Recombinant Brucella suis biovar 1 Flagellar M-ring protein FliF (fliF)

What expression systems are most effective for producing recombinant FliF protein?

The most commonly utilized expression system for producing recombinant Brucella FliF is Escherichia coli. As detailed in current research protocols, the full-length gene (encoding amino acids 1-580) is typically cloned with an N-terminal histidine tag to facilitate purification . When designing expression constructs, researchers should consider:

• Optimizing codon usage for E. coli

• Using inducible promoters (such as T7) for controlled expression

• Including a tag (commonly His-tag) for purification

• Growing cultures under controlled conditions to maximize protein yield

While E. coli is the predominant system, other studies suggest that modifying the host strain to reduce flagella formation may improve recombinant protein yields. For instance, knocking out genes like flhC (a master regulator of flagella assembly) in combination with ptsG deletion has been shown to increase yields of recombinant proteins by up to 1.81-fold through redirection of cellular energy resources .

What are the optimal storage and reconstitution conditions for recombinant FliF protein?

Based on established protocols, the following conditions are recommended for handling recombinant FliF protein:

These conditions help maintain protein stability and activity for experimental applications . For maximum reproducibility in functional studies, it is advisable to prepare single-use aliquots rather than subjecting the protein to multiple freeze-thaw cycles.

What is the role of FliF in Brucella flagellar assembly?

FliF is a fundamental structural component that forms the MS (membrane and supramembrane) rings of the flagellar basal body in Brucella. Research indicates that:

• FliF oligomerization is one of the first steps in flagellar basal body assembly

• The protein contains three Ring-Building Motif (RBM) domains that adopt different symmetries in the assembled structure, with RBM3 forming a 34-mer ring and RBM2 adopting a 23-mer configuration

• FliF serves as a scaffold to recruit the C-ring through interaction with FliG and the export apparatus

• In Brucella melitensis, FliF is classified as a class II flagellar gene and is regulated by the flagellar two-component regulator FtcR

Understanding FliF assembly is particularly important given the unexpected symmetry mismatches observed within the MS-ring structure, which may serve multiple functions in flagellar biology .

How does the oligomerization of FliF domains contribute to MS-ring assembly in Brucella?

Recent structural studies reveal a complex oligomerization process for FliF domains that is critical for proper MS-ring assembly:

• The RBM3 domain plays a dominant role in initiating oligomerization, forming stable ring structures independently

• RBM1 appears to bind to RBM2, preventing its premature oligomerization

• RBM3 acts to prevent the inhibitory interaction between RBM1 and RBM2, promoting proper assembly

• The complete MS-ring incorporates asymmetric structures, with RBM3 forming a 34-mer stoichiometry while RBM2 forms a 23-mer ring, creating an intriguing symmetry mismatch within a single protein

Experimental approaches to study this process include negative-stain electron microscopy and cryo-EM analysis of purified RBM domains individually and in combination. For instance, mixing RBM3 with RBM1-RBM2 constructs results in tubular structures composed of stacked rings, demonstrating the interaction between these domains .

This oligomerization process differs from that of the related SctJ protein in the injectisome, suggesting distinct regulatory mechanisms for these macromolecular machines despite their evolutionary relationship .

What experimental approaches can effectively measure FliF interaction with other flagellar proteins?

Several complementary methodologies can be employed to characterize FliF interactions:

Research indicates that FliF interacts directly with FliG to form the MS-C ring interface, and these interactions are crucial for proper flagellar assembly and function. Additional interactions with FlhA from the export apparatus have also been reported in some bacterial species, suggesting a cooperative assembly process .

How do transcriptional regulators control FliF expression in Brucella species?

The regulation of FliF expression involves a complex network of transcriptional regulators:

• FtcR (Flagellar Two-Component Regulator) is the master regulator of flagellar genes in Brucella melitensis and directly controls the expression of class II genes, including fliF

• VjbR, a LuxR-type quorum sensing regulator, is required for transcription of several flagellar genes including fliF, flhA, motB, and flgE

• BlxR, another LuxR-type regulatory protein, also influences flagellar gene expression

• RpoE1, an extracytoplasmic function sigma factor, indirectly affects FliF expression by modulating the promoter activity of FtcR

These regulatory pathways are integrated with broader bacterial adaptation mechanisms. For example, Hfq, an RNA chaperone protein, has been found to bind to numerous mRNAs and small regulatory RNAs in Brucella, potentially providing an additional layer of post-transcriptional regulation for flagellar genes .

Experimental approaches to study these regulatory networks include:

• Chromatin immunoprecipitation (ChIP) to identify direct binding of regulators to promoter regions

• Reporter gene assays to measure promoter activities under different conditions

• RNA-seq and transcriptome analysis to map regulatory networks

• Construction of deletion mutants to assess phenotypic effects

What is the relationship between FliF expression and Brucella virulence in host infection models?

The flagellar system, including FliF, plays multifaceted roles in Brucella virulence:

• Studies have demonstrated that flagellar gene expression is growth phase-dependent, with increased expression in late-log phase cultures compared to stationary phase, correlating with enhanced invasiveness in epithelial cells

• Transcriptional regulators that control flagellar genes, such as VjbR and BlxR, are also essential for the virulence of Brucella in animal models

• FliF, as part of the flagellar apparatus, contributes to multiple virulence-associated functions:

  • Cell invasion and host colonization

  • Biofilm formation

  • Growth and cell division processes

  • Modulation of host immune responses

Researchers studying the relationship between FliF expression and virulence should consider:

• Using both cellular (macrophage) and animal infection models

• Constructing defined mutants with deletion or controlled expression of fliF

• Examining both acute and chronic phases of infection

• Analyzing tissue-specific bacterial loads and histopathological changes

How can recombinant FliF protein be utilized in vaccine development against brucellosis?

