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1 Institute of Medical Sciences, University of Toronto, Toronto, ON, Canada
2 Molecular Structure and Function Program, Hospital for Sick Children Research Institute, Toronto, ON, Canada
3 Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, Canada
4 Department of Biochemistry, University of Toronto, Toronto, ON, Canada
Correspondence
Lori L. Burrows
burrowl{at}mcmaster.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). Of these, the ATPases of the T2S and T4P systems are more closely related to one another than to the T3S or T4S ATPases; the phylogeny of T2S and T4P protein families is reviewed by Planet et al. (2001). The T4P system is typically associated with adherence and surface-associated ‘twitching’ motility, but in some bacteria has also been shown to participate in protein secretion (Forsberg & Guina, 2007; Hager et al., 2006; Kirn et al., 2003; Kirn & Taylor, 2005; Kennan et al., 2001; Han et al., 2007).
Unlike the T2S and T4S systems, which have a single motor ATPase, the T4P system has two, and sometimes three, motor ATPases (Burrows, 2005). The two-protein systems typically have an extension ATPase (pilin polymerase) and a retraction ATPase (pilin depolymerase). In three-protein systems, such as that found in the opportunistic and nosocomial pathogen Pseudomonas aeruginosa, the typical extension and retraction ATPases (PilB and PilT, respectively, in P. aeruginosa) are accompanied by a PilT paralogue called PilU (Mattick, 2002; Whitchurch et al., 1991; Whitchurch & Mattick, 1994; Turner et al., 1993). The larger (62 kDa) PilB protein is most similar to T2S ATPases, while the smaller PilT (38 kDa) and PilU (42 kDa) proteins are peculiar to T4P systems (Planet et al., 2001). In many bacteria, including P. aeruginosa, T4P mediate twitching motility, a form of surface-associated translocation in which movement occurs via cycles of pilus assembly and disassembly (Skerker & Berg, 2001), thought to require ATP hydrolysis by PilB and PilT, respectively. The role of the PilT paralogue PilU in twitching motility is not clearly understood, but loss of this protein results in the unusual phenotypic combination of pilus-specific bacteriophage susceptibility (typically associated with functional, retractable pili) and loss of twitching motility (typically associated with loss of pilus extension or pilus retraction) (Whitchurch & Mattick, 1994).
The bacterial secretion NTPases are members of the P-loop NTPase family, which can be separated into two principal classes: the kinase–GTPase (KG) division and the additional strand, catalytic E (ASCE) division (Iyer et al., 2004). ASCE division members, including the bacterial secretion NTPases, use a conserved glutamic acid residue to abstract a proton from water for nucleophilic attack on the 
-phosphate of the ATP ligand. ATP hydrolysis results in conformational changes within the protein, translating chemical energy into mechanical events that result in folding/unfolding or assembly/disassembly of protein complexes. All P-loop NTPases are characterized by highly conserved Walker A (WA; GxxGxGKT/S) and Walker B (WB; hhhhDE) motifs (Walker et al., 1982). The WA residues form an extensive hydrogen-bonding network with the phosphate tail of the ATP substrate, while the WB motif participates in the coordination of the Mg2+ co-factor required for ATP hydrolysis (Robien et al., 2003; Yeo et al., 2000).
Secretion NTPases of the T2S and T4P systems, including PilB, PilT and PilU, function as toroidal homohexamers and, in addition to WA and WB motifs, contain unique Asp Box and His Box motifs (Planet et al., 2001). Site-directed mutagenesis of the WA motif has been used before to inactivate PilB and PilT from P. aeruginosa (Aukema et al., 2005; Turner et al., 1993), but similar studies of PilU have not, to our knowledge, been reported. While the functions of the WA and WB motifs are well characterized, those of the Asp and His Box motifs remain largely unexplored, despite their broad conservation in the secretion NTPase family. Based on limited structural and mutagenesis data, residues of the Asp and His Boxes are likely to contribute to nucleotide binding and/or sensing as well as subunit–subunit interactions (Satyshur et al., 2007; Robien et al., 2003). In this work we demonstrate that the three motor proteins in P. aeruginosa are bona fide ATPases, and we use a site-directed mutagenesis approach to show that for their function all three require specific invariant residues within the characteristic secretion NTPase motifs. Due to their wide conservation among these proteins, we focussed specifically on the three acidic residues within the Asp Box motif, and the two His within the eponymous His Box motif, providing experimental support for hypotheses regarding the function of these residues.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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. Both Escherichia coli and P. aeruginosa were grown on 1.5 % Luria agar plates or in Luria broth, containing, where appropriate, antibiotics at the following concentrations: tetracycline, 15 µg ml–1 for E. coli and 50 µg ml–1 for P. aeruginosa; gentamicin, 15 µg ml–1 for E. coli and 30 µg ml–1 for P. aeruginosa. For twitching assays, the agar concentration was reduced to 1 %.
