Identification of a novel transport system in Borrelia burgdorferi that links the inner and outer membranes.

Borrelia burgdorferi, the spirochete that causes Lyme disease, is a diderm organism that is similar to Gram-negative organisms in that it contains both an inner and outer membrane. Unlike typical Gram-negative organisms, however, B. burgdorferi lacks lipopolysaccharide (LPS). Using computational genome analyses and structural modeling, we identified a transport system containing six proteins in B. burgdorferi that are all orthologs to proteins found in the lipopolysaccharide transport (LPT) system that links the inner and outer membranes of Gram-negative organisms and is responsible for placing LPS on the surface of these organisms. While B. burgdorferi does not contain LPS, it does encode over 100 different surface-exposed lipoproteins and several major glycolipids, which like LPS are also highly amphiphilic molecules, though no system to transport these molecules to the borrelial surface is known. Accordingly, experiments supplemented by molecular modeling were undertaken to determine whether the orthologous LPT system identified in B. burgdorferi could transport lipoproteins and/or glycolipids to the borrelial outer membrane. Our combined observations strongly suggest that the LPT transport system does not transport lipoproteins to the surface. Molecular dynamic modeling, however, suggests that the borrelial LPT system could transport borrelial glycolipids to the outer membrane.

Co-immunoprecipitation for Identifying Protein-Protein Interactions in Borrelia burgdorferi.

Co-immunoprecipitation can be utilized to study protein-protein interactions from various environments, cell types, or tissues. Herein, we describe a co-immunoprecipitation protocol that can be used to examine protein complexes found in the pathogenic spirochete Borrelia burgdorferi. The method outlined here has successfully identified known and unknown members of borrelial protein complexes and is an efficient method for studying protein interactions in this pathogenic spirochete.

Outer Membrane Proteins BB0405 and BB0406 Are Immunogenic, but Only BB0405 Is Required for Borrelia burgdorferi Infection.

We recently identified the Borrelia burgdorferi outer membrane protein (OMP) BB0406 and found that the gene encoding this OMP was cotranscribed with the gene encoding the OMP BB0405. Interestingly, BB0405 and BB0406 share 59% similarity and are grouped into the same B. burgdorferi paralogous gene family. Given their overall similarity, it is plausible that both OMPs have similar or overlapping functions in this pathogenic spirochete. BB0405 was recently shown to be required for mammalian infection despite the observations that BB0405 is poorly immunogenic and not recognized during mouse or human infection. BB0405 orthologs have also been shown to bind the complement regulator protein factor H. Therefore, to better elucidate the role of BB0405 and its paralog BB0406 during infection and in serum resistance, we examined both proteins in animal infection, factor H binding, and serum sensitivity assays. Our combined results suggest that BB0405- and BB0406-specific antibodies are borreliacidal and that both OMPs are immunogenic during nonhuman primate infection. Additionally, while BB0405 was found to be required for establishing mouse infection, BB0406 was not found to be essential for infectivity. In contrast to data from previous reports, however, neither OMP was found to bind human factor H or to be required for enhancing serum resistance of B. burgdorferi in vitro.

The TamB ortholog of Borrelia burgdorferi interacts with the ß-barrel assembly machine (BAM) complex protein BamA.

Two outer membrane protein (OMP) transport systems in diderm bacteria assist in assembly and export of OMPs. These two systems are the ß-barrel assembly machine (BAM) complex and the translocation and assembly module (TAM). The BAM complex consists of the OMP component BamA along with several outer membrane associated proteins. The TAM also consists of an OMP, designated TamA, and a single inner membrane (IM) protein, TamB. Together TamA and TamB aid in the secretion of virulence-associated OMPs. In this study we characterized the hypothetical protein BB0794 in Borrelia burgdorferi. BB0794 contains a conserved DUF490 domain, which is a motif found in all TamB proteins. All spirochetes lack a TamA ortholog, but computational and physicochemical characterization of BB0794 revealed it is a TamB ortholog. Interestingly, BB0794 was observed to interact with BamA and a BB0794 regulatable mutant displayed altered cellular morphology and antibiotic sensitivity. The observation that B. burgdorferi contains a TamB ortholog that interacts with BamA and is required for proper outer membrane biogenesis not only identifies a novel role for TamB-like proteins, but also may explain why most diderms harbor a TamB-like protein while only a select group encodes TamA.

