Recombinant Aquifex aeolicus Flagellar M-ring protein (fliF)

What is the structural role of FliF protein in the bacterial flagellar system?

FliF forms the MS-ring complex that serves as the mounting plate for the flagellar motor in bacterial flagella. This protein creates a structural foundation anchored in the inner membrane, upon which other components of the flagellar motor, including the rotor proteins (FliG, FliM, and FliN), are assembled. The MS-ring complex is essential for proper flagellar assembly and function, acting as both a structural foundation and a component of the rotational machinery. The architecture of the flagellar motor in A. aeolicus involves a sophisticated arrangement of proteins where FliF interfaces with FliG, which subsequently interacts with FliM through specific conserved motifs .

How does Aquifex aeolicus FliF differ from homologous proteins in mesophilic bacteria?

A. aeolicus FliF, like many proteins from this hyperthermophile, possesses adaptations for extreme thermal stability. These typically include a higher proportion of charged amino acids forming salt bridges, increased hydrophobic interactions in the protein core, fewer thermolabile residues, and more compact folding. The protein maintains structural homology with mesophilic counterparts but differs in amino acid composition to enable function at temperatures approaching 95°C. These adaptations reflect the evolutionary position of Aquifex as one of the most thermophilic bacterial genera, occupying ecological niches originally thought to be exclusive to archaeal organisms .

What is known about the stoichiometry of FliF in the flagellar motor of A. aeolicus?

While specific stoichiometric data for A. aeolicus FliF is not directly reported in the available literature, comparative analysis with related flagellar systems suggests approximately 26 subunits of FliF form the MS-ring. This creates a stoichiometric mismatch with the C-ring proteins, particularly FliM, which typically has around 34 subunits in the assembled structure . This numerical difference is functionally significant as it contributes to the mechanical properties of the motor and influences the rotational characteristics of the flagellum. The stoichiometric relationships between flagellar proteins have important implications for understanding the mechanisms of torque generation and switching of rotational direction.

What are the optimal expression systems for recombinant A. aeolicus FliF production?

For recombinant expression of A. aeolicus FliF, a modified pET expression system in E. coli strains optimized for membrane protein expression is recommended. Based on successful approaches with other A. aeolicus proteins, the following methodology is advised:

• Clone the fliF gene from A. aeolicus genomic DNA using PCR with primers containing appropriate restriction sites (typically NdeI and BamHI)

• Ligate the amplified gene into a pET vector (such as pET21a) with a C-terminal His-tag for purification

• Transform into an expression strain such as BL21(DE3) or C43(DE3), which is better suited for membrane protein expression

• Induce expression at lower temperatures (18-25°C) with reduced IPTG concentrations (0.1-0.5 mM) to promote proper folding

This approach has proven successful for other A. aeolicus proteins, including LpxC and various ferredoxins .

What purification challenges are specific to A. aeolicus FliF and how can they be addressed?

Purification of recombinant A. aeolicus FliF presents several challenges:

• Membrane protein solubilization: FliF requires careful detergent selection. A screening approach using detergents like DDM, LDAO, or LMNG is recommended, starting with initial solubilization in 1% DDM.

• Aggregation prevention: Introduction of point mutations similar to the C181A mutation used for A. aeolicus LpxC may reduce aggregation . Additionally, inclusion of stabilizing agents such as glycerol (10-15%) in all buffers helps maintain protein stability.

• Thermostability during purification: Unlike mesophilic proteins, A. aeolicus FliF may exhibit improved folding at elevated temperatures. Consider performing certain purification steps at 40-50°C to promote correct folding.

• Protein yield optimization: Co-expression with chaperones or fusion partners may improve yields. Truncation constructs removing flexible regions can sometimes improve expression while maintaining core functional domains.

What crystallization strategies have proven successful for A. aeolicus membrane proteins like FliF?

Crystallization of membrane proteins from A. aeolicus requires specialized approaches:

• Lipidic cubic phase (LCP) crystallization: This method has been successful for membrane proteins from extremophiles, maintaining a lipid environment that better mimics native conditions.

• Detergent screening: Systematic screening of detergents is crucial, with shorter-chain detergents often promoting better crystal contacts. For A. aeolicus proteins, detergents stable at higher temperatures should be prioritized.

• Thermal stability screening: Using differential scanning fluorimetry to identify buffer conditions that maximize thermal stability before crystallization trials.

• Co-crystallization with binding partners: For FliF, co-crystallization with segments of interacting proteins (such as FliG) may stabilize the structure and facilitate crystallization, similar to the approach used for the FliG-FliM complex .

