• As with any prokaryote other than archaea’s cell wall, it surrounds the cell and mediates the interaction of the cell with its environment.
• It can also be used to maintain cell shape, protect against viruses, heat, acidity, or alkalinity.
• It can appear like a microsieve during the formation of pores-like structures and thus enable or disable transport processes.
• Sometimes, cells wall components may make up over 10% of total cellular protein.
• A wide range of cell envelope structures and compounds have been described and documented so far.
• The S-layer is the archaeal cell wall with the highest frequency.
• Other archaeal cell wall structures are pseudomurein, methanochondroitin, glutaminylglycan, sulfated heteropolysaccharides and protein sheaths and they are sometimes associated with additional proteins and protein complexes like the STABLE protease or the bindosome.
• Recent advancements in electron microscopy have also shown the existence of an outer (most) cellular membrane within several archaeal group, similar to the Gram-negative cell walls within bacteria.

Archaeal Cell Wall Introduction

• Archaea are the third major lineage, or “domain”, after bacteria and eukaryotes.
• Since the discovery of archaeal cells walls and their first descriptions, the S-layer has been suggested as the most important and ancient archaeal envelope structure.
• The S-layer is a paracrystalline 2D crystallinity that forms paracrystalline crystal lattices with the p1- to p4-, and p6-symmetry. It is a particularly important component of organisms known as extremophiles.
• The Sulfolobales, for example, thrive at 65 to 75 degrees Celsius and a pH of around 2. They can grow in hot sulfuric acids, and the only barrier between the outside world and the cytoplasm is the S layer with a distinctive p3-symmetry.
• Pyrolobus fumarii is also the current record holder of the highest temperature for growth at 113 degrees Celsius. He is enclosed by an S-layer that has, in this instance, p4symmetry.
• It is currently accepted that the S layer was the first archaeal cell-wall to develop.
• In recent years, however, a growing number of archaeal cells envelopes have been described in a different way from Slayers and pseudomurein to methanochondroitin and protein sheaths.
• These outermost layers are made up of single and double membranes, mucous layers, and polysaccharides.
• Two microorganisms that do not have an S-layer or other solid cell walls rank among the extremophiles. They seem to have adapted to extreme environments without the protection of a S-layer: Ferroplasma Acidiphilum, and Thermoplasma species.

One Membrane Archaea

• Even though they live in extreme conditions (e.g. Ferroplasma acidiphilum, Thermoplasma species can survive in low pH levels of 1-2 and 60 degrees Celsius. They do not have any cell walls.
• These organisms are bounded only by one membrane and exhibit a pleomorphic shape.
• Thermoplasma acidophilum cells were shown to retain their integrity via mannose-rich glycoproteins, lipoglycans, and plasma membrane anchors. These forms a protective glycocalix at cell surface.
• Ferroplasma acidamanus membranes were found to contain tetraetherlipids that form monolayer membranes rather than bilayer membranes. This allows the cells to survive in acidic environments.
• These monolayer membranes are stable due to their greater resistance to hydrolysis.
• In this regard, it is possible that effective proton pumps or acid stable membrane proteins could also play a significant role.

Double Membranes of Archaea 

• Different phyla of archaea can contain microorganisms with double membranes as their outermost structures.
• It is interesting that all the double-membraned Archaea mentioned so far in literature have in common the close interaction of other microorganisms.
• The architecture of double-membraned arhea is similar to the cell envelope of Gram negative bacteria.
• Three layers make up the Gram-negative cell envelope: the outer membrane, peptidoglycan and inner membrane.
• The outer membrane is composed of a bilayer that consists of an inner leaflet with phospholipids, and an outer leaflet with glycolipids (e.g. lipopolysaccharides).
• The outer membrane also contains two major classes of proteins: mainly lipoproteins and b-barrel protein.
• The inner membrane, however, forms a phospholipid bilayer.
• The periplasm, an aqueous chamber, is enclosed by both the inner-and outer membranes.
• The Gram-negative bacteria periplasmic space is approximately 10% of total cell volume. It contains a thin layer of peptidoglycan and soluble proteins, such as hydrolytic enzymes.
• These proteins are essential for substrate initial degradation, binding proteins involved in transport of substances and receptors for Chemotaxis.
• While they look very similar, the archaeal double-membrane is different from the Gram negative cell envelope in terms of biochemical composition and the function of the various compartments.
• The space between the inner and outer layers of archaea is called pseudoperiplasmic space. It is filled with pseudo-periplasm.
• The following Archaea contain Double Membranes; Crenarchaeon Ignicoccus hospitalis, ARMAN archaea (archaeal Richmond Mine acidophilic nanoorganisms), Methanomassiliicoccus luminyensis.

What is S-Layer?

