Introduction
Pseudomonas aeruginosa is a Gram-negative opportunistic bacterial pathogen that often
infects compromised individuals, such as those suffering from burn wounds, cancer,
AIDS, and cystic fibrosis (1). Essential to its virulence is the presence of lipopolysaccharide
(LPS), which is the predominant lipid species in the outer leaflet of the outer membrane
(OM) and as such is an integral component of the bacterial cell surface. The importance
of LPS has been demonstrated with respect to many virulence traits, including inactivation
of surface-deposited C3b complement (2), formation of a layer of protection against
serum-mediated lysis (3), and aminoglycoside antibiotic binding (4). More recently,
LPS in P. aeruginosa has been shown to affect the interaction of the bacterium with
the cystic fibrosis transmembrane (TM) regulator (5), the secretion of type III effectors
(6), and the formation of biofilm architecture (7).
P. aeruginosa LPS is composed of three distinct regions, namely, the proximal lipid
A moiety (which anchors the molecule in the OM), the core oligosaccharide (which possesses
important phosphate substituents), and the distal O-antigen (O-Ag) polysaccharide
(8). Two glycoforms of O-Ag, known as A and B band, are synthesized by P. aeruginosa,
with the latter being the immunodominant cell surface antigen. Differences in B-band
O-Ag composition and structure are responsible for the classification of P. aeruginosa
into 20 serotypes (9).
Previous work by our group has provided genetic evidence demonstrating that B-band
LPS is synthesized via the Wzy-dependent pathway, in which several integral inner
membrane (IM) proteins are believed to function in stepwise assembly of the mature
glycoform in P. aeruginosa PAO1 (10, 11). According to the Wzy-dependent model, LPS
biosynthesis begins in the cytoplasm, where trisaccharide repeat units are built on
undecaprenyl pyrophosphate (Und-PP) in the inner leaflet of the IM (12). Individual
Und-PP-linked trisaccharide units are subsequently translocated by the O-Ag flippase
Wzx to the outer leaflet of the IM (10). Periplasmic polymerization of trisaccharide
repeat units at the reducing terminus is then mediated by the O-Ag polymerase Wzy
to preferred modal lengths governed by the chain length regulator proteins Wzz1 and
Wzz2 (13). Completed O-Ag chains are ligated to lipid A-core by the O-Ag ligase WaaL
to form B-band LPS (14).
The essential nature of the above-mentioned proteins with respect to B-band LPS biosynthesis
has been demonstrated for P. aeruginosa, with the loss of function of Wzx (10), Wzy
(11), or WaaL (14) correlating with an abrogation of B-band LPS biosynthesis. However,
due to the intrinsic difficulties of working with integral IM proteins, structural
characterization of these proteins and their homologues in various organisms has primarily
been based on in silico analysis of the amino acid sequence. In this study, in order
to gain a better understanding of the various domains of each protein with possible
functional importance, we mapped the complete IM topology of Wzx, Wzy, and WaaL from
P. aeruginosa PAO1 through phoA-lacZα dual-reporter fusion to random-length and targeted
3′ gene truncation libraries.
The dual-reporter approach was employed in order to yield different alkaline phosphatase
(AP) and β-galactosidase (BG) enzyme activity ratios contingent on its subcellular
localization (15). Detection of only high AP activity would indicate a periplasmically
localized truncation, whereas detection of only high BG activity would signify cytoplasmic
localization of a given truncation. However, if the terminal residue of the truncation
were localized within α-helical transmembrane segments (TMS), a combination of both
AP and BG activities would be observed. On the basis of this premise, various truncation
clones were initially isolated via growth on dual-indicator agar plates supplemented
with both AP- and BG-specific chromogenic substrates, resulting in pigmented colonies
indicating potential periplasmic (blue), cytoplasmic (red), or TM (purple) truncations.
To determine the length of gene truncation, constructs from pigmented colonies were
sequenced, thereby allowing for the construction of a topology map for each protein.
Subsequent quantification of AP and BG enzyme activities was carried out for representative
residues to substantiate the subcellular localization of various domains. Together,
this allowed for the construction of unbiased topology maps for Wzx, Wzy, and WaaL.
To independently verify the orientation of each protein within the IM, full-length
gene fusions to green fluorescent protein (GFP) were created. This investigation represents
the first simultaneous structural characterization of all three integral IM proteins
essential to LPS assembly and based directly on experimental evidence. Importantly,
our data have revealed previously unknown TM and loop domains and pointed to their
potential significance in the stepwise biogenesis of B-band LPS in P. aeruginosa PAO1.
RESULTS
Truncation library screening.
Random 3′ gene truncation libraries of wzx, wzy, and waaL from P. aeruginosa PAO1,
fused to phoA-lacZα, were generated as described in Materials and Methods. Following
the sequencing of clones from colored colonies, various 3′ truncation fusions were
obtained for wzx, wzy, and waaL, which were used to generate topology maps via the
HMMTOP 2.0 program (16). Targeted clones were constructed for regions of the proteins
lacking sufficient random truncation coverage, yielding combined totals of 76, 105,
and 66 unique truncations for wzx, wzy, and waaL, respectively. The subcellular localizations
of the various helices and loops within these maps were further verified and validated
via quantification of both AP and BG enzyme activity for representative residues by
use of established methods (17, 18).
Topology of the O-Ag flippase Wzx.
Random truncation libraries were generated for wzx, with the aim of obtaining optimal
coverage of the entire protein. However, it was observed that sufficient random coverage
was not possible at positions in the protein downstream of I255. As such, to ensure
initial unbiased coverage of the protein, an “interval-scanning” approach was taken
in which truncations were generated every 7 amino acids downstream of I255, after
which targeted truncations were made to elucidate remaining regions of Wzx. The N
and C termini of the O-Ag flippase were revealed to be localized in the cytoplasm,
with 12 TMS in between (Fig. 1). Three relatively large periplasmic loops flanked
by the first six TMS were identified. The cytoplasmic face of the protein was found
to contain several large loop regions, constrained by TMS X2 and X3 (20 amino acids),
X6 and X7 (47 amino acids), and X10 and X11 (18 amino acids). This is in contrast
to results from in silico prediction analysis, which did not reveal the extents of
the various cytoplasmic loops, in particular the third, and underestimated the number
of charged amino acids within the TM region by over 55% (data not shown).
