Recombinant Ralstonia solanacearum Hypersensitivity response secretion protein hrcV (hrcV)
Description
Introduction to HrcV Protein in Ralstonia solanacearum
Ralstonia solanacearum is ranked as the second most important bacterial plant pathogen globally, causing devastating bacterial wilt disease in more than 200 plant species, including economically important crops . The pathogenicity of R. solanacearum relies heavily on its Type III Secretion System (T3SS), which is encoded by the hypersensitivity response and pathogenicity (hrp) genes . Within this system, the HrcV protein serves as a critical component of the secretion apparatus, facilitating the translocation of type III effectors (T3Es) into host plant cells . These effectors subsequently suppress plant defense mechanisms and promote bacterial colonization. The HrcV protein belongs to the FHIPEP (flagella/hypersensitivity response/invasion proteins export pore) family, a conserved group of proteins involved in protein secretion across bacterial membranes .
Significance in Bacterial Pathogenicity
The HrcV protein is essential for the virulence of R. solanacearum. Studies have consistently demonstrated that mutants defective in hrcV are non-pathogenic on susceptible hosts such as tomato and eggplant . These mutants fail to elicit the hypersensitive response in resistant plants and cannot cause disease in susceptible hosts, underscoring the critical role of HrcV in the infection process . Furthermore, HrcV- mutants show significantly reduced colonization ability in plant vascular tissues, which is a key factor in the progression of bacterial wilt disease .
The FHIPEP Motif and Its Functional Significance
A distinguishing feature of HrcV is the presence of the conserved FHIPEP (flagella/hypersensitive response/invasion proteins export pore) motif . This motif is located in a cytoplasmic loop between transmembrane helices four and five and plays a crucial role in protein function . Research on the Xanthomonas HrcV homolog demonstrated that mutations in the FHIPEP motif abolish the protein's function but do not affect its interaction with effector proteins . This suggests that the FHIPEP domain is specifically involved in the interaction with T3SS components and early substrates rather than with the effector proteins themselves.
Functional Role of HrcV in Type III Secretion System
The HrcV protein serves as an integral component of the T3SS machinery, playing multiple roles in the secretion and translocation of bacterial effector proteins.
Interaction with T3SS Components
HrcV interacts with several other components of the T3SS apparatus, forming part of a complex molecular machine. Protein interaction studies have identified strong associations between HrcV and other Hrc proteins, including HrcS, HrcR, HrcT, HrcJ, and HrcU, with interaction scores above 0.99 . These interactions are essential for assembling a functional T3SS capable of delivering effector proteins into host cells.
The protein interaction network of HrcV is summarized in the following table:
| Interacting Partner | Protein Family | Interaction Score |
|---|---|---|
| HrcS | Hrp conserved transmembrane protein | 0.998 |
| HrcR | FliP/MopC/SpaP family | 0.997 |
| HrcT | Hrp conserved transmembrane protein | 0.997 |
| HrcJ | Hrp conserved lipoprotein | 0.995 |
| HrcU | Type III secretion exporter family | 0.993 |
| HrcN | Hrp conserved protein | 0.981 |
| HrcQ | Hrp conserved protein | 0.972 |
| HrpW | Hrp transmembrane protein | 0.955 |
| HrcC | T3SS outer membrane component | 0.936 |
| FliN | Flagellar motor switch protein | 0.933 |
Substrate Recognition and Translocation Function
The cytoplasmic domain of HrcV (HrcVC) plays a critical role in recognizing and binding to T3SS substrates . Studies on the Xanthomonas HrcV homolog have shown that this domain interacts with early T3SS substrates like HrpB2, the pilus protein HrpE, and various effector proteins . These interactions are essential for the ordered secretion of proteins through the T3SS.
In R. solanacearum, HrcV is specifically required for the translocation of effector proteins such as the AvrA avirulence protein into tobacco cells . While HrcV is not necessary for the secretion of effector proteins into the extracellular environment, it is essential for their delivery across the plant cell wall and membrane into the host cytoplasm . This translocation function distinguishes HrcV as a key player in the infection process.
Recombinant Production of HrcV Protein
Recombinant production has made HrcV protein available for detailed structural and functional studies, as well as for various research applications.
Expression Systems and Protein Characteristics
The full-length HrcV protein from R. solanacearum can be expressed as a recombinant protein in Escherichia coli expression systems . Commonly, the protein is produced with an N-terminal His-tag to facilitate purification . The recombinant protein maintains the full 690 amino acid sequence of the native protein, preserving its structural and functional characteristics.
The specifications of commercially available recombinant HrcV protein include:
| Parameter | Specification |
|---|---|
| Species | Ralstonia solanacearum |
| Source | E. coli |
| Tag | His |
| Protein Length | Full Length (1-690) |
| Form | Lyophilized powder |
| Purity | >90% as determined by SDS-PAGE |
| Applications | SDS-PAGE |
Development and Applications of HrcV Mutants
HrcV mutants have proven to be valuable tools for studying bacterial pathogenicity mechanisms and for developing strategies to control bacterial wilt disease.