• Construct optimization: Creation of truncated constructs that remove flexible regions while preserving core domains, similar to the approach used for A. aeolicus LpxC where a C-terminal truncation (Δ272-282) improved crystallization properties .

How can cryo-EM be applied to study the structure of recombinant A. aeolicus FliF in its assembled form?

Cryo-electron microscopy offers advantages for studying assembled flagellar structures:

• Sample preparation: Recombinant FliF reconstituted into nanodiscs or amphipols provides a native-like membrane environment suitable for cryo-EM analysis.

• Data collection parameters: For A. aeolicus FliF:

  • Voltage: 300 kV

  • Defocus range: -1.0 to -3.0 μm

  • Total dose: 50-60 e-/Ų

  • Frame collection: 40-50 frames per micrograph

• Image processing workflow:

  • Motion correction using MotionCor2

  • CTF estimation with CTFFIND4

  • Particle picking using reference-free approaches

  • 2D and 3D classification to identify homogeneous populations

  • High-resolution refinement with imposed symmetry based on the known stoichiometry

• Validation approaches: Resolution assessment using gold-standard FSC (0.143 criterion) and model validation through independent map reconstruction.

What methods are most effective for studying FliF interactions with other flagellar proteins in A. aeolicus?

Several complementary approaches are recommended:

• Pull-down assays: Using His-tagged FliF as bait to identify interacting partners. This method successfully identified interactions between FliG central domain and FliM in A. aeolicus, and similar approaches can be applied to FliF .

• Disulfide crosslinking: Strategic introduction of cysteine residues at predicted interaction interfaces allows for in vivo crosslinking experiments. This approach was successfully used to verify the interaction between FliM and FliG domains in Thermotoga maritima, a thermophile related to A. aeolicus .

• Surface plasmon resonance (SPR): Quantitative measurement of binding kinetics and affinities between purified FliF and its binding partners, with immobilization strategies optimized for membrane proteins.

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): Particularly valuable for mapping interaction surfaces between FliF and other flagellar components without requiring crystallization.

• Bacterial two-hybrid systems: Modified for thermophilic proteins by using thermostable reporter systems or performing assays at intermediate temperatures compatible with both the host organism and maintaining native interactions.

How does the interaction between FliF and FliG in A. aeolicus compare to that in mesophilic bacteria?

The FliF-FliG interaction in A. aeolicus likely exhibits several thermophile-specific features:

• Interaction strength: The interaction domains likely contain additional stabilizing elements such as salt bridges and hydrophobic contacts to maintain association at elevated temperatures.

• Interface composition: Comparative sequence analysis suggests the interaction interface contains a higher proportion of charged and hydrophobic residues compared to mesophilic counterparts, a common adaptation in thermophilic protein-protein interfaces.

• Domain arrangement: The domain organization of FliG in A. aeolicus involves three domains (N, M, and C), with the N and M domains likely interacting with FliF. The specific positioning may differ from mesophilic systems, with the FliG M domain potentially adopting a more compact conformation relative to FliG N when engaged in interactions with FliF .

• Functional implications: These adaptations ensure flagellar function at extreme temperatures while preserving the mechanical properties necessary for motor rotation and switching.

How can structural information from A. aeolicus FliF contribute to understanding flagellar assembly evolution?

Structural characterization of A. aeolicus FliF provides unique evolutionary insights:

• Ancestral features: As a deep-branching bacterial lineage, A. aeolicus may preserve ancestral features of the flagellar system . Comparative structural analysis between FliF from A. aeolicus and other organisms can reveal conserved core elements essential for function versus lineage-specific adaptations.

• Thermoadaptation signatures: Identifying specific structural elements that contribute to thermostability helps distinguish between adaptations for extreme environments and conserved functional features.

• Evolutionary model testing: Structural data can test competing hypotheses about Aquifex's evolutionary position - either basal like Thermotogae (Fig. 1a) or related to Epsilonproteobacteria (as suggested in some analyses) .

• Horizontal gene transfer assessment: The flagellar system components in thermophiles may have been subject to lateral gene transfer events, which structural comparison can help identify through chimeric features or unexpected similarity patterns.

What approaches can be used to study the dynamics of A. aeolicus FliF in the assembled MS-ring?

Studying the dynamics of assembled FliF presents unique challenges that require specialized techniques:

• Molecular dynamics simulations: Performing high-temperature MD simulations (90-95°C) of the assembled MS-ring provides insights into thermally-induced conformational changes and stability mechanisms. Comparison with simulations of mesophilic counterparts at lower temperatures can reveal thermoadaptation principles.

