Recombinant Brucella suis biovar 1 Type IV secretion system protein virB10 (virB10)

What is the structure and function of VirB10 in the Brucella T4SS?

VirB10 is a key structural component of the Type IV secretion system in Brucella suis biovar 1. Functionally, it acts as an ATP energy sensor that couples inner membrane energy consumption to substrate transfer through the secretion channel .

The protein structure includes:

• N-terminal cytoplasmic region

• Transmembrane (TM) α-helix anchored in the inner membrane

• Proline-rich region (PRR) that spans the periplasm

• C-terminal β-barrel domain that interacts with the outer membrane complex

VirB10 spans the entire cell envelope, forming a critical bridge between the inner and outer membrane components of the T4SS. Upon sensing ATP utilization by the VirD4 and VirB11 ATPases, VirB10 undergoes a conformational change necessary for stable assembly of the T4SS "core" complex . This energy-mediated structural transition is required for substrate passage through the distal portion of the translocation channel.

How is recombinant B. suis VirB10 expressed and purified for research purposes?

The standard protocol for producing recombinant VirB10 involves:

Expression System:

• E. coli expression systems (typically BL21 Star DE3 cells) are most commonly used

• The full-length sequence (1-391 amino acids) is typically fused to an N-terminal His tag for purification purposes

Expression Protocol:

• Clone the VirB10 open reading frame into an expression vector (e.g., pET101/D-TOPO)

• Transform into E. coli cells and grow in LB medium with appropriate antibiotics

• Induce protein expression using IPTG (typically 0.5 mM)

• Optimize culture conditions to enhance soluble protein production

A refined method that yields soluble protein involves:

• Initial growth in LB medium with glucose until OD₆₀₀ of 3-5

• Switching to M9 minimal medium supplemented with glucose

• Inducing expression at 4°C for 28 hours to minimize protein aggregation and reduce heat shock protease activity

Purification Method:

• Lyse cells and collect soluble fraction

• Purify using nickel affinity chromatography

• Further purify via size exclusion chromatography if needed

• Store in Tris/PBS-based buffer with 6% trehalose at pH 8.0

For reconstitution, the lyophilized protein should be dissolved in deionized sterile water to a concentration of 0.1-1.0 mg/mL, with 5-50% glycerol added for long-term storage at -20°C/-80°C .

What are the key domains of VirB10 and how do they contribute to its function?

VirB10 contains distinct domains with specific functions in the T4SS architecture and activity:

Mutational studies have revealed two functional classes of VirB10 mutations:

• Mutations that block both substrate transfer and pilus production (Tra⁻, Pil⁻)

• Mutations that permit substrate transfer but block pilus biogenesis (Tra⁺, Pil⁻)

This domain structure allows VirB10 to function as a dynamic bridge between inner and outer membrane complexes of the T4SS, responding to ATP energy to regulate both secretion and pilus formation.

What phenotypes are associated with virB10 mutations in Brucella?

Studies of VirB10 mutations in Brucella have revealed several important phenotypes:

Virulence Phenotypes:

• VirB10 mutants show reduced virulence in mice models

• Non-polar VirB10 mutants and VirB11 deletion mutants are cleared from mice by day 21 post-infection

• VirB10 mutants display reduced capacity to survive and replicate in both professional and non-professional phagocytic cells

Immunological Phenotypes:

• Mutant strains elicit protective immunity and may serve as vaccine candidates

• VirB10 mutants induce specific antibody responses (IgG1, IgG2a, IgG2b, and IgM)

• Both wild-type and mutant strains induce similar levels of different antibody isotype profiles

• VirB10 mutants show a distinct cytokine response pattern compared to wild-type strains:

  • Wild-type strains induce rapid and strong IFN-γ response that diminishes by 96 hours

  • Mutants induce mild IFN-γ responses that remain constant over time

Cellular Function Phenotypes:

• VirB10 mutations affect the ability of Brucella to establish and maintain infection

• The absence of VirB10 disrupts the ability of the T4SS to secrete effector molecules required for intracellular survival

• Mutations in VirB10 lead to defective secretion apparatus assembly

This range of phenotypes demonstrates the critical role of VirB10 in Brucella pathogenesis and suggests that VirB10 mutants could be promising candidates for live attenuated vaccines against brucellosis.

What methodologies are most effective for studying VirB10 conformational changes in response to ATP?

