Download Structural biology of bacterial pathogenesis

Structural biology of bacterial pathogenesis
Han Remaut1,2,3 and Gabriel Waksman1,2,3,4,
Recent years have seen a rapid increase in structural information
on proteins implicated in bacterial pathogenesis. The different
modes by which bacteria establish contact with their host
tissues are exemplified by the structures of bacterial adhesins in
complex with their cognate host receptor. A more detailed
structural understanding of the various Gram-negative
secretion systems has emerged with the determination of the
structures of type I and type IV secretion system components,
and with the elucidation of the mechanism of fibre formation
in the chaperone-usher pathway of pilus biogenesis. Finally,
the structures of complexes of secreted virulence factors
bound to their host targets have unravelled the mechanisms
by which bacterial pathogens exploit cellular processes to
their advantage.
Addresses
1
Institute of Structural Molecular Biology, 2School of Crystallography,
Birkbeck College, Malet Street, London WC1E 7HX, UK
3
Department of Biochemistry and Molecular Biology, University College
London, Gower Street, London WC1E 6BT, UK
4
Department of Biochemistry and Molecular Biophysics, Washington
University School of Medicine, 660 South Euclid Avenue, Saint Louis,
MO 63110, USA
e-mail: [email protected]
ment to their target tissues for successful infection.
Surface-exposed adhesion molecules, termed ‘adhesins’,
establish initial attachment to the host, often in a specific
manner. First-line attachment through adhesins is sufficient to trigger host responses such as cytoskeleton
reorganisation or to enable virulence mechanisms mediated by the various secretion systems to come into action
[1,2]. These secretion systems secrete toxins and effector
proteins into the extracellular milieu or directly into the
host cells.
This review seeks to give an overview of the current
understanding of pathogenesis from a structural point of
view, focusing on adhesins, and the secretion systems
used to present adhesins to the bacterial cell surface and
to secrete virulence factors. For reasons of clarity and
given the limited space, Gram-positive bacterial adhesion
and secretion systems will not be addressed. Recently
determined Gram-positive adhesin structures not discussed here include the internalin isologues InlA, InlB
and InlH, and the InlA–human E-cadherin (hEC1) complex [3–5,6], the Staphylococcus fibronectin-binding
adhesins ClfA and SdrG [7,8], and collagen-binding
adhesins Cbd19 and Cna [9,10].
Current Opinion in Structural Biology 2004, 14:161–170
This review comes from a themed issue on
Macromolecular assemblages
Edited by R Anthony Crowther and BV Venkataram Prasad
0959-440X/$ – see front matter
ß 2004 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.sbi.2004.03.004
Abbreviations
AAD
all-a-helical domain
ABC
ATP-binding cassette
CTD
C-terminal domain
GAP
GTPase-activating protein
GEF
guanine exchange factor
GlcNAc N-acetyl-D-glucosamine
OM
outer membrane
NBD
nucleotide-binding domain
NTD
N-terminal domain
Nte
N-terminal extension
PRR
proline-rich repeat
T3SS
type III secretion system
T4SS
type IV secretion system
Introduction
Pathogenic bacteria apply a versatile and flexible repertoire of mechanisms by which they exert influence over
their hosts. Yet, studies have unravelled common themes.
In the first instance, most bacteria need to initiate attachwww.sciencedirect.com
Bacterial adhesins
A common way for bacteria to accomplish adhesion is
through the use of pili — fibrous protein organelles
produced on the surface of bacteria. In the case of type
I and P pili, prototypical pili produced by the chaperoneusher pathway (Figure 1), a single two-domain adhesin
is present at the distal end of the multisubunit pilus
[11,12]. Structures of the uropathogenic and enterotoxic
Escherichia coli adhesins FimH, PapG (uropathogenic) and
GafD/F17-G (enterotoxic) show that, despite their lack
of sequence similarity, their receptor-binding domains
share a similar b-barrel jellyroll fold [13–18]. However,
the receptor-binding sites of these adhesins differ markedly and occupy different parts of the structure
(Figure 2a). Mannose-specific FimH binds its sugar
receptor in a deep, negatively charged pocket formed
by the loops at the tip of its receptor-binding domain [13].
