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Journal of Bacteriology, February 2004, p. 1215-1219, Vol. 186, No. 4
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.4.1215-1219.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Institute of Microbiology, ETH Zürich, 8092 Zürich, Switzerland
Received 2 September 2003/ Accepted 11 November 2003
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-helix and are often encoded next to or in close vicinity to the effector protein (9, 17). In contrast, the chaperone Spa15 of Shigella spp. is encoded within an operon encoding essential components of the TTS apparatus and binds to not just one but several effector proteins which do not show sequence similarities (16). Due to these special features, Spa15 is thought to represent a new class of TTS chaperone (16, 17). The Salmonella pathogenicity island 1 (SPI-1) of Salmonella enterica serovar Typhimurium encodes the protein InvB, which is homologous to Spa15 of Shigella spp. InvB is a chaperone for the SPI-1-encoded effector SipA/SspA (2). Recently members of our group have shown that InvB also binds to SopE and SopE2, two effector proteins encoded outside of SPI-1 but secreted in a SPI-1-dependent manner (7a). Secretion and translocation of SipA, SopE, and SopE2 depend on InvB. Based on this observation, we hypothesized that InvB might be required for secretion of additional effector proteins of serovar Typhimurium.
To address this question, we expressed a glutathione S-transferase (GST)-InvB fusion protein (pM672) (7a) in the mutant strain M574 (invB::aphT
sopE
sopE2
sopB
sipA) (7a), which lacks all known InvB binding effector proteins and the chromosomally encoded invB. This strain also lacks the effector protein gene sopB. However, SopB/SigD is transported via its own cognate chaperone, PipC/SigE (6). Therefore, the sopB mutation was not expected to affect any InvB-effector protein interactions. M574 (pM672) was grown overnight in Luria broth containing 0.3 M NaCl, diluted 1:20 into fresh medium, and grown for another 4 h at 37°C (referred to as SPI-1 inducing conditions). Cells were lysed in a French pressure cell, and GST-InvB and bound proteins were purified on glutathione-Sepharose beads from the cleared cell lysate. Aliquots from every step of the purification procedure were analyzed on a Coomassie brilliant blue-stained SDS gel. A polypeptide with an apparent molecular weight of 80 kDa was copurified with GST-InvB (Fig. 1). The band was excised from the gel, trypsin digested, and eluted as described recently (7a). The protein was identified by matrix-assisted laser desorption ionization-mass spectrometry fingerprint analysis as SopA (12 matching peptides, 21% covered sequence), a known effector protein, which is encoded outside of SPI-1 but translocated in a SPI-1-dependent manner (22). Although the biochemical activity of SopA is still unknown, it was shown to play a role in bovine enterocolitis models (22, 23).
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FIG. 1. Pull-down assay to isolate InvB binding proteins. GST-InvB (34 kDa) and bound proteins were purified from cleared cell lysate by incubation with glutathione (GSH)-Sepharose beads. Bound proteins larger than the GST-InvB fusion protein were analyzed by SDS-PAGE and Coomassie brilliant blue staining. wc, whole culture before harvesting of the cells; FP pe, resuspended pelleted cell debris after lysis using a French pressure cell; FP sup, cleared French pressure cell lysate; GSH sup, cleared cell lysate after binding of GST-InvB and its associated proteins; washing, supernatant after the first and seventh wash of the GSH-Sepharose beads; co-purified, GSH-Sepharose beads.
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TABLE 1. Strains and plasmids used in this work
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FIG. 2. InvB is coimmunoprecipitated with SopAM45. A lysate of the sopAM45-expressing strain M612 (top panel) was incubated with a monoclonal mouse anti-M45 ( -M45) antibody and protein A-Sepharose beads. Samples from the precipitation procedure were analyzed by Western blotting using a polyclonal rabbit anti-InvB ( -InvB) antiserum (recognizes InvB [15 kDa] and another unidentified 18-kDa Salmonella protein) and a mouse anti-M45 antibody (recognizes SopAM45). To demonstrate specificity, a coimmunoprecipitation experiment was performed with M712 (bottom panel) (Table 1). The 18-kDa protein cross-reacting with the anti-InvB antiserum was not coprecipitated in either strain, which confirmed the specificity of the coimmunoprecipitation experiment. Lane a, whole culture before harvesting the cells (wc); lane b, resuspended bacterial pellet (pe); lane c, cleared cell lysate after incubation with anti-M45 antibody and removal of nonspecific aggregates by centrifugation ( -M45 sup); lane d, supernatant after incubation with protein A-Sepharose beads (prot. A sup); lane e, supernatant after the fourth wash of the protein A-Sepharose beads (wash 4); lane f, proteins bound to the protein A-Sepharose beads.