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): This technique can map regions with different solvent accessibility and backbone dynamics in the assembled structure, providing experimental validation for computational models.

• Site-directed spin labeling and EPR spectroscopy: Strategic placement of spin labels can provide information about distances and dynamics between specific regions of FliF in the assembled MS-ring.

• Temperature-dependent fluorescence methodologies: Using specifically placed fluorescent probes to monitor conformational changes as a function of temperature can reveal the dynamic properties of FliF in conditions mimicking the native environment of A. aeolicus.

How can inclusion body formation be minimized when expressing recombinant A. aeolicus FliF?

Inclusion body formation is a common challenge when expressing recombinant membrane proteins like FliF. Several strategies can reduce this issue:

• Expression temperature optimization: Lower induction temperatures (16-20°C) slow protein production rate, allowing more time for proper membrane insertion, despite being counterintuitive for a thermophilic protein.

• Induction modulation: Using lower IPTG concentrations (0.1-0.2 mM) or auto-induction media provides gentler expression conditions that favor proper folding.

• Host strain selection: Specialized strains like C41(DE3), C43(DE3), or Lemo21(DE3) are engineered specifically for membrane protein expression.

• Co-expression strategies:

  • Chaperone co-expression (GroEL/GroES, DnaK/DnaJ)

  • Co-expression with FliF-interacting partners

  • Co-expression with membrane-insertion facilitating proteins

• Construct engineering: Creating fusion proteins with solubility-enhancing partners (MBP, SUMO) or removing aggregation-prone regions can significantly improve soluble expression.

What strategies can address the challenges of obtaining homogeneous preparations of A. aeolicus FliF for structural studies?

Obtaining homogeneous protein preparations suitable for structural studies requires:

• Optimized detergent exchange protocol:

  • Initial solubilization in stronger detergents (DDM, FC-12)

  • Gradual exchange to milder detergents (LMNG, GDN) during purification

  • Detergent concentration maintenance above CMC throughout all steps

• Strategic chromatography sequence:

  • IMAC using Ni-NTA resin with extended washing steps

  • Ion exchange chromatography exploiting the unique charge distribution of A. aeolicus FliF

  • Size exclusion chromatography as final polishing step

• Sample monodispersity assessment:

  • Dynamic light scattering to monitor aggregation

  • Negative-stain EM to visualize protein homogeneity

  • FSEC (fluorescence-detection size exclusion chromatography) to optimize conditions

• Stability screening: Systematic testing of buffer components, pH ranges, ionic strengths, and additives using Thermofluor assays to identify conditions maximizing thermal stability and homogeneity.

What methods can demonstrate the functional properties of recombinant A. aeolicus FliF?

Assessing functional properties of recombinant FliF requires:

• In vitro reconstitution assays:

  • Reconstitution of FliF into liposomes or nanodiscs

  • Visualization of MS-ring formation by negative-stain EM

  • Sequential addition of other flagellar components to assess proper assembly interactions

• Thermostability profiling:

  Temperature (°C)	Expected Retention of Secondary Structure (%)
  25	100
  50	98-100
  75	90-95
  85	85-90
  95	70-80

• Binding assays with interaction partners:

  • Microscale thermophoresis (MST) to measure binding affinities with FliG

  • Co-sedimentation assays to assess complex formation

  • FRET-based approaches to monitor interactions in near-native conditions

• Complementation studies: Testing whether recombinant A. aeolicus FliF can complement motility defects in FliF-deficient strains of model organisms like E. coli (likely requiring chimeric constructs due to species differences).

How can the assembly properties of A. aeolicus FliF be studied under thermophilic conditions?

Studying assembly under thermophilic conditions requires specialized approaches:

• Temperature-controlled in vitro reconstitution:

  • Using temperature-stable lipids (archaeal or synthetic thermostable lipids)

  • Monitoring assembly processes at elevated temperatures (70-95°C)

  • Time-course negative-stain EM visualization of assembly intermediates

• Thermostable fluorescent labeling strategies:

  • Site-specific incorporation of thermostable fluorophores

  • FRET-based monitoring of subunit association at elevated temperatures

  • Fluorescence correlation spectroscopy to analyze oligomerization state

• Analytical ultracentrifugation under thermophilic conditions:

  • Specialized cells and optical systems for high-temperature operation

  • Determination of oligomerization state and assembly kinetics

  • Comparative analysis with mesophilic counterparts

• Real-time light scattering analysis:

  • Monitoring assembly processes through changes in light scattering profiles

  • Determination of critical concentrations for assembly initiation

  • Effect of solution conditions on assembly efficiency