Recombinant Salmonella paratyphi B ATP synthase subunit a (atpB)

How does atpB contribute to Salmonella pathogenesis?

The atpB gene product plays a significant role in Salmonella virulence through its function in energy metabolism and bacterial adaptation to host environments. Research has demonstrated that mutations in atpB lead to substantial attenuation in multiple Salmonella serovars, including Typhimurium, Dublin, and Gallinarum, across various host systems .

Mechanistically, atpB contributes to pathogenesis in several ways:

• Maintenance of intracellular pH: ATP synthase is critical for bacterial adaptation to acidic environments encountered within host cells, particularly phagocytes.

• Energy provision for virulence factor expression: The ATP generated by the F1Fo complex provides energy for numerous virulence mechanisms.

• Interaction with virulence regulators: The ATP synthase subunit interacts with virulence proteins such as MgtC, which inhibits ATP synthase activity during infection .

Studies have shown that atpB mutants exhibit reduced invasiveness in vivo, decreased survival within phagocytic cells, and impaired acid tolerance, all of which contribute to attenuated virulence .

What are the optimal storage conditions for recombinant Salmonella paratyphi B atpB protein?

For optimal stability and activity, recombinant Salmonella paratyphi B ATP synthase subunit a should be stored according to the following protocol:

• Short-term storage (1-2 weeks): Store working aliquots at 4°C in a Tris-based buffer containing 50% glycerol.

• Long-term storage: Maintain at -20°C, with extended storage preferably at -80°C to prevent protein degradation.

• Avoid repeated freeze-thaw cycles, as this can significantly reduce protein activity and structural integrity.

• For experimental use, prepare small working aliquots that can be used within one week .

The protein is typically supplied in a stabilizing buffer optimized for this specific membrane protein, which helps maintain its native conformation and functional properties .

How does the interaction between MgtC and atpB affect Salmonella virulence regulation?

The interaction between MgtC and atpB represents a sophisticated regulatory mechanism in Salmonella virulence. MgtC, a virulence factor expressed within macrophages, directly binds to and inhibits the F1Fo ATP synthase, with atpB being a key target of this interaction .

This inhibitory interaction serves several critical functions:

• Cytoplasmic pH regulation: By inhibiting ATP synthase, MgtC helps Salmonella maintain its cytoplasmic pH near 7 when experiencing the mildly acidic environment inside macrophages .

• Metabolic adaptation: This inhibition reduces cellular ATP levels, which correlates with decreased rRNA transcription when cytosolic conditions prevent functional ribosome assembly .

• Virulence gene expression: MgtC prevents degradation of PhoP, a master regulator of Salmonella pathogenesis, thereby enhancing virulence gene expression .

Interestingly, Salmonella produces CigR, an anti-virulence protein that competes with atpB for binding to MgtC. When CigR binds MgtC, it prevents MgtC from inhibiting the ATP synthase, thereby establishing a regulatory threshold. The virulence program is only activated when MgtC protein levels exceed those of CigR .

Experimental data shows that cigR mutants exhibit lower ATP levels and ATPase activity compared to wild-type Salmonella, which are opposite to the phenotypes displayed by mgtC mutants. This finding confirms that CigR functions by counteracting MgtC's inhibition of the ATP synthase .

What approaches can be used to study atpB mutations for vaccine development?

Developing attenuated Salmonella vaccines through atpB mutation requires a systematic approach:

• Mutation Generation Strategies:

  • Site-directed mutagenesis targeting conserved functional domains

  • Whole gene deletion using homologous recombination

  • Point mutations in critical residues identified through structural analysis

• Attenuation Assessment Protocol:

  • In vitro growth curves in various media conditions

  • Acid tolerance response testing (pH 3.0-5.0)

  • Intracellular survival assays in macrophage cell lines

  • Animal virulence models specific to the Salmonella serovar

Studies have demonstrated that mutations in the atp operon (including atpB and atpH) consistently produce high levels of attenuation across multiple Salmonella serovars, making them excellent candidates for broad-spectrum vaccine development .

When developing atpB-based vaccines, researchers should consider the following factors:

• Stability of attenuation in vivo

• Immunogenicity of the attenuated strain

• Potential for reversion to virulence

• Balance between attenuation and immunostimulatory properties

How can researchers differentiate between the direct and indirect effects of atpB mutations on Salmonella virulence?

