Recombinant Brucella abortus Type IV secretion system protein virB3 (virB3)
Description
Introduction to Brucella and the Type IV Secretion System
Brucella abortus is a gram-negative facultative intracellular parasitic bacterium that causes brucellosis, a zoonotic disease affecting both animals and humans. The pathogen's virulence largely depends on its ability to survive and replicate within host cells, a process critically mediated by the Type IV secretion system (T4SS) encoded by the virB operon . This sophisticated molecular machinery comprises twelve genes (virB1-virB12) that form a multi-protein complex functioning as a molecular syringe to deliver bacterial effector proteins directly into host cells .
The T4SS plays a crucial role in Brucella's intracellular lifecycle by subverting cellular pathways and modulating host immune responses. During infection, VirB T4SS is rapidly activated, reaching maximum activity within five hours, and subsequently downregulated once the replication niche is established . This temporal regulation is essential for the biogenesis of the endoplasmic reticulum-derived replicative Brucella-containing vacuole (rBCV), which provides a protected environment for bacterial multiplication .
Physical and Biochemical Properties
The recombinant form of VirB3 is typically produced with an N-terminal histidine tag to facilitate purification and has been successfully expressed in E. coli expression systems . When purified, the protein demonstrates stability in lyophilized form and can be reconstituted in appropriate buffers for experimental applications.
| Property | Description |
|---|---|
| Protein Length | 116 amino acids (Full length) |
| Expression System | E. coli |
| Tag | N-terminal His-tag |
| Form | Lyophilized powder |
| Purity | >90% (SDS-PAGE) |
| Storage Buffer | Tris/PBS-based buffer, 6% Trehalose, pH 8.0 |
| Reconstitution | Deionized sterile water (0.1-1.0 mg/mL) |
| Long-term Storage | -20°C/-80°C with 5-50% glycerol |
Functional Significance of VirB3 in Brucella Pathogenesis
Extensive research has established VirB3 as a critical factor in Brucella virulence and pathogenesis. Studies utilizing nonpolar deletion mutants have demonstrated that virB3 is essential for bacterial survival in macrophages and persistence in mice .
Role in Intracellular Survival and Virulence
Deletion of the virB3 gene significantly reduces B. abortus survival in J774A.1 mouse macrophage-like cells, with effects comparable to deletion of the entire virB locus . Furthermore, virB3 deletion markedly reduces the bacterium's ability to persist in mouse spleens at 8 weeks post-infection, confirming its essential role in establishing persistent infection . This evidence collectively positions VirB3 as a critical virulence determinant.
Influence on T4SS Assembly and Stability
One of the most intriguing aspects of VirB3 function is its apparent role in maintaining the structural integrity of the T4SS complex. Experimental data indicate that deletion of virB3 leads to reduced levels of other VirB proteins, including VirB5, VirB8, VirB9, VirB10, and VirB11 . This suggests that VirB3 may function as a stabilizing component, potentially through direct protein-protein interactions that support the assembly or maintenance of the secretion apparatus.
| Deleted Gene | Survival in Macrophages | Persistence in Mouse Spleen | Effect on Other VirB Proteins |
|---|---|---|---|
| virB3 | Severely Reduced | Reduced | Reduced levels of VirB5, VirB8, VirB9, VirB10, VirB11 |
| virB4 | Severely Reduced | Reduced | Reduced VirB5 levels |
| virB7 | Severely Reduced | Normal | Reduced levels of VirB8, VirB9, VirB10, VirB11 |
Contribution to Brucella Intracellular Cycle
The VirB T4SS plays sequential roles throughout Brucella's intracellular lifecycle, from initial vacuole remodeling to post-replication stages. Research using conditionally controlled expression systems has revealed that early VirB production is essential for optimal rBCV biogenesis and bacterial replication . VirB3, as an integral component of this system, contributes to these processes by enabling the secretion of effector proteins that modulate host cell functions.
