Archaeal Cell Walls


Next to the bacterial and eukaryal domains, Archaea form one of the three domains of life. Within the last decade, the phylogenetic tree of archaea has dramatically expanded and nowadays Archaea are found in almost every habitat, from extreme to moderate. The adaptation to these diverse environments might have resulted in the remarkably high variety of different archaeal cell envelope types. One major difference to bacteria is the lack of murein or a lipopolysaccharide (LPS)‐containing outer membrane.

The cell wall of many Archaea is formed by a proteinaceous surface (S‐) layer. S‐layer proteins have the intrinsic ability to form two‐dimensional crystals, which can have an oblique (p2), square (p4) or hexagonal (p3 or p6) symmetry. All currently studied archaeal S‐layer proteins were found to be modified by the attachment of N‐linked and, in some cases, additionally by O‐linked glycans. However, also Archaea have been characterised which lack an S‐layer and possess instead a second outermost membrane or sugar polymers like pseudomurein, methanochondroitin, or heteropolysaccharides as their cell envelope. These polymeric cell wall structures can either form the sole cell wall structure or be supported by an additional S‐layer cover.

Key Concepts

  • Archaea do have peptidoglycan, it is called pseudomurein and different in its composition to murein.
  • Most studied Archaea have one cytoplasmic membrane, but more and more species are found which have two membranes.
  • Archaeal cell envelopes lack murein or a lipopolysaccharide (LPS)‐containing outer membrane.
  • Many Archaea possess a glycosylated proteinaceous surface layer (S‐layer) as their sole cell wall structure.
  • In some Archaea, the cell wall is composed of glycan polymers, like glutaminylglycan, heterosaccharide, methanochondroitin, or pseudomurein, which can be further supported by an S‐layer.

Keywords: archaea; cell envelope; glutaminylglycan; glycosylation; heteropolysaccharide; methanochondroitin; pseudomurein; S‐layer (glyco‐)protein

Figure 1. Diversity of archaeal cell envelopes. Abbreviations: CM, cytoplasmic membrane; GC, glycocalyx; HP, heteropolysaccharide sacculus; MC, methanochondroitin matrix; OM, outermost membrane; PM, pseudomurein; PS, proteinaceous sheath; SL, S‐layer.
Figure 2. The different S‐layer symmetries. Single S‐layer proteins are depicted. S‐layer proteins that form the respective symmetry units are depicted in red.
Figure 3. Schematic depiction of the N‐ and O‐glycosylation pathways in the crenarchaeaon S. acidocaldarius (a) and the model of the pseudomurein biosynthesis pathway (b). (a) The N‐glycan biosynthesis starts at the cytoplasmic side of the membrane where nucleotide‐ or lipid‐phosphate‐ activated monosaccharide sugar precursors (e.g. UDP‐GlcNAc, or dolichol‐phosphate‐mannose) are sequentially added onto the lipid carrier dolichol phosphate (Euryarchaeota) or dolichol pyrophosphate (Crenarchaeota) by multiple specific glycosyltransferases (yellow). The fully assembled DolP(P)‐linked N‐glycan is translocated across the cytoplasmic membrane by an unknown flippase and the oligosaccharide is transferred by the oligosaccharyltransferase AglB (blue) onto a secreted target protein (Sec secretion system, green; S‐layer, brown) onto specific aspartic acid residues within N‐glycosylation sequins (Asp‐X‐Ser/Thr). So far no biosynthesis study on the O‐glycosylation process in Archaea has been performed. It is proposed that before protein secretion, specific O‐glycosyltransferases sequentially transfer nucleotide‐activated sugar precursors onto the hydroxyl group of Ser or Thr residues on to the target protein. (b) Putative biosynthetic pathway of pseudomurein. Based on homologs of the bacterial MraY and MurG in pseudomurein synthesising archaea, a new pathway has been here predicted. Similar to the bacterial peptidoglycan the biosynthesis starts with the formation of N‐acetyl‐l‐talosaminuronic acid (TalNAcA), which is then attached with Glc, Ala, and Lys residues. This TalNAcA pentapeptide and tetrapeptide is transferred onto a lipid carrier, by a MraY homolog enzyme and further enlarged by the attachment of N‐acetyl‐d‐glucosamine or N‐acetyl‐d‐galactosamine via a β‐1,3 linkage. Flipped across the membrane and assembled by transglycosyadases and transpeptidases into the pseudomurein polymer. During transpeptidation, one C‐terminal alanine residue is predicted to split off.
Figure 4. Chemical structure of pseudomurein. Adapted from Kandler and König .
Figure 5. Proposed structure of the repeating units (regions 1–3) of the cell wall polymer of Natronococcus occultus. Based on Niemetz et al. .
Figure 6. Chemical structure of methanochondroitin. From Kreisl and Kandler .


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Further Reading

Baumeister W and Lembcke G (1992) Structural features of archaebacterial cell envelopes. Journal of Bioenergetics and Biomembranes 24 (6): 567–575.

Claus H and König H (2010) Cell envelopes of methanogens. In: Claus H and König H (eds) Prokaryotic Cell Wall Compounds, chapter 7, pp 231–252. Springer: Heidelberg.

Eichler J and Adams MW (2005) Posttranslational protein modification in Archaea. Microbiology and Molecular Biology Reviews 69 (3): 393–425.

Jarrell KF, Ding Y, Meyer BH, et al. (2014) N‐linked glycosylation in Archaea: a structural, functional, and genetic analysis. Microbiology and Molecular Biology Reviews 78 (2): 304–341.

König H (1988) Archaeobacterial cell envelopes. Canadian Journal of Microbiology 34 (4): 395–406.

Rachel R (2010) Cell envelopes of crenarchaeota and nanoarchaeota. In: Claus H and König H (eds) Prokaryotic Cell Wall Compounds, chapter 9, pp 271–291. Springer: Heidelberg.

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Meyer, Benjamin H, and Albers, Sonja‐Verena(Jul 2020) Archaeal Cell Walls. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000384.pub3]