FIG 1
Topological map of Wzx from P. aeruginosa PAO1 based on phoA-lacZα fusion analysis
(GenBank accession no. 15598349). Colored residues represent the amino acid positions
of each truncation used. Residue colors denote the subcellular localization of a given
truncation: blue, periplasm; purple, TM; and red,
cytoplasm. Truncation letter colors denote the method of truncation generation: white,
random; green, interval scanning; black, targeted (periplasm and cytoplasm); orange,
targeted (TM). All TMS are labeled (X1 to X12). The AP/BG enzyme normalized activity
ratios (NARs) for representative residues (see Table S1 in the supplemental material)
are displayed in rectangles. Amino acid identity is displayed above/below each NAR
for quantified residues.
Topology of the O-Ag polymerase Wzy.
Fourteen TMS were identified in Wzy, with four intervening periplasmic loops and two
intervening cytoplasmic loops of possible functional significance (Fig. 2). Of the
periplasmic loops characterized, that between TMS Y9 and Y10 is the largest at 42
amino acids, followed by a comparable 36-residue loop localized between TMS Y5 and
Y6. Two sizeable cytoplasmic loops were also uncovered, with the first having a length
of 24 residues, flanked by TMS Y4 and Y5. The second spans 21 amino acids and is flanked
by TMS Y12 and Y13. The former amino acid stretch was partially shifted into a TM
helix while the latter was completely absent from the cytoplasm upon analysis of in
silico prediction data (data not shown).
FIG 2
Topological map of Wzy from P. aeruginosa PAO1 based on analysis of 105 phoA-lacZα
fusions (GenBank accession no. 15598350). Colored residues represent the amino acid
positions of each truncation used. Residue colors denote the subcellular localization
of a given truncation: blue, periplasm; purple, TM; red, cytoplasm. Truncation letter
colors denote the method of truncation generation: white, random; black, targeted
(periplasm and cytoplasm); orange, targeted (TM). All TMS are labeled (Y1 to Y14).
The AP/BG enzyme NARs for representative residues (see Table S2 in the supplemental
material) are displayed in rectangles. Amino acid identity is displayed above/below
each NAR for quantified residues.
Topology of the O-Ag ligase WaaL.
Topology mapping of WaaL from P. aeruginosa PAO1 revealed an integral IM protein with
12 TMS (Fig. 3). One large periplasmic loop domain was identified, localized between
TMS L9 and L10, composed of 48 amino acids, and containing the functionally critical
H303 residue (19). Adjacent to this main periplasmic loop is a second, smaller loop,
containing 14 amino acids, flanked by TMS L11 and L12. The length of TMS L8 was extended,
following the software program output (see Materials and Methods), to 26 residues
to account for experimental data obtained based on both color scoring and enzyme activity
data for the random fusion clone K233. As such, this TMS is only 1 residue longer
than the default upper limit of 25 residues per TMS constraining the HMMTOP 2.0 algorithm
(16). PhoA-LacZα fusion screening of WaaL truncations also revealed that the cytoplasmic
face of the protein contains a sizeable loop of 30 amino acids between TMS L6 and
L7. This tract of amino acids was predicted to begin in the periplasm and end within
a TM helix when purely de novo prediction algorithm analysis was performed in the
absence of experimental input (data not shown).
FIG 3
Topological map of WaaL from P. aeruginosa PAO1 based on analysis of 66 phoA-lacZα
fusions (GenBank accession no. 15600192). Colored residues represent the amino acid
positions of each truncation used. Residue colors denote the subcellular localization
of a given truncation: blue, periplasm; purple, TM; red, cytoplasm. Truncation letter
colors denote the method of truncation generation: white, random; black, targeted
(cytoplasm); orange, targeted (TM). All TMS are labeled (L1 to L12). The AP/BG enzyme
NARs for representative residues (see Table S3 in the supplemental material) are displayed
in rectangles. Amino acid identity is displayed above/below each NAR for quantified
residues.
Protein orientation analysis.
The use of C-terminal GFP tagging has been established as an additional experimental
approach for determining the subcellular localization of a given domain in integral
IM proteins (20), as GFP will not fluoresce in the periplasm (21). To confirm the
IM orientation of Wzx, Wzy, and WaaL, translational fusions to GFP were created and
expressed in P. aeruginosa PAO1. All constructs displayed pronounced fluorescence
across the entire cell population, indicating cytoplasmic localization of the C terminus
in Wzx, Wzy, and WaaL (Fig. 4). These results served to independently verify the orientation
determined through PhoA-LacZα topology mapping (Fig. 1, 2, and 3). When expressed
in its respective chromosomal knockout in P. aeruginosa PAO1, each GFP fusion was
able to restore the synthesis of B-band LPS (see Fig. S1 in the supplemental material).
Synthesis of A-band LPS, deficient in the absence of a functional ligase, was also
restored upon expression of WaaL-GFP in the waaL mutant background (data not shown).
FIG 4
Fluorescence micrographs of P. aeruginosa PAO1 expressing C-terminal GFP fusions of
Wzx, Wzy, and WaaL from respective pHERD26T clones. Images were captured at ×400 as
described in Materials and Methods. FM, fluorescence micrograph; DIC, differential
interference contrast. White bar = 15 µm.
DISCUSSION
Membrane proteins represent over a quarter of all known proteins and play integral
roles in a vast array of cellular process, including import/export of substrates,
energy production, signal transduction events, and assembly of cellular components.
However, due to the inherent difficulties associated with expressing, purifying, and
crystallizing membrane proteins, of the more than 61,000 entries in the Protein Data
Bank to date, just over 250 represent unique membrane protein structures (White Laboratory
[http://blanco.biomol.uci.edu/Membrane_Proteins_xtal.html]). In the absence of crystallographic
data, topological mapping of membrane proteins can provide valuable information on
the sizes and subcellular localizations of various domains potentially contributing
to their overall function. This rationale has led us to examine the membrane topologies
of Wzx, Wzy, and WaaL from P. aeruginosa PAO1.
