Integral outer membrane proteins: structure assembly and function

Question 1

what are the functions of integral OMPs

Answer 1

• Transport of small molecules by uptake or efflux and secretion of macromolecules
• Catalysis
• structural (e.g binding of outer to peptidoglycan)
• Adhesion (receptors)

Question 2

Explain the Beta barrel

Answer 2

• most integral outer membranes proteins for beta barrel
• A beta barrel is made from an antiparallel beta sheet which wraps around on itself to form a
  closed barrel shape
  -The outer ‘waist’ of the barrel is hydrophobic, so the protein stably integrates into the outer
  membrane

Question 3

Explain Porins

Answer 3

• Passive pore-forming OMPs
• Allow passage of low molecular weight solutes (salts, monosaccharides, antibiotics)
• Some selectivity towards charge (anions/cations) and substrate (eg sugars)
• 16 and 18 stranded beta barrels
• Some form trimers
• OmpF is a 16 stranded barrel and forms trimer
• Loop region folds down into lumen of barrel this regulates size of molecules passing in
• Charged residues in eyelet region regulate preference for anion or cations

Question 4

Explain TonB-coupled Receptors

Answer 4

• Active, rather than passive transport
• Large family of transporters responsible for uptake of iron, heme and vitamin B
• Energy obtained from the proton motive force across the inner membrane through the
periplasmic protein TonB
• A 22-stranded beta barrel structure with a ‘plug’ domain which fills the barrel lumen

• Binding of TonB induces BtuB plug of transport cobalamin in E.coli

Question 5

Explain secretion by FhaC

Answer 5

• FhaC is a member of a large family of OMPs (Omp85) which promote secretion and
insertion of proteins into the OM
• FhaC mediates the secretion of filamentous haemagluttinin (FHA)
• FhaC forms a 16-strand β-barrel structure
• the lumen of the barrel is occluded by a 20 residue α-helix and a large extracellular loop
(L6) which is essential for substrate secretion
• Polypeptide Transport-Associated, or POTRA domains, are α/β modules which extend
into the periplasm and bind substrates to assist translocation

• possible mechanism involves displacement of L6 and threading the nascent unfolded polypeptide chain through the barrel

Question 6

Explain catalysis models

Answer 6

• Few few enzymes known to be integral OMPs
  • Two examples are:
  OmpT protease
  phospholipase

Question 7

Explain structural models

Answer 7

• Harder functional category to define, but there is evidence that several OMPs interact
directly with peptidoglycan, possibly helping to stabilize the OM relative to the
peptidoglycan layer.
• An example is the E coli protein OmpA

Question 8

Explain Secretion models

Answer 8

• More complex OMPs associated with secretion
  • Not all are transmembrane beta barrels
  • Not all well characterised at the structural level

Question 9

What are secretins

Answer 9

• Multidomain integral OMPs
  • Involved in type II, type III secretion and type IV pilus biogenesis
  • Form large oligomers and mediate passage of pili or protein complexes (eg cholera
  toxin) across the OM

Question 10

Explain adhesion with the example Opa-CEACAM

Answer 10

Opacity, or Opa proteins, are 8-stranded beta barrel OMPs from Neisseria meningitidis

• They bind to CarcinoEmbryonic Antigen-related Cell Adhesion Molecules (CEACAMs)
• CEACAMs are glycoproteins displayed on the surface of mucosal epithelial cells
• CEACAM recognition is thought to assist in colonization of mucosa

Question 11

How are OMP formed

Answer 11

• Folding and insertion of OMPs into the OM is promoted by the Bam complex
• There are five Bam proteins, BamA to BamE
• BamA is related in sequence to FhaC and may work in a similar way
• Refolding is also assisted by DegP, which forms a large cage-like structure to support
OMP refolding

Question 12

Medical relevance of OMP

Answer 12

• Some porins exhibit sequence hypervariability in their surface-exposed loops (example,
PorA from Neisseria meningitidis)
• Sequence variance can be used for typing bacteria
• PorA variability is caused by host immune selection pressure
• PorA has also been shown to act as a major protective antigen in meningitis vaccines
based on outer membrane vesicles
• Other porins have been shown to modulate antibiotic resistance, through mutations to
residues lining the pore