Several complementary techniques have proven effective for investigating the ATP-dependent conformational changes of VirB10:

Protease Accessibility Assay:

• Comparing protease susceptibility of VirB10 in energized cells versus ATP-depleted cells

• Cells can be depleted of ATP using arsenate treatment

• The pattern of protease digestion fragments reveals structural changes in response to cellular energy status

Cross-linking Studies:

• Chemical cross-linking with formaldehyde (FA) can identify close contacts between VirB10 and other T4SS components

• FA cross-linking has revealed that ATP energy at the inner membrane activates a structural transition in VirB10 required for outer membrane channel formation

• Changes in cross-linking patterns between energized and energy-depleted conditions provide insights into conformational changes

Co-immunoprecipitation Under Different Energy Conditions:

• Anti-VirB10 antibodies can be used to co-precipitate VirB10 and its binding partners

• Comparing immunoprecipitation results between untreated cells and those treated with energy-depleting agents (e.g., CCCP or arsenate)

• This approach has demonstrated that the ATP-dependent conformation of VirB10 is required for complex formation with VirB9 in vivo

Molecular Dynamics (MD) Simulations:

• MD simulations of wild-type VirB10 C-terminal domain (CTD) compared to ATP-insensitive mutants like G272R

• RMSDev and RMSF analyses to identify regions showing conformational flexibility

• MD simulations typically run for 1 μs using NPT equilibration followed by NVT ensemble

• These computational approaches have identified specific loops (e.g., AP loop, L1, L2, L3) that show differential flexibility between wild-type and mutant proteins

Cysteine Scanning Accessibility Method (SCAM):

• Introducing cysteine residues at specific positions in VirB10

• Testing accessibility to membrane-impermeable thiol-reactive reagents

• This approach has helped determine whether VirB10 functions through a "shuttling" mechanism or remains stably anchored at the inner membrane

How can VirB10 mutants be designed to specifically disrupt pilus formation without affecting secretion?

Designing VirB10 mutants that selectively disrupt pilus formation while maintaining secretion function requires targeting specific domains based on structure-function studies:

N-terminal Modifications:

• Deletion of the cytoplasmic domain (ΔN18): Supports T-DNA and plasmid transfer at reduced levels while disrupting pilus formation

• Two-residue insertions (Ala-Cys) every five residues in the N-terminal 50 residues can selectively impact pilus biogenesis

α-helical Projection Mutations:

• Deletions of α-helical projections extending from the β-barrel domain specifically disrupt pilus biogenesis without affecting substrate transfer

• These mutants cause release of pilin monomers to the milieu, suggesting a role in pilus assembly rather than secretion

Targeted Point Mutations:

• G272R mutation in the C-terminal domain: Confers substrate release but blocks pilus production (Tra⁺, Pil⁻)

• This mutation renders VirB10 conformationally insensitive to cellular ATP depletion, suggesting it "locks" the protein in an energy-activated, open conformation

• Mutations near the AP pore can specifically affect pilus formation

Transmembrane Domain Modifications:

• TM domain insertion mutations can maintain substrate transfer capability while blocking pilus production

• Substituting the VirB5 signal sequence for the N-terminal 46 residues of VirB10 results in abundant accumulation of the C-terminal fragment in the periplasm and disrupts pilus formation

Experimental Validation Methods:

• Assess T-DNA transfer efficiency using plant tumor formation assays

• Verify pilus formation by:

  • Shearing assays to recover T pili, followed by Tricine-SDS-PAGE

  • Colony immunoblotting to detect surface-exposed VirB2 and VirB5

• Examine protein-protein interactions through co-immunoprecipitation with other T4SS components

Understanding these domain-specific functions has important implications for designing targeted interventions against bacterial pathogenesis or for developing improved vaccine candidates.

What are the key considerations in using VirB10 as a vaccine candidate against brucellosis?

VirB10 has shown promise as a vaccine candidate against brucellosis, with several important considerations for vaccine development:

Immunogenicity Considerations:

• VirB10 is highly conserved among Brucella strains and species of the order Rickettsiales

• It induces both humoral and cell-mediated immune responses

• VirB10 vaccination in mice has shown significant reduction in bacterial load following challenge with A. phagocytophilum

• Protection appears to be IFN-γ dependent, suggesting a Th1-type immune response

Vaccine Delivery Strategies:
A prime-boost immunization approach has proven effective:

This regimen has been shown to improve and broaden protective immune responses against various pathogens, including Rickettsiales species .

