Myxococcus xanthus spore coat protein S may have a similar structure to vertebrate lens βγ-crystallins

Abstract

The Gram-negative bacterium Myxococcus xanthus has a complex life cycle¹ during which large amounts of a protein of relative molecular mass (M_r) 19,000, known as protein S, are assembled into a spore surface coat by a process that specifically requires calcium ions^2,3. 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 long⁴. Although protein S resembles calmodulin both in its internally duplicated structure and its ability to bind calcium⁴, it apparently has a β-sheet secondary structure⁵ rather than the helix–loop–helix motifs that characterize the calmodulin family^6–8. We now show that protein S has a striking homology with the β- and γ-crystallins of the vertebrate eye lens^9–11 which are β-sheet proteins with internally duplicated structures. This implies that the β- and γ-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 crystalline, which are found closely packed in the lens fibre cells, where their stability is essential for maintenance of transparency.