Consensus computational network analysis for identifying candidate outer membrane proteins from Borrelia spirochetes.

BACKGROUND: Similar to Gram-negative organisms, Borrelia spirochetes are dual-membrane organisms with both an inner and outer membrane. Although the outer membrane contains integral membrane proteins, few of the borrelial outer membrane proteins (OMPs) have been identified and characterized to date. Therefore, we utilized a consensus computational network analysis to identify novel borrelial OMPs. RESULTS: Using a series of computer-based algorithms, we selected all protein-encoding sequences predicted to be OM-localized and/or to form ß-barrels in the borrelial OM. Using this system, we identified 41 potential OMPs from B. burgdorferi and characterized three (BB0838, BB0405, and BB0406) to confirm that our computer-based methodology did, in fact, identify borrelial OMPs. Triton X-114 phase partitioning revealed that BB0838 is found in the detergent phase, which would be expected of a membrane protein. Proteolysis assays indicate that BB0838 is partially sensitive to both proteinase K and trypsin, further indicating that BB0838 is surface-exposed. Consistent with a prior study, we also confirmed that BB0405 is surface-exposed and associates with the borrelial OM. Furthermore, we have shown that BB0406, the product of a co-transcribed downstream gene, also encodes a novel, previously uncharacterized borrelial OMP. Interestingly, while BB0406 has several physicochemical properties consistent with it being an OMP, it was found to be resistant to surface proteolysis. Consistent with BB0405 and BB0406 being OMPs, both were found to be capable of incorporating into liposomes and exhibit pore-forming activity, suggesting that both proteins are porins. Lastly, we expanded our computational analysis to identify OMPs from other borrelial organisms, including both Lyme disease and relapsing fever spirochetes. CONCLUSIONS: Using a consensus computer algorithm, we generated a list of candidate OMPs for both Lyme disease and relapsing fever spirochetes and determined that three of the predicted B. burgdorferi proteins identified were indeed novel borrelial OMPs. The combined studies have identified putative spirochetal OMPs that can now be examined for their roles in virulence, physiology, and disease pathogenesis. Importantly, the studies described in this report provide a framework by which OMPs from any human pathogen with a diderm ultrastructure could be cataloged to identify novel virulence factors and vaccine candidates.

Characterization of the ß-barrel assembly machine accessory lipoproteins from Borrelia burgdorferi.

BACKGROUND: Like all diderm bacteria studied to date, Borrelia burgdorferi possesses a ß-barrel assembly machine (BAM) complex. The bacterial BAM complexes characterized thus far consist of an essential integral outer membrane protein designated BamA and one or more accessory proteins. The accessory proteins are typically lipid-modified proteins anchored to the inner leaflet of the outer membrane through their lipid moieties. We previously identified and characterized the B. burgdorferi BamA protein in detail and more recently identified two lipoproteins encoded by open reading frames bb0324 and bb0028 that associate with the borrelial BamA protein. The role(s) of the BAM accessory lipoproteins in B. burgdorferi is currently unknown. RESULTS: Structural modeling of B. burgdorferi BB0028 revealed a distinct ß-propeller fold similar to the known structure for the E. coli BAM accessory lipoprotein BamB. Additionally, the structural model for BB0324 was highly similar to the known structure of BamD, which is consistent with the prior finding that BB0324 contains tetratricopeptide repeat regions similar to other BamD orthologs. Consistent with BB0028 and BB0324 being BAM accessory lipoproteins, mutants lacking expression of each protein were found to exhibit altered membrane permeability and enhanced sensitivity to various antimicrobials. Additionally, BB0028 mutants also exhibited significantly impaired in vitro growth. Finally, immunoprecipitation experiments revealed that BB0028 and BB0324 each interact specifically and independently with BamA to form the BAM complex in B. burgdorferi. CONCLUSIONS: Combined structural studies, functional assays, and co-immunoprecipitation experiments confirmed that BB0028 and BB0324 are the respective BamB and BamD orthologs in B. burgdorferi, and are important in membrane integrity and/or outer membrane protein localization. The borrelial BamB and BamD proteins both interact specifically and independently with BamA to form a tripartite BAM complex in B. burgdorferi. A working model has been developed to further analyze outer membrane biogenesis and outer membrane protein transport in this pathogenic spirochete.

