S-layer

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S-layer

An S-layer (surface layer) is a part of the cell envelope

Cell envelope

The cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative....
commonly found in bacteria

Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
, as well as among archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
. It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two dimensional structure is built via self-assembly

Self-assembly

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An S-layer (surface layer) is a part of the cell envelope

Cell envelope

Bacteria

Archaea

Self-assembly

Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction....
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 . 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 . 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
Gram-negative

Gram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color....
bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
S-layers are associated to the LPS
Lipopolysaccharide

Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals....
via ionic, carbohydrate-carbohydrate, protein carbohydrate interactions and/or protein-protein interactions.
In Gram-positive
Gram-positive

Gram-positive Bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink....
bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
whose S-layers contain surface layer homology (SLH) domains the binding occurs to the peptidoglycan
Peptidoglycan

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall....
and to a secondary cell wall
Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cell . It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism....
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
Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cell . It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism....
polymer.
In Gram-negative
Gram-negative

Gram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color....
archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
S-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane.
In Gram-positive
Gram-positive

Gram-positive Bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink....
archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
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
Phagocytosis

File:Phagocytosis in three steps.pngPhagocytosis is the cell process of Phagocytes and Protists of engulfing solid particles by the cell membrane to form an internal phagosome, which is a food vacuole, or pteroid....
resistance against low pH
PH

pH is a measure of the Acid or Base of a solution. It is defined as the cologarithm of the Activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations....
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