A leading approach for examining membrane protein topology involves the fusion of
C-terminal reporter tags at various lengths of the amino acid sequence, with independent
PhoA (periplasmically active) and LacZ (cytoplasmically active) fusions often created;
fusion of LacZ to periplasmic residues may yield low BG activity (22) or toxicity
to cells (23). Due to the requirement of separate fusions, the expression levels of
reciprocal PhoA and LacZ fusions must be normalized for comparison of enzyme values
at the same truncation residue. The activity of either AP (PhoA) or BG (LacZ) can
be monitored by the breakdown of enzyme-specific chromogenic substrates, thus allowing
for rapid analysis of fusion activity (17). To date, all published data on the full
membrane topologies of Wzx (24), Wzy (25), and WaaL (26) proteins from various Gram-negative
species, as well as their homologues (27, 28), have employed the use of separate PhoA
and LacZ truncation fusions. Furthermore, these studies were invariably aimed at validating
the results of consensus TMS localizations based on multiple in silico topology predictions.
The generation of random fusions was carried out in only three of these studies, which
still relied primarily on purpose-built fusions (24–26). In the remaining studies,
targeted fusions were created based exclusively on in silico analyses (27, 28). To
overcome the limitations of separate PhoA and LacZ fusions, we employed a chimeric
PhoA-LacZα dual-reporter system capable of displaying both AP and BG activity (15)
to map the topologies of Wzx, Wzy, and WaaL from P. aeruginosa PAO1. Functional LacZ
was reconstituted only by the host-encoded ω fragment of LacZ in the cytoplasm, with
no periplasmic toxicity associated with the LacZα moiety. This reporter system was
originally developed by Alexeyev and Winkler (15) to map the membrane topology of
the ATP/ADP translocase Tlc from Rickettsia prowazekii.
In this investigation, we took the approach opposite to that used in all previously
published topology characterizations of homologous O-Ag assembly proteins, i.e., we
generated unbiased random and interval-scanning libraries of wzx, wzy, and waaL fused
to the phoA-lacZα dual-reporter construct. These were then screened based on both
pigmentation phenotype and enzyme activity quantitation. On the basis of the results
obtained from these initial fusion libraries, preliminary topology maps were derived
using segment localization data via the HMMTOP 2.0 server, which generates outputs
based directly on experimental data (16). To remedy ambiguous regions where coverage
based on either the random truncation or the “interval-scanning” approach was lacking,
we constructed targeted truncations to help clarify their localizations, allowing
for the generation of finalized topology maps for Wzx, Wzy, and WaaL.
Targeted fusions generally yielded higher absolute enzyme activity values than their
random truncation library-generated counterparts (see Tables S1 to S3 in the supplemental
material). The full-length wzx, wzy, and waaL constructs fused to phoA-lacZα, which
were used to create their respective random truncations, were cloned into the same
pPLE01 source vector as that used for the individual targeted truncations for each
gene. However, creation of the targeted truncations introduced 2 amino acids between
the terminal residue of the truncation and the reporter motif. These residues were
unavoidable, as they were translated from the 3′ PstI restriction endonuclease site
used for cloning of the targeted fusions immediately upstream of the reporter. While
the same 3′ restriction site was used to linearize the full-length gene fusions in
advance of exonuclease III treatment for random library generation, the ensuing enzyme
treatment regimen resulted in removal of the PstI site, thus eliminating the two residues
described above. As such, compared to the truncations that were actively made, the
reporter motif in the randomly generated truncations may have been more sterically
constrained; this would have resulted in lower absolute enzyme values upon quantitation
due to lowered substrate accessibility by either reporter moiety. Regardless of the
method of generation, when the various activities across the three different proteins
were normalized as percentages of the maximum activity of a particular set, general
trends could be inferred from the AP/BG normalized activity ratio (NAR) for a specific
residue. NARs of <0.1, between 0.1 and 10, and >10 were found to coincide with cytoplasmic,
TM, and periplasmic residues, respectively, allowing for localization data comparison
between Wzx, Wzy, and WaaL.
Wzx.
Wzx and its homologues are members of the polysaccharide transporter (PST) family
of proteins, present in both Gram-negative and Gram-positive organisms as well as
in Archaea (29). PST family members have 10 to 14 predicted TMS (29), consistent with
our experimentally derived model of 12 TMS. The first topological characterization
of a Wzx homologue, PssL from Rhizobium leguminosarum bv. trifolii strain TA1, also
yielded a model consisting of 12 TMS (28), as did a recent study of Wzx from Salmonella
enterica serovar Typhimurium group B (Wzx
Se
) (24). However, these models were based on purpose-built PhoA and LacZ fusions made
to support in silico predictions and, as described below, overlooked certain key features.
The cytoplasmic loop residue compositions for Wzx from P. aeruginosa (Wzx
Pa
) are such that no motifs resembling conventional ATPase consensus sequences are present.
Furthermore, the extent of loop lengths on this face of the protein would suggest
an elaborate tertiary-structure architecture in the cytoplasm, which is reinforced
by secondary-structure-propensity analysis of the various loop domains (see Fig. S2
in the supplemental material). This would be consistent with the predicted function
of Wzx in mediating translocation of lipid-linked oligosaccharides, a process which
would likely be initiated in the cytoplasm. Due to its size, cytoplasmic loop 3 (CL3X)
of Wzx
Pa
in particular may play an important role during the translocation step, perhaps functioning
as part of a gating mechanism governing entry on the cytoplasmic face of the protein.
Four of the random truncations near the membrane interface were color scored as purple,
but quantitation of their enzyme activities as well as those of flanking residues
revealed AP/BG NARs reflective of cytoplasmic localization consistent with all three
proteins studied. Background breakdown of the AP-specific substrate was likely due
to kinking of the reporter moiety towards the membrane through the secondary structure,
as the aforementioned purple clones are immediately downstream of a Pro residue. Conversely,
the periplasmic face of the protein contains only 3 loops of possible functional importance,
the largest of which is only 16 amino acids in length, alluding to a less intricate
tertiary structure for this face of the protein (see Fig. S2 in the supplemental material).