• Except for archaea, which are only surrounded by a cell membrane without additional cell walls, the S-layer or surface layer is the most common variation of cell wall.
• S-layer proteins are often highly glycosylated, with a high proportion of the total protein mass being caused by the glycan side chain.
• The high sugar content protects proteins from degradation and enhances their stability in extreme environments such as high temperature and acidity.
• In an entropically driven process, the S-layer protein arranges itself in a pseudocrystal that is 2-dimensionally and consistently arranged on the cell surface.
• It is the most common cell wall variant of archaea so far, particularly within the Crenarchaea. However, it can also be found in many bacterial species, including cyanobacteria.
• The S-layer may be made up of one major (glycoprotein) protein (large subunit), as well as an amino acid stretch which forms the anchor in membrane or pseudoperiplasmic Murein.
• Sometimes the S-layer can be formed by two proteins. The anchor is usually the smaller subunit of the second protein (Veith and al. 2009).
• Major S-layer protein (with a protein mass ranging from 40 to 325kDa) is creating a 2D pseudocrystalline array with distinct symmetry at the cell surface.
• The stalk-like anchoring structure and the S-layer array are superimposing the cytoplasmic Membrane and thereby creating a quasi-periplasmic space between the cytoplasmic Membrane and S-layer. This is in contrast to the (pseudo-periplasm), which is the space between the inner and outer membrane.
• In extreme cases, this quasi-periplasm may be as wide as 70 nm. This is what happened with Staphylothermus marineus.
• Tetrabrachion is the name given to the stalks that anchor the S-layer in the second case.
• S-layer arrays’ central crystal unit is composed of either two, three, four, or six subunits. This leads to p2- or p4-, respectively.
• The lattice constants for 2D crystals vary between 11 and 30 nanometers.
• Sometimes, the symmetry and spacing between center and center were found to be genus-specific as could be seen for the Sulfolobales. All of the S-layers described had glycosylated S layers with p3-symmetry. The lattice constants were around 21 nm.
• High levels of protein stability against hydrolysis and degradation under harsh environmental conditions leads directly to potential functions of S-layer proteins such as protection from the environment.
• Some crenarchaea, such as the Sulfolobales must withstand pH 2 and temperatures between 60 and 80 degC.
• Other functions of S-layer protein include maintaining cell integrity under alternating osmotic conditions, (osmoprotection), as well as supporting or maintaining cell shape.
• S-layers may also play other functions, such as being molecular sieves and/or generating an extra cellular compartment like the periplasm of Gram-negative bacteria.
• Quasiperiplasm is the space between the cytoplasmic membrane, the umbrella-like envelope of the S-layer and the cytoplasmic membrane.
• The S-layer is a protective layer that protects against viruses. It prevents the entry or attachment of potentially harmful particles.

Cell Wall Structures of Methanogenic Archaea

• Methanogens are part of the euryarchaeal Kingdom and can be divided into five orders: Methanobacteriales (Methanococcales), Methanomicrobiales (Methanomicrobiales) and Methanopyrales (Methanopyrales).
• They are a diverse group in the archaeal domain and show a wide range of phylogeny as well as physiology, morphology, and morphology.
• This diversity is also evident in the structure and compositions of methanogenetic cells, such as pseudomurein and certain S-Layers, as well as methanochondroitin and proteinaceous sheaths.

Pseudomurein

• Most bacterial species have a peptidoglycan muein in their cell walls. This is a polymer made of sugars and amino acid that forms a net-like layer outside of the cell.
• Because of structural similarities, such as the 50% glycan/50% peptide constructions, pseudomurein was given to the cell walls of Methanobacteriales & Methanopyrales.
• Pseudomurein is made up of chains of disaccharides, which are cross-linked using peptide subunits.
• In general, the glycan strand is built up by alternating units of β(1-3)-linked N-acetyl-d-glucosamine and N-acetyl-l-talosaminuronic acid.
• Glycan strands are then cross-linked with peptide subunits made of L-amino acid (in most cases glutamic, alanine and lysine) only.
• The lack of d-amino acids, the use of N-acetyl-l-talosaminuronic acid instead of Nacetyl-muramic acid, the β(1-4)-linkage to N-acetylglucosamine and the increased occurrence of a- and e-peptide bonds represent the main differences to the bacterial murein As the linkage between the two glycan components is β(1-3) there is the possibility of a replacement of N-acetyl-glucosamine by Nacetyl-galactosamine.

Methanochondroitin

• Methanochondroitin can only be found in Methanosarcina species aggregated cells. The shape-preserving structure is represented by the methanochondroitin layers.
• It is possible to disintegrate cells by increasing the osmolality, the concentrations of divalent Cations such as Ca2+ and Mg2+, or when nutrients are lacking. Methanosarcina barkeri shows an example of this, with a 20-fold reduction in glucuronic acid which is a key component of the methanochondroitin layers.
• Methanochondroitin isn’t the only cell wall structure found in Methanosarcina. Some species have an additional S-Layer that lies between the cytoplasmic membrane layer and the methanochondroitin.
• Structurally, methanochondroitin is built from proteoglycans, with the trimer [-)β-d-GlcA-(1-3)-β-d-GalNAc-(1-4)-β-d-GalNAc-(1-]n as the fundamental glycan structure.
• This structure is made up of a fibrillar polymer, and creates a compact or loose matrix.
• This structure was called methanochondroitin because it is similar to the eukaryotic methanochondroitin sulfate. However, the two main differences between the structures and chondroitin are the absence of sulphate residues (methanochondroitin) and the different molar ratios of GalNAc to LcA.

References

• Albers, SV., Meyer, B. The archaeal cell envelope. Nat Rev Microbiol 9, 414–426 (2011). https://doi.org/10.1038/nrmicro2576
• Meyer, B.H. and Albers, S.-V. (2022). Archaeal Cell Walls. In eLS, John Wiley & Sons, Ltd (Ed.). https://doi.org/10.1002/9780470015902.a0000384.pub3
• Klingl A, Pickl C, Flechsler J. Archaeal Cell Walls. Subcell Biochem. 2019;92:471-493. doi: 10.1007/978-3-030-18768-2_14. PMID: 31214995.
• https://en.wikipedia.org/wiki/Archaea#Cell_wall_and_archaella
• https://courses.lumenlearning.com/wm-biology2/chapter/archaea-vs-bacteria/