PapG, on the other hand, binds globoside series of glycopeptides presented on the kidney surface; the sugar is
bound in a shallow binding pocket formed by three
strands and a loop, and located along the side of the
molecule [15]. Isologous GafD and F17-G (in F17 and G
fimbriae) bind the terminal N-acetyl-D-glucosamine
(GlcNAc) residues of glycoproteins. Structures of GafD
and F17-G in complex with GlcNAc reveal a shallow
sugar-binding site along the side of the molecule that is
unrelated to those of PapG or FimH [16,17]. Together,
Current Opinion in Structural Biology 2004, 14:161–170
162 Macromolecular assemblages
Figure 1
Type I
Type III
GSP
Chaperone-usher
PapG
Type IV
Precipitation-nucleator
Type V
(autotransporter)
CsgA
Type II
PrgI,(J)
PapA
E
OM
TolC
CsgB
NalP
PapC
CsgG
HlyD
IM
C
HlyB
ATP
ADP
C
InvG,H
PulD
PulS
B7
Vir
B9, B10
PulC
PulF-O
PapD
P
B8
C
N
B2 B5
B6
SecYEG
SecA
ATP N
ADP
SP
PrgH,K
PulE
SecB
B11
C
ATP
ADP
N
Sic
ATP
ADP
C
ATP
ADP
D4
N
Current Opinion in Structural Biology
Schematic overview of the major protein secretion systems in Gram-negative bacteria. The secretion systems are represented by the following
pathway models: hemolysin secretion for type I secretion; P pili assembly for the chaperone-usher pathway; NalP for the type V pathway
(autotransporters); Csg curli for the precipitation-nucleator pathway; pullulanase secretion for type II secretion; and Salmonella and Agrobacterium
annotation for the type III and type IV pathways, respectively. Type I, type III and type IV secretion systems secrete proteins in an energized step
without a periplasmic intermediate. The chaperone-usher, autotransporter, precipitation-nucleator and type II pathways have periplasmic
intermediates that are transported through the general secretory pathway (GSP). The N- or C-terminal signal peptides of the exported proteins
are indicated by green rectangles (removed only in the GSP by a signal peptidase [SP]). C, cytoplasmic space; E, extracellular space; IM, inner
membrane; OM, outer membrane; P, periplasm.
these structures underline the flexibility with which
different bacterial strains can establish tropism in infection through the use of a common scaffold for receptor
binding.
Type IV pili subunits (type IV pilins) show a different
mode of receptor binding [19–21,22]. The common
scaffold of the type IV pilins, assembled by a type-IIrelated secretion machinery (Figure 1), is composed of
an elongated a helix packed against an a/b domain at its
C-terminal end. The packing architecture of these pilus
subunits differs between species, but in each case
involves helical packing of the a/b domains, so that the
hydrophobic extended a helices are aligned along the
helical axis and form the inner core of the pilus [19,22].
Although each subunit has a receptor-binding site, the
variation in subunit structure and helical packing gives
rise to differences in their exposure. In Pseudomonas PAK
pili, only the subunits at the distal end of the pilus have
their binding pockets exposed, whereas in TCP pili in
Vibrio they have their functional residues exposed all
along the pilus [22]. In Neisseria MS11 pili, the receptor-binding site is housed in separate subunits at the tip of
the pilus [19].
Current Opinion in Structural Biology 2004, 14:161–170
Both the chaperone-usher and type IV pilin secretion
pathways are dedicated to the secretion of adhesins.
However, other secretion pathways are also known to
participate in adhesin assembly. Bordetella pertussis P.69
pertactin is an example of an autotransporter adhesin [23].
