Structural diversity and functional versatility in Gram-positive S-layers

Surface layers (S-layers) are highly ordered, paracrystalline protein lattices that form the outermost envelope of many bacteria and archaea. Although widespread, their functions and structural organisation are only recently being elucidated at molecular resolution. This review addresses S-layer structure and function in Gram-positive bacteria, with emphasis on emerging insights into lattice-forming self-assembly and the mechanisms by which S-layers attach to the cell wall. Recent work highlights the role of Gram-positive S-layers as exoskeletons that provide organisation and mechanical stabilisation of the cell envelope, and discusses their involvement in host interactions and stress tolerance. Although crystallinity is central for S-layer function, these lattices must also incorporate intrinsic plasticity to accommodate cell curvature and permit cell growth and division. S-layers and associated proteins are increasingly thought to act as active organisers of the cell envelope, and can undergo shedding or remodelling in response to changing environmental conditions.