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. 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. This structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer protein can represent up to 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 2–8 nm in diameter. The terminology "S-layer" was used the first time in 1976. 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"). For a brief summary on the history of S-layer research see "References". A comprehensive historical account of the development of fundamental and applied S-layer research is given in the following current review.

Location of S-layers

In Gram-negative bacteria, S-layers are associated to the lipopolysaccharides via protein–carbohydrate interactions. In Gram-positive bacteria whose S-layers often contain surface layer homology (SLH) domains, the binding occurs to the peptidoglycan and to a secondary cell wall 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 wall polymer. In Lactobacilli the binding domain may be located at the C-terminus. In Gram-negative archaea, S-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane. In Gram-positive archaea, 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 many archaeal species the S-layer is the only cell wall component and, therefore, is important for mechanical and osmotic stabilization. The S-layer is considered to be porous, which contributes to many of its functions. A most relevant general function of S-layers of both, bacteria and archaea, seems to be their excellent anti-fouling properties. In Archaea that possess S-Layers as the exclusive cell wall component, a general function of S-layer lattices is that of a cell shape-determining/maintaining scaffold. For an overview of functions of S-layers, see. The spectrum of functions associated with S-layers include:

protection against bacteriophages, Bdellovibrios, and phagocytosis resistance against low pH barrier against high-molecular-weight substances (e.g., lytic enzymes) adhesion (for glycosylated S-layers) stabilization of the membrane (e.g. the SDBC in Deinococcus radiodurans) resistance against electromagnetic stress (e.g. ionizing radiations and high temperatures) provision of adhesion sites for exoproteins provision of a periplasmic compartment in Gram-positive prokaryotes together with the peptidoglycan and the cytoplasmic membranes biomineralization molecular sieve and barrier function A great example of a bacterium which utilizes the biological functions of the S-layer is Clostridioides difficile. In C. difficile, the S-layer has helped with biofilm formation, host cell adhesion, and immunomodulation through cell signaling of the host response.

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