The structure of its N-terminal, functional domain (residues 1–539) consists of an elongated 16-stranded parallel
b helix from which several loops protrude laterally. These
loops contain a proline-rich repeat (PRR) of sequence
(GGXXP)5 following an Arg-Gly-Asp (RGD) sequence,
both known to form the cell-attachment sites of various
mammalian adhesion proteins. Another PRR is present in
the C-terminal, immunodominant region of the protein.
Invasin and intimin are structurally related adhesins
anchored to the outer membrane and assembled by the
Sec-dependent system (Figure 1). Whereas invasin targets host receptors of the b1 integrin family, intimin binds
Tir, a bacterial receptor inserted into the host cell membrane by a type III secretion system. In both Yersinia
pseudotuberculosis invasin and enteropathogenic E. coli
intimin, a C-type lectin-like receptor-binding domain is
separated from a membrane-embedded N-terminal
domain by several tandem Ig-like repeats, four in invasin
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Bacterial pathogenesis Remaut and Waksman 163
Figure 2
of the lectin-like domain (Figure 2b) [24,26]. Similar to
the picture revealed for the chaperone-usher pili adhesins
(above), the invasin and intimin structures demonstrate
the use of a common scaffold to mediate different modes
of adhesion.
(a)
Yet another mode of host–pathogen interaction is demonstrated by Neisseria meningitides OpcA. OpcA is an integral
outer membrane protein that mediates adhesion to
epithelial and endothelial cells by binding to vitronectin
and proteoglycans. OpcA consists mainly of a tenstranded transmembrane b barrel [27]. A positively
charged receptor-binding site is formed by loops protruding from the outer surface of the barrel.
FimH
GafD
PapG
(b)
D1
D2
D3
D4
D5
Secretion systems in Gram-negative bacteria
Figure 1 summarizes the major secretion pathways in
Gram-negative bacteria and structural information on the
various systems is discussed below. For recent reviews on
bacterial secretion systems, the reader is directed to
[1,2,28,29].
Type I secretion
D3
D0
D1
D2
Tir IBD
Current Opinion in Structural Biology
Structures of bacterial adhesion molecules. (a) Comparison of the
N-terminal receptor-binding domains of the E. coli chaperone-usher
adhesins FimH, GafD and PapG (from left to right) in complex with
their ligands D-mannose, GlcNAc and GbO4, respectively (shown as
ball-and-stick models). Despite the low sequence similarity (22–27%
identity), the receptor-binding domains adopt a similar Ig-like jellyroll
motif. The labelled strands are structural elements of the Ig core. The
receptor-binding sites of the different proteins are formed by
unrelated structural elements of the similar scaffold. Strands and helices
are coloured red and blue, respectively. (b) Comparison of the
structurally related adhesins Yersinia pseudotuberculosis invasin
(upper panel) and E. coli intimin (lower panel). The C-terminal lectin-like
domain (D5 and D3 in invasin and intimin, respectively, represented by
blue strands and red helices) is preceded by several Ig repeats (light
blue; D1–D4 and D0–D2, respectively), which are attached to an Nterminal membrane-spanning domain. The receptor-binding sites of
invasin and intimin are indicated in green. Both molecules use unrelated
parts of the lectin-like fold for receptor binding. (The D0 domain of
intimin is homology modelled and is absent from the determined
structure.) Tir IBD indicates the location of the Tir-intimin binding
domain.
(D1–D4) and three in intimin (D0–D2) (Figure 2b) [24–
26]. The distal Ig-like repeat (D4 and D2 in invasin and
intimin, respectively) maintains close interactions with
the terminal lectin-like domain. The receptor-binding
interfaces of intimin and invasin map to opposite sides
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Type I or ABC (ATP-binding cassette) protein-mediated
secretion systems, involved in multidrug efflux and toxin
export, comprise three components: an inner membrane
ABC protein, an outer membrane (OM) protein and a
membrane fusion protein that connects the two during
protein translocation [30,31]. Type I protein secretion
proceeds without a periplasmic intermediate. The OM
component, visualized in the E. coli TolC structure, forms
a transperiplasmic channel-tunnel composed of a 40 Å
transmembrane b-barrel (the channel) and a 100 Å periplasmic a-helical barrel (the tunnel) (Figure 3) [32]. In
contrast to other OM-spanning b-barrels, which are
formed by a single polypeptide, the TolC channel is
trimeric, each TolC subunit contributing four strands.