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Protein expression and purification.
The pET28a expression vectors were transformed into E. coli BL21 Codon Plus (Stratagene) and used to inoculate a 5 ml LB–Kan (50 µg kanamycin ml–1) culture. The overnight culture was used to inoculate 1 l LB–Kan and the cells were allowed to grow at 37 °C until OD600 reached 0.6–0.7, at which time protein expression was induced by adding IPTG to a final concentration of 0.5 mM. The temperature was then decreased to 16 °C and, after overnight incubation, the cells were harvested by centrifugation at 18 000 g for 30 min at 4 °C. Cells were stored at –20 °C until required.
The frozen cells were resuspended in 40 ml buffer A (20 mM Tris/HCl, pH 8.5, 150 mM sodium citrate) and one Complete EDTA-free protease inhibitor cocktail tablet (Roche) was added prior to cell lysis. The cell suspension was sonicated for seven 30 s pulses, with 60 s cooling on ice between each pulse, then centrifuged at maximum speed in a microcentrifuge for 30 min at 4 °C. The supernatant was mixed with 2 ml Ni–NTA agarose (Qiagen) at 4 °C for 2 h. The resin was then packed into a column and washed twice, first with buffer A containing 5 mM imidazole and subsequently with the same buffer containing 20 mM imidazole. The protein was eluted from the column in two fractions. The first fraction, eluted with buffer A and 100 mM imidazole, contained some protein impurities, while the second fraction, eluted with buffer A and 300 mM imidazole, yielded protein of higher purity, as estimated by SDS-PAGE (not shown). The second fraction was dialysed overnight at 4 °C in buffer A and concentrated to a final volume of approximately 1 ml. The protein concentration was determined using the Bradford assay (Pierce) with BSA (Pierce) as the standard. The protein was used immediately in the ATPase assays. To facilitate the interpretation of the results, the same purification protocol and buffer conditions (20 mM Tris/HCl, pH 8.5, and 150 mM sodium citrate) were used for all proteins.
ATPase assays.
ATP hydrolysis was monitored spectrophotometrically using the EnzCheck phosphate assay kit (Invitrogen). The inorganic phosphate released during ATP and 2-amino-6-mercapto-7-methyl-purine riboside (MESG) hydrolysis are converted by purine phosphorylase (PNP) to ribose 1-phosphate and 2-amino-6-mercapto-7-methylpurine, which results in an absorbance shift from 330 to 360 nm. The assay was performed in triplicate for each protein. The 500 µl reaction mix contained between 9 and 95 µg purified protein, 25 µl 20x reaction mix (1 M Tris/HCl, pH 7.5, 20 mM MgCl2) supplemented with 5 mM MgCl2, 0.2 mM MESG, 1 U PNP and 1 mM ATP. After incubation for 30 min at 24 °C the A360 was determined. The results were normalized for non-enzymic ATP degradation by subtracting the reading from that of a control reaction containing ATP but no ATPase. BSA was used as a negative control. The total amount of phosphate released was then determined by comparing the normalized absorbance of the protein sample to a phosphate standard curve.
Site-directed mutagenesis of conserved residues in PilB, PilT and PilU.
