Protein S (Myxococcus xanthus)

Protein S is a protein found in Myxococcus xanthus. Its name derives from being the "S" band in an alphabetical ordering of proteins run from Myxococcus xanthus cell contents on a SDS-denaturing gel.[2] Its study was initially prompted by the huge increase in Protein S production during sporulation of Myxococcus xanthus.[3]

Protein S
PDB 1prs EBI.jpg
Structure of Protein S.[1]


Though it has been purported as recently as 1994 that Protein S enables myxospores of Myxococcus xanthus higher resistance to endure heating, desiccation, UV radiation and sonication [4] no such evidence exists. The work that [4] cites claims no such evidence:

Protein S is not essential for spore variability and resistance: protein S-deficient spores are viable and are as resistant to heat and sonication as complete spores; glycerol spores lack protein S but are resistant to sonication and heat

— Inoyue et al.[3]

and there are other studies that also found that there is no increase in resistance when Protein S is eliminated.[5]



Protein S is structured into two domains. The two domains are highly homologous and have a Greek key structure. The domains share high similarity with other crystallin proteins.[6]

Binding SiteEdit

Protein S binds two 2 mol of calcium per mol of protein with a binding dissociation constant of 27 and 76 μM according to dialysis experiments. In the same study mutagenesis experiments revealed arginine replacements to residue 40 or residue 129 can reduce the binding affinity.[7] Since then, an NMR structure determine positions for the two binding sites at residues 10 and 71 and also at residues 99 and 159. However, these binding sites were based on a cluster analysis of side-chain oxygen atoms and on results from site-directed mutagenesis studies and not yet experimentally verified.[1] More recently, an X-ray crystal structure on the truncated N-terminal domain revealed crystallographic evidence of two binding sites in the N-terminal domain. These binding sites are at residues 7,37,39 and 76 as well as at residues 36,77, and 79, which agree with the mutagenesis experiments.[8] However, if the N-terminal domain can bind two Ca2+ then either only the N-terminal domain binds calcium, or, the full Protein S can bind more than two mol of calcium per mol of protein. Since both these claims have been experimentally shown to not be true[7] then the exact binding site of Protein S cannot yet fully be described.


  1. ^ a b Bagby S, Harvey TS, Eagle SG, Inouye S, Ikura M (February 1994). "NMR-derived three-dimensional solution structure of protein S complexed with calcium". Structure. 2 (2): 107–22. doi:10.1016/s0969-2126(00)00013-7. PMID 8081742.
  2. ^ Inouye M, Inouye S, Zusman DR (February 1979). "Gene expression during development of Myxococcus xanthus: pattern of protein synthesis". Developmental Biology. 68 (2): 579–91. doi:10.1016/0012-1606(79)90228-8. PMID 108160.
  3. ^ a b Inouye M, Inouye S, Zusman DR (January 1979). "Biosynthesis and self-assembly of protein S, a development-specific protein of Myxococcus xanthus". Proceedings of the National Academy of Sciences of the United States of America. 76 (1): 209–13. doi:10.1073/pnas.76.1.209. PMC 382907. PMID 284334.
  4. ^ a b Bagby S, Harvey TS, Eagle SG, Inouye S, Ikura M (May 1994). "Structural similarity of a developmentally regulated bacterial spore coat protein to beta gamma-crystallins of the vertebrate eye lens". Proceedings of the National Academy of Sciences of the United States of America. 91 (10): 4308–12. doi:10.1073/pnas.91.10.4308. PMC 43774. PMID 8183906.
  5. ^ Downard JS, Kupfer D, Zusman DR (June 1984). "Gene expression during development of Myxococcus xanthus. Analysis of the genes for protein S". Journal of Molecular Biology. 175 (4): 469–92. doi:10.1016/0022-2836(84)90180-3. PMID 6204058.
  6. ^ Wistow G, Summers L, Blundell T (1985). "Myxococcus xanthus spore coat protein S may have a similar structure to vertebrate lens beta gamma-crystallins". Nature. 315 (6022): 771–3. doi:10.1038/315771a0. PMID 3925350.
  7. ^ a b Teintze M, Inouye M, Inouye S (January 1988). "Characterization of calcium-binding sites in development-specific protein S of Myxococcus xanthus using site-specific mutagenesis". The Journal of Biological Chemistry. 263 (3): 1199–203. PMID 3121626.
  8. ^ Wenk M, Baumgartner R, Holak TA, Huber R, Jaenicke R, Mayr EM (March 1999). "The domains of protein S from Myxococcus xanthus: structure, stability and interactions". Journal of Molecular Biology. 286 (5): 1533–45. doi:10.1006/jmbi.1999.2582. PMID 10064714.