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Abstract

Microbial surface layer (S-layer) proteins assemble into two-dimensional (2D) crystalline lattices on the cell surface of many species of Archaea and Bacteria and represent the outermost cell wall component, except for the existence of a carbohydrate capsule. Despite their widespread occurrence, the function of S-layers remained obscure. Analysing the S-layer structure on the one hand and the interaction of the 2D crystal with the underlying cell membrane on the other, reveals the S-layers' cell wall function and the mechanism of cell stabilisation in Archaea. This basic function is not obvious in Bacteria. Here, experimental data suggest a major role in mediating and controlling interactions of S-layers with the microbes' environment. The hybrid functional role of S-layers will be understood more clearly if structural research is combined with the investigation of S-layer interactions with the adjacent cell envelope components and the complex environmental factors.