Single amino acid replacement of conserved residues in the WA (G135S), Asp Box (E159Q, D160N, E163Q), Walker B (E204Q) and His Box (H222A, H229A) motifs of PilT, using our previously generated yfp–pilT construct (Chiang et al., 2005) as a template, were performed with the QuikChange mutagenesis kit (Stratagene) as recommended by the manufacturer. Oligonucleotide sequences used are summarized in Table 2. Briefly, all PCR reactions were performed at 55 °C annealing for 50 s and 68 °C extension for 20 min, over a total of 18 cycles. Similar constructs with mutations G135S, E159Q, D160N, E163Q, Q167E, E204Q, H222A and H229A were made in the PilU component of the previously generated yellow fluorescent protein (YFP)–PilU fusion construct (Chiang et al., 2005).
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), precluding functional analyses of the fusion. Therefore, an untagged version of PilB expressed from the same vector as the YFP–PilT and YFP–PilU fusions was mutated at the equivalent WA (G331S), Asp Box (E354Q, D355N, E358Q), WB (E396Q) and His Box (H414A, H421A) residues. The integrity of each mutant construct was verified by DNA sequence analysis. Plasmid constructs were transformed into chemically competent E. coli for amplification and purification and subsequently introduced into relevant P. aeruginosa strains (mutant and wild-type) by electroporation (see below). For assays of the ATPase activity of mutant PilT proteins, the same mutagenic primers were used to mutagenize the PilT pET28a construct (described above), and the mutant proteins were expressed, purified and assayed as described above.
Complementation of motor protein mutants.
Wild-type or mutant strains of P. aeruginosa were streaked for single colonies on LB agar plates. Three to five colonies were washed and resuspended in 1 ml sterile distilled water, and 200 µl of the suspension was immediately transformed with 100–150 ng DNA using a 0.2 cm path-length electroporation cuvette (EquiBio cat. no. ECU-102) and the E. coli Pulser device (Bio-Rad). Transformants were selected on LB plates containing appropriate antibiotics at 37 °C overnight.
Twitching motility assays.
Assessment of twitching motility was performed as described elsewhere (Semmler et al., 1999), and the resulting zones of twitching motility were visualized by carefully removing the agar and staining the bacteria adhering to the polystyrene Petri plate with 1 % crystal violet for 10 min at room temperature, followed by a brief rinse with tap water to remove unbound dye. ImageJ software (NIH) was used to measure and calculate average areas of the resulting twitching zones to acquire quantitative comparative data.
Western blot analysis to assess protein stability.
Overnight cultures of the P. aeruginosa pilB, pilT or pilU mutants carrying the cognate complementation constructs were standardized to OD600=0.6. A 200 µl volume of each of the suspensions was centrifuged at maximum speed in a microfuge to harvest the cells. The cells were lysed in 150 µl 1x SDS-PAGE sample buffer and 5 µl lysate was separated on a 12.5 % SDS-PAGE minigel, then transferred to nitrocellulose. A second gel was stained with Coomassie brilliant blue to verify that equivalent amounts of lysate were loaded in each lane. The blots were blocked with 5 % skimmed milk in sterile PBS, pH 7.4, at 4 °C overnight. The proteins of interest were detected using a rabbit polyclonal antibody to PilB or a mAb to GFP (JL-8, Clontech) for the YFP–PilT and YFP–PilU fusions. Both primary antibodies were used at a 1 : 5000 dilution in PBS for 1 h at room temperature. The secondary antibodies were goat anti-rabbit or goat anti-mouse alkaline phosphatase conjugates, also used at 1 : 5000 dilution in PBS for 1 h at room temperature. The blots were developed with nitro blue tetrazolium and 5-bromo-4-chloro-3-indolyl phosphate, as recommended by the manufacturer (Sigma).
Analysis of sheared surface proteins.