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invB) (2), and M623 (invC::aphT), which all harbor an M45 epitope-tagged sopA gene in the chromosome (Table 1). The strains were grown under SPI-1 inducing conditions, and SopAM45 secretion was analyzed by Western blotting as described elsewhere (7a). SopAM45 was secreted from the wild-type strain M619 but not from the secretion-deficient strain M623 (invC::aphT), lacking the ATPase InvC (8), and the
invB strain M618 (Fig. 3, upper panel). The latter secretion defect could be complemented using the invB expression vector pM250, which expresses invB under control of an arabinose-inducible promoter (7a). Reprobing with a rabbit polyclonal antiserum raised against amino acids 49 to 543 of the SPI-1 effector protein SptP verified that the invB deletion had no general effect on the TTSS (Fig. 3, middle panel).
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FIG. 3. invB-dependent expression and secretion of SopAM45. Pelleted bacteria corresponding to 300 µl of culture and proteins recovered from 2 ml of culture supernatant of strains M619 (sopAM45), M623 (sopAM45 invC::aphT), M618 (sopAM45 invB), and M618/pM250 (sopAM45 invB pInvB) were analyzed by Western blotting using an anti-M45 ( -M45) antibody. The blot was reprobed with a polyclonal anti-SptP ( -SptP) antiserum to verify that a deletion of invB had no general effect on the TTSS and an anti-DnaK ( -DnaK) antiserum to confirm that the same amounts were loaded onto each lane.
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invB strain M618 than in the wild-type strain, M619 (Fig. 3). This indicated that InvB might play a role in stabilization or expression of SopA. To examine the cytoplasmic stability of SopAM45, one has to consider that InvB might have two functions: stabilization of cytoplasmic SopA and transport of SopA via the SPI-1 TTSS. If significant amounts of SopA protein become transported to the outside during the course of the assay, this fraction might become protected from degradation by Salmonella proteases. To exclude this, we have analyzed the role of invB in stabilization of cytoplasmic SopAM45 in secretion-deficient strains. The invB open reading frame is overlapping with the invC open reading frame. Therefore, it was not possible to combine the
invB and invC::aphT alleles (Fig. 3; Table 1) by P22 transduction. For this reason we constructed the secretion-deficient spaO::aphT strain (M629), lacking an essential subunit of the export apparatus (3) encoded 2.7 kb downstream of invB, and the
invB spaO::aphT double mutant (M630).
We then analyzed the cytosolic stability of SopAM45 in the
invB strain M618 (sopAM45
invB; does not secrete SopAM45 [Fig. 3]), the secretion-deficient mutant M629 (sopAM45 spaO::aphT), and the double mutant M630 (sopAM45 spaO::aphT
invB) (Table 1). As a control we also examined the cytosolic stability of SopAM45 in the wild-type strain M619 (sopAM45). M619, M618, M629, and M630 were grown under SPI-1 inducing conditions, and protein biosynthesis was inhibited by addition of spectinomycin (final concentration, 200 µg/ml). Aliquots were removed 0, 5, 20, 40, and 90 min after spectinomycin addition. Western blot analysis of bacterial pellets revealed that SopAM45 degradation was slightly accelerated in the absence of InvB (Fig. 4A, compare M629 and M630 or M618 and M629). In the wild-type strain background (M619), the amount of bacterium-associated SopAM45 was slightly higher than in the secretion-deficient strain M629 at the beginning of the experiment (0' to 20') but decreased faster (Fig. 4A). As discussed above, this is probably due to cumulative effects of secretion of SopAM45 from M619 into the culture supernatant and degradation. For this reason we could not base any conclusions about the role of InvB in SopA stabilization on this strain.