The TolC channel-tunnel has an internal diameter of
35 Å, is open to the cell exterior and decreases in diameter
to a nearly closed gate at the periplasmic end of the abarrel. The a-helices associate in a coiled-coil ‘knobsinto-holes’ interaction and mutagenesis studies provide a
model for the widening of the aperture of the tunnel
entrance through an iris-like realignment of the helices
during recruitment by substrate-engaged inner membrane complexes [33].
Chaperone-usher pathway
The chaperone-usher pathway is used to produce pili on
the surface of bacteria. It requires a periplasmic chaperone that stabilizes pilus subunits (or pilins) in the periplasm and primes them for fibre formation at the OM
usher [12,34,35]. A recent series of structures of the PapD,
FimC and Caf1M chaperones in complex with the PapK,
PapE, FimH and Caf1 pilins reveals details of how the
chaperone is able to facilitate pilin folding and stabilization, and cap the pilin–pilin interaction surfaces, avoiding
Current Opinion in Structural Biology 2004, 14:161–170
164 Macromolecular assemblages
Caf1M–Caf1–Caf1 complex (Figure 4) [37,38]. In
addition, these complexes rationalize how pilus subunits
are primed for fibre formation in the absence of an NTPdriven energy step. The chaperones trap the pilus subunits in a high-energy folding intermediate that relaxes to
its ground state only upon donor-strand exchange
(Figure 4).
Figure 3
(a)
(b)
OM
Type III secretion and effectors
P
At present, high-resolution structures of components of
the injectosome, the type III secretion system (T3SS)
machinery (Figure 1), are lacking. Electron microscopy
imaging, however, reveals a modular architecture reminiscent of the flagellar basal body, in which the hook is
replaced by an extracellular needle that is involved in
protein secretion and direct injection into eukaryotic
host cells via pore-forming components at its distal
end [39–42].
Current Opinion in Structural Biology
Ribbon representation of the TolC outer membrane component of the
type I secretion pathway (a) viewed sideways and (b) viewed from
the closed periplasmic tunnel end. A single monomer in the trimer is
differentiated by colour: blue strands and red helices in the
transmembrane b-barrel and the periplasmic tunnel, and green
helices and dark-blue strands in the small domain on the side of the
periplasmic tunnel. OM, outer membrane; P, periplasm.
untimely polymerisation [13,36,37,38]. The chaperone consists of two Ig-fold domains that form an L-shaped
molecule. The pilus subunit comprises an incomplete Ig
fold, lacking the seventh G b-strand (Figure 4). As a
result, a deep hydrophobic groove is formed on the pilin
surface. In the chaperone–pilus subunit complexes, the
missing strand in the pilin fold is ‘donated’ by the
chaperone, which inserts its G1 b-strand (i.e. the G strand
of the N-terminal Ig-fold domain of the chaperone) into
the pilin groove in a mechanism termed ‘donor-strand
complementation’ (Figure 4).