Surface proteins (mainly flagella and pili) were isolated using the methods of Castric (1995), with modifications. Bacteria were streaked in a grid pattern on LB agar plates containing 30 mg gentamicin l–1 and 0.2 % (w/v) L-arabinose, and incubated overnight at 37 °C. One or two plates per sample were used. The bacteria were gently scraped from the agar surface using a sterile coverslip and resuspended in 2 ml sterile PBS (pH 7.4) per plate, and surface proteins were sheared by vigorous vortexing for 30 s. The suspension was transferred to two 1.5 ml microcentrifuge tubes and centrifuged for 5 min at maximum speed to pellet the cells, which were retained for whole-cell-lysate Western immunoblots (below). The supernatant was transferred to a new tube and centrifuged for an additional 25 min at maximum speed at room temperature. To precipitate the sheared proteins, one-tenth volume each of 5 M NaCl and 30 % (w/v) PEG (molecular mass 
8000 Da) were added to the supernatant, and the samples were incubated on ice for 60 min. Samples were centrifuged at maximum speed in a microcentrifuge for 25 min at 4 °C. After discarding the supernatant, the resulting pellets were resuspended in 2x SDS-PAGE loading dye [125 mM Tris, pH 6.8, 2 % (w/v) 2-mercaptoethanol, 20 % (v/v) glycerol, 0.001 % (w/v) bromophenol blue, 4 % (w/v) SDS], boiled for 5 min and resolved on a 15 % 1D SDS-PAGE minigel with a pre-stained Benchmark Protein Ladder (Invitrogen). The proteins were visualized using Coomassie blue dye.
Fluorescence microscopy.
Single colonies were streaked on 1.5 % LB agar plates with the appropriate antibiotics then incubated for 18 h at 37 °C. The samples were prepared by mixing a single colony with 10 µl PBS (pH 7.1) on a glass slide using a sterile toothpick. Polylysine-coated coverslips (Sigma Diagnostics P8920) were used to mount the cells for photography. Using fluorescence illumination, slides were viewed at x100 magnification (oil immersion) with a Zeiss Axiovert 200M microscope. Open Lab V 3.1.3 (Improvision) software was used to collect fluorescence images. Images were processed with PhotoShop 7.0.1 (Adobe).
Bacteriophage susceptibility assay.
Bacterial strains to be tested for phage susceptibility were streaked in a single line on an LB plate and 1 µl of a PO4 bacteriophage suspension, containing approximately 109 p.f.u. ml–1, was spotted onto the centre of the streak. The plates were subsequently incubated at 37 °C for 18 h. Phage-resistant strains showed no disruption of growth above and below the point of phage inoculation, while susceptible strains showed no growth in the centre of the streak, due to lysis of the cells by the bacteriophage.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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75 % over 240 aa) between the C-terminal ATPase domains of PilT and PilU, a stable bipolar chimera containing the N terminus of PilT fused to the C terminus of PilU could not complement twitching in either pilT or pilU mutants (Chiang et al., 2005). This result suggested that the C terminus of PilU does not contain a functional ATPase domain. Although PilT proteins from Aquifex aeolicus, P. aeruginosa and Neisseria gonorrhoeae have been demonstrated elsewhere to have ATPase activity (Aukema et al., 2005; Herdendorf et al., 2002; Maier et al., 2002), the ATPase activities of PilB and PilU have not been assayed previously. Therefore, we sought to test the ATPase activities of purified PilB, PilT and PilU in vitro. P. aeruginosa PilB, PilT and PilU were N-terminally tagged with 6xHis, expressed in E. coli and purified by affinity chromatography. The amount of protein obtained in the soluble fraction was sufficient to allow their enzymic characterization using an assay similar to the one performed for Legionella pneumophila DotB (Sexton et al., 2004). PilB, PilT and PilU all exhibited low levels of ATPase activity, with the highest specific activity, 10.79±1.09 nmol Pi min–1 (mg protein)–1, observed for PilB, and the lowest, 5.02±0.78 nmol Pi min–1 (mg protein)–1, for PilU. PilT had specific activity of 7.43±0.14 nmol Pi min–1 (mg protein)–1, intermediate between PilB and PilU (Table 3). These in vitro activities are consistent with those reported for other purified bacterial secretion ATPases, including EspE at 5.6 nmol min–1 (mg protein)–1, PilT from Aquifex at 15.7±0.9 nmol min–1 (mg protein)–1, DotB from L. pneumophila at 6.4 nmol min–1 (mg protein)–1 and TrwD from plasmid R388 at 4.5 nmol min–1 (mg protein)–1 (Sexton et al., 2004; Aukema et al., 2005; Herdendorf et al., 2002; Rivas et al., 1997; Camberg & Sandkvist, 2005).