Contrary to the mainly in silico-based topology models of Wzx proteins put forth by
others, we observed that 9 of the 12 TMS present in Wzx
Pa
contain at least one charged residue, with some possessing as many as seven charged
amino acids (see Fig. S5 in the supplemental material). Interestingly, the only other
investigation involving the topology of a Wzx O-Ag flippase (Wzx
Se
) proposed a membrane topology model in which only 5 charged amino acids were predicted
among the total of 12 proposed TMS (24). Rather, the locations of the various TMS
appear to have been constrained by the localization of the various Arg, Asp, Glu,
His, and Lys residues. As with the above-mentioned characterization of PssL from R. leguminosarum
bv. trifolii, the proposed topology of Wzx
Se
was based on the initial generation of an in silico-derived map, with only 12 reciprocal
PhoA and LacZ fusions created to verify the predictions.
The distribution of so many charged residues within TMS of Wzx
Pa
provides a possible explanation to further elucidate the mechanism of its function.
The negatively charged sugar units of O-Ag repeats in P. aeruginosa, for instance,
that of strain PAO1 (serotype O5) containing two mannuronic acid residues and one
fucosamine residue (9), would need to be translocated through the hydrophobic environment
of the IM bilayer before polymerization by Wzy. This process would be more energetically
favorable if the TMS of Wzx were arranged in a pore-like structure with charged residues
lining the interior, particularly since the majority of the charged amino acids within
the TMS are cationic. With the use of the RHYTHM server (http://proteinformatics.charite.de/rhythm/)
to identify potential TMS-TMS and TMS-membrane contact points in membrane proteins,
our analysis of TMS X1 to X12 revealed numerous candidates for each interaction environment
(see Fig. S5 in the supplemental material). By extension, it is possible to generate
a preliminary model of the potential Wzx
Pa
channel within the IM and identify the charged residues that may line the interior.
While the observations described above are consistent with the predicted role of Wzx
Pa
, further investigation is required to fully understand the actual translocation mechanism,
whether it be active or passive.
The substrate specificity of Wzx has been proposed to be dependent on the identity
of the first sugar of the O-Ag subunit, as evidenced by cross-complementation experiments
in which nonnative Wzx proteins were able to restore wild-type LPS synthesis if the
organism from which they were derived contained the same sugar as the initial unit
of its O-Ag repeat (30). However, the basis for substrate specificity remains unknown.
Prior work in an Escherichia coli K-12 system has shown that a full-length O-Ag repeat
unit is not required for translocation by Wzx (31). These data suggest that recognition
of the initiating sugar is important for flippase function. As described above, the
charge property of the potential Wzx channel, as well as the overall amino acid composition
of the various loops, may confer substrate specificity. Interestingly, periplasmic
loop 2 in Wzx
Pa
(PL2X) contains an RX10G tract of amino acids (see Table S4 in the supplemental material).
This amino acid motif is discussed in detail below for Wzy and WaaL, with respect
to its potential role in interaction with the initiating sugar of the O-Ag repeat
unit.
Wzy.
The data from this study indicate that Wzy
Pa
(438 amino acids) contains 14 TMS, with two large periplasmic loops flanked either
by TMS Y5 and Y6 or by TMS Y9 and Y10 (Fig. 2). Truncations in the tail end of the
latter loop were found to yield purple colony coloration (F306, G311, and R316); however,
determination of the AP/BG NAR indicated that these residues each displayed a NAR
consistent with all periplasmic residues studied and as such were assigned to the
periplasm. Initially, this would appear at odds with the proposed 12-TMS model for
Wzy from Shigella flexneri (Wzy
Sf
) (382 amino acids). However, ClustalW alignment results for Wzy
Pa
and Wzy
Sf
indicate that specific subcellular regions of these proteins correspond with each
other until the termination point of periplasmic loop 5 (PL5). Although the amino
acid sequence of Wzy
Pa
is longer than that of Wzy
Sf
, the positions of analogous TM and loop portions of the proteins were found to align
well when compared to one another, with two additional TM domains in Wzy
Pa
accounting for the extra amino acids (data not shown). Differences in the numbers
of TMS and the sizes of various periplasmic loops between the two homologues may reflect
differences in substrate specificities (32), as O-Ag units in all serotypes of S. flexneri
(except type 6) are tetrasaccharide repeats beginning with N-acetyl-d-glucosamine
(33), whereas those in P. aeruginosa PAO1 are trisaccharide repeats beginning with
N-acetyl-d-fucosamine (9).
To date, Wzy protein structure remains poorly characterized, and as such, information
relating to catalytic mechanisms of O-Ag polymerization is lacking. It has been proposed
that Wzy and WaaL may function in similar ways, as each protein requires O-Ag polysaccharides
bound to Und-PP and is subsequently able to transfer O-Ag to a sugar acceptor (26).
For Wzy, the sugar acceptor would be another O-Ag repeat unit bound to Und-PP. Conceptually,
Wzy would have to interact with the same molecular structure in two different ways,
similar to the model proposed by Bastin et al. (34). The first method of interaction
would require prolonged docking and retention of the extending O-Ag chain, either
still bound to Und-PP or independently associated with the protein. The former scenario
would be more plausible, as remaining bound to the lipid carrier would undoubtedly
facilitate the retention of the growing chain in the IM before the completion of chain
polymerization. The second mode of interaction would entail association with the incoming
O-Ag repeat still bound to Und-PP. The latter interaction would need to be more short-term
to allow for either (i) the recycling of the new Und-PP and the transfer of the newly
arrived O-Ag subunit to the reducing terminus of the growing chain bound to the initial
Und-PP or (ii) the shuttling of the incoming Und-PP-linked repeat to the prolonged
docking site to extend the O-Ag chain and displace the previous Und-PP carrier. In
both of the above-mentioned general interaction scenarios, it is conceivable that
similar recognition motifs would be required. Upon comparison against each other via
ClustalW alignment, PL3Y and PL5Y are of comparable size and contain many conserved
or structurally equivalent residues (see Fig. S6 in the supplemental material). Polymerase
proteins such as Wzy have also been found to contain a conserved HX10G so-called “polymerase
motif,” though an exact motif such as this is not present in Wzy
Pa
. This motif was also identified in WaaL proteins (26). In WaaL from P. aeruginosa
PAO1 (WaaL
Pa
), the periplasmic His residue of the HX10G motif (H303) was found to be critically
important for function (19). On the basis of complementation experiments, His could
be replaced by Arg, creating an RX10G tract and demonstrating the functional equivalency
of His and Arg residues in this setting. Our investigation of Wzy
Pa
topology has revealed the presence of RX10G motifs in both PL3Y and PL5Y (see Table S4
in the supplemental material). PSIPRED analysis (35) of the PL3Y and PL5Y secondary
structures indicates analogous positioning of both RX10G tracts on their respective
loops, with each beginning in the longer α-helical domain and running to the flexible
linker separating the two α helices of each loop (see Fig. S3 in the supplemental
material). Taken together, this conserved tract of amino acids with similar secondary-structure
patterning between PL3Y and PL5Y may represent the same motif present in two different
tertiary-structure contexts within the same protein.