Structural modeling and physicochemical characterization provide evidence that P66 forms a ß-barrel in the Borrelia burgdorferi outer membrane.

The Borrelia burgdorferi outer membrane (OM) contains numerous surface-exposed lipoproteins but a relatively low density of integral OM proteins (OMPs). Few membrane-spanning OMPs of B. burgdorferi have been definitively identified, and none are well characterized structurally. Here, we provide evidence that the borrelial OMP P66, a known adhesin with pore-forming activity, forms a ß-barrel in the B. burgdorferi OM. Multiple computer-based algorithms predict that P66 forms a ß-barrel with either 22 or 24 transmembrane domains. According to our predicted P66 topology, a lysine residue (K487) known to be sensitive to trypsin cleavage is located within a surface-exposed loop. When we aligned the mature P66 amino acid sequences from B. burgdorferi and B. garinii, we found that K487 was present only in the B. burgdorferi P66 protein sequence. When intact cells from each strain were treated with trypsin, only B. burgdorferi P66 was trypsin sensitive, indicating that K487 is surface exposed, as predicted. Consistent with this observation, when we inserted a c-Myc tag adjacent to K487 and utilized surface localization immunofluorescence, we detected the loop containing K487 on the surface of B. burgdorferi. P66 was examined by both Triton X-114 phase partitioning and circular dichroism, confirming that the protein is amphiphilic and contains extensive (48%) ß-sheets, respectively. Moreover, P66 also was able to incorporate into liposomes and form channels in large unilamellar vesicles. Finally, blue native PAGE (BN-PAGE) revealed that under nondenaturing conditions, P66 is found in large complexes of ∼400 kDa and ∼600 kDa. Outer surface lipoprotein A (OspA) and OspB both coimmunoprecipitate with P66, demonstrating that P66 associates with OspA and OspB in B. burgdorferi. The combined computer-based structural analyses and supporting physicochemical properties of P66 provide a working model to further examine the porin and integrin-binding activities of this OMP as they relate to B. burgdorferi physiology and Lyme disease pathogenesis.

The role of Borrelia burgdorferi outer surface proteins.

Human pathogenic spirochetes causing Lyme disease belong to the Borrelia burgdorferi sensu lato complex. Borrelia burgdorferi organisms are extracellular pathogens transmitted to humans through the bite of Ixodes spp. ticks. These spirochetes are unique in that they can cause chronic infection and persist in the infected human, even though a robust humoral and cellular immune response is produced by the infected host. How this extracellular pathogen is able to evade the host immune response for such long periods of time is currently unclear. To gain a better understanding of how this organism persists in the infected human, many laboratories have focused on identifying and characterizing outer surface proteins of B. burgdorferi. As the interface between B. burgdorferi and its human host is its outer surface, proteins localized to the outer membrane must play an important role in dissemination, virulence, tissue tropism, and immune evasion. Over the last two decades, numerous outer surface proteins from B. burgdorferi have been identified, and more recent studies have begun to elucidate the functional role(s) of many borrelial outer surface proteins. This review summarizes the outer surface proteins identified in B. burgdorferi to date and provides detailed insight into the functions of many of these proteins as they relate to the unique parasitic strategy of this spirochetal pathogen.