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), allowing us to use it as a model for the P. aeruginosa motor proteins. The A. aeolicus PilT protein (PilTAa), as well as related T2S ATPases EpsE (Vibrio cholerae) and HP0525 (Helicobacter pylori), have a characteristic structure composed of globular N- and C-terminal domains connected by a highly flexible linker domain (Satyshur et al., 2007; Robien et al., 2003; Savvides et al., 2003). The C-terminal domains containing the NTPase motifs are highly conserved among the motor proteins, allowing us to use the ATP-bound form of PilTAa and the AMP–PNP-bound form of EpsE as structural models with confidence. Examination of the seven genomes of P. aeruginosa currently available revealed that the PilT and PilU proteins are 100 % identical at the amino acid level in every strain (not shown). The PilB proteins are also highly conserved (94–100 % identical to PilB from laboratory strain PAO1), with most amino acid differences in the N-terminal domain.
To dissect the roles of the secretion NTPase motifs in motor protein function, site-directed mutagenesis of conserved residues in WA, WB, Asp and His Boxes (indicated in Fig. 1a) was performed on YFP–PilT, YFP–PilU and untagged PilB, and the ability of the mutant proteins to complement twitching motility in the cognate P. aeruginosa mutants was tested. Use of the mutated YFP–PilT and –PilU fusion proteins for complementation of their respective mutants allowed the simultaneous detection of protein localization and function in vivo. Since the YFP-tagged PilB construct used previously was not functional (i.e. could not complement a pilB mutant for twitching motility; Chiang et al., 2005), we performed mutagenesis on an untagged version of the protein. Of particular interest were the three acidic residues within the Asp Box motif, and the two invariant His residues within the eponymous His Box motif. The WA Gly residue selected for mutation has been shown previously to inactivate both PilB and XcpR, the T2S NTPase in P. aeruginosa (Turner et al., 1993), while the WB Glu selected for mutagenesis is the key catalytic residue for ASCE family NTPases (Iyer et al., 2004). The positions of the targeted residues in the P. aeruginosa ATPases relative to the ligand are indicated in Fig. 1(b).
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), with the single exception of the PilB H414A mutant, which appeared to have a small deletion or truncation not due to DNA sequence alterations, and was therefore not further examined. The results of twitching motility and polar localization assays (for PilT and PilU) of pilB, pilT and pilU mutant strains complemented with their respective wild-type or mutated constructs are summarized in Table 4.
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; Yeo et al., 2000). In contrast, the functions of the conserved Asp and His Box motifs are less clear. The role of these motifs in the in vivo function of the P. aeruginosa ATPases was tested using a twitching motility assay. Mutation of the WA or WB motif resulted in loss of the ability to restore twitching motility for all three proteins (Fig. 3a).
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. Mutation of the first residue (Glu) affected the function of all three proteins, as determined by decreased twitching motility. The PilB E354Q mutant had the most severe deficit, with twitching areas reduced to 34 % of those of the positive control (unmodified protein), while the PilT and PilU mutants were less affected, with 62 and 46 % of the control twitching level, respectively (Fig. 3b). Mutation of the conserved Asp residue (Asp Box 2) had a limited effect on the function of PilB, twitching areas corresponding to 82 % of that of the control. This residue is naturally mutated to Asn in the PilB protein of P. aeruginosa PA7, which is able to twitch (not shown), supporting the minor effect of this mutation on function. In contrast, mutation of this Asp residue significantly impaired the function of PilU, with only 18 % of control twitching (Fig. 3b). Twitching motility in the strain complemented with the PilT D160N mutant was approximately half that of the control. The third of three conserved acidic residues, corresponding to E176 of PilTAa and E298 of EpsE (Fig. 1b), was essential for twitching motility in all three proteins (Fig. 3).
Mutating the first of two invariant His residues in the His Box motif resulted in PilB truncation when examined by Western blotting (Fig. 2), and this mutant was not further evaluated. PilT and PilU mutant proteins were stable but not markedly affected by this mutation; twitching in the YFP–PilT H222A mutant was reduced by approximately 50 % relative to the positive control (Fig. 3a). In contrast, the second His residue was essential for twitching motility in all three T4P motor proteins (Fig. 3a).
