Abstract
SummaryMany prokaryotic cells are covered by an ordered, proteinaceous, sheet-like structure called a surface layer (S-layer). S-layer proteins (SLPs) are usually the highest copy number macromolecules in prokaryotes, playing critical roles in cellular physiology such as blocking predators, scaffolding membranes, and facilitating environmental interactions. Using electron cryomicroscopy of two-dimensional sheets, we report the atomic structure of the S-layer from the archaeal model organism Haloferax volcanii. This S-layer consists of a hexagonal array of tightly interacting immunoglobulin-like domains, which are also found in SLPs across several classes of archaea. Cellular tomography reveal that the S-layer is nearly continuous on the cell surface, completed by pentameric defects in the hexagonal lattice. We further report the atomic structure of the SLP pentamer, which shows markedly different relative arrangements of SLP domains needed to complete the S-layer. Our structural data provide a framework for understanding cell surfaces of archaea at the atomic level.
Highlights
All prokaryotes interact with their environment through molecules on the cell surface
Cryo-electron microscopy (EM) structure of the csg hexamer To understand cell-surface organization in haloarchaea, we aimed to resolve the atomic structure of the model surface layer (S-layer) from H. volcanii
We used single-particle cryo-EM data analysis to resolve a global 3.5-Aresolution structure of the hexameric repeating unit of the S-layer within the sheets (Figures S1E–S1J; Table S1) and used it to build an atomic model of csg (Figures 1C and 1D; Video S1; Method details)
Summary
All prokaryotes interact with their environment through molecules on the cell surface. S-layers are built by repeated interactions of S-layer proteins (SLPs), which are often the most abundant cellular molecules, representing up to 15% of the entire protein content of a cell (Bharat et al, 2021; Fagan and Fairweather, 2014; Pum et al, 2013). Owing to their plentitude in prokaryotes, estimates place SLPs as one of the most abundant classes of proteins on Earth (Pum et al, 2013). At the fundamental molecular level, relatively little is known structurally about these enigmatic sheet-forming proteins (Bharat et al, 2021)
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