Production of Recombinant VirB3 Protein
Advances in recombinant protein technology have enabled the successful production of VirB3 for research and diagnostic applications. The full-length protein (amino acids 1-116) can be expressed in E. coli with an N-terminal histidine tag for affinity purification .
Expression and Purification Methods
Standard recombinant protein expression techniques involving bacterial expression systems have been employed to produce VirB3. The protein is typically purified to greater than 90% homogeneity as determined by SDS-PAGE analysis . Following purification, the protein is often lyophilized in a stabilizing buffer containing trehalose to maintain its structural integrity during storage.
Applications of Recombinant VirB3 Protein in Research and Diagnostics
The availability of purified recombinant VirB3 has facilitated various applications in both basic research and clinical diagnostics, providing valuable tools for understanding Brucella pathogenesis and improving disease detection.
Diagnostic Applications
Recent proteomics studies have identified VirB3 among several T4SS proteins that demonstrate potential as serological diagnostic antigens for brucellosis . Recombinant VirB3 exhibits particularly high specificity in diagnostic assays, although its sensitivity may be slightly lower than traditional diagnostic antigens such as lipopolysaccharide (LPS) and Rose Bengal antigen .
| VirB Protein | Diagnostic Sensitivity | Diagnostic Specificity | Notes |
|---|---|---|---|
| VirB3 | Moderate | High | Best specificity among VirB proteins |
| VirB4 | High | Moderate-High | Highest sensitivity |
| VirB9 | High | Moderate-High | High sensitivity |
| Traditional LPS | Very High | Very High | Gold standard |
While recombinant VirB3 may not outperform established diagnostic antigens, its specificity makes it valuable for confirmatory testing or as part of multi-antigen diagnostic panels that could improve the accuracy of brucellosis diagnosis .
Research Applications
Recombinant VirB3 serves as an essential tool for investigating T4SS assembly, structure, and function. The protein enables studies of protein-protein interactions within the secretion complex, identification of structural domains critical for function, and development of potential inhibitors that could disrupt T4SS activity.
Post-Replication Roles of VirB T4SS
Research using functionally controllable T4SS has revealed that this secretion system plays important roles not only in establishing the replicative niche but also in post-replication stages of the Brucella intracellular cycle . These findings suggest that VirB3, as a component of T4SS, may contribute to the conversion of replicative BCVs into autophagic vacuoles (aBCVs) that facilitate bacterial egress and subsequent infections.
Therapeutic Potential
The essential role of VirB3 in Brucella virulence positions it as a promising target for therapeutic intervention. Future research may focus on developing small-molecule inhibitors or peptide-based agents that disrupt VirB3 function, potentially leading to novel treatments for brucellosis that circumvent the challenges associated with antibiotic resistance.
Comparative Analysis with Other T4SS Components
The VirB system in Brucella comprises multiple proteins with distinct but interconnected functions. Understanding the relative contributions of each component provides context for appreciating VirB3's specific role.
Functional Hierarchy Among VirB Proteins
Studies of nonpolar deletions in the virB locus have established a hierarchy of importance among the VirB proteins. VirB3, along with VirB2, VirB4, VirB5, VirB6, VirB8, VirB9, VirB10, and VirB11, is essential for virulence in mice, while VirB1, VirB7, and VirB12 appear dispensable for persistence in this model . This differential requirement suggests specialized roles for each protein within the T4SS complex.
Interdependence Among T4SS Components
The observation that deletion of virB3 affects the stability of other VirB proteins highlights the interdependence among T4SS components . This interconnectedness suggests that VirB3 may function early in the assembly pathway, potentially serving as a nucleation point or stabilizing factor for subsequent protein recruitment.
Product Specs
Note: We prioritize shipping the format currently in stock. However, if you have specific format requirements, please indicate them in your order notes. We will fulfill your request if possible.
Note: All protein shipments are standardly packaged with blue ice packs. If you require dry ice shipping, please inform us in advance as additional charges will apply.