Effect of ATPase mutations on surface piliation
Loss of twitching motility can arise from lack of pilus assembly, or from loss of pilus retraction. SDS-PAGE analysis of sheared surface protein preparations from each mutant was performed to determine whether the twitching motility defects observed could be correlated with changes in the levels of surface piliation (Fig. 4). The results for the PilB mutants correlated well with the observed twitching phenotypes; of the seven mutations generated, only the D355N mutant retained significant twitching motility (Fig. 3), and only this strain had levels of surface pilin comparable to those of the strain complemented with the unmodified pilB allele. The PilT and PilU mutants are unable to retract their pili and thus appear hyperpiliated. Complementation with mutant PilT or PilU proteins unable to support twitching motility resulted in large amounts of surface pili, which were generally reduced in strains carrying alleles that support twitching (Fig. 4). For reasons that are not clear, the PilU E204Q (WB) mutant had approximately wild-type levels of surface pili but was not able to twitch, similar to the pilU mutant strain.
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). Complementation of the phage-resistant pilB and pilT mutants with modified proteins that supported twitching motility resulted in concomitant restoration of phage susceptibility in all cases (Table 4). Similarly, complementation of the pilB and pilT mutants with constructs that did not restore twitching resulted in continued phage resistance. The complemented pilU mutants exhibited phage susceptibility regardless of their twitching phenotype, consistent with the fact that the non-twitching pilU mutant continues to be phage-susceptible.
Effect on ATPase activity of mutations that abrogate twitching in all three ATPases
To determine whether those specific mutations that caused complete loss of twitching motility in all three motor proteins were associated with loss of ATPase activity in vitro, we used PilT as a model protein. Site-directed mutagenesis was used to introduce the G135S (WA), E163Q (Asp Box 3), E204Q (WB) and H229A (His Box 2) mutations into the E. coli expression construct encoding His-tagged P. aeruginosa PilT. The mutated proteins were expressed and purified, and their ATPase activities tested as described above. Compared to the wild-type PilT protein, all four mutant proteins showed reduced or undetectable ATPase activities (Table 3), confirming that loss of complementation in vivo was correlated with reduced ATPase activity in vitro.
Subcellular localization of mutant YFP fusions
We have demonstrated previously that YFP–PilT exhibits bipolar localization, while the localization of YFP–PilU is unipolar (Chiang et al., 2005). All mutant fusion proteins tested retained their characteristic pattern of polar localization, with one exception. The WA mutant of YFP–PilT exhibited delocalized fluorescence in the pilT mutant background, while the same mutation in YFP–PilU did not affect unipolar localization in the pilU mutant background (Fig. 5). When the WA mutant of YFP–PilT was introduced into the wild-type, the non-functional fusion protein was localized to both poles, similar to the wild-type pattern (Fig. 5). These results are consistent with those for the truncated constructs we reported previously (Chiang et al., 2005), which retained their specific patterns of polar localization even when the secretion NTPase motif regions were deleted. Because these data suggest an interaction between active and inactive subunits, all mutated constructs unable to complement twitching motility in their cognate mutant background were introduced into the PAK wild-type strain to test for dominant-negative effects. None of the mutant constructs impaired twitching motility in the wild-type strain (data not shown).
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DISCUSSION |
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). Zinc would not be predicted to enhance the activity of the P. aeruginosa PilT and PilU proteins, as they lack the tetracysteine motif required for its coordination (Crowther et al., 2005; Robien et al., 2003). We have shown previously that PilB requires PilC for polar localization (Chiang et al., 2005); however, it has not yet been determined which region(s) of P. aeruginosa PilC interact with PilB, nor have potential partners for PilT and PilU yet been identified.