In addition to the two main periplasmic loops, we have also identified a large, 24-residue
cytoplasmic loop (CL2Y) flanked by TMS Y4 and Y5 (Fig. 2). This loop contains the
sequence RENQGRRMLVLLS (see Table S4 in the supplemental material), which is remarkably
similar to the consensus Walker B ATPase sequence of R/KXXXGXXXLhhhhD (X, any amino
acid; h, hydrophobic residue; underlined, consensus match), which forms a β-sheet
structure (36). Incidentally, PSIPRED secondary-structure analysis of CL2Y has revealed
a high propensity for β-strand formation in this loop domain (see Fig. S3 in the supplemental
material). This cytoplasmic domain is connected directly to PL3Y through TMS Y5, a
helix with charge properties. The other sizeable cytoplasmic loop in Wzy
Pa
(CL6Y) also has a high propensity for the secondary structure, with α helices sandwiched
by β-stranded regions (see Fig. S3 in the supplemental material). Functional characterization
of both loops is currently under way.
WaaL.
Consistent with WaaL homologues from different bacterial species (26), in silico secondary-structure
analysis predicted the presence of a large characteristic periplasmic loop in WaaL
Pa
, in which several amino acids appeared to be conserved; these were subjected to alanine-scanning
mutagenesis. Among these mutants, only the H303A mutation was unable to restore the
production of B-band LPS in a waaL knockout background. Upon screening of a panel
of H303G, H303N, and H303R mutants, only the H303R mutation was able to preserve the
function of WaaL
Pa
, likely due to the ability of Arg to carry a positive charge, similar to what was
found for His (19). The H303 residue is the leading residue in the HX10G motif described
above for Wzy, which could also function in the form of an RX10G motif. Through WaaL
topology mapping, we have confirmed the presence of a large, 48-residue periplasmic
loop between TMS L9 and L10 (PL5L) containing the catalytically essential H303 residue.
Subsequent to the investigation by our group, an analogous catalytically essential
His residue (H337) was identified in WaaL from E. coli (WaaL
Ec
) (37). This demonstrates the importance of the conserved positively charged residue
in the ligation of O-Ag to the lipid A-core, though the length of the periplasmic
loop in WaaL
Ec
on which this residue is located has not been conclusively determined (37). Using
computer modeling of amino acids constituting the proposed loop domain as well as
22% of the downstream TMS, the same authors proposed that the conserved acidic residue
would form part of a putative “catalytic center” created by the tertiary structure
of the extended principal periplasmic loop (as described above), resulting in two
pairs of perpendicular α helices. In contrast, PL5L from WaaL
Pa
possesses a high level of propensity for the formation of a single α helix, as well
as a partial β-strand character, with a high degree of flexibility (see Fig. S4 in
the supplemental material). The validation of either model awaits the detailed biophysical
determination of its three-dimensional (3-D) structure.
Abeyrathne and Lam also examined both truncated and chimeric versions of WaaL
Pa
(19). C-terminally truncated versions of WaaL
Pa
with truncations comprising amino acids 1 to 101, 1 to 156, 1 to 232, 1 to 301, and
1 to 353 were generated, but none were able to complement a waaL mutant. On the basis
of the topology characterized in our investigation, the version with a truncation
comprising amino acids 1 to 353 would have contained the required catalytic H303 residue
and intact PL5L, yet this version was nonfunctional. This suggests an important functional
role for the region of WaaL
Pa
comprising amino acids 354 to 401, which likely contributes to stabilization of the
protein and/or participation in the catalytic mechanism. Chimeric fusions to WaaL
Ec
were also constructed, and the only one able to restore a wild-type level of B-band
O-antigen production contained WaaL
Pa
residues 1 to 360 and a C-terminal fragment of WaaL
Ec
residues 389 to 419. As determined from the topology map, the former stretch of amino
acids would have encompassed a portion of PL6L that contained a Glu and an Asn residue,
with this portion in its entirety containing several charged residues. The latter
tract of amino acids from WaaL
Ec
is predicted by an in silico analysis to form an 18-residue TM helix, which would
maintain an Arg residue in an analogous loop position and contribute an Asp residue
in place of the Lys, Glu, and His residues present in the native WaaL
Pa
PL6L sequence. Since this final periplasmic loop of WaaL
Pa
is charged, sizeable, flexible, and predicted to possess secondary structure (see
Fig. S4 in the supplemental material), this loop may be analogous to EL4, a predicted
periplasmic loop adjacent to the principal loop from WaaL
Ec
found to contain a charged residue (R216) important for O-Ag ligation (37).
WaaL from Vibrio cholerae O1 (WaaL
Vc
) has a sequence length (398 amino acids) comparable to those of WaaL
Pa
(401 amino acids) and WaaL
Ec
(419 amino acids), with the largest periplasmic loop located from S234 to L327 in
a primary-structure position similar to those for both WaaL
Pa
(R259 to E306) and WaaL
Ec
(N259 to E342). However, WaaL
Vc
was proposed to contain only 10 TMS, each at a maximum length of only 8 amino acids,
with all intervening loops being relatively large (26). In comparison, the average
thickness of the IM in E. coli K-12 and P. aeruginosa PAO1 has been measured at 59 Å
(38). Moreover, a higher level of membrane insertion instability was observed for
artificial TM helices, as the number of residues decreased incrementally from 19 to
15 within a model lipid bilayer (39). Combined with inconsistent PhoA fusion quantitation
values (e.g., periplasmic D79), these data cast doubt on the number and boundaries
of TMS described for WaaL
Vc
and suggest that further model refinement is required.
