Bacterial outer membrane

Source: Wikipedia, the free encyclopedia.
Structure of gram-negative cell envelope
Lipopolysaccharide-assembly, LptC-related
Identifiers
SymbolLptC
PfamPF06835
Pfam clanCL0259
InterProIPR010664
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
Lipopolysaccharide-assembly
Identifiers
SymbolLptE
TCDB
1.B.42
OPM superfamily412
OPM protein4q35
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary

The bacterial outer membrane is found in gram-negative bacteria. Gram-negative bacteria form two lipid bilayers in their cell envelopes - an inner membrane (IM) that encapsulates the cytoplasm, and an outer membrane (OM) that encapsulates the periplasm.[1]

The composition of the outer membrane is distinct from that of the inner

endotoxin - and in some bacteria such as E. coli it is linked to the cell's peptidoglycan by Braun's lipoprotein
.

Porins can be found in this layer.[2]

Outer membrane proteins

Outer membrane proteins are membrane proteins with key roles associated with bacterial cell structure and morphology; cell membrane homeostasis; the uptake of nutrients; protection of the cell from toxins including antibiotics; and virulence factors including adhesins, exotoxins, and biofilm formation.[3][4] There are a number of outer membrane proteins that are specifically virulence-related.

Outer membrane proteins consist of two major classes of protein - transmembrane proteins and lipoproteins. The transmembrane proteins form channels or pores in the membrane called porins, and actively pumping efflux channels.[5]

The outer membranes of a bacterium can contain a huge number of proteins. In E. Coli for example there are around 500,000 in the membrane.[5]

Bacterial outer membrane proteins typically have a unique beta barrel structure that spans the membrane. The beta barrels fold to expose a hydrophobic surface before their insertion into the outer membrane. Beta barrels vary in sequence and size that ranges from 8 to 36 beta strands. A subset of OMPs have a perisplasmic or an extracellular link to their beta barrel structure. [3] An outer membrane protein is translocated across the inner membrane through ‘’Sec’’ machinery, and finally inserted to the outer membrane by the barrel assembly machinery complex.

Biogenesis

The

O-antigen repeat units are synthesized at the cytoplasmic face of the inner membrane and are separately exported via two independent transport systems, namely, the O-antigen transporter Wzx (RfbX) and the ATP binding cassette (ABC) transporter MsbA that flips the lipid A-core moiety from the inner leaflet to the outer leaflet of the inner membrane.[6][7][8][9][10] O-antigen repeat units are then polymerised in the periplasm by the Wzy polymerase and ligated to the lipid A-core moiety by the WaaL ligase.[11][12]

The

cellular compartment suggests a model for how the LPS assembly pathway is organised and ordered in space.[12]

LptC is required for the translocation of lipopolysaccharide (LPS) from the inner membrane to the outer membrane.[12] LptE forms a complex with LptD, which is involved in the assembly of LPS in the outer leaflet of the outer membrane and is essential for envelope biogenesis.[12][13][14]

Clinical significance

If

host–pathogen interface, implicated in translocation of gram-negative microbial biochemical signals to host or target cells.[citation needed
]

See also

References

This article incorporates text from the public domain Pfam and InterPro: IPR010664