BB0324 and BB0028 are constituents of the Borrelia burgdorferi ß-barrel assembly machine (BAM) complex.

BACKGROUND: Similar to Gram-negative bacteria, the outer membrane (OM) of the pathogenic spirochete, Borrelia burgdorferi, contains integral OM-spanning proteins (OMPs), as well as membrane-anchored lipoproteins. Although the mechanism of OMP biogenesis is still not well-understood, recent studies have indicated that a heterooligomeric OM protein complex, known as BAM (ß-barrel assembly machine) is required for proper assembly of OMPs into the bacterial OM. We previously identified and characterized the essential ß-barrel OMP component of this complex in B. burgdorferi, which we determined to be a functional BamA ortholog. RESULTS: In the current study, we report on the identification of two additional protein components of the B. burgdorferi BAM complex, which were identified as putative lipoproteins encoded by ORFs BB0324 and BB0028. Biochemical assays with a BamA-depleted B. burgdorferi strain indicate that BB0324 and BB0028 do not readily interact with the BAM complex without the presence of BamA, suggesting that the individual B. burgdorferi BAM components may associate only when forming a functional BAM complex. Cellular localization assays indicate that BB0324 and BB0028 are OM-associated subsurface lipoproteins, and in silico analyses indicate that BB0324 is a putative BamD ortholog. CONCLUSIONS: The combined data suggest that the BAM complex of B. burgdorferi contains unique protein constituents which differ from those found in other proteobacterial BAM complexes. The novel findings now allow for the B. burgdorferi BAM complex to be further studied as a model system to better our understanding of spirochetal OM biogenesis in general.

The hybrid histidine kinase Hk1 is part of a two-component system that is essential for survival of Borrelia burgdorferi in feeding Ixodes scapularis ticks.

Two-component systems (TCS) are principal mechanisms by which bacteria adapt to their surroundings. Borrelia burgdorferi encodes only two TCS. One is comprised of a histidine kinase, Hk2, and the response regulator Rrp2. While the contribution of Hk2 remains unclear, Rrp2 is part of a regulatory pathway involving the spirochete's alternate sigma factors, RpoN and RpoS. Genes within the Rrp2/RpoN/RpoS regulon function to promote tick transmission and early infection. The other TCS consists of a hybrid histidine kinase, Hk1, and the response regulator Rrp1. Hk1 is composed of two periplasmic sensor domains (D1 and D2), followed by conserved cytoplasmic histidine kinase core, REC, and Hpt domains. In addition to its REC domain, Rrp1 contains a GGDEF motif characteristic of diguanylate cyclases. To investigate the role of Hk1 during the enzootic cycle, we inactivated this gene in two virulent backgrounds. Extensive characterization of the resulting mutants revealed a dramatic phenotype whereby Hk1-deficient spirochetes are virulent in mice and able to migrate out of the bite site during feeding but are killed within the midgut following acquisition. We hypothesize that the phosphorelay between Hk1 and Rrp1 is initiated by the binding of feeding-specific ligand(s) to Hk1 sensor domain D1 and/or D2. Once activated, Rrp1 directs the synthesis of cyclic dimeric GMP (c-di-GMP), which, in turn, modulates the expression and/or activity of gene products required for survival within feeding ticks. In contrast to the Rrp2/RpoN/RpoS pathway, which is active only within feeding nymphs, the Hk1/Rrp1 TCS is essential for survival during both larval and nymphal blood meals.

The OspE-related proteins inhibit complement deposition and enhance serum resistance of Borrelia burgdorferi, the lyme disease spirochete.