Investigation of the mechanism for O-Ag ligation to lipid A-core also revealed that
WaaL
Pa
was able to hydrolyze ATP in vitro at a rate in accordance with Michaelis-Menten kinetics
(19). As ATP is not believed to exist in the periplasm (40), this would entail a cytoplasmic
domain capable of ATP hydrolysis. However, in silico topology analysis for WaaL
Pa
did not reveal the presence of any cytoplasmic loops having either sufficient size
or recognizable ATPase motif sequence conservation (19). The recent in silico topology
prediction for WaaL
Ec
yielded analogous results, revealing the lack of a sizeable cytoplasmic motif that
would be capable of hydrolyzing ATP. Interestingly, in the present study of WaaL
Pa
, we have discovered the presence of a large, 30-residue cytoplasmic loop flanked
by TMS L6 and L7 (Fig. 3) and thus revealed a candidate cytoplasmic motif for the
observed ATPase activity of the protein in vitro. As purification of WaaL
Pa
yielded a dimer (19), interaction of the two cytoplasmic domains from monomers of
the protein may be required to form a contiguous quaternary structure capable of hydrolyzing
ATP. In silico analysis predicted that this cytoplasmic stretch of residues in WaaL
Pa
began in the periplasm and ended within a TM helix, thus demonstrating the limitations
of relying solely on in silico topology predictions and reinforcing the benefits of
obtaining experimentally based entire-protein topology maps for downstream functional
characterization of membrane proteins.
The above-mentioned loop does not possess a high degree of amino acid sequence similarity
to conventional ATPase motifs, although a TGYG tract is present at amino acids 158
to 161 (see Table S4 in the supplemental material), corresponding to a conserved portion
of the consensus GXS/TGXGKS/TS/T Walker A/phosphate-binding loop (P-loop) sequence
(41). Deviant Walker A motifs in which the downstream Lys residue, carrying a positive
charge, can be located 2 residues upstream of the TGXG motif have also been found
(42). The residue located 2 positions upstream of the TGYG motif is a positively charged
Arg residue, possibly analogous in position and function to Lys in deviant Walker
A domains. Walker A domains generally contain an uncapped α helix preceded by a loop
rich in Gly residues (41). Secondary-structure propensity indicates that CL3L forms
a flexible coiled-coil structure in the region of TGYG, followed immediately by a
helix motif (see Fig. S4 in the supplemental material), consistent with general secondary-structure
characteristics of Walker A motifs. CL3L in WaaL
Pa
is currently being characterized in our laboratory.
Conclusion.
Among all three proteins studied, numerous motifs known to promote oligomerization
of TM helices have been identified. The most well known is the GX3G motif, in which
two Gly residues, spaced 4 residues apart, promote packing of TMS. Ala and Ser residues
have been shown to substitute for Gly in the above-mentioned motif, now referred to
as the GASRight motif for right-handed α helices (43). This motif possesses high propensity
for the formation of a flat surface capable of docking against grooves created by
bulkier amino acids also spaced 4 residues apart in a nearby helix (43). From our
investigation, numerous GASRight motifs can be found in Wzx, Wzy, and WaaL (see Table S4
in the supplemental material). Since antiparallel pairs of α helices tend to form
between sequential TMS (43), the GASRight motif content of Wzx, Wzy, and WaaL TMS
may provide a clue as to the nature of the local packing and domain organization for
the various proteins. As such, tracts of amino acids separated by large distances
of primary structure may interact through packing and tertiary-structure events to
form a functional domain.
While the Wzy-dependent model for LPS biosynthesis generally fits with observed phenotypes
for mutants at various stages of the process, the exact functional mechanisms of each
stage have yet to be elucidated. In this investigation, we have discovered the presence
of novel domains in both TM and soluble subcellular localizations for three essential
proteins in this pathway. While Wzx, Wzy, and WaaL catalyze different events during
the assembly process, these proteins are conceptually required to interact with an
equivalent molecular structure, in the form of O-Ag, albeit in differing capacities.
Remarkably, we have uncovered the presence of a common periplasmic motif in all of
the proteins that are required for the Wzy-dependent assembly pathway. The first manifestation
of the RX10G motif in the Wzy-dependent assembly pathway can be found in PL2X of Wzx
Pa
. It is also found in both PL3Y and PL5Y of Wzy
Pa
. An RX10G motif is also located on PL5L, the functionally essential loop of WaaL
Pa
(see Table S4 in the supplemental material). In each of these cases, the initial Arg
residue is followed immediately by another charged (for Wzx and Wzy) or highly polar
(for WaaL) amino acid. Intriguingly, the equivalent motif is present twice in Wzz1
and seven times in Wzz2 from P. aeruginosa PAO1, both proteins that would function
to regulate the modal chain length of O-Ag polymerization carried out by Wzy
Pa
. While the oligomeric state of full-length Wzz proteins has been determined (44)
and X-ray crystallographic data exist for the soluble periplasmic domain (45), the
mechanism by which Wzz proteins interact with O-Ag and regulate the chain length is
presently unknown. The concept of a common interaction motif for a specific O-Ag subunit
would serve as a unifying thread between Wzx, Wzy, Wzz, and WaaL in a given species
and merits further investigation.
In conclusion, results from this study have revealed the membrane topology of Wzx,
Wzy, and WaaL from P. aeruginosa PAO1 at a resolution unmatched by previous attempts
for homologous proteins. Rather than relying on in silico topology predictions to
frame ensuing fusion creation, we first generated localization data based on experimental
evidence and then used these data to create a final topology map for Wzx, Wzy, and
WaaL. This has allowed us to eliminate the inherent bias of predetermined TMS localization
and, in doing so, revealed the positions of novel TMS as well as the locations and
extents of previously unidentified periplasmic and cytoplasmic loop domains in line
with the proposed functions for each protein. This investigation will serve as a springboard
for detailed functional characterization of these proteins, which will undoubtedly
further our understanding of such a widely conserved yet poorly understood biosynthesis
pathway for virulence-associated polysaccharides in bacteria.
