Chapter 7 - Bacterial surface layer glycoproteins and “non-classical” secondary cell wall polymers

Abstract

This chapter describes the components, general features, biosysnthesis, functions, and genetics of cell surface layers, S-layers. S-Layers are common structures of the prokaryotic cell envelope with a lattice-like appearance. They are formed by a self-assembly process and are attached to the bacterial cell via a special class of secondary cell wall polymers. S-layers are recognized as one of the most common cell surface structures in bacteria and represent an almost universal feature of archaea. In most archaea, the S-layer is either integrated into or penetrating the plasma membrane. In Gram-positive bacteria and archaea, the S-layer lattice assembles on the surface of the rigid peptidoglycan or the pseudomurein, respectively. In Gram-negative bacteria, a specific fraction of “smooth” lipopolysaccharide (LPS) is required for attachment of the S-layer to the outer membrane. S-layer synthesis and assembly resemble a very efficient system since, at a generation time of ∼20 minutes, at least 500 copies of a single (glyco)protein species have to be synthesized per second, translocated to the cell surface and incorporated into the S-layer lattice. Bacterial S-layer glycoproteins possess a tripartite structure that compares to LPS O-antigens. Archaeal S-layer (glyco)proteins are generally characterized by a predominance of nonpolar amino acid residues, but some species contain an increased portion of charged residues. S-layer proteins can undergo diverse co- or post-translational modifications.