S-layer
Encyclopedia
An S-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 cell
. 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. 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.
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:
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. These are differentiated by their Gram staining characteristics....
commonly found in bacteria
Bacteria
Bacteria are a large domain of prokaryotic 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...
.
It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly
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 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. 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.
Fixation of S-layers in the cell wall
- In Gram-negativeGram-negativeGram-negative bacteria are 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...
bacteriaBacteriaBacteria are a large domain of prokaryotic 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 lipopolysaccharides via ionic, carbohydrate–carbohydrate, protein–carbohydrate interactions and/or protein–protein interactions. - In Gram-positiveGram-positiveGram-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...
bacteriaBacteriaBacteria are a large domain of prokaryotic 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 peptidoglycanPeptidoglycanPeptidoglycan, 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. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
and to a secondary cell wallCell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
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 wallCell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
polymer. - In Gram-negativeGram-negativeGram-negative bacteria are 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...
archaeaArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, S-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane. - In Gram-positiveGram-positiveGram-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...
archaeaArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, the S-layer proteins bind to pseudomurein or to methanochondroitin.
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, BdellovibrioBdellovibrioBdellovibrio is a genus of Gram-negative, obligate aerobic bacteria.One of the more notable characteristics of this genus is that members parasitize other Gram-negative bacteria by entering into their periplasmic space and feeding on the biopolymers, e.g. proteins and nucleic acids, of their hosts...
s, and phagocytosis - resistance against low pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
- 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