MATERIALS AND METHODS
DNA manipulations.
Plasmid DNA was isolated with a GenElute miniprep kit (Sigma). Products of PCR amplifications
and restriction digestions were cleaned with a QIAquick PCR cleanup kit (Qiagen).
The resultant output from sequenced clones was analyzed via the open reading frame
(ORF) finder program created by the National Center for Biotechnology Information
(Bethesda, MD). The oligonucleotide primer sequences used in this study are available
upon request.
Construction of PhoA-LacZα and GFP translational fusions.
The strains and plasmids used in this study are listed in Table 1. The phoA-lacZα
reporter sequence from pMA632 (15) was cloned between the EcoRV and HindIII sites
of pBluescript II SK(+) under the control of the lac promoter to create pPLE01. Full-length
genes were PCR amplified from P. aeruginosa PAO1 genomic DNA and cloned upstream of
the reporter construct in pPLE01 by use of either the XbaI and BamHI sites (for wzx)
or the SacI and XbaI sites (for wzy and waaL). Random exonuclease III-generated truncation
fusion libraries were created as previously described by Alexeyev and Winkler (15).
E. coli DH10B was used as an α-complementing host strain. Fusion junction sequencing
for random truncations was carried out at the Laboratory Services division, University
of Guelph. Targeted truncation fusions for a respective gene were created in the same
manner as for the full-length constructs used for library generation, by cloning of
PCR products of various 3′ truncation positions upstream of phoA-lacZα. To construct
the GFP fusions, PCR products (amplified from P. aeruginosa PAO1 genomic DNA) were
cloned upstream of the gfp-his
8 reporter in the pWaldo-GFPd vector (46) for wzx, wzy, and waaL to yield pWaldo-wzx-GFP,
pWaldo-wzy-GFP, and pWaldo-waaL-GFP. For fluorescence analysis of P. aeruginosa strains,
the pWaldo clones were used as templates for PCR amplification of the GFP fusion constructs
prior to pHERD26T cloning (47). All PCR amplifications were performed using KOD Hot
Start DNA polymerase (Novagen). All digestions and ligations were performed with enzymes
from Invitrogen or NEB.
TABLE 1
Bacterial strains and plasmids
Strain or plasmid
Genotype, phenotype, or relevant characteristic(s)
c
Reference or source
Strain
P. aeruginosa
PAO1
Serotype O5; A+ B+
Laboratory stock
PAO1waaL
waaL::Gmr derived from strain PAO1; A− B−
14
PAO1wzx
wzx::Gmr derived from strain PAO1; A+ B−
10
PAO1wzy
wzy::Gmr derived from strain PAO1; A+ B−
11
E. coli DH10B
F−
araD139 Δ(ara leu)7697 ΔlacX74
galU galK rpsL deoR ϕ80dlacZΔM15 endA1
nupG
recA1
mcrA Δ(mrr hsdRMS mcrBC)
Laboratory stock
Plasmid
pBluescript II SK(+)
Escherichia expression vector (Plac); Apr
Fermentas
pHERD26T
Pseudomonas expression vector (PBAD
araC); Tetr
47
pMA632
Plasmid encoding phoA-lacZα dual-reporter construct; Apr
15
pPLE01
a
pBluescript II SK+ with phoA-lacZα from pMA632; Apr
This study
pWaldo-GFPd wzx
Plasmid carrying gfp-his
8; Kmr
46
pHERD26T-wzx-GFP
pHERD26T carrying wzx-gfp-his
8; Tetr
This study
pPLE01-wzx
b
Full-length wzx (encoding aa 1-411) fused to phoA-lacZα; Apr
This study
pWaldo-wzx-GFP wzy
pWaldo carrying wzx-gfp-his
8; Kmr
This study
pHERD26T-wzy-GFP
pHERD26T carrying wzy-gfp-his
8; Tetr
This study
pPLE01-wzy
b
Full-length wzy (encoding aa 1-438) fused to phoA-lacZα; Apr
This study
pWaldo-wzy-GFP waaL
pWaldo carrying wzy-gfp-his
8; Kmr
This study
pHERD26T-waaL-GFP
pHERD26T carrying waaL-gfp-his
8; TetR
This study
pPLE01-waaL
b
Full-length waaL (encoding aa 1-401) fused to phoA-lacZα; Apr
This study
pWaldo-waaL-GFP
pWaldo carrying waaL-gfp-his
8; Kmr
This study
a
All targeted truncations for wzx, wzy, and waaL were cloned upstream of phoA-lacZα
in pPLE01 and are represented in Fig. 1, 2, and 3.
b
All random truncation libraries for wzx, wzy, and waaL were generated using the respective
full-length fusion vectors and are represented in Fig. 1, 2, and 3.
c
Superscript “+” or “−” after A or B denotes the presence or absence of the particular
O polysaccharide, respectively. Resistance (r) is shown for gentamicin (Gm), ampicillin
(Ap), tetracycline (Tet), and kanamycin (Km). aa, amino acids.
Color scoring and enzyme quantitation.
Ligation recovery cultures for truncation libraries were plated on defined-medium
agar plates supplemented with ampicillin (100 µg/ml; Sigma), isopropyl-β-d-thiogalactopyranoside
(IPTG) (1 mM; Roche), 5-bromo-4-chloro-3-indolyl phosphate (BCIP) (80 µg/ml; Sigma),
and 6-chloro-3-indolyl-β-d-galactoside (Red-Gal) (100 µg/ml; Research Organics) as
previously described (15). Subcultures from overnight inoculations were grown to mid-exponential
phase in Miller’s LB broth (Invitrogen) supplemented with ampicillin and IPTG. Quantitation
of AP and BG activity was carried out as previously described, including compensation
for spectrophotometric absorbance caused by cell debris (17), with cells for Wzy BG
assays treated with 50 µL of B-PER II permeabilization reagent (Thermo), instead of
chloroform-SDS, to increase permeabilization in order to compensate for low protein
expression levels. All NAR values presented are a result of quadruplicate independent
enzyme assays. Residue localization data based on color scoring and enzyme activity
analysis were entered into the HMMTOP version 2.0 prediction algorithm in order to
determine the positions and lengths of TMS on the basis of experimental results (16).
