Recombinant Renibacterium salmoninarum ATP synthase subunit b (atpF)

What is Renibacterium salmoninarum and why is it significant in research?

Renibacterium salmoninarum is a Gram-positive, intracellular pathogen that causes Bacterial Kidney Disease (BKD) in several fish species in both freshwater and seawater environments. It poses a significant threat to healthy and sustainable production of salmonid fish worldwide . The pathogen is particularly challenging for researchers because it is difficult to culture in vitro, genetic manipulation is challenging, and current therapies and preventative strategies have limited effectiveness . The complete genome of R. salmoninarum ATCC 33209 has been sequenced, revealing a 3,155,250-bp circular chromosome with 3,507 predicted open-reading frames (ORFs), approximately 21% of which have been inactivated through various genetic mechanisms .

What is ATP synthase subunit b (atpF) and what is its function in bacterial systems?

ATP synthase subunit b (atpF) is a critical component of the F-type ATPase system that produces ATP from ADP in the presence of a proton gradient across the membrane. The protein functions within the F₀ domain containing the membrane proton channel . During catalysis, ATP synthesis in the catalytic domain of F₁ is coupled via a rotary mechanism of the central stalk subunits to proton translocation, with atpF playing an essential role in this energy conversion process . In R. salmoninarum, the atpF protein contributes to the peripheral stalk structure that connects the F₁ and F₀ domains, helping maintain the structural integrity of the ATP synthase complex during ATP production.

What expression systems are recommended for producing recombinant R. salmoninarum atpF?

For recombinant expression of R. salmoninarum atpF, Escherichia coli BL21(DE3) cells represent the most widely used expression system due to their high efficiency and ease of genetic manipulation . When designing expression constructs, researchers should consider including fusion tags that facilitate both purification and detection. A recommended approach includes:

• Using a pET-based vector system with a T7 promoter for high-level expression

• Incorporating a 6×His tag for purification via IMAC (immobilized metal affinity chromatography)

• Adding fluorescent protein tags (such as EGFP or mCherry) when visualization is required

• Potentially including a TAT-HA (trans-activator of transduction-hemagglutinin) tag for mammalian cell transduction studies

The expression construct should be carefully designed to maintain the correct folding of atpF, which may require optimization of induction conditions (temperature, IPTG concentration, and induction time).

What purification strategies yield the highest purity of recombinant atpF?

The most effective purification strategy for recombinant atpF protein involves a multi-step approach:

• Initial Capture: Immobilized metal affinity chromatography (IMAC) using Ni-nitrilotriacetic acid (Ni-NTA) columns is highly effective when the protein contains a 6×His tag .

• Intermediate Purification: Size exclusion chromatography (SEC) to separate monomeric atpF from aggregates and contaminants of different molecular weights.

• Polishing: Ion exchange chromatography (IEX) based on the predicted isoelectric point of atpF to remove closely related impurities.

Buffer optimization is critical, typically requiring:

• 50 mM Tris-HCl (pH 8.0)

• 300 mM NaCl

• 10% glycerol to enhance stability

• Protease inhibitors during initial extraction steps

For membrane-associated proteins like atpF, including 0.1% non-ionic detergent (such as n-dodecyl-β-D-maltoside) in buffers may improve solubility and reduce aggregation during purification.

How can researchers verify the structural integrity of purified recombinant atpF?

Verification of structural integrity requires multiple complementary approaches:

• SDS-PAGE and Western Blotting: To confirm the correct molecular weight and immunoreactivity

• Circular Dichroism (CD) Spectroscopy: To assess secondary structure elements and proper folding

• Thermal Shift Assays: To evaluate protein stability and domain integrity

• Limited Proteolysis: To confirm the expected domain organization

• Dynamic Light Scattering (DLS): To assess homogeneity and detect aggregation

For functional verification, ATP hydrolysis assays in reconstituted liposomes can determine if the recombinant atpF can associate with other ATP synthase components to form a functional complex.

How can recombinant atpF be used to study R. salmoninarum pathogenesis?

Recombinant atpF can serve as a valuable tool in understanding R. salmoninarum pathogenesis through several experimental approaches:

• Immunological Studies: Purified atpF can be used to raise antibodies for tracking bacterial localization in infected tissues

• Protein-Protein Interaction Studies: Pull-down assays using tagged atpF to identify host proteins that interact with this bacterial component

• Immunization Trials: Evaluating the potential of atpF as a vaccine candidate against BKD

• Virulence Assessment: Creating atpF mutants or utilizing recombinant atpF to block native protein function and assess effects on bacterial survival and virulence

The role of atpF in bacterial energy metabolism makes it particularly relevant for studying survival mechanisms of R. salmoninarum within host cells, as intracellular pathogens must adapt their metabolism to the host environment .

What experimental models are appropriate for studying atpF function in the context of bacterial kidney disease?

Several experimental models can be employed to study atpF function in BKD:

• In vitro cellular models:

  • Fish cell lines (especially kidney-derived)

  • Primary kidney macrophages from salmonid species

  • Co-culture systems mimicking host-pathogen interactions

• Ex vivo tissue models:

  • Precision-cut kidney slices from susceptible species

  • Explant cultures maintaining tissue architecture

• In vivo models:

  • Lumpfish (Cyclopterus lumpus) - recently demonstrated as susceptible to R. salmoninarum

  • Traditional salmonid models (Atlantic salmon, rainbow trout)

  • Zebrafish for high-throughput preliminary studies

When using lumpfish as a model, researchers should consider the demonstrated infection kinetics: intraperitoneal injection of R. salmoninarum (1×10⁹ cells dose⁻¹) results in approximately 65% survival rate, with mortality stabilizing after 50 days post-infection, though the pathogen persists in tissues until at least 98 days post-infection .

