S-layer

An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria.

[1][2] The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) identical proteins or glycoproteins.

[6] The general use was accepted at the "First International Workshop on Crystalline Bacterial Cell Surface Layers, Vienna (Austria)" in 1984, and in the year 1987 S-layers were defined at the European Molecular Biology Organization Workshop on “Crystalline Bacterial Cell Surface Layers”, Vienna as “Two-dimensional arrays of proteinaceous subunits forming surface layers on prokaryotic cells” (see "Preface", page VI in Sleytr "et al. 1988"[7]).

In many archaeal species the S-layer is the only cell wall component and, therefore, is important for mechanical and osmotic stabilization.

[19] While ubiquitous among Archaea, and common in bacteria, the S-layers of diverse organisms have unique structural properties, including symmetry and unit cell dimensions, due to fundamental differences in their constituent building blocks.

Subsequently, S-layer fusion proteins with specific functional domains (e.g. enzymes, ligands, mimotopes, antibodies or antigens) allowed to investigate completely new strategies for functionalizing surfaces in the life sciences, such as in the development of novel affinity matrices, mucosal vaccines, biocompatible surfaces, micro carriers and encapsulation systems, or in the material sciences as templates for biomineralization.

Schematic illustration of the supramolecular architecture of the major classes of prokaryotic cell envelopes containing surface (S) layers. S-layers in archaea with glycoprotein lattices as exclusive wall component are composed either of mushroom-like subunits with pillar-like, hydrophobic trans-membrane domains (a), or lipid-modified glycoprotein subunits (b). Individual S-layers can be composed of glycoproteins possessing both types of membrane anchoring mechanisms. Few archaea possess a rigid wall layer (e.g. pseudomurein in methanogenic organisms) as intermediate layer between the plasma membrane and the S-layer (c). In Gram-positive bacteria (d) the S-layer (glyco)proteins are bound to the rigid peptidoglycan-containing layer via secondary cell wall polymers. In Gram-negative bacteria (e) the S-layer is closely associated with the lipopolysaccharide of the outer membrane. Figure and figure legend were copied from Sleytr et al. 2014, [ 2 ] which is available under a Creative Commons Attribution 3.0 International (CC BY 3.0) licence .