Generation of HrcV- Mutants through Insertional Mutagenesis
HrcV- mutants of R. solanacearum can be created through insertional mutagenesis, a technique involving the integration of a plasmid into the hrcV gene through homologous recombination . This method disrupts the gene, resulting in a non-functional HrcV protein. The procedure typically involves cloning an internal fragment of the hrcV gene into a suitable vector, which is then introduced into R. solanacearum . The integration of the plasmid is confirmed through diagnostic PCR targeting the junction region between the bacterial chromosome and the inserted plasmid .
This method of creating insertional mutants is described as "simple, time-saving, and cost-efficient" and can be used for developing mutants for various genes in R. solanacearum . The stability of these mutants has been confirmed through in vitro studies showing that the plasmid integration remains stable for several generations .
Pathogenicity and Colonization Studies
HrcV- mutants of R. solanacearum display significantly altered pathogenicity and colonization patterns compared to wild-type strains. These mutants are non-pathogenic on susceptible hosts like tomato and eggplant, failing to cause wilt symptoms even when introduced directly into the vascular system through petiole inoculation . Furthermore, the colonization ability of HrcV- mutants is greatly reduced, with bacterial populations remaining below the threshold required to initiate wilt (typically 10^8 CFU) .
Interestingly, studies on the colonization patterns of HrcV- mutants in tomato have revealed complex interactions when co-inoculated with wild-type strains. Both strains can be found together in infected root tips and lateral root emergence sites, but subsequently invade separate xylem vessels in the root system . At the hypocotyl level, three vascular colonization patterns have been observed: exclusive colonization by each strain or simultaneous presence of both strains in separate xylem vessels . The presence of the HrcV- mutant strain has been shown to reduce the population density of the wild-type strain, suggesting potential applications in biological control of bacterial wilt .
Applications in Type III Effector Validation
One of the most valuable applications of HrcV- mutants is in validating putative Type III effectors through translocation studies . Since these mutants can express but cannot translocate effector proteins into host cells, they provide an excellent system for confirming that a candidate protein is indeed delivered via the T3SS . This application has been crucial in identifying and characterizing the extensive repertoire of effector proteins in R. solanacearum, which includes over 70 different T3Es in some strains .
Comparative Analysis of HrcV Across Bacterial Species
The HrcV protein belongs to a conserved family of inner membrane proteins found in various plant and animal pathogenic bacteria with Type III secretion systems.
Homology with Other Bacterial Secretion System Components
HrcV shows significant homology with proteins from other bacterial species, including the YscV protein from Yersinia spp. and the FlhA protein from flagellar systems . In plant pathogenic bacteria, close homologs include the HrcV protein from Xanthomonas spp. . These homologies reflect the evolutionary conservation of the T3SS across diverse bacterial lineages.
Functional conservation between HrcV homologs has been demonstrated experimentally. For instance, the hrpF gene of Xanthomonas campestris pv. campestris can partially restore the HR-inducing ability of popF1 popF2 mutants of R. solanacearum, suggesting functional conservation between the T3SS translocators in these bacteria .
Variability Within Ralstonia solanacearum Strains
Within the R. solanacearum species complex, HrcV shows conservation but also some variation between different strains and phylotypes. For example, while strain GMI1000 (phylotype I) contains a typical HrcV protein, strain UW551, which belongs to a different phylotype, possesses two putative translocator proteins with different characteristics . This variability may reflect adaptations to different host ranges or environmental conditions.
The natural transformability of R. solanacearum further contributes to genetic diversity, with studies showing that horizontal gene transfer can occur between different strains . This process could potentially lead to the acquisition of new variants of T3SS components, including HrcV, contributing to the evolution of virulence in this bacterial species.
Future Perspectives in HrcV Research
Research on the HrcV protein continues to advance our understanding of bacterial pathogenicity mechanisms and opens new avenues for controlling bacterial wilt disease.
Applications in Disease Control Strategies
The non-pathogenic nature of HrcV- mutants, combined with their ability to colonize plant vascular tissues and reduce wild-type pathogen populations, suggests potential applications in biocontrol strategies . Further research on the mechanisms underlying this antagonistic effect could lead to the development of effective biological control agents for bacterial wilt disease.
Additionally, understanding the structure-function relationships of HrcV could inform the design of chemical compounds targeting this protein, potentially leading to new bactericides specific to plant pathogenic bacteria with minimal environmental impact.
Product Specs
Note: While we prioritize shipping the format currently in stock, please specify your format preference during order placement for customized preparation.
Note: Our proteins are shipped with standard blue ice packs. Dry ice shipping requires prior arrangement and incurs additional charges.
The specific tag type is determined during production. If you require a specific tag, please inform us; we will prioritize its development.
Target Background
KEGG: rso:RSp0863
Q&A
What is the role of hrcV protein in Ralstonia solanacearum pathogenicity?
The hrcV protein is a transmembrane protein that forms a critical component of the hrp Pili, the needle-like apparatus involved in translocation of Type Three Effectors (T3Es) into host plant cells . It functions as an essential structural component of the Type Three Secretion System (T3SS), which is a major virulence determinant in R. solanacearum .
