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.^[3] 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.^[4] 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.^[5]

The terminology “S-layer” was used the first time in 1976.^[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]). For a brief summary on the history of S-layer research see "References". ^[2][8]