Generally, the shelf life of liquid form is 6 months at -20°C/-80°C. The shelf life of lyophilized form is 12 months at -20°C/-80°C.
Target Background
KEGG: bmf:BAB2_0066
Q&A
What is the Brucella abortus virB locus and how is virB3 organized within it?
The virB locus in Brucella abortus contains 12 open reading frames, designated virB1 through virB12, which collectively encode a Type IV secretion system. This genetic machinery is homologous to conjugation systems found in other bacteria. The virB3 gene is positioned within this cluster and encodes an essential component of the secretion apparatus . The entire virB locus functions as an integrated system, with most components being necessary for proper secretion function and virulence.
What is the functional significance of virB3 in Brucella abortus pathogenesis?
The virB3 protein is essential for Brucella abortus virulence. Research has definitively demonstrated that virB3 deletion significantly compromises the bacterium's ability to survive within macrophages and persist in host tissues. Experimental evidence shows that virB3, along with virB2, virB4, virB5, virB6, virB8, virB9, virB10, and virB11, is critical for B. abortus to establish and maintain infection in mouse models . Unlike some other virB components (virB1, virB7, and virB12), which appear dispensable for certain virulence traits, virB3 is absolutely required for both intracellular survival and in vivo persistence.
How does virB3 differ functionally from other virB components?
While many virB proteins (virB2-6, virB8-11) share the characteristic of being essential for both in vitro macrophage survival and in vivo virulence, virB3 belongs to this core group of indispensable T4SS components . Interestingly, not all virB proteins share this dual essentiality. For instance, virB1 only impacts macrophage survival without significantly affecting in vivo persistence, while virB7 and virB12 appear largely dispensable for both virulence traits . This functional differentiation suggests that virB3 plays a central structural or operational role in the T4SS that cannot be compensated by other components.
What techniques are effective for creating nonpolar deletions of virB3?
Creating nonpolar deletions of virB3 is critical for precise functional analysis without disrupting downstream gene expression. Researchers typically employ targeted gene replacement strategies using suicide vectors carrying the deletion construct. The process involves:
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PCR amplification of regions flanking the virB3 gene
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Fusion of these fragments to create a deletion construct
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Cloning into a suicide vector with a selectable marker
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Introduction into B. abortus via conjugation or electroporation
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Selection for double crossover events using appropriate markers
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Confirmation of deletion via PCR and/or Southern blot analysis
This approach ensures that the deletion affects only the virB3 gene without disrupting the expression of downstream genes in the virB operon . Such nonpolar mutations are essential for attributing phenotypes specifically to the absence of virB3 rather than polar effects on other virB genes.
What cellular and animal models are most appropriate for studying virB3 function?
Based on established research protocols, the following models have proven effective for studying virB3 function:
Cellular Models:
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J774A.1 mouse macrophage-like cells: These provide a reliable system for assessing intracellular survival and replication of B. abortus strains with virB3 mutations .
Animal Models:
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Mouse infection model: Intraperitoneal inoculation of mice with 1 × 10^5 to 5 × 10^5 CFU of B. abortus strains allows assessment of bacterial persistence in spleen and other organs over 8-week periods . This model effectively demonstrates the attenuated phenotype of virB3 deletion mutants.
When using these models, it's important to include appropriate controls, such as wild-type B. abortus and a complete virB locus deletion mutant, to provide reference points for interpreting the specific effects of virB3 deletion.
How can recombinant virB3 protein be produced for biochemical and structural studies?
Though the search results don't directly address recombinant virB3 production, insights can be drawn from methodologies used for other recombinant viral proteins. The approach would likely involve:
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Amplification of the virB3 gene by PCR from B. abortus genomic DNA
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Addition of appropriate restriction sites for directional cloning
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Insertion into an expression vector (such as pET or pUC series)
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Expression in a prokaryotic host system (E. coli strains optimized for protein expression)
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Protein purification via affinity chromatography using appropriate tags
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Characterization using techniques such as SDS-PAGE and Western blotting
From the methodology described for other recombinant proteins, researchers typically optimize codons, expression conditions, and purification protocols to maximize yield and purity .