Pilus subunits have an approximately 11-residue N-terminal peptide (named N-terminal extension [Nte]) that is
not part of the pilin fold and contains a conserved motif of
alternating hydrophobic residues, required for fibre formation. These observations led to a model for pilin–pilin
interaction in which the chaperone is replaced by an
incoming pilin through exchange of the chaperone G1
complementing strand for the Nte of the pilin next in
the assembly line. This mechanism of ‘donor-strand
exchange’ was caught in action in the structure of
PapENtd (i.e. a PapE subunit in which the Nte had been
deleted to prevent self-polymerisation) in complex with
the 11-residue PapK Nte peptide, and the structure of a
Current Opinion in Structural Biology 2004, 14:161–170
By contrast, the structural biology of type III effectors
has made rapid progress in recent years and has revealed
a mechanism of functional mimicry whereby several
components interact with key cellular regulators,
thereby hijacking host cell processes to their advantage
[43]. Secretion through the T3SS requires specific chaperones that recognize an N-terminal chaperone-binding domain in the effector protein. Remarkably,
structures of the chaperone-binding domains of the
Salmonella and Yersinia effectors SptP and YopE bound
to their respective chaperones, SicP and SycE, show a
rather unconventional chaperone–effector interaction
[44,45]. The chaperone-binding domains bind in an
extended, unfolded form by inserting two distinct portions of their sequence in equivalent hydrophobic
grooves on both molecules of a chaperone dimer
(Figure 5a). In analogy with the related flagellar system,
this non-globular state may be relevant for travel
through the T3SS, given the limited diameter of the
needle complex (25 Å) [46]. Structures of CesT, SigE
and SycE confirm a general fold for T3SS chaperones,
despite the low degree of sequence similarity [47–49].
The way in which effector molecules interact with host
metabolism is demonstrated by the co-crystal structures
of the Salmonella and Pseudomonas guanine exchange
factor (GEF) SopE, and GTPase-activating proteins
(GAPs) SptP and ExoS with their human Rho GTPase
targets, Cdc42 and Rac1 (Figure 5) [50,51,52]. The
SopE78–420 effector domain binds the regulatory switch
I and II regions of Cdc42, leaving the nucleotide-binding
pocket open to the solvent (Figure 5b). A loop connecting
the two a-helical bundles in SopE inserts between both
switch regions and pushes switch I into a distorted conformation. Furthermore, the SopE–switch II interaction
induces a peptide flip that disrupts the Cdc42 magnesium-binding site. In concert, these actions displace GDP
from inactive GDP-bound Cdc42, leaving it free for
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Bacterial pathogenesis Remaut and Waksman 165
Figure 4
(a)
(c)
Caf1’’
Caf1’Nte
Caf1M
(b)
Caf1’
Current Opinion in Structural Biology
Donor-strand exchange in chaperone-usher pilus assembly. (a) Ribbon representation and (b) topology diagram of the PapENtd (cyan)—PapD
chaperone complex (at left; for PapD, only the G1 donor strand is depicted, in yellow) and the PapENtd (cyan)—PapK Nte peptide (purple) complex
(at right). Upon exchange, the G1 strand of the chaperone, running parallel to the pilin F strand, is replaced by the Nte of a preceding pilin, now
running antiparallel to the F strand. The structures demonstrate the repacking of residues in the pilin core and highlight how the collapse of the
pilin domain to its final folded state drives fibre formation. (c) Structure of the Caf1M–Caf1–Caf1 complex capturing both donor-strand
complementation and donor-strand exchange in action. The two-domain Caf1M chaperone is coloured cyan, with the exception of the A1 and G1
strands, which are involved in donor-strand complementation (shown in dark blue). The donor-strand complemented Caf1 subunit (Caf10 ) is coloured
dark green, whereas the donor-strand exchanged Caf1 subunit (Caf100 ) is coloured green. The Nte of the Caf10 subunit, exchanged with the Caf100
subunit, is coloured red. Isolation of the Caf1M–Caf1–Caf1 complex was made possible through mutating Ala9 to arginine, which prevents further
polymerisation of Caf1 subunits.
activation by the cytoplasmic GTP pool. In contrast, the
GAPs activate the intrinsic GTPase activity of Rho
GTPases. Structures of ExoS, SptP, YopE, and the
ExoS–Rac1 and SptP–Rac1 complexes show a four-helix
bundle for the bacterial GAP domain [51–54]. In the
GAP–Rac1 complexes, two of the helices interact with
Rac1, inserting a catalytically essential arginine (146 and
209 in ExoS and SptP, respectively) into the active site of
the GTPase, where it interacts with the b-phosphate
oxygen and the fluoride of the aluminium trifluoride
g-phosphate equivalent. By doing so, ExoS and SptP
are able to stabilize the negative charge that develops
on the phosphoryl leaving group. Other GAP–Rac1 interactions include the ordering of switch I, and the positioning of Gln61 in switch II so that it can hydrogen bond with
and activate an attacking water molecule. Through these
interactions, GAPs are able to increase GTPase activity
many-fold.