Recombinant FliF protein represents a promising candidate for vaccine development against brucellosis for several reasons:

• As a conserved structural protein, FliF contains epitopes that may elicit protective immune responses

• Similar flagellar proteins, such as FlgJ, have shown protective effects when used to vaccinate mice against B. abortus infection

• The non-LPS nature of FliF avoids the cross-reactivity issues associated with LPS-based diagnostics and vaccines

For vaccine development, several approaches can be considered:

When developing FliF-based vaccines, researchers should assess both antibody and cell-mediated immune responses, as both are critical for protection against intracellular pathogens like Brucella. Additionally, comparative studies with other Brucella antigens (such as Omp25, Omp31, and BP26) would be valuable to determine optimal antigen combinations .

What methodological approaches can resolve contradictory findings about FliF function across different Brucella species?

Research findings on FliF function and regulation sometimes differ between Brucella species and experimental systems. To address these contradictions, several methodological approaches should be considered:

• Standardized expression systems: Develop consistent protocols for expressing and purifying FliF from different Brucella species using identical tags and expression conditions to enable direct comparisons.

• Comparative genomics and structural biology:

  • Perform detailed sequence alignment and structural modeling of FliF across species

  • Identify conserved and variable regions that might explain functional differences

  • Use cryo-EM to determine species-specific structural configurations

• Isogenic mutant construction:

  • Generate clean deletion and complementation mutants across multiple species

  • Use the same genetic tools and markers to minimize technical variables

  • Include appropriate controls to validate genotypes and phenotypes

• Multi-species phenotypic assays:

  • Develop standardized protocols for assessing flagellar assembly, motility, and biofilm formation

  • Use identical growth conditions and media formulations

  • Implement quantitative measurements rather than qualitative observations

• Host-pathogen interaction models:

  • Test multiple Brucella species in the same host cell models

  • Use both human and animal-derived cell lines relevant to natural infections

  • Develop ex vivo tissue models that better recapitulate the in vivo environment

For example, contradictory findings regarding spontaneous MS-ring assembly have been reported between Salmonella Typhimurium (where FliF overexpression leads to spontaneous assembly) and Vibrio alginolyticus (where this behavior is not observed) . Similarly, regulatory mechanisms involving FlhA differ between E. coli and other species . Resolving these contradictions requires explicit consideration of species-specific factors and careful experimental design.

How does the structure of recombinant FliF compare to the native protein in Brucella suis?

Comparing recombinant and native FliF protein structures is essential for validating experimental findings. Key considerations include:

• Post-translational modifications:

  • Native FliF may undergo modifications absent in recombinant systems

  • Mass spectrometry analysis of native FliF isolated from Brucella can identify these modifications

  • Site-directed mutagenesis of potential modification sites can assess their functional significance

• Structural integrity and folding:

  • Circular dichroism spectroscopy can compare secondary structure content

  • Limited proteolysis can probe domain organization and accessibility

  • NMR or X-ray crystallography of both proteins can provide high-resolution structural comparisons

• Oligomerization properties:

  • Size-exclusion chromatography and multi-angle light scattering can assess native oligomerization states

  • Negative-stain electron microscopy can visualize assembled structures

  • In vitro assembly assays can compare ring-forming capabilities

• Functional assays:

  • Binding assays with interaction partners (e.g., FliG, FlhA)

  • Complementation of fliF mutants with recombinant protein

  • Antibody recognition profiles of native versus recombinant proteins

Research has shown that the N-terminal His-tag used in recombinant FliF does not typically interfere with oligomerization or basic structure formation, but may affect subtle aspects of protein-protein interactions or assembly kinetics that should be considered when interpreting experimental results .

What innovative techniques can be applied to study FliF dynamics during Brucella infection?

Understanding the dynamic behavior of FliF during infection requires sophisticated approaches:

• Live-cell imaging techniques:

  • CRISPR-based tagging of genomic fliF with fluorescent proteins

  • Super-resolution microscopy to visualize flagellar assembly in situ

  • Single-molecule tracking to monitor FliF movement and incorporation into structures

• Temporal gene expression control:

  • Inducible or repressible promoter systems to control fliF expression during infection

  • Destabilized protein variants for rapid turnover studies

  • Optogenetic tools for spatiotemporal control of expression

• Biosensor development:

  • FRET-based sensors to detect FliF interactions in real-time

  • Split fluorescent protein complementation to visualize assembly

  • Förster resonance energy transfer (FRET) to measure protein proximity

• Advanced proteomics approaches:

  • Proximity labeling (BioID, APEX) to identify near-neighbors during infection

  • Pulse-chase proteomics to measure protein turnover rates

  • Cross-linking mass spectrometry to capture transient interactions

• Host-pathogen interface analysis:

  • Correlative light and electron microscopy to connect molecular events with ultrastructural changes

  • Tissue-clearing techniques combined with light-sheet microscopy for 3D visualization in intact tissues

  • Multi-parameter flow cytometry to correlate FliF expression with infection outcomes

These approaches would help address fundamental questions about when and where FliF is expressed during infection, how it incorporates into flagellar structures, and how these processes contribute to Brucella pathogenesis and intracellular survival .