S ‐Layers

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. Key Concepts Microbial S‐layers have a hybrid functional role, that is, mediating cell stability and controlling cell surface properties. The architecture of S‐layers and their close interactions with the cell membrane determine and explain the S‐layers' cell wall function in Archaea . Attracting and repelling surface characteristics of S‐layers in Bacteria mediate the interactions with the environment and contribute to the proliferation and survival of cells. The investigation of S‐layer functions requires the combined analysis of the S‐layer structure and its impact on the cell envelope components as well as of interactions with environmental factors.

Dividing the Archaeal Way: The Ancient Cdv Cell-Division Machinery

“…The Sulfolobales S-layer is a cell-wall-like layer, and the sole protector against the turgor pressure (Engelhardt, 2007 ; Albers and Meyer, 2011 ). For a thin porous S-layer to function as a mechanical protection against turgor, the crystalline distance between adjacent S-layer monomers should be small, on the order of a few nm (Engelhardt, 2016 ). Otherwise, the membrane will bulge out.…”

Archaeal cell surface biogenesis

“…In addition, it creates a pseudoperiplasmic space around the cell by acting as a permeability filter (Baumeister and Lembcke 1992 ). The importance of the S-layer to cope with cell turgor has been reported (Engelhardt 2016 ). Two notable functions have been proposed for the S-layer: its ‘antifouling’ property (nano-lotus-effect), which minimizes unspecific binding and could be important for functions such as nutrient uptake.…”

“…The S-layer covers the whole cell and is in most cases the only constituent of the cell envelope outside the cytoplasmic membrane (Albers and Meyer 2011 ; Sleytr et al. 2014 ; Engelhardt 2016 ; Rodrigues-Oliveira et al. 2017 ).…”

“…Accordingly, the S-layer is a metabolically expensive product that places a strong demand on the protein biosynthesis and protein secretion machineries, a cost that indicates that the S-layer must confer a strong selective advantage (Sleytr et al. 2014 ; Engelhardt 2016 ). S-layer proteins make two types of contact: one between subunits to allow lateral assembly and the other to underlying cell structures, which in archaea is commonly the cytoplasmic membrane (Sleytr et al.…”

“…S-layer proteins make two types of contact: one between subunits to allow lateral assembly and the other to underlying cell structures, which in archaea is commonly the cytoplasmic membrane (Sleytr et al. 2014 ; Engelhardt 2016 ). The 2D crystals formed by S-layers have various symmetries, which can be summarized as oblique (p1, p2), square (p4) or hexagonal (p3, p6), with hexagonal assemblies found to be predominant in archaea.…”