Recently determined structures of T3SS effectors not
discussed here include the N-terminal domain of the
protein tyrosine phosphatase (PTP) YopH, known to bind
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phosphate in conjunction with its role as a chaperonebinding domain [55–57]; the leucine-rich repeat protein
YopM [58]; and SipA, which directly interacts with host
actin [59].
Type IV secretion
Type IV secretion systems (T4SSs) are ubiquitous in
bacterial pathogens. They are ancestrally related to bacterial conjugation systems and consist of at least 12
proteins, named VirB1–11 and VirD4 [60]. The T4SS
machinery is powered by three ATPases or nucleotidebinding proteins: VirB4, VirB11 and VirD4 (Figures 1 and
6). The core components of the system include VirB6–10.
An extracellular appendage or pilus consisting of a major
component, VirB2, and a minor component, VirB5,
appears to be attached to the core machinery through
VirB7. The pilus may serve as an attachment device.
Recently determined structures of free and nucleotidebound HP0525, the VirB11 homologue from Helicobacter
pylori, yield a complete view of its inner workings
[61,62] (Figure 6). The HP0525 monomer contains
Current Opinion in Structural Biology 2004, 14:161–170
166 Macromolecular assemblages
Figure 5
(a)
the HP0525ADP or HP0525ATPgS complex structures.
While the CTD rings remain unaffected, the NTDs
undergo rigid-body rotations. Based on the structural
and functional evidence, a four-step cycling mechanism
was proposed for the VirB11 ATPases [62]. The only
significant structural similarity HP0525 has with other
proteins in the Protein Data Bank is with the p97 AAA
ATPase, a protein involved in homotypic membrane
fusion and organelle biogenesis, suggesting that VirB11
ATPases are perhaps involved in similar functions in the
assembly and disassembly of the T4SS machinery.
SycE
N
(b)
YopE dimer
Cdc42
Switch II
C
TrwB, the VirD4 homologue from the conjugative
plasmid R388, is also a hexameric T4SS component,
but with a very different assembly [63,64] (Figure 6).
The TrwB71–507 monomer has an orange-segment shape
and consists of two domains: a cytosol-oriented all-ahelical domain (AAD) and a membrane-proximal nucleotide-binding domain (NBD). The assembly of TrwB is
similar to that of hexameric helicases. A central channel
runs from the cytosolic pole (formed by the AADs) to the
membrane pole (formed by the NBDs), ending at the
putative transmembrane pore.
SopE
Switch I
Switch II SptP GAP domain
Rac1
SptP PTP domain
Switch I
Current Opinion in Structural Biology
Structures of type III effectors bound to their cognate chaperone or
cellular targets. (a) Structure of the Yersinia SycE chaperone dimer
(cyan) in complex with the YopE recognition domain (red). The
chaperone-binding domain is bound to the chaperone dimer in an
extended, unfolded state. (b) Ribbon representations of the
Salmonella type III effectors (colour ramped from blue to green from
N to C terminus) SopE78–420 (upper panel) and SptP (lower panel)
bound to their human Rho GTPase targets Cdc42 and Rac1 (gold),
respectively. Both SopE78–420 and the SptP N-terminal GAP domain
(blue) interact with the switch I and switch II regions (coloured magenta)
of the Rho GTPases. The C-terminal domain (green) of SptP is a protein
tyrosine phosphatase (PTP) domain, the function of which is not
discussed in this review.
two contiguous domains that form a nucleotide-binding
site at their interface. In the functional hexamer, the Nand C-terminal domains (NTD and CTD) form two
rings, which together form a dome-like chamber open
on one side (the NTD ring) and closed on the other (the
CTD ring).
