S-layer
An S-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 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 " 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". 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 domains, the binding occurs to the peptidoglycan and to a secondary cell wall polymer. 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
- protection against bacteriophages, Bdellovibrios, and phagocytosis
- resistance against low pH
- barrier against high-molecular-weight substances
- adhesion
- stabilization of the membrane
- resistance against electromagnetic stress
- 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
S-layer structure
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. Sequence analyses of S-layer proteins have predicted that S-layer proteins have sizes of 40-200 kDa and may be composed of multiple domains some of which may be structurally related. Since the first evidence of a macromolecular array on a bacterial cell wall fragment in the 1950s, S-layer structures have been investigated extensively by electron microscopy. These studies have provided useful information on overall S-layer morphology.In general, S-layers exhibit either oblique, square or hexagonal lattice symmetry. Depending on the lattice symmetry, each morphological unit of the S-layer is composed of one, two, three, four, or six identical protein subunits. The center-to-center spacing between these subunits range from 4 to 35 nm.
For example, high-resolution structures of an archaeal S-layer protein of the Methanosarcinales S-layer Tile Protein family and a bacterial S-layer protein, from Geobacillus stearothermophilus PV72, have been determined by X-ray crystallography. In contrast with existing crystal structures, which have represented individual domains of S-layer proteins or minor proteinaceous components of the S-layer, the MA0829 and SbsB structures have allowed high resolution models of the M. acetivorans and G. stearothermophilus S-layers to be proposed. These models exhibit hexagonal and oblique symmetry, for M. acetivorans and G. stearothermophilus S-layers, respectively, and their molecular features, including dimensions and porosity, are in good agreement with data from electron microscopy studies of archaeal and bacterial S-layers.
Finally, in connection with questions of structure-function investigations on S-layers, it should be mentioned that the recent introduction of SymProFold, which utlizes the high accuracy of AlphaFold-Multimer predictions to derive symmetrical assemblies from protein sequeces has proven to be a groundbreaking method for the accurate structural prediction of S-layer arrays. The predicted models could be vallidated using available experimental data at the cellular level, and additional crystal structures were obtained to confirm the symmetry and interfaces of numerous SymProFold assemblies. Thus, this methodological approach to the structural elucidation of S-layers opens possibilities for exploring functionalities and designing targeted applications in diverse fields such as nanotechnology, biotechnology, nanomedicine, and environmental sciences.