Monitoring the Assembly of a Secreted Bacterial Virulence Factor Using Site-specific Crosslinking

This article describes a method to detect and analyze dynamic interactions between a protein of interest and other factors in vivo. Our method is based on ...

National Institutes of Health

The ferric uptake regulator protein Fur regulates iron-dependent gene expression in bacteria. In Helicobacter pylori it has been shown to regulate iron-activated and iron-repressed genes. In this study, we show that H. pylori Fur protein regulates transcription from its own sigma 80 promoter P fur in response to iron. Footprinting analysis shows that Fur binds at three distinct operators at P fur overlapping and proximal to the promoter elements. Site-directed mutagenesis of the proposed iron-binding site of the protein results in derepression of P fur and the loss of iron regulation. In vivo oligomerization assays reveals that the C-terminus of Fur is necessary for multimerization of the protein and that the mutations do not affect this activity. Molecular and phenotypic analysis of the mutant proteins provides evidence that the iron-binding site controls the specific affinity of Fur for the operators at P fur and hence its repressive ability. In summary, the data presented are consistent with a model in which Fur acts as a rheostat of transcription to autoregulate its own expression in response to iron, which in turn controls expression of iron-induced and iron-repressed genes, providing maintenance of homeostasis.

Autoregulation of Helicobacter pylori Fur revealed by functional analysis of the iron-binding site.

Fur is a well-known iron-responsive repressor of gene transcription, which is used by many bacteria to respond to the low-iron environment that pathogens encounter during infection. The fur gene in Neisseria meningitidis has been described as an essential gene that may regulate a broad array of genes. We succeeded in obtaining an N. meningitidis mutant with the fur gene knocked out and used it to undertake studies of fur-mediated iron regulation. We show that expression of both Fur and the transferrin binding protein Tbp2 is iron regulated and demonstrate that this regulation is Fur mediated for the Tbp2 protein. Footprinting analysis revealed that Fur binds to two distinct sites upstream of its coding region with different affinities and that these binding sites overlap two promoters that differentially control transcription of the fur gene in response to iron. The presence of two independently regulated fur promoters may allow meningococcus to fine-tune expression of this regulator controlling iron homeostasis, possibly during infection.

The iron-responsive regulator fur is transcriptionally autoregulated and not essential in Neisseria meningitidis.

The crgA gene of Neisseria meningitidis, which codes for a LysR-type regulator, is divergently oriented with respect to the mdaB gene, which codes for a hypothetical NADPH-quinone oxidoreductase. Transcriptional studies of the intergenic region between crgA and mdaB showed that two overlapping and divergent promoters, P(crgA) and P(mdaB), control transcription of these genes. Deletion of the crgA gene led to a strong increase in transcription from the P(crgA) promoter and a concomitant strong decrease in transcription from the P(mdaB) promoter, indicating that CrgA acts both as an autorepressor of transcription at its own promoter and as an activator of transcription at the mdaB promoter. Addition of alpha-methylene-gamma-butyrolactone (MBL), an inducer of NADPH-quinone oxidoreductase, to wild-type N. meningitidis cells specifically resulted in further activation of transcription of the P(mdaB) promoter and more repression of transcription of the P(crgA) promoter. No such regulation was observed when MBL was added to crgA-deficient cells, indicating that the transcriptional response to MBL is CrgA mediated. Under the same experimental conditions, no regulation of transcription by either CrgA or MBL was detected at the pilus and capsule genes. The role of CrgA in the regulation of gene expression during the infectious cycle of N. meningitidis is discussed.

CrgA is an inducible LysR-type regulator of Neisseria meningitidis, acting both as a repressor and as an activator of gene transcription.

Two important metal-responsive regulators, NikR and Fur, are involved in nickel and iron homeostasis and controlling gene expression in Helicobacter pylori. To date, they have been implicated in the regulation of sets of overlapping genes. We have attempted here dissection of the molecular mechanisms involved in transcriptional regulation of the NikR and Fur proteins, and we investigated protein-promoter interactions of the regulators with known target genes. We show that H. pylori NikR is a tetrameric protein and, through DNase I footprinting analysis, we have identified operators for NikR to which it binds with different affinities in a metal-responsive way. Mapping of the NikR binding site upstream of the urease promoter established a direct role for NikR as a positive regulator of transcription and, through scanning mutagenesis of this binding site, we have determined two subsites that are important for the binding of the protein to its target sequence. Furthermore, by alignment of the operators for NikR, we have shown that the H. pylori protein recognizes a sequence that is distinct from its well-studied orthologue in Escherichia coli. Moreover, we show that NikR and Fur can bind independently at distinct operators and also compete for overlapping operators in some coregulated gene promoters, adding another dimension to the previous suggested link between iron and nickel regulation. Finally, the importance of an interconnection between metal-responsive gene networks for homeostasis is discussed.

