An S-layer (surface layer) is a part of the
cell envelopecommonly found in bacteria, as well as among archaea. It consists of a monomolecular layer composed of identical proteinsor glycoproteins. This two dimensional structure is built via self-assemblyand encloses the whole cell surface. Thus, the S-layer protein can represent up to 10-15% of the whole protein content of a cellcite journal |author=Messner P, Sleytr U |title=Crystalline bacterial cell-surface layers |journal=Adv. Microb. Physiol. |volume=33 |issue= |pages=213–75 |year=1992 |pmid=1636510 |doi=10.1016/S0065-2911(08)60218-0] cite journal |author=Pum D, Messner P, Sleytr U |title=Role of the S layer in morphogenesis and cell division of the archaebacterium Methanocorpusculum sinense |journal=J. Bacteriol. |volume=173 |issue=21 |pages=6865–73 |year=1991 |pmid=1938891] cite journal |author=Sleytr U, Messner P, Pum D, Sára M |title=Crystalline bacterial cell surface layers |journal=Mol. Microbiol. |volume=10 |issue=5 |pages=911–6 |year=1993 |pmid=7934867 |doi=10.1111/j.1365-2958.1993.tb00962.x] . S-layer proteins are poorly 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 cite journal |author=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 |title=Applications of S-layers |journal=FEMS Microbiol. Rev. |volume=20 |issue=1-2 |pages=151–75 |year=1997 |pmid=9276930] . 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 centre to centre spacings (or unit cell dimensions) between these subunits range between 2.5 and 35 nm.
Fixation of S-layers in the cell wall
Gram-negative bacteriaS-layers are associated to the LPSvia ionic, carbohydrate-carbohydrate, protein carbohydrate interactions and/or protein-protein interactions.
Gram-positive bacteriawhose S-layers contain surface layer homology (SLH) domains the binding occurs to the peptidoglycanand to a secondary cell wallpolymer (e.g. teichuronic 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 wallpolymer.
Gram-negative archaeaS-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane.
Gram-positive archaeathe S-layer proteins bind pseudomurein or to methanochondritin.
Biological functions of the S-layer
As 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 stabilisation. Additional functions associated with S-layers include:
* protection against
* resistance against low
* barrier against high molecular weight substances (e.g. lytic
* adhesion (for
* stabilisation of the membrane
* provide adhesion sites for exoproteins
* provide a periplasmic compartment in Gram-positive prokaryotes together with the peptidoglycan and the cytoplasmic membrane
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