9.B.324. The Pore-forming S-layer protein (S-layer) Family
Crystalline bacterial and archaeal cell surface layers (S-layers) represent the outermost cell envelope component in a broad range of bacteria and archaea (Pum and Sleytr 2014). They are monomolecular arrays composed of a single protein or glycoprotein species and represent the simplest proteinaceous biological membranes. They are highly porous protein mesh works with unit cell sizes in the range of 3 to 30 nm, and pore sizes of 2 to 8 nm. S-layers are usually 5 to 20 nm thick in bacteria but up to 70 nmthick in archaea. S-layer proteins are one of the most abundant biopolymers on earth. One of their key features, is the intrinsic capability of isolated native and recombinant S-layer proteins to form self-assembled mono- or double layers in suspension, at solid supports, the air-water interface, planar lipid films, liposomes, nanocapsules, and nanoparticles. Reassembly is entropy-driven as an example of matrix assembly following a multistage pathway in which the process of S-layer protein folding is directly linked with assembly into extended clusters. Basic research on the structure, synthesis, genetics, assembly, and function of S-layer proteins laid the foundation for their application in novel approaches in biotechnology, biomimetics, synthetic biology, and nanotechnology (Pum and Sleytr 2014).