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FIG. 4. Effect of an invB deletion on the stability of SopAM45. Amounts of cytosolic SopAM45 at different time points after addition of spectinomycin were analyzed by Western blotting using a mouse anti-M45 ( -M45) and polyclonal rabbit anti-SptP ( -SptP) to verify that a deletion of invB had no general effect on the stability of effectors. To ensure equal loading of the lanes, the blot was reprobed using an anti-DnaK ( -DnaK) antibody.
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To verify that the invB expression level is not altered in the secretion-deficient mutants, we performed a Western blot analysis using the strains M619 (sopAM45), M618 (sopAM45
invB), M629 (sopAM45 spaO::aphT), M630 (sopAM45 spaO::aphT
invB), and M623 (sopAM45 invC::aphT) (Table 1). This analysis confirmed that the amount of cytosolic InvB is not altered in the secretion-deficient mutants M629 (sopAM45 spaO::aphT) and M623 (sopAM45 invC::aphT) and that InvB is absent from the invB deletion strains M618 (sopAM45
invB) and (sopAM45 spaO::aphT
invB) (Fig. 5).
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FIG. 5. Western blot analysis of invB expression level. Bacteria were grown under SPI-1 inducing conditions. Bacteria recovered from 200 µl of culture of strains M619 (sopAM45), M618 (sopAM45, invB), M629 (sopAM45 spaO::aphT), M630 (sopAM45 invB spaO::aphT), and M623 (sopAM45 invC::aphT) were analyzed by Western blotting using an anti-InvB antiserum ( -InvB). The blot was reprobed with a monoclonal anti-OmpC antibody ( -OmpC) to verify that equivalent amounts of lysate were loaded onto each lane.
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invB), M637 (spaO::aphT), M638 (
invB spaO::aphT), and M639 (invC::aphT) were constructed by P22 transduction of the sopA::pM265 allele (Table 1).
Thus, we could use ß-galactosidase assays to study sopA promoter activity. The ß-galactosidase activity was determined in at least eight independent experiments, and statistical analysis was performed using the exact Mann-Whitney U test. We found that sopA transcription was in the same order of magnitude for all strains, analyzed (Fig. 6). Disruption of invB did not decrease ß-galactosidase activity. Rather, ß-galactosidase activity was slightly but significantly increased in M636 (
invB) (P < 0.001), M639 (invC::aphT) (P = 0.001), and M638 (
invB spaO::aphT) (P < 0.001) (Fig. 6). Therefore, the decreased SopAM45 protein levels in the cytoplasm of an invB mutant (Fig. 3) are attributable to a slightly decreased protein stability but not to transcriptional down regulation. However, the reasons for the slight augmentation of transcription in M636, M638, and M639 remain to be analyzed.
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FIG. 6. Effect of an invB deletion on transcription of sopAM45. Transcription of sopAM45 was measured using transcriptional lacZ reporter constructs in standard ß-galactosidase activity assays. ß-Galactosidase activities were determined in at least eight independent experiments. Bars indicate the median.
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TTS chaperones have been divided into three classes: class I, chaperones which associate with effector proteins; class II, chaperones which associate with translocators; and class III, chaperones of the flagellar system (17). Due to their unique features, InvB and its homologs Spa15 (Shigella spp.), YsaK (Yersinia spp.), and InvB (Sodalis spp.) are thought to represent a new family of TTS chaperones. Therefore, they have been assigned to the new subclass IB, which represents chaperones that bind several different effectors (17). This classification was based on experimental evidence from Shigella flexneri (16). Interestingly, Page and Parsot have hypothesized that InvB, like Spa15, might also associate with different unrelated proteins (15). This was confirmed by our findings that InvB is a chaperone not only for SipA (2) but also for SopE, SopE2 (7a), and SopA (this work).
The project was funded in part by the Swiss National Foundation.
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