the prokaryotes. The biological function of protein S, as a closely packed, stable protein in a relatively dehydrated environment, has implications for the functions of crystallins, which are found closely packed in the lens fibre cells, where their stability is essential for maintenance of transparency.
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000302:isolation & purification*; D001665:Binding Sites; D002118:Calcium; D003459:Crystallins; D008958:Models, Molecular; D008970:Molecular Weight; D009231:Myxococcales / Q000378:metabolism*; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; D003459:Crystallins; C046032:Myxococcus spore coat proteins; D002118:Calcium