Borrelia burgdorferi, the Lyme disease spirochete, binds the host complement inhibitors factor H (FH) and FH-like protein 1 (FHL-1). Binding of FH/FHL-1 by the B. burgdorferi proteins CspA and the OspE-related proteins is thought to enhance resistance to serum-mediated killing. While previous reports have shown that CspA confers serum resistance in B. burgdorferi, it is unclear whether the OspE-related proteins are relevant in B. burgdorferi serum resistance when OspE is expressed on the borrelial surface. To assess the role of the OspE-related proteins, we overexpressed them in a serum-sensitive CspA mutant strain. OspE overexpression enhanced serum resistance of the CspA-deficient organisms. Furthermore, FH was more efficiently bound to the B. burgdorferi surface when OspE was overexpressed. Deposition of complement components C3 and C5b-9 (the membrane attack complex), however, was reduced on the surface of the OspE-overexpressing strain compared to that on the CspA mutant strain. These data demonstrate that OspE proteins expressed on the surface of B. burgdorferi bind FH and protect the organism from complement deposition and subsequent serum-mediated destruction.

CspA-mediated binding of human factor H inhibits complement deposition and confers serum resistance in Borrelia burgdorferi.

Borrelia burgdorferi has developed efficient mechanisms for evading the innate immune response during mammalian infection and has been shown to be resistant to the complement-mediated bactericidal activity of human serum. It is well recognized that B. burgdorferi expresses multiple lipoproteins on its surface that bind the human complement inhibitors factor H and factor H-like protein 1 (FH/FHL-1). The binding of FH/FHL-1 on the surface of B. burgdorferi is thought to enhance its ability to evade serum-mediated killing during the acute phase of infection. One of the key B. burgdorferi FH/FHL-1 binding proteins identified thus far was designated CspA. While it is known that CspA binds FH/FHL-1, it is unclear how the interaction between CspA and FH/FHL-1 specifically enhances serum resistance. To better understand how CspA mediates serum resistance in B. burgdorferi, we inactivated cspA in a virulent strain of B. burgdorferi. An affinity ligand blot immunoassay and indirect immunofluorescence revealed that the CspA mutant does not efficiently bind human FH to its surface. Consistent with the lack of FH binding, the CspA mutant was also highly sensitive to killing by human serum. Additionally, the deposition of complement components C3, C6, and C5b-9 was enhanced on the surface of the CspA mutant compared to that of the wild-type strain. The combined data lead us to conclude that the CspA-mediated binding of human FH confers serum resistance by directly inhibiting complement deposition on the surface of B. burgdorferi.

Borrelia burgdorferi complement regulator-acquiring surface protein 2 does not contribute to complement resistance or host infectivity.

Borrelia burgdorferi, the pathogen of Lyme disease, cycles in nature through Ixodes ticks and mammalian hosts. At least five Complement Regulator-Acquiring Surface Proteins (BbCRASPs) are produced by B. burgdorferi, which are thought to assist spirochetes in host immune evasion. Recent studies established that BbCRASP-2 is preferentially expressed in mammals, and elicits robust antibody response in infected hosts, including humans. We show that BbCRASP-2 is ubiquitously expressed in diverse murine tissues, but not in ticks, reinforcing a role of BbCRASP-2 in conferring B. burgdorferi defense against persistent host immune threats, such as complement. BbCRASP-2 immunization, however, fails to protect mice from B. burgdorferi infection and does not modify disease, as reflected by the development of arthritis. An infectious BbCRASP-2 mutant was generated, therefore, to examine the precise role of the gene product in spirochete infectivity. Similar to wild type B. burgdorferi, BbCRASP-2 mutants remain insensitive to complement-mediated killing in vitro, retain full murine infectivity and induce arthritis. Quantitative RT-PCR assessment indicates that survivability of BbCRASP-2-deficient B. burgdorferi is not due to altered expression of other BbCRASPs. Together, these results suggest that the function of a selectively expressed B. burgdorferi gene, BbCRASP-2, is not essential for complement resistance or infectivity in the murine host.