How does virB3 contribute to Brucella abortus survival in macrophages?
The virB3 protein, as part of the Type IV secretion system, is essential for B. abortus survival within macrophages. Experimental evidence demonstrates that deletion of virB3 significantly reduces bacterial persistence in J774A.1 mouse macrophage-like cells, similar to the reduction seen with deletion of the entire virB locus . While the exact molecular mechanisms remain under investigation, the T4SS likely functions by:
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Facilitating the delivery of bacterial effector proteins into host cells
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Modifying the intracellular trafficking pathway to prevent lysosomal fusion
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Creating and maintaining a replicative niche within the host cell
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Manipulating host cell signaling to prevent antimicrobial responses
The virB3 protein likely serves as a structural component of the secretion apparatus, contributing to the assembly or stability of the T4SS complex.
What is known about the persistence of virB3 mutants in animal models?
Deletion of virB3 markedly reduces the ability of B. abortus to persist in the spleens of mice at 8 weeks post-infection . This attenuation is similar to that observed with deletions of virB2, virB4, virB5, virB6, virB8, virB9, virB10, and virB11, but contrasts with virB7, virB1, and virB12 mutants, which show varying degrees of in vivo persistence . The significant attenuation of virB3 mutants in the mouse model underscores the critical role of this protein in establishing chronic infection, making it a potential target for vaccine development or therapeutic intervention.
How might recombinant virB3 be utilized for immunodiagnostic purposes?
Drawing parallels from recombinant viral protein applications in diagnostics, recombinant virB3 could potentially be utilized for developing:
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ELISA-based diagnostic tests to detect anti-virB3 antibodies in infected hosts
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Lateral flow immunochromatographic assays for rapid field diagnosis
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Protein microarray platforms for multiplex detection of brucellosis
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Immunohistochemical reagents for tissue section analysis
The development of such diagnostic applications would require validating the immunogenicity of recombinant virB3 and determining the presence and persistence of anti-virB3 antibodies during different stages of infection .
What are the prospects for using virB3 in subunit vaccine development?
Based on methodologies described for other viral proteins, virB3 could be investigated as a subunit vaccine candidate through:
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Expression and purification of recombinant virB3 protein
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Characterization of its immunogenic properties
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Formulation with appropriate adjuvants
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Evaluation of protective immunity in animal models
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Assessment of humoral and cell-mediated immune responses
Given that virB3 is essential for virulence, antibodies or cell-mediated responses targeting this protein might interfere with the establishment of infection. The potential for using virB3 as part of a subunit vaccine would depend on identifying immunodominant epitopes and demonstrating protective efficacy .
What are the current limitations in understanding virB3 structure-function relationships?
Current research limitations include:
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Limited structural information about virB3 and its interactions with other T4SS components
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Incomplete understanding of the precise role of virB3 in T4SS assembly and function
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Challenges in expressing and purifying functional recombinant virB3 protein
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Limited data on virB3 protein-protein interactions within the secretion apparatus
Future research should focus on crystallographic or cryo-EM studies of virB3 alone and in complex with other T4SS components to elucidate its structural role in the secretion apparatus.
How might systems biology approaches enhance our understanding of virB3 function?
Systems biology approaches could provide valuable insights into virB3 function through:
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Transcriptomic analysis of host cells infected with wild-type versus virB3 mutant strains
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Proteomic identification of host proteins interacting with virB3 or affected by its presence
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Metabolomic profiling to identify metabolic pathways influenced by virB3-dependent processes
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Network analysis to place virB3 within the broader context of bacterial virulence systems
These approaches could reveal previously unrecognized roles of virB3 in pathogenesis and potentially identify new targets for therapeutic intervention.
