S-layer
An S-layer (surface layer) is a part of the cell envelope commonly found in bacteria, as well as among archaea .[1] It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer protein can represent up to 10–15% of the whole protein content of a cell [2] [3] .[4] S-layer proteins are poorly conserved or not conserved at all, and can differ markedly even between related species. Depending on species, the S-layers have a thickness between 5 and 25 nm and possess identical pores with 2–8 nm in diameter.[5] S-layers exhibit either an oblique (p1, p2), square (p4) or hexagonal (p3, p6) lattice symmetry. Depending on the lattice symmetry, the S-layer is composed of one (P1), two (P2), three (P3), four (P4), or six (P6) identical protein subunits, respectively. The center-to-center spacings (or unit cell dimensions) between these subunits range between 2.5 and 35 nm.
[edit] Fixation of S-layers in the cell wall
- In Gram-negative bacteria, S-layers are associated to the lipopolysaccharides via ionic, carbohydrate–carbohydrate, protein–carbohydrate interactions and/or protein–protein interactions.
- In Gram-positive bacteria whose S-layers contain surface layer homology (SLH) domains, the binding occurs to the peptidoglycan and to a secondary cell wall polymer (e.g., teichoic acids). In the absence of SLH domains, the binding occurs via electrostatic interactions between the positively charged N-terminus of the S-layer protein and a negatively charged secondary cell wall polymer.
- In Gram-negative archaea, S-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane.
- In Gram-positive archaea, the S-layer proteins bind to pseudomurein or to methanochondroitin.
[edit] Biological functions of the S-layer
For many bacteria, the S-layer represents the outermost interaction zone with their respective environment. Its functions are very diverse and vary from species to species. In Archaea the S-layer is the only cell wall component and, therefore, is important for mechanical stabilization. Additional functions associated with S-layers include:
- protection against bacteriophages, Bdellovibrios, and phagocytosis
- resistance against low pH
- barrier against high-molecular-weight substances (e.g., lytic enzymes)
- adhesion (for glycosylated S-layers)
- stabilisation of the membrane
- provision of adhesion sites for exoproteins
- provision of a periplasmic compartment in Gram-positive prokaryotes together with the peptidoglycan and the cytoplasmic membranes
[edit] References
- ^ S-layers on cell walls of cyanobacteria Micron Volume 33, Issue 3, January 2002, Pages 257–277; doi:10.1016/S0968-4328(01)00031-2
- ^ Messner P, Sleytr U (1992). "Crystalline bacterial cell-surface layers". Adv. Microb. Physiol. 33: 213–75. doi:10.1016/S0065-2911(08)60218-0. PMID 1636510.
- ^ Pum D, Messner P, Sleytr U (1991). "Role of the S layer in morphogenesis and cell division of the archaebacterium Methanocorpusculum sinense". J. Bacteriol. 173 (21): 6865–73. PMC 209039. PMID 1938891. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=209039.
- ^ Sleytr U, Messner P, Pum D, Sára M (1993). "Crystalline bacterial cell surface layers". Mol. Microbiol. 10 (5): 911–6. doi:10.1111/j.1365-2958.1993.tb00962.x. PMID 7934867.
- ^ Sleytr U, Bayley H, Sára M, Breitwieser A, Küpcü S, Mader C, Weigert S, Unger F, Messner P, Jahn-Schmid B, Schuster B, Pum D, Douglas K, Clark N, Moore J, Winningham T, Levy S, Frithsen I, Pankovc J, Beale P, Gillis H, Choutov D, Martin K (1997). "Applications of S-layers". FEMS Microbiol. Rev. 20 (1–2): 151–75. doi:10.1016/S0168-6445(97)00044-2. PMID 9276930.