The structure of apo-HP0525 reveals an asymmetric
hexameric assembly that is significantly different from
Current Opinion in Structural Biology 2004, 14:161–170
The interactions of VirD4 proteins with other proteins
and their location strongly suggest that these proteins
do indeed act as a coupling factor between the relaxosome (see below) and the T4SS machinery. The ability
of VirD4 proteins to bind DNA and the possibility that
they may be able to harness the energy derived from
NTP binding and hydrolysis (perhaps cofactor aided)
suggest that they may play a direct role in DNA transport, and help thread the DNA towards a transmembrane T4SS pore and/or drive the DNA through it. A
main drawback of this model is that the hole (8 Å) on
the cytoplasmic side is too narrow for single-stranded
DNA to pass through it. Also, ATP binding does not
appear to be sufficient to trigger conformational
changes. Conformational changes may, however, be
induced by interaction with relaxosome components
or other cellular proteins.
The structure of TraC, the VirB5 orthologue from the E.
coli conjugative plasmid pKM101, reveals a single-domain
protein with a mostly a-helical elongated structure [65]
(Figure 6). Three long a-helices form the backbone of the
structure. This backbone supports an a-helical, loose
appendage formed by four short helices. Recent efforts
to assign the TraC function from its structure provided
functional evidence suggesting that VirB5 proteins mediate some of the pilus functions, such as phage attachment
and cell adhesion [65].
The structural biology of T4SS effectors is still relatively
new, except for the structure of the pertussis toxin, which
has been known for some time [66–68]. More recently,
the structure of the catalytic moiety of a relaxase has been
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Bacterial pathogenesis Remaut and Waksman 167
Figure 6
C
B5
Surface structures
B2
OM
N
B7
B9
B10
P
Core complex
B8
B6
IM
B4
B11
D4
ATP
ADP
Current Opinion in Structural Biology
Schematic representation of a model of the T4SS machinery, annotated according to the Agrobacterium VirB nomenclature. The pop-up
diagrams show the type IV components that have been structurally characterized (clockwise from upper right): the E. coli VirB5 orthologue TraC,
the H. pylori VirB11 homologue HP0525 and the E. coli VirD4 homologue TrwB. In the latter two structures, monomers have been differentiated
by colour. Secondary structures in TraC are indicated. IM, inner membrane; OM, outer membrane; P, periplasm.
determined [69]. The relaxase is part of the relaxosome, a
multiprotein complex that nicks and reacts covalently
with conjugative plasmid DNA, thereby initiating its
transport by the conjugative T4SS. This structure sheds
light on the enzymatic activity of the relaxase, but, as for
pertussis toxin, molecular details of effector transport
remain elusive.
Conclusions
The past three years have seen exciting developments in
our understanding of the molecular basis of bacterial
pathogenesis. Not only are the modes of action of bacterwww.sciencedirect.com
ial toxins and effectors being unravelled, but also our
understanding of the complex architecture and function
of bacterial secretion systems is increasing. Future efforts
will no doubt be directed towards the visualization of
entire secretion machineries using cryo-electron microscopy and image reconstruction. This will require the
development of sophisticated biochemical tools. Purification of the entire type III system has already been
achieved and efforts are being deployed to unravel its
molecular architecture at high resolution. Future developments will include similar work on other secretion
systems.
Current Opinion in Structural Biology 2004, 14:161–170
168 Macromolecular assemblages
Update
The recent structure of the in vitro folded translocator
domain of Neisseria meningitidis NalP [71] provides the
first structural insight into the mechanism of type V or
autotransporter assembly.
Adding to this break-through in our understanding of
bacterial autotransporters, the recent structures of the
collagen-binding domain of Yersinia adhesin YadA [72]
and the high-affinity receptor-binding domain (HiaBD1)
of Haemophilus influenza adhesin Hia [73] provide structural information on the details of pathogen–host interactions mediated by type V autotransported adhesins.
Acknowledgements
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This work was funded by grant 065932 from the Wellcome Trust, grant
58149 from the Medical Research Council and grant AI049950 from the
National Institutes of Health. Ribbon diagrams were generated with
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