The demonstration that PilB, PilT and PilU have ATPase activity in vitro was consistent with the loss of function of each protein (as measured by the inability to complement twitching motility, and for PilT, loss of activity) when either of the WA or WB motif was mutated. WA mutations in the related proteins PilQ of the R64 thin pilus system and DotB of Legionella pneumophila (T4S) have been shown to be dominant-negative, whereas VirB11 (T4S) WA mutants have not (Bhattacharjee et al., 2001; Sakai et al., 2001; Stephens et al., 1995). Turner et al. (1993) have shown that a WA mutant of XcpR (P. aeruginosa T2S ATPase) causes dominant-negative effects on secretion in the wild-type, but that the identical mutation in PilB does not affect the presence of external T4P in a wild-type strain, as assessed by electron microscopy. This finding was confirmed by our work, as neither WA nor WB mutants of PilB impaired twitching motility when expressed in the wild-type. Similarly, PilT and PilU proteins with mutations in these motifs did not affect twitching in wild-type transformants.
These results suggest that only a subset of monomers in any of the three ATPases need be functional for pilus extension and retraction to occur. Recent studies of BfpD have shown that while all six members of a BfpD hexamer bind ATP, only alternating subunits interact with the BfpE peptide (Crowther et al., 2005), a finding that is consistent with current models of the pilus as a three-start helix (Ramboarina et al., 2005; Craig et al., 2004, 2006). Structural studies of A. aeolicus PilT have revealed an interesting intermediate quaternary structure that contains a combination of subunits in partially open, open and closed conformations (Satyshur et al., 2007), showing that heterogeneity in the conformational state of individual subunits within the hexameric assembly is likely to be a common feature of these proteins.
The WA mutant of YFP–PilT, but not YFP–PilU, delocalized from the cell pole. Although the factors required for the bipolar localization of PilT are unknown, it is conceivable that substrate binding is required for correct interaction with the hypothetical polar retention protein or complex. Shiue et al. (2006) have found that binding of ATP by the T2S ATPase XpsE (a PilB homologue) is necessary for its oligomerization and subsequent interaction with its membrane anchor protein, XpsL. In the absence of ATP, or upon introduction of mutations that abrogate the binding of ATP, XpsE is monomeric and does not associate with XpsL. Other ATPases in the T4S family, such as HP0525 or TrbB (from conjugative plasmid RP4), have been observed to undergo ATP-independent oligomerization (Savvides et al., 2003; Krause et al., 2000), suggesting that this step can precede ligand binding, allowing the formation of localization-competent hexamers. It is possible that substrate binding is required for efficient oligomerization of PilT and that the delocalized cytoplasmic fluorescence arose from non-interacting monomers, though this was not tested directly. YFP–PilT WA mutant proteins expressed in the wild-type were correctly localized (Fig. 5c), suggesting that formation of stable mixed oligomers occurs despite the lack of dominant-negative effects on twitching motility, supporting the premise that only a subset of monomers need be active for twitching motility to occur.
Although the Asp and His Box motifs are widely conserved in the secretion NTPase family, their specific contributions to protein function have not been extensively investigated. In this work, we focused on the invariant acidic residues in the Asp Box motif, and on the invariant His residues in the His Box motif. We examined the molecular environment of the corresponding conserved residues in the crystal structures of the A. aeolicus PilT protein (Satyshur et al., 2007), and the T2S ATPase EpsE (Robien et al., 2003). The first invariant Asp Box 1 residue is E172 in PilTAa and E292 in EpsE. In EpsE, this amino acid is less than 3. Å (0.32 nm) from several residues, including V307, Q304 and Y275. Interestingly, all three of these residues are conserved in PilB but not PilT or PilU, and mutation of Asp Box 1 has more profound effects on PilB than on the other two proteins. The Y275 residue is close enough to form a hydrogen bond with the main chain nitrogen of the essential E296 residue (Asp Box 3 in Fig. 1b).
The second conserved Asp residue (Asp Box 2; D173 in PilTAa, D293 in EpsE, Fig. 1b) is required in all three twitching ATPases for normal twitching motility, although PilU function is most severely affected by its mutation. Site-directed mutagenesis of the corresponding Asp residue in the T2S protein PulE results in reduced secretion, while mutation of the Asp residue in R64 PilQ reduces both ATPase activity and mating efficiency in liquid (Sakai et al., 2001; Possot et al., 1992). Examination of the EpsE structure shows that the D293 residue forms a salt bridge to R336, stabilizing its guanidinium moiety. This Arg, which is conserved among all three twitching ATPases, interacts with the carboxylate of E334 in the WB motif, which was shown in this work to be essential for the function of all three ATPases. Disruption of the orientation of the WB Glu could cause the observed reductions in activity, with differences in the local environments in the three proteins modulating the severity of the mutation.
