S-layer

GO:0030115 · ENVELOPE · ENVELOPE_LAYER · PROPOSED

A paracrystalline monomolecular array of a single (glyco)protein that self-assembles on the outermost surface of the cell envelope, with oblique, square or hexagonal lattice symmetry. Nearly universal in Archaea, where it is 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

Components

Component	Type	Grounding	Stoichiometry	Essential	Role
S-layer protein (SLP)	PROTEIN		one protein species per lattice, typically 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	`InterPro:IPR001119`		CONDITIONAL	Anchors bacterial SLPs 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			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

Physical properties

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

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.

Source: data/structures/envelope/s_layer.yaml