In vitro analysis of protein-operator interactions of the NikR and fur metal-responsive regulators of coregulated genes in Helicobacter pylori.

Bacterial autotransporters consist of an N-terminal 'passenger domain' that is transported into the extracellular space by an unknown mechanism and a C-terminal 'beta-domain' that forms a beta-barrel in the outer membrane. Recent studies have revealed that fully assembled autotransporters have an unusual architecture in which a small passenger domain segment traverses the pore formed by the beta-domain. It is unclear, however, whether this configuration forms prior to passenger domain translocation or results from the translocation of the passenger domain through the beta-domain pore. By examining the accessibility of tobacco etch virus protease sites and single-cysteine residues in the passenger domain of the Escherichia coli O157:H7 autotransporter EspP at different stages of protein biogenesis, we identified a novel pre-translocation intermediate whose topology resembles that of the fully assembled protein. This intermediate was isolated in the periplasm in cell fractionation experiments. The data strongly suggest that the EspP beta-domain and an embedded polypeptide segment are integrated into the outer membrane as a single pre-formed unit. The data also provide indirect evidence that at least some outer membrane proteins acquire considerable tertiary structure prior to their membrane integration.

Incorporation of a polypeptide segment into the beta-domain pore during the assembly of a bacterial autotransporter.

Mechanisms for coping with oxidative stress (OS) are crucial for the survival of pathogenic Neisseria spp. in the human host. In this study we investigate the mechanism by which OxyR finely regulates the catalase gene (kat) in Neisseria meningitidis. Detailed transcriptional analyses show that catalase is transcribed from a single promoter that is induced by H(2)O(2) in an OxyR-dependent manner and two key cysteine residues are essential for this. OxyR also represses the kat promoter: kat expression in the null mutant is at a constitutive intermediary level higher than uninduced, but lower than H(2)O(2)-induced levels in the wild type. Our data are consistent with a model in which OxyR binds to the kat promoter and exerts: (i) repression of transcription in the absence of OS signal and (ii) activation of the promoter in response to OS signal. This direct double-edged mechanism may ensure tight regulatory control of kat expression ensuring catalase is synthesized only when needed. In addition, our results provide an explanation for the altered OS resistance phenotypes seen in Neisseria mutant strains where, paradoxically, the oxyR mutants are more resistant than the wild type in oxidative killing assays.

OxyR tightly regulates catalase expression in Neisseria meningitidis through both repression and activation mechanisms.

Autotransporters are a superfamily of virulence factors produced by Gram-negative bacteria consisting of a large N-terminal extracellular domain ("passenger domain") and a C-terminal beta barrel domain ("beta domain"). The mechanism by which the passenger domain is translocated across the outer membrane (OM) is unknown. Here we show that the insertion of a small linker into the passenger domain of the Escherichia coli O157:H7 autotransporter EspP effectively creates a translocation intermediate by transiently stalling translocation near the site of the insertion. Using a site-specific photocrosslinking approach, we found that residues adjacent to the stall point interact with BamA, a component of a heterooligomeric complex (Bam complex) that catalyzes OM protein assembly, and that residues closer to the EspP N terminus interact with the periplasmic chaperones SurA and Skp. The EspP-BamA interaction was short-lived and could be detected only when passenger domain translocation was stalled. These results support a model in which molecular chaperones prevent misfolding of the passenger domain before its secretion and the Bam complex catalyzes both the integration of the beta domain into the OM and the translocation of the passenger domain across the OM in a C- to N-terminal direction.

Interaction of an autotransporter passenger domain with BamA during its translocation across the bacterial outer membrane.

Autotransporters are bacterial virulence factors consisting of an N-terminal "passenger domain" that is secreted in a C- to-N-terminal direction and a C-terminal "β domain" that resides in the outer membrane (OM). Although passenger domain secretion does not appear to use ATP, the energy source for this reaction is unknown. Here, we show that efficient secretion of the passenger domain of the Escherichia coli O157:H7 autotransporter EspP requires the stable folding of a C-terminal ≈17-kDa passenger domain segment. We found that mutations that perturb the folding of this segment do not affect its translocation across the OM but impair the secretion of the remainder of the passenger domain. Interestingly, an examination of kinetic folding mutants strongly suggested that the ≈17-kDa segment folds in the extracellular space. By mutagenizing the ≈17-kDa segment, we also fortuitously isolated a unique translocation intermediate. Analysis of this intermediate suggests that a heterooligomer that facilitates the membrane integration of OM proteins (the Bam complex) also promotes the surface exposure of the ≈17-kDa segment. Our results provide direct evidence that protein folding can drive translocation and help to clarify the mechanism of autotransporter secretion.

Secretion of a bacterial virulence factor is driven by the folding of a C-terminal segment.

