An S-layer (surface layer) is a part of the cell envelope commonly found in bacteria, as well as among archaea. 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 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

* In Gram-negative bacteria S-layers are associated to the LPS 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. 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 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 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 bacteriophages 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
* 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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