Differentiating between direct and indirect effects of atpB mutations requires a multi-faceted experimental approach:

• Complementation Studies:

  • Express wild-type atpB from a plasmid in the mutant strain

  • Compare phenotype restoration to confirm direct causality

  • Use point mutations in key functional domains to identify essential residues

• Protein-Protein Interaction Analysis:

  • Perform co-immunoprecipitation experiments to identify interaction partners

  • Use bacterial two-hybrid systems to confirm direct protein interactions

  • Conduct pull-down assays with purified components to verify direct binding

• Metabolic Profiling:

  • Compare ATP levels, proton motive force, and intracellular pH between wild-type and mutant strains

  • Measure expression of genes affected by energy status using qRT-PCR

  • Perform metabolomic analysis to identify downstream metabolic alterations

• Temporal Analysis:

  • Study the expression patterns of atpB and interacting proteins (like MgtC and CigR) over time during infection

  • Compare when phenotypic changes occur relative to protein expression changes

Research has shown that the effects of atpB mutation on virulence are likely both direct (through impaired energy production) and indirect (through altered expression of virulence factors). For example, experiments with the regulators CigR and MgtC demonstrate that when MgtC protein amounts exceed those of CigR (approximately 4 hours after induction), ATP levels decrease significantly, indicating that timing of protein expression is critical for virulence regulation .

What are the optimized protocols for expressing and purifying recombinant Salmonella paratyphi B atpB?

Successful expression and purification of recombinant Salmonella paratyphi B atpB requires specialized techniques for membrane proteins:

• Expression System Selection:

  • E. coli C41(DE3) or C43(DE3) strains specifically designed for membrane protein expression

  • Vector systems with tunable promoters (e.g., T7lac) to control expression levels

  • Inclusion of appropriate fusion tags (His-tag, MBP, or SUMO) to aid solubility and purification

• Optimized Expression Protocol:

  • Culture growth at lower temperatures (16-20°C) after induction

  • Use of reduced inducer concentrations (0.1-0.4 mM IPTG)

  • Extended expression periods (16-24 hours)

  • Supplementation with membrane-stabilizing additives

• Membrane Protein Extraction:

  • Gentle cell lysis using French press or sonication

  • Membrane fraction isolation through differential centrifugation

  • Solubilization using appropriate detergents (DDM, LDAO, or C12E8)

  • Detergent screening to maintain protein stability and activity

• Purification Strategy:

  • Immobilized metal affinity chromatography (IMAC) as initial capture step

  • Size exclusion chromatography for further purification

  • Buffer optimization containing glycerol and specific detergent concentrations

  • Quality assessment using SDS-PAGE, Western blotting, and activity assays

The purified protein should be stored in Tris-based buffer with 50% glycerol at -20°C for short-term use or -80°C for long-term storage .

How can researchers effectively measure the functional activity of recombinant atpB in vitro?

Assessing the functional activity of recombinant atpB requires techniques that can measure its role in the ATP synthase complex:

• ATP Hydrolysis (ATPase) Activity:

  • Spectrophotometric assay coupling ATP hydrolysis to NADH oxidation

  • Measurement of inorganic phosphate release using colorimetric methods

  • Comparison of activity in the presence and absence of specific inhibitors

• Proton Translocation Measurement:

  • Reconstitution of purified protein into liposomes

  • Monitoring pH changes using pH-sensitive fluorescent dyes

  • Assessment of membrane potential using voltage-sensitive probes

• Protein-Protein Interaction Assays:

  • Surface plasmon resonance (SPR) to determine binding kinetics with partners like MgtC

  • Microscale thermophoresis to measure binding affinities

  • FRET-based assays to monitor interactions in real-time

• Structural Integrity Assessment:

  • Circular dichroism spectroscopy to confirm secondary structure

  • Limited proteolysis to verify proper folding

  • Thermal shift assays to determine protein stability

Experimental data from wild-type and mutant Salmonella strains has shown that changes in ATP synthase activity correlate with virulence phenotypes. For example, cigR mutants exhibited lower ATPase activity than wild-type Salmonella, while mgtC mutants showed higher activity, demonstrating the regulatory relationship between these proteins and ATP synthase function .

