cell outer membrane

GO:0009279 · Envelope · Membrane · Proposed

The outermost lipid bilayer of a diderm cell envelope. In canonical Gram-negative bacteria it is asymmetric, with phospholipid enriched in the inner leaflet and lipopolysaccharide enriched in the outer leaflet, and contains beta-barrel proteins that mediate transport and assembly. [DOI:10.1101/cshperspect.a000414]

Synonyms: outer membrane of cell

Xrefs: uniprot.location:SL-0040 — Cell outer membrane

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

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
lipopolysaccharide-rich outer leaflet	Lipid	part of				CONDITIONAL	Creates the characteristic asymmetric permeability barrier in canonical Gram-negative bacteria. `DOI:10.1101/cshperspect.a000414` Silhavy et al. describe outer-membrane lipid asymmetry.
outer-membrane beta-barrel proteins	Protein	part of				ESSENTIAL	Provide selective transport and envelope assembly functions. `DOI:10.1101/cshperspect.a000414` Porins and assembly proteins are core outer-membrane components.

Taxonomic distribution

NCBITaxon:2 Bacteria — VARIABLE: A diderm feature; monoderm bacteria lack an outer membrane. [DOI:10.1101/cshperspect.a000414]

Canonical examples

NCBITaxon:562 Escherichia coli — Canonical model for outer-membrane composition and biogenesis. [DOI:10.1101/cshperspect.a000414]

Functions

selective permeability barrier — Limits entry of harmful compounds while porins admit selected solutes.
- DOI:10.1101/cshperspect.a000414 The outer membrane protects against environmental insults.

Mechanism graphs

Lpt and BAM assemble the asymmetric permeability barrier (ASSEMBLY)

The Lpt transenvelope machine delivers LPS to the cell surface, while the BAM complex folds and inserts beta-barrel proteins into the outer membrane. LPS concentrated in the outer leaflet and OMP beta barrels together form the asymmetric permeability barrier that distinguishes the diderm envelope.

Subject	Predicate	Object	Evidence
Lpt transenvelope LPS transport machine	transports to the outer leaflet	lipopolysaccharide	`DOI:10.1074/jbc.R117.802512` Sperandeo, Martorana and Polissi 2017 review LPS transport across the envelope and assembly at the outer-membrane outer leaflet.
lipopolysaccharide	forms	cell outer membrane	`DOI:10.1101/cshperspect.a000414` Silhavy, Kahne and Walker 2010 describe the outer membrane as a lipid bilayer with phospholipids confined to the inner leaflet and LPS in the outer leaflet.
beta-barrel assembly machinery complex	folds and inserts	outer-membrane beta-barrel proteins	`DOI:10.1146/annurev-biochem-061408-144611` Hagan, Silhavy and Kahne 2011 review beta-barrel membrane-protein assembly by the Bam complex.
outer-membrane beta-barrel proteins	embed in	cell outer membrane	`DOI:10.1101/cshperspect.a000414` Silhavy, Kahne and Walker 2010 describe nearly all integral transmembrane outer-membrane proteins as beta barrels.
cell outer membrane	enables	selective permeability barrier	`DOI:10.1101/cshperspect.a000414` Silhavy, Kahne and Walker 2010 tie the tightly packed LPS leaflet to the hydrophobic barrier and beta-barrel porins to selected small-molecule diffusion.

LPS and beta-barrel porins tune selective permeability (FUNCTION)

Lipopolysaccharide forms a densely packed outer leaflet that slows hydrophobic toxin entry, while beta-barrel porins provide size- and chemistry-limited routes for hydrophilic solutes, jointly creating the selective permeability barrier of the outer membrane.

Subject	Predicate	Object	Evidence
lipopolysaccharide	packs into	tightly packed LPS surface layer	`DOI:10.1128/MMBR.67.4.593-656.2003` Nikaido 2003 reviews how LPS organization makes the outer membrane a low-permeability barrier.
tightly packed LPS surface layer	excludes	hydrophobic toxic compounds	`DOI:10.1128/MMBR.67.4.593-656.2003` Nikaido 2003 explains that the outer leaflet blocks hydrophobic solutes that would otherwise partition through ordinary phospholipid bilayers.
beta-barrel porin channels	channel	hydrophilic solutes	`DOI:10.1128/MMBR.67.4.593-656.2003` Nikaido 2003 reviews porin-mediated diffusion across Gram-negative outer membranes.
beta-barrel porin channels	filter	hydrophilic solutes	`DOI:10.1128/MMBR.67.4.593-656.2003` Nikaido 2003 describes porin permeability in terms of size exclusion, channel specificity, and diffusion rate.
tightly packed LPS surface layer	cooperates with	beta-barrel porin channels	`DOI:10.1101/cshperspect.a000414` Silhavy, Kahne and Walker 2010 connect outer-leaflet LPS and beta-barrel porins to the outer membrane's permeability-barrier function.
beta-barrel porin channels	enables	selective permeability barrier	`DOI:10.1128/MMBR.67.4.593-656.2003` Nikaido 2003 reviews how porin diffusion and outer-leaflet LPS jointly determine Gram-negative outer-membrane permeability.

Evidence

DOI:10.1101/cshperspect.a000414 The bacterial cell envelope.

Curation history

2026-08-30T19:30:00Z codex · CREATED_RECORD — Added as a literature-backed foundational seed. Identity checked against the 2026-07-01 Gene Ontology release; requires human review.
2026-08-31T03:48:23Z uniprot_sl · ADD_UNIPROT_SL_XREF — Added uniprot.location:SL-0040 (Cell outer membrane); mapping read from the GO line of UniProt subcell.txt.
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.
2026-09-04T01:09:39Z codex · ADD_CAUSAL_GRAPH — Added an ASSEMBLY graph for Lpt-driven LPS transport, BAM-driven beta-barrel OMP assembly, and the resulting asymmetric outer-membrane permeability barrier, with additional DOI evidence on Lpt and BAM.
2026-09-04T04:03:00Z codex · ADD_CAUSAL_GRAPH — Added a FUNCTION graph for LPS- and beta-barrel-porin-mediated selective permeability, grounded LPS nodes with ChEBI, and added Nikaido 2003 evidence for hydrophobic exclusion and porin diffusion.

Source: data/structures/envelope/cell_outer_membrane.yaml