Expression and Purification Considerations:

• Producing soluble recombinant VirB10 requires optimized expression conditions

• Methods that avoid inclusion body formation are preferred to maintain native protein folding

• Bacterial expression systems have been effective, but may require:

  • Growth in minimal media

  • Induction at low temperatures (4°C)

  • Extended expression periods (28h)

Immune Response Patterns:

• VirB10 vaccination induces a distinct cytokine profile compared to wild-type infection

• Wild-type strains induce rapid and strong IFN-γ response with marked reduction at 96h

• VirB10 mutants induce mild IFN-γ responses that remain consistent over time

• Antibody responses include IgG1, IgG2a, IgG2b, and IgM isotypes

Protective Efficacy Data:

• In mouse models, VirB10 vaccination resulted in significantly lower bacterial loads (Mean ± SEM peak bacterial load of 530 ± 159.9) compared to control groups (1380 ± 311.6)

• Some vaccinated mice completely cleared the infection

• The protection appears to be IFN-γ dependent but doesn't correlate directly with antibody titers

Combination with Other Antigens:

• Combining VirB10 with other T4SS components (VirB9-1, VirB9-2) in a multi-subunit vaccine has been tested

• While individual VirB10 showed protection, the mixture of all three components did not significantly enhance protection

How do computational approaches advance our understanding of VirB10 structural dynamics?

Computational methods have provided significant insights into the structure-function relationship of VirB10, particularly regarding its energy-sensing mechanism and conformational changes:

Homology Modeling Approaches:

• Constructing VirB10 models based on homologous proteins like E. coli TraF/VirB10

• The model of A. tumefaciens VirB10 was built with 40% sequence identity over 217 residues

• Modeller v9.21 with symmetry operation is used to generate tetradecameric assemblies

• The quality of models is assessed using PROCHECK and PROSA

Molecular Dynamics Simulations:

• Simulations typically run for 1 μs production following equilibration

• NPT equilibration followed by NVT ensemble

• Bond lengths constrained using M-SHAKE

• r-RESPA integrator with 2 fs time step for short-range interactions

• GPU acceleration enables ~15 ns/day simulation rates

Analysis of Simulation Results:

• Root Mean Square Deviation (RMSDev) plots reveal coordinate deviations across the protein backbone

• Root Mean Square Fluctuation (RMSF) analysis identifies regions of high mobility

• Principal Component Analysis (PCA) to identify dominant modes of motion

• These analyses have shown that wild-type VirB10 exhibits greater flexibility in loops extending from the β-barrel compared to G272R mutant

Pore Analysis:

• CHAP program to analyze pore dimensions and characteristics

• Calculation of hydrophobicity profiles along the pore

• Visualization of pores using VMD software

Electrostatic Analysis:

• Calculation of electrostatic potentials using Adaptive Poisson-Boltzmann Solver (APBS)

• Comparison between wild-type and mutant proteins to identify changes in charge distribution

• The G272R mutation has been shown to alter the electrostatic properties at a critical interface

Energy Calculations:

• Free energy calculations to evaluate the impact of mutations

• ΔΔG calculations using in silico site-directed mutagenesis methods

• ICM-Pro suite for implementing these analyses

Integration with Experimental Data:

• Computational predictions validated through experimental approaches like protease accessibility assays

• Simulation results providing mechanistic explanations for phenotypes observed in mutational studies

• Conformational states identified in simulations correlating with functional states of the T4SS

What epitopes of VirB10 show the greatest promise for multi-epitope vaccine development?

Immunoinformatics approaches have identified several promising epitopes in Brucella VirB10 that show potential for multi-epitope vaccine development:

Identified Epitope Categories:

• Several types of epitopes have been identified in VirB10 using immunoinformatics approaches:

  • Cytotoxic T lymphocyte (CTL) epitopes

  • Helper T lymphocyte (HTL) epitopes

  • Linear B cell epitopes

  • Conformational B cell epitopes

Key Considerations for Epitope Selection:

• Epitopes must have high antigenicity scores

• Selected epitopes should be conserved across Brucella species

• Epitopes should induce both cellular and humoral immunity

• Non-allergenic epitopes are preferred for vaccine safety

Multi-epitope Vaccine Design Strategy:

• Selected epitopes are spