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Structure of Bacterial
Outer Membrane Proteins
Bhaskar GangulyPh.D. ScholarAnimal Biotechnology
LaboratoryCollege of Veterinary & Animal SciencesG.B.P.U.A.
& T., PantnagarINDIA - 263145
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INTRODUCTION
20 % of all known proteins are found in
membranes
Consist of transmembrane - helices;characterized by 20 - 30
non-polar a.a. with apredominance of aliphatic side chains at
the
center and aromatic residues at both the ends
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Introduction
Outer membranes of Gram Negative
Bacteria lack helical proteins. Instead, - barrel proteins are
present in outer
membranes; characterized by an alternating
sequence of polar & non-polar residues.
Polar
residues
Non-polar residues
Contain all-next-neighbor anti-parallel - sheets.
However, - barrel proteins are
absent from the outermembranes of mitochondrion
andchloroplasts.
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STRUCTURE OF OMPs
Determined from X-ray crystallographic studies
Requires 3-D crystals of purified proteins
Difficulties:
Limitations on the amount of recombinant
OMPs that can be incorporated within a fixedmembrane volume
Altered membrane properties may bedeleterious to the host
Solution:
Cytosolic expression of the OMP andreconstitution into
miscelles
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STRUCTURAL FEATURES OF OMPs
n {no. of strands} S {Shear number}
R {radius}
{tilt angle}
R = [(Sa)2 + (nb)2]0.5 / 2
tan = Sa / nbR = nb / 2 cos
a = 3.3 ; b = 4.4
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n = 16
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1.
2.3.
4.
5.
6.
7.
8.
9.10.
11.
12.
13.
14.
S = 14
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n S n+4 n is always even (n = 8 barrels are common)
S is generally positive and always even
- strands are anti-parallel and connected to nextneighbor
- barrel surface bears aliphatic side chains; rims ofthe barrel
are lined by aromatic side chains
Structural features of OMPs
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FUNCTIONS OF OMPs
OmpX: Stress modulator; binds foreign proteins OmpT: protease;
role in pathogenicity
OmpLA: phospholipase; forms channels in
outer membrane for secretion of colicins andvirulence
factors
Porins: e.g. OmpF, PhoE, OmpC, etc. {trimers
consisting of three parallel - barrels} OmpF osmoporin, low salt
concentration
PhoE phosphate uptake
OmpC osmoporin, high salt concentration
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Functions of OMPs
Maltoporin: Glucose oligomers Sucrose porin: Sucrose
transport
FhuA, FepA: iron transporters; interact with
TonB of inner membrane that draws energyfrom cytosolic ATP
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n = 16, S = 20, nearly circular cross-section, commonlyassociate
to form trimers, pore size 10 , allowmolecules of about 600 Da
n = 18, trimeric, smaller cross-section than n = 18
porins, high selectivity, pore size 6 n = 22, iron transporters,
monomeric, very large cross-
section, filled with N-terminal 150 residue domain
PORINS
Hydrophobic interfacebehaves like a
cytoplasmic - Barrel
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- Barrel is a robust and stablestructure
Large external loops present in - Barrels of OmpA were
replaced
by short-cuts The deletion mutants were
functionally inactive; lackedbiological functions in
F-conjugation and as a receptorfor Bacteriophages
However, the trans-membrane - Barrel structure was retained
STABILITY STUDIES
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