While the transcriptional regulation of T3Es is controlled by hrpB, the actual secretion of these effectors depends on the pili formed by the hrcV gene . Experimental evidence has demonstrated that mutations in hrcV result in non-pathogenic strains that fail to secrete T3Es into the external environment, even though the effectors may still be expressed .
How are hrcV mutants of R. solanacearum typically generated for research purposes?
Insertional mutagenesis is the most commonly employed method for developing hrcV mutants. The procedure involves:
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Amplification of an internal fragment of the hrcV gene (typically 763 bp)
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Cloning this fragment into a suitable vector (e.g., pTZ57R/T)
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Selection of transformants where the plasmid has integrated into the bacterial chromosome through homologous recombination
This method has been demonstrated to be simple, reliable, and produces stable mutations that persist for several generations even without antibiotic selection pressure . The integration can be confirmed using diagnostic primers designed to amplify across the junction between genomic DNA and the integrated plasmid, with successful hrcV disruption confirmed by a 1624 bp band when using M13 reverse primers .
What phenotypic characteristics distinguish hrcV mutants from wild-type R. solanacearum?
The key phenotypic differences between hrcV mutants and wild-type strains include:
Interestingly, while hrcV mutants show reduced colonization ability compared to wild-type strains when inoculated alone, their growth can be significantly increased in the presence of pathogenic strains .
How does an HrcV⁻ mutant colonize plant xylem vessels differently from wild-type strains, and what implications does this have for biological control?
The colonization pattern of HrcV⁻ mutants in plant vascular tissues follows a distinct pattern that differs significantly from wild-type strains. Microscopic studies using dual-marked strains (lacZ and uidA) have revealed three primary vascular colonization patterns when both strains are present:
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Exclusive colonization by the mutant strain
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Exclusive colonization by the wild-type strain
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Simultaneous presence of both strains in separate xylem vessels
The relative frequencies of these patterns depend on the inoculation technique employed. In root tissues, both strains can initially be found together at infection sites like root tips and lateral root emergence points, but subsequently invade separate xylem vessels . This spatial separation appears to be a key factor in the biological control potential of HrcV⁻ mutants.
When plants are inoculated with both wild-type and mutant strains, the population density of wild-type bacteria is lower than in plants inoculated with wild-type alone, suggesting competitive inhibition . Surprisingly, growth of the HrcV⁻ mutant is significantly enhanced in the presence of the pathogenic strain , indicating a complex interaction between these populations.
The method of inoculation significantly impacts the effectiveness of biological control. Transplanting tomato plants into soil amended with clay micro-granules impregnated with the HrcV⁻ mutant provides higher and more reproducible colonization than direct watering with bacterial suspensions . This method also yields a higher percentage of exclusive colonization by the mutant strain, which correlates with improved disease control .
What methodologies are most effective for expressing and purifying recombinant hrcV protein for structural and functional studies?
Recombinant expression of full-length R. solanacearum hrcV protein can be achieved using the following methodology:
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Expression System Selection: E. coli is the preferred heterologous expression system for recombinant hrcV protein production .
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Vector Construction:
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Protein Expression Conditions:
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Optimal induction conditions should be determined empirically
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Expression at lower temperatures (15-25°C) may improve proper folding of this transmembrane protein
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Purification Protocol:
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Quality Assessment:
For reconstitution of lyophilized protein, it is recommended to centrifuge the vial briefly prior to opening and reconstitute the protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL with 5-50% glycerol for long-term storage .
How does natural transformation in R. solanacearum impact the genetic stability of hrcV mutants and what implications does this have for laboratory and field research?
R. solanacearum is naturally transformable both in vitro and in planta, capable of acquiring free exogenous DNA . This characteristic has significant implications for the stability of hrcV mutants:
What are the regulatory pathways controlling hrcV expression in R. solanacearum and how do they interact with other virulence mechanisms?
The expression of hrcV, as part of the T3SS in R. solanacearum, is regulated through a complex network:
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Primary Regulatory Pathway:
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Novel Regulatory Elements:
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Integration with Other Virulence Mechanisms:
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Expression Timing and Conditions:
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Environmental factors (pH, temperature, plant-derived signals) modulate expression
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Expression is typically induced upon contact with plant cells
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The quorum sensing system influences expression timing in relation to bacterial population density
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Understanding these regulatory networks is crucial for designing effective research approaches and potential biocontrol strategies targeting the T3SS system.
How do genomic variations across the R. solanacearum species complex affect hrcV structure and function?
The R. solanacearum species complex (RSSC) displays significant genomic diversity that impacts hrcV and other T3SS components:
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Phylogenetic Distribution:
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The RSSC is divided into four phylotypes (I-IV), with further classification into species: R. solanacearum, R. pseudosolanacearum, and R. syzygii
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Phylotype II is the most diverse and may represent the ancestral group of the RSSC
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The core genome (present in all strains) represents approximately 60% of the average RSSC genome
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Genetic Variation Impact on hrcV:
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Recombination Effects:
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Methodological Considerations:
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When characterizing hrcV function across the species complex, researchers should consider:
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Understanding these genomic variations is crucial for accurately interpreting hrcV function across different R. solanacearum strains and for developing broadly effective control strategies.