The third of three invariant acidic residues in the Asp Box (corresponding to E176 in PilTAa and E296 in EpsE, Fig. 1b) was essential for twitching motility in all three T4P AAA+ proteins, and its mutation markedly reduced the in vitro ATPase activity of PilT. The corresponding residue in the related RecA protein (Glu96) has been proposed to activate an attacking water molecule during ATP hydrolysis (Story & Steitz, 1992). In both EpsE and PilT, this Glu is closest to the -phosphate of the ligand (AMP–PNP or ATP, respectively), while in the ADP-bound structure of PilT, there is a water molecule in the approximate position of the -phosphate, less than Å (0.4 nm) from E176 (Satyshur et al., 2007). The position of this residue and its requirement for the functioning of all three proteins tested suggest that it participates in catalysis.
Mutation of the first of two invariant His residues in the His Box motif resulted in a truncated PilB protein, precluding further analysis of its function in that protein. The YFP–PilT H222A mutant had twitching motility reduced by approximately 50 % relative to the unmodified protein (Fig. 3a), while YFP–PilU was not significantly impaired by this mutation. An H222R mutation in PilTPa has been shown by Satyshur et al. (2007) to abrogate twitching motility altogether. This residue is oriented away from the ligand-binding site and probably participates in subunit–subunit interactions (Fig. 6). Because the interaction surfaces of each ATPase in its hexameric state are formed in part by the less-conserved N-terminal domains Robien et al. (2003), mutation of this residue may have less impact in some proteins than others due to differences in local environment. The smaller, non-polar Ala side chain substituted here appears less perturbing to PilT activity than the large positively charged Arg side chain used by Satyshur et al. (2007).
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). This proximity makes it possible for His Box 2 to act as a phosphate sensor, as has been proposed for other ATPases such as RecA (Story & Steitz, 1992), with rotation of the side chain occurring upon hydrolysis. Mutation of the corresponding residue in plasmid R64 PilQ (a PilB homologue) reduced ATPase activity and DNA transfer efficiency; the other His residue was not mutated in that study (Sakai et al., 2001).
Of the three T4P motor proteins (PilB, PilT and PilU) in P. aeruginosa that are required for twitching motility, the function of PilU remains the most enigmatic. From previous studies of the function of PilB and PilT in P. aeruginosa and other bacteria, it is clear that they play antagonistic roles in the extension and retraction of T4P. In contrast, PilU and its homologues are less commonly identified in T4P-expressing bacteria, suggesting that it is not a universal component of the T4P apparatus (Chiang et al., 2005). In P. aeruginosa, pilU mutants appear hyperpiliated, similar to pilT mutants; therefore, PilU is thought to be involved in pilus retraction. The inability of WA and WB mutants of PilU to complement twitching motility in a pilU mutant is consistent with in vitro assays that show that purified PilU has ATPase activity. Therefore, we conclude that P. aeruginosa requires three distinct, functional ATPases for twitching motility to occur. However, the continued sensitivity of pilU mutants to phage killing suggests that unlike pilT mutants, they are capable of some level of pilus retraction (mediated by PilT), although it is not sufficient to enable twitching motility to occur.
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ACKNOWLEDGEMENTS |
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Edited by: J. G. Shaw
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Received 18 July 2007;
revised 20 September 2007;
accepted 21 September 2007.
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null mutant, as well as the pilB mutant complemented with an unmodified or mutant copy of pilB in trans, were probed with a rabbit polyclonal antibody to PilB. The levels of PilB expressed from the plasmid are similar to those observed in the wild-type. YFP–PilT, YFP–PilU and mutant variants thereof were probed with a mAb to YFP (Clontech). M, 72 kDa molecular mass marker.
