S-layer

GO:0030115 · Envelope · Envelope layer · Proposed

A paracrystalline monomolecular array of one or more (glyco)protein species that self-assemble on the outermost surface of the cell envelope, with oblique, square or hexagonal lattice symmetry. S-layers are common in Archaea, where they are often the sole wall component, and widespread but patchily distributed in Bacteria. [DOI:10.1111/1574-6976.12063]

Image (not hosted — CC_BY_NC_4_0): CaltechDATA qgffm-kzf88 (Atlas of Bacterial and Archaeal Cell Structure, 2.6 Surface layer, file 2_6_Ccrescentus.mp4)

Electron cryotomogram video of a Caulobacter crescentus cell; the Atlas section shows the paracrystalline S-layer outside the outer membrane. CRYO_ET · Caulobacter vibrioides (NCBITaxon:155892) · Catherine Oikonomou & Grant Jensen, Atlas of Bacterial and Archaeal Cell Structure (Caltech Library), CC BY-NC 4.0; tomogram collected by Ariane Briegel · CC_BY_NC_4_0 · source: CELL_STRUCTURE_ATLAS CaltechDATA qgffm-kzf88 (Atlas of Bacterial and Archaeal Cell Structure, 2.6 Surface layer, file 2_6_Ccrescentus.mp4) · DOI:10.22002/D1.1355

Synonyms: surface layer; crystalline cell surface layer

Xrefs: uniprot.location:SL-0262 — S-layer

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Stoichiometry	Essential	Role
S-layer protein (SLP)	Protein	part of		typically one dominant protein species; subunits are usually 40–200 kDa	ESSENTIAL	The lattice-forming subunit; self-assembles in vitro. `DOI:10.1111/1574-6976.12063` Sleytr et al. 2014 review SLP self-assembly and lattice types.
S-layer homology (SLH) domain	Protein	part of	`InterPro:IPR001119`		CONDITIONAL	In many Gram-positive bacteria, tandem SLH domains within the S-layer protein anchor it to pyruvylated secondary cell-wall polymer; other lineages use different anchoring modules. `DOI:10.1038/nrmicro3213` Fagan & Fairweather 2014 describe SLH-mediated anchoring.
secondary cell-wall polymer (SCWP)	Polysaccharide	part of			CONDITIONAL	Peptidoglycan-linked anchor polymer recognised by the SLH domain. `DOI:10.1111/1574-6976.12063` Sleytr et al. 2014.

Taxonomic distribution

NCBITaxon:2157 Archaea — COMMON: Frequently the only envelope layer outside the membrane. [DOI:10.1038/nrmicro2576]
NCBITaxon:2 Bacteria — VARIABLE: Present across many phyla but a strain-level trait, and readily lost on laboratory passage. [DOI:10.1111/1574-6976.12063]

Canonical examples

NCBITaxon:155892 Caulobacter vibrioides — RsaA S-layer; the best-characterised Gram-negative S-layer. [DOI:10.1038/nrmicro3213]
NCBITaxon:1422 Geobacillus stearothermophilus — SbsA/SbsB S-layers, the classic self-assembly system. [DOI:10.1111/1574-6976.12063]

Functions

cell shape maintenance and envelope stabilisation — Contributes to shape and osmotic stability, especially in Archaea lacking a rigid wall.
- DOI:10.1038/nrmicro2576 Albers & Meyer 2011.
molecular sieving and surface interaction — Lattice pores set a size cut-off; the outer face mediates adhesion and phage or immune interactions.
- DOI:10.1038/nrmicro3213 Fagan & Fairweather 2014.

Associated traits

CONFERS traitmech:000064 — S-layer S-layer
- DOI:10.1038/nrmicro3213 Fagan and Fairweather 2014 define and review the bacterial S-layer phenotype.

Physical properties

THICKNESS: 5-25 UO:0000018 [DOI:10.1111/1574-6976.12063]

Mechanism graphs

S-layer proteins self-assemble into a surface lattice (ASSEMBLY)

Secreted S-layer proteins self-assemble into a crystalline two-dimensional lattice that coats the outer face of the cell. In many Gram-positive S-layers, SLH domains within the lattice-forming protein bind secondary cell-wall polymers to retain the lattice on the peptidoglycan-linked envelope surface.

Subject	Predicate	Object	Evidence
S-layer homology domain	is part of	S-layer (glyco)protein subunit	`DOI:10.1038/nrmicro3213` Fagan and Fairweather 2014 review SLH-domain-mediated anchoring of bacterial S-layer proteins.
S-layer (glyco)protein subunit	self-assembles into	S-layer	`DOI:10.1111/1574-6976.12063` Sleytr et al. 2014 review S-layer self-assembly and lattice symmetry.
S-layer	coats	cell envelope outer surface	`DOI:10.1038/nrmicro3213` Fagan and Fairweather 2014 describe bacterial S-layers as cell-surface coats.
secondary cell-wall polymer	localizes to	cell envelope outer surface	`DOI:10.1111/1574-6976.12063` Sleytr et al. 2014 review the peptidoglycan-linked secondary cell-wall polymers that anchor S-layer proteins in many Gram-positive bacteria.
S-layer homology domain	binds	secondary cell-wall polymer	`DOI:10.1038/nrmicro3213` Fagan and Fairweather 2014 describe the interaction between SLH domains and pyruvylated secondary cell-wall polymers.
secondary cell-wall polymer	anchors	S-layer	`DOI:10.1038/nrmicro3213` Fagan and Fairweather 2014 review non-covalent S-layer anchoring through SLH domains and secondary cell-wall polymers.

Evidence

DOI:10.1111/1574-6976.12063 Sleytr et al. 2014, "S-layers - principles and applications".
DOI:10.1038/nrmicro3213 Fagan & Fairweather 2014, "Biogenesis and functions of bacterial S-layers".
DOI:10.1038/nrmicro2576 Albers & Meyer 2011, "The archaeal cell envelope".

Curation history

2026-08-29T00:00:00Z claude · CREATED_RECORD — Drafted composition, distribution and functions from the cited reviews. No mechanism graph yet. Not yet human-reviewed.
2026-08-29T03:00:00Z claude · ADD_IMAGE — Added link-only Cell Structure Atlas cryo-ET video (CaltechDATA DOI:10.22002/D1.1355, CC BY-NC 4.0, Caulobacter crescentus NCBITaxon:155892); rights verified against the CaltechDATA API.
2026-08-30T02:14:17Z uniprot_sl · ADD_UNIPROT_SL_XREF — Added uniprot.location:SL-0262 (S-layer); mapping read from the GO line of UniProt subcell.txt.
2026-08-30T19:29:55Z codex · REVIEW_COMPOSITION_AND_ASSEMBLY — Qualified the definition for one-or-more S-layer protein species; clarified that the SLH entry denotes SLH-domain-containing S-layer proteins rather than a free protein component; added evidence to the TraitMech link and an evidence-backed self-assembly graph.
2026-08-31T06:12:34Z claude · SET_COMPONENT_ROLE — component_role became required (#77) so the constituent/machinery distinction is machine-readable rather than prose. Set per component; ASSOCIATED_MACHINERY only where the protein acts on the already-assembled structure.
2026-09-04T01:23:31Z codex · ENHANCE_CAUSAL_GRAPH — Expanded the S-layer assembly graph with SLH-domain and secondary-cell-wall polymer anchoring nodes and evidence-backed edges.

Source: data/structures/envelope/s_layer.yaml