What controls should be included in experiments involving recombinant atpF?

Robust experimental design for studies involving recombinant atpF should include:

• Negative controls:

  • Buffer-only treatments

  • Irrelevant recombinant proteins with similar tags and production methods

  • Heat-denatured atpF to control for non-specific effects

• Positive controls:

  • Known immunogenic proteins from R. salmoninarum

  • Complete bacterial cells for comparative studies

  • Commercial ATP synthase components from related organisms

• Technical validation controls:

  • Multiple biological replicates (minimum n=6 for immunological studies)

  • Tag-only protein constructs to account for tag-related effects

  • Gradient of protein concentrations to establish dose-response relationships

These controls help distinguish specific atpF-related effects from non-specific or technical artifacts.

How can computational tools like AlphaMissense be applied to analyze atpF variants?

AlphaMissense represents a cutting-edge computational approach for predicting the pathogenicity of missense variants in proteins . For atpF research, this tool can be applied to:

• Identify Functional Hotspots: By analyzing predicted pathogenicity scores across the protein sequence to identify regions critical for function

• Assess Conservation: Comparing variant pathogenicity predictions across atpF from different bacterial species to identify conserved functional domains

• Structural Impact Assessment: Combining AlphaMissense predictions with structural data to visualize how variants might affect protein folding and function

When applying AlphaMissense to atpF analysis, researchers should note the tool's varying performance across different protein types - it shows MCC (Matthews Correlation Coefficient) scores predominantly between 0.6 and 0.74, with lower performance on disordered protein regions . The tool's effectiveness can be optimized by focusing on high-confidence AlphaFold segments of the protein structure.

What advanced techniques can characterize atpF interactions with other ATP synthase components?

These techniques can be complementarily employed to build a comprehensive understanding of how atpF integrates into the ATP synthase complex and contributes to its function.

How can researchers investigate the role of atpF in R. salmoninarum survival within host cells?

Investigating the role of atpF in intracellular survival requires specialized approaches:

• Conditional Knockdown Systems:

  • Inducible antisense RNA targeting atpF

  • CRISPR interference (CRISPRi) for partial gene repression

  • Temperature-sensitive mutants if available

• Functional Complementation:

  • Trans-complementation with wild-type and mutant atpF variants

  • Heterologous expression in surrogate bacterial systems

• Metabolic Analysis:

  • ATP/ADP ratio measurements during infection

  • Membrane potential assessment using fluorescent probes

  • Respirometry to measure oxygen consumption

• Imaging Approaches:

  • Fluorescently tagged atpF to track localization during infection

  • Super-resolution microscopy to visualize ATP synthase assembly

  • Live-cell imaging to monitor dynamic processes

These approaches should be combined with gene expression analysis of both pathogen and host to correlate atpF function with adaptation to the intracellular environment.

How does R. salmoninarum infection affect host immune responses, and how might atpF contribute?

R. salmoninarum infection elicits a complex immune response in host fish. Studies in lumpfish have shown that:

• Early infection (28 days post-infection):

  • Upregulation of cytokines (il1β, il8a, il8b)

  • Increased expression of pattern recognition receptors (tlr5a)

  • Elevated interferon-induced effectors (rsad2, mxa, mxb, mxc)

  • Enhanced iron regulation (hamp) and acute phase reactants (saa5)

• Concurrent immunosuppression:

  • Downregulation of cell-mediated adaptive immunity genes (cd4a, cd4b, ly6g6f, cd8a, cd74) at 28 days post-infection

  • This reveals the immune suppressive nature of R. salmoninarum

• Late infection (98 days post-infection):

  • Significant upregulation of cd74, suggesting eventual induction of cell-mediated immune response

The ATP synthase complex, including atpF, may contribute to this immunomodulation through:

• Providing energy for bacterial persistence

• Potentially serving as a pathogen-associated molecular pattern (PAMP)

• Contributing to membrane integrity necessary for bacterial survival during immune attack

What methodologies are recommended for studying atpF immunogenicity?

To comprehensively evaluate atpF immunogenicity, researchers should employ:

• In silico epitope prediction:

  • B-cell epitope prediction algorithms

  • MHC binding prediction for T-cell epitopes

  • Cross-referencing with known immunogenic epitopes in related bacteria

• In vitro assays:

  • Peripheral blood leukocyte stimulation with recombinant atpF

  • Cytokine expression profiling following exposure

  • Antibody binding assays using sera from infected fish

• In vivo studies:

  • Immunization trials with purified atpF with appropriate adjuvants

  • Challenge studies to assess protection levels

  • Adoptive transfer experiments to determine protective immune components

Experimental designs should include time-course studies with sampling at multiple timepoints (e.g., 28 and 98 days post-infection) to capture the dynamic nature of the immune response observed in previous studies .

How can researchers differentiate between atpF effects and those of other ATP synthase components?

Differentiating the specific effects of atpF from other ATP synthase components requires:

These approaches should be guided by the known interaction network of ATP synthase components, including atpA (ATP synthase alpha chain), atpB (ATP synthase A chain), atpE (ATP synthase C chain), atpF (ATP synthase B chain), and atpH (ATP synthase delta chain) .