Architecture and modular assembly of Sulfolobus S-layers revealed by electron cryotomography

Abstract

Surface protein layers (S-layers) often form the only structural component of the archaeal cell wall and are therefore important for cell survival. S-layers have a plethora of cellular functions including maintenance of cell shape, osmotic, and mechanical stability, the formation of a semipermeable protective barrier around the cell, and cell–cell interaction, as well as surface adhesion. Despite the central importance of S-layers for archaeal life, their 3-dimensional (3D) architecture is still poorly understood. Here we present detailed 3D electron cryomicroscopy maps of archaeal S-layers from 3 different Sulfolobus strains. We were able to pinpoint the positions and determine the structure of the 2 subunits SlaA and SlaB. We also present a model describing the assembly of the mature S-layer.

Molecular Logic of Prokaryotic Surface Layer Structures

“…Some intensely studied archaeal species are highlighted, with the lattice symmetry and predicted anchoring sequence shown in brackets (*denotes S-layers with some structural biology data available segments in many archaeal SLPs [13,23]. Notably, in some organisms, the lattice-forming and anchoring segments are harbored by two distinct SLPs, for example, in the archaeon Sulfolobus islandicus [23]. While the lattice-forming segments are remarkably divergent in their sequences, the anchoring segments show significantly lower sequence variability.…”

“…In species of the order Methanobacteriales (e.g., M. fervidus), however, the cytoplasmic membrane is surrounded by a cell wall composed of pseudopeptidoglycan, also known as pseudomurein, and the S-layer is associated with this pseudomurein layer. While crenarchaeal SLPs, such as those found in the extremophile S. islandicus, are generally anchored to the membrane via a C-terminal transmembrane segment [23], most euryarchaeal SLPs appear to be anchored through the covalent attachment of a lipid moiety to their C-terminal end [39][40][41][42]. Euryarchaeal SLPs typically contain a C-terminally located tripartite segment that comprises a highly conserved PGF (proline-glycine-phenylalanine) motif, a transmembrane helix, and a cluster of basic residues.…”

“…studies can now shed light on how S-layers are assembled on cells (Figure 5), applied to archaeal [23], Gram-positive [7], and Gram-negative bacterial S-layers [18]. Resolutions achievable in cryo-ET are not quite high enough currently; however, potent microscopy and image processing tools [66][67][68][69] are beginning to be applied to S-layer structural biology.…”

Viral Hijack of Filamentous Surface Structures in Archaea and Bacteria

“…Archaea do not have a cell wall of murein. Instead, many archaea are covered in a protective Surface (S) layer, built of multiple copies of 1–2 glycosylated proteins that form a semi-crystalline sheet [ 67 , 68 , 69 ]. In several archaea, the S-layer is the sole constituent of the cell wall, or the S-layer forms the cell wall in combination with other polymers.…”

The Cell Membrane of Sulfolobus spp.—Homeoviscous Adaption and Biotechnological Applications

“…The salmon colored area represents the pseudoperiplasmic space. In Sulfolobus spp., the membrane is predominantly comprised of membrane spanning tetraether lipids [25,31].…”