Fluorescence microscopy.
P. aeruginosa PAO1 overnight cultures expressing GFP fusions were subcultured and
grown to exponential phase in LB broth at 37°C containing 90 µg/ml tetracycline (Sigma)
with 0.1% l-arabinose. Culture aliquots were sedimented, resuspended in 1× phosphate-buffered
saline (PBS), and spotted on a glass slide. No fixation to the slide was carried out,
as GFP fluorescence may become labile following various fixative treatments. Cells
were imaged at ×400 magnification using a Zeiss Axiovert 200 M fluorescence microscope,
with excitation at 470 nm and emission collection at 525 nm, using the Improvision
Openlab 5 software package (PerkinElmer).
SUPPLEMENTAL MATERIAL
FIG. S1 LPS phenotype analysis. Chromosomal knockout mutants of wzx, wzy, and waaL
in P. aeruginosa PAO1 are all unable to produce B-band LPS; however, the waaL mutant
has been shown to produce full-length B-band O-Ag linked to Und-PP, as the biosynthetic
machinery leading to the ligation step of the B-band LPS assembly pathway is still
functional. Cells of wild-type P. aeruginosa PAO1 as well as individual wzx, wzy,
and waaL chromosomal knockout mutants, containing either empty pHERD26T or the respective
complementation GFP fusion vectors, were grown as described in Materials and Methods.
LPS samples were prepared and visualized as previously described (19). (A) SDS-PAGE
and silver-staining analysis of LPS prepared from wzx, wzy, and waaL knockout mutants
as well as wild-type (strain PAO1) and complemented strains. WT, wild-type PAO1; (−),
mutant strain lacking plasmid; E, mutant control containing pHERD26T empty vector;
(+), mutant strain complemented with the respective pHERD26T clone expressing a C-terminal
GFP fusion to Wzx, Wzy, or WaaL. (B) Western immunoblot for visualizing the presence
of B-band O-Ag by use of monoclonal primary antibody MF 15-4, produced by our group
in a previous study (J. S. Lam, L. A. MacDonald, M. Y. Lam, L. G. Duchesne, and G.
G. Southam, Infect. Immun. 55:1051-1057, 1987). Secondary antibody treatment was carried
out using an anti-mouse goat F(ab′)2–alkaline phosphatase conjugate (Jackson ImmunoResearch)
diluted to 1:2,000. Download
FIG. S2 PSIPRED secondary-structure analysis of large cytoplasmic (CL) and periplasmic
(PL) loop domains from Wzx (X), with three residues from each flanking TMS included.
As seen from the analysis of the potential secondary structure of CL1X, CL3X, and
CL5X, an elaborate secondary structure is predicted to exist on the cytoplasmic face
of the protein. Download
FIG. S3 PSIPRED secondary-structure analysis of large cytoplasmic (CL) and periplasmic
(PL) loop domains from Wzy (Y), with three residues from each flanking TMS included.
CL2Y contains a tract of amino acids similar to the R/KXXXGXXXLhhhhD Walker B consensus
motif (J. E. Walker, M. Saraste, M. J. Runswick, and N. J. Gay, EMBO J. 1:945–951,
1982). PL3Y and PL5Y contain similar tracts of residues, and as such, each possesses
bihelical propensities. Download
FIG. S4 PSIPRED secondary-structure analysis of large cytoplasmic (CL) and periplasmic
(PL) loop domains from WaaL (L), with three residues from each flanking TMS included.
CL3L is the candidate motif for the observed ATPase activity of the protein; this
motif contains the propensity for a structure similar to a conventional Walker A domain,
in which an uncapped α helix is preceded by a Gly-rich loop motif (P. M. Jones and
A. M. George, FEMS Microbiol. Lett. 179:187–202, 1999). PL5L is essential for WaaL
function (P. D. Abeyrathne and J. S. Lam, Mol. Microbiol. 65:1345–1359, 2007) and
contains both rigid α-helical and β-stranded character properties as well as a degree
of flexibility in the intervening regions. PL6L contains several charged residues
and appears to be very flexible. As such, this loop, adjacent to PL5L, may also be
important for the ligation of full-length O-Ag to the lipid A core. Download
FIG. S5 Helical-wheel diagrams of TMS from Wzx displaying charge properties as well
as RHYTHM server analysis of potential helix-helix and helix-membrane contact points
(A. Rose, S. Lorenzen, A. Goede, B. Gruening, and P. W. Hildebrand, Nucleic Acids
Res. 37:W575–W580. 2009). The RHYTHM server was run at “very high” confidence, with
output generated using the “channels” position-specific matrix. The residue fill color
key represents charge properties: orange, nonpolar; green, polar and uncharged; red,
acidic; blue, basic. The residue outline color key for RHYTHM analysis includes pink
(predicted helix-helix contact) and purple (predicted helix-membrane contact). All
helices are oriented as if looking downwards from the periplasm, with residue size
decreasing/increasing accordingly from the starting arrow (→). In particular, TMS
X8, 10, and 12 possess charged/highly polar faces with no potential membrane-helix
contacts. Download
FIG. S6 ClustalW alignment of PL3Y and PL5Y from Wzy from P. aeruginosa PAO1, including
three residues from each flanking α-helical TMS. Color key: blue, identical; teal,
high structural equivalence; grey, loose structural equivalence. Both loop regions
contain identical and structurally equivalent amino acids, alluding to a common motif
between the two. Download
TABLE S1 Normalized activities of AP and BG Wzx truncation fusions to PhoA-LacZα.
TABLE S2 Normalized activities of AP and BG Wzy truncation fusions to PhoA-LacZα.
TABLE S3 Normalized activities of AP and BG WaaL truncation fusions to PhoA-LacZα.
TABLE S4 Protein sequence motifs identified in this study.