What techniques are most effective for studying atpB-protein interactions in Salmonella?

Several techniques have proven valuable for investigating atpB interactions with other proteins:

• Co-immunoprecipitation (Co-IP):

  • Use of specific antibodies against atpB or its interaction partners

  • Cross-linking approaches to capture transient interactions

  • Mass spectrometry analysis of co-precipitated proteins

  • Western blot validation of specific interaction partners

• Bacterial Two-Hybrid Systems:

  • Adaptation of bacterial two-hybrid assays for membrane protein interactions

  • Use of split-ubiquitin systems specifically designed for membrane proteins

  • Quantification of interaction strength through reporter gene expression

• Microscopy-Based Approaches:

  • Fluorescence colocalization studies using fluorescent protein fusions

  • FRET microscopy to detect direct protein interactions in bacterial cells

  • Super-resolution microscopy to precisely locate interaction complexes

• Biochemical Validation:

  • Pull-down assays using purified components

  • Competition assays to determine binding sites and affinities

  • Mutagenesis studies to identify critical residues for interaction

Research has demonstrated that MgtC interacts directly with atpB, and this interaction is disrupted by the anti-virulence protein CigR. The MgtC N92T variant was defective for interaction with atpB, highlighting the importance of specific residues in mediating protein-protein interactions .

How do mutations in atpB affect Salmonella survival within macrophages?

ATP synthase mutations significantly impact Salmonella survival within macrophages through multiple mechanisms:

• Intracellular pH Regulation:

  • Wild-type Salmonella maintains near-neutral cytoplasmic pH despite acidic macrophage environments

  • atpB mutants exhibit impaired ability to regulate internal pH, leading to decreased survival

  • Cytoplasmic pH measurements show that cigR mutants (which have enhanced MgtC inhibition of ATP synthase) maintain higher pH than wild-type Salmonella inside macrophages

• Energy Production:

  • ATP synthesis is crucial for powering various virulence mechanisms

  • atpB mutants show reduced ATP levels, limiting energy available for survival processes

  • Experiments demonstrate that atpB mutations cause significant attenuation in macrophage survival assays

• Virulence Gene Expression:

  • ATP status affects expression of various virulence factors

  • atpB mutants show altered PhoP-dependent gene expression

  • cigR mutants (with enhanced MgtC-mediated inhibition of ATP synthase) produce more PhoP-activated mRNAs than wild-type Salmonella

• Acid Tolerance Response:

  • atpB mutants show impaired adaptation to acidic conditions rather than general acid sensitivity

  • This defect significantly impacts survival within the acidified phagosome environment

Experimental data shows that atpB mutants are less invasive in vivo and demonstrate reduced survival within phagocytic cells across multiple Salmonella serovars, confirming the critical role of ATP synthase in intracellular survival .

What is the relationship between atpB and temporal regulation of virulence in Salmonella infection?

The temporal regulation of atpB function during Salmonella infection involves a sophisticated interplay with other proteins:

• Expression Timing Dynamics:

  • CigR protein is constitutively expressed through a PhoP-independent promoter before infection

  • MgtC expression is induced under infection-like conditions (low Mg2+, acidic pH)

  • Experimental data shows that CigR protein amounts are >20 times higher than MgtC's at early times after induction

  • MgtC protein levels increase dramatically starting at 3 hours post-induction and exceed CigR levels by 4 hours

• Functional Consequences:

  • At early infection stages (0-3 hours), CigR prevents MgtC from inhibiting ATP synthase

  • As infection progresses (4+ hours), MgtC levels exceed CigR, allowing inhibition of ATP synthase

  • This creates a threshold effect where MgtC-dependent phenotypes only manifest when MgtC protein amounts exceed CigR's

• Metabolic Transition:

  • Early infection: High ATP levels and ATPase activity

  • Late infection: Reduced ATP synthesis, altered metabolism, enhanced virulence gene expression

• Gene Expression Regulation:

  • The cigR gene is transcribed both constitutively and as part of the MgtC operon

  • This dual regulation ensures proper temporal control of ATP synthase inhibition

  • PhoP-dependent transcription becomes the dominant source of cigR expression in later infection stages

This temporal regulation mechanism ensures that Salmonella only commits to the metabolically costly virulence program after persistent exposure to host conditions .