古細菌の細胞壁

Archaeal cell walls lack peptidoglycan, the key component in most bacterial cell walls. Instead, archaea use diverse materials including S-layers, pseudomurein, and polysaccharides. This structural flexibility allows archaea to thrive in extreme environments ranging from deep-sea hydrothermal vents to highly saline or acidic habitats, demonstrating their ecological versatility.

An S-layer is the most common archaeal cell wall type, consisting of interlocked protein or glycoprotein molecules anchored to the plasma membrane. These proteins self-assemble into highly ordered crystalline lattices with symmetrical patterns such as hexagonal or tetragonal arrangements. This precise geometry enhances structural integrity and facilitates selective permeability, allowing nutrient passage while excluding harmful substances.

Pseudomurein is a polymer found in methanogenic archaea that resembles bacterial peptidoglycan but contains critical differences. It features β-1,3 glycosidic bonds instead of β-1,4 bonds, making it resistant to lysozyme. Additionally, pseudomurein uses L-amino acids rather than D-amino acids in cross-links, disrupting penicillin effectiveness and providing structural stability in extreme environments.

Archaeal cell walls incorporate specialized components suited to harsh conditions. Methanosarcina species possess a polysaccharide layer of methanochondroitin for flexibility, while Halococcus species have negatively charged polysaccharides with sulfate groups that bind sodium ions, preventing cellular dehydration in hypersaline habitats. These adaptations enable survival across diverse extreme environments.

Pseudomurein's backbone contains alternating units of N-acetylglucosamine and N-acetyltalosaminuronic acid joined by β-1,3 glycosidic bonds. These chains are interconnected by peptide cross-links made of L-amino acids. This unique composition, particularly the use of N-acetyltalosaminuronic acid instead of N-acetylmuramic acid, provides additional resistance to enzymatic attack by antibacterial agents.

Most archaea have cell walls, but some species lack them entirely. Ignicoccus, for example, has no traditional cell wall and instead relies on an outer membrane rich in protein complexes for energy production and nutrient exchange. This variation demonstrates the evolutionary innovation and structural diversity within the archaeal domain.

Beyond S-layers and pseudomurein, some archaeal cell walls include extra protective layers. Methanosarcina species have a polysaccharide layer of methanochondroitin, while Methanospirillum species possess additional external protein layers. These supplementary structures enhance cell wall resilience and structural integrity, enabling archaea to withstand diverse environmental stresses.