S-Layer

As an S-layer (from the surface layer eng -. Surface layer), Crystalline cell wall, S- layer (s), or envelope proteins referred to membrane-like surface structures, which are formed by many bacteria and archaea on their cell wall.

Structure of the S-layer

In contrast to the cell membrane, which is composed of lipids, of the S-layer usually made of a single protein (or glycoprotein ) species formed. These protein monomers are by self-organization in a position to form layers with a symmetrically arranged grid. Due to the regular arrangement of these layers we also speak of ( two-dimensional ) crystalline structures or organic crystals. In general, the S-layer proteins are non-covalently bound to other cell wall components with one another. There were - depending on the organism - very different S-layer insulated; some species are even capable of forming a plurality of different S-layer as required. In the previously identified S-layers, the monomers have a molecular weight of 40 to 200 kDa. The thickness of the structures formed therefrom is from 5 to 20 nm

Function of the S-layer

The S-layer is usually the outermost (or as in the case of some archaea, the only one ) cell wall component represents and can perform different functions depending on the organism. It is believed that the S-layer in addition to the form-building function in many cases also to protect from damaging environmental influences (eg biomineralization ), but also against phages or in the case of pathogenic bacteria, serves phagocytosis. In addition, the S-layer is a virulence factor of some bacterial strains, for example, it results in Campylobacter spp. the in vivo variability ( antigenic shift ) and prevents the binding of C3b. In many cases, however, the purpose of these additional cell-wall component is unknown; under laboratory conditions, some species lose the ability to form S-layer.

Self-organization

The possibility of S-layers of self-organization gives the S-layer monomers, the ability to bind to the cell wall and other monomers. It is believed that the bonding strength of the individual monomers to each other is greater than the bond to the underlying cell wall component. In experiments with the isolated coat protein monomer was shown that S-layer are dependent on a natural cell surface for recrystallization is not, but also on other surfaces, at phase boundaries, or may recrystallize spontaneously in suspension. The information for the formation of a regular S-layer layer therefore must be included in each monomer.