Autotransporters are bacterial virulence factors that consist of an N-terminal extracellular ("passenger") domain and a C-terminal β barrel domain ("β domain") that resides in the outer membrane. Here we used an in vivo site-specific photocrosslinking approach to gain insight into the mechanism by which the β domain is integrated into the outer membrane and the relationship between β domain assembly and passenger domain secretion. We found that periplasmic chaperones and specific components of the β barrel assembly machinery (Bam) complex interact with the β domain of the Escherichia coli O157:H7 autotransporter extracellular serine protease P (EspP) in a temporally and spatially regulated fashion. Although the chaperone Skp initially interacted with the entire β domain, BamA, BamB, and BamD subsequently interacted with discrete β domain regions. BamB and BamD remained bound to the β domain longer than BamA and therefore appeared to function at a later stage of assembly. Interestingly, we obtained evidence that the completion of β domain assembly is regulated by an intrinsic checkpoint mechanism that requires the completion of passenger domain secretion. In addition to leading to a detailed model of autotransporter biogenesis, our results suggest that the lipoprotein components of the Bam complex play a direct role in the membrane integration of β barrel proteins.

Sequential and spatially restricted interactions of assembly factors with an autotransporter beta domain.

Many mitochondrial proteins are synthesized with N-terminal presequences in the cytosol. The presequence translocase of the inner mitochondrial membrane (TIM23) translocates preproteins into and across the membrane and associates with the matrix-localized import motor. The TIM23 complex consists of three core components and Tim21, which interacts with the translocase of the outer membrane (TOM) and the respiratory chain. We have identified a new subunit of the TIM23 complex, the inner membrane protein Mgr2. Mitochondria lacking Mgr2 were deficient in the Tim21-containing sorting form of the TIM23 complex. Mgr2 was required for binding of Tim21 to TIM23(CORE), revealing a binding chain of TIM23(CORE)-Mgr2/Tim21-respiratory chain. Mgr2-deficient yeast cells were defective in growth at elevated temperature, and the mitochondria were impaired in TOM-TIM23 coupling and the import of presequence-carrying preproteins. We conclude that Mgr2 is a coupling factor of the presequence translocase crucial for cell growth at elevated temperature and for efficient protein import.

Mgr2 promotes coupling of the mitochondrial presequence translocase to partner complexes.

Autotransporters are bacterial virulence factors that contain an N-terminal extracellular ("passenger") domain and a C-terminal β barrel ("β") domain that anchors the protein to the outer membrane. The β domain is required for passenger domain secretion, but its exact role in autotransporter biogenesis is unclear. Here we describe insights into the function of the β domain that emerged from an analysis of mutations in the Escherichia coli O157:H7 autotransporter EspP. We found that the G1066A and G1081D mutations slightly distort the structure of the β domain and delay the initiation of passenger domain translocation. Site-specific photocrosslinking experiments revealed that the mutations slow the insertion of the β domain into the outer membrane, but do not delay the binding of the β domain to the factor that mediates the insertion reaction (the Bam complex). Our results demonstrate that the β domain does not simply target the passenger domain to the outer membrane, but promotes translocation when it reaches a specific stage of assembly. Furthermore, our results provide evidence that the Bam complex catalyzes the membrane integration of β barrel proteins in a multistep process that can be perturbed by minor structural defects in client proteins.

Mechanistic link between β barrel assembly and the initiation of autotransporter secretion.

The presequence translocase of the inner mitochondrial membrane (TIM23 complex) is essential for importing cleavable preproteins into mitochondria. The preproteins contain amino-terminal targeting sequences that are removed by the mitochondrial processing peptidase (MPP). Some preproteins carry bipartite presequences that are cleaved twice, by MPP and the inner membrane protease (IMP). Here, we report that the TIM23 complex is altered in mitochondria lacking the IMP subunit Imp1 although none of the TIM23 components contains a bipartite presequence. We show that the TIM23 subunit Mgr2 is processed by IMP, but not by MPP. The cytosolic precursor of Mgr2 contains a carboxy-terminal sequence that promotes targeting to mitochondria, but impairs stable assembly and function of the mature TIM23 complex. IMP removes the carboxy-terminal targeting sequence and thus promotes proper assembly of the TIM23 complex. Our results reveal carboxy-terminal processing as a new mechanism in the biogenesis of the mitochondrial inner membrane.

Raffaele Ieva

Genetics and Biochemistry Branch of the National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health

Monitoring the Assembly of a Secreted Bacterial Virulence Factor Using Site-specific Crosslinking

Raffaele Ieva

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Genetics and Biochemistry Branch of the National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health

Monitoring the Assembly of a Secreted Bacterial Virulence Factor Using Site-specific Crosslinking

Genetics and Biochemistry Branch of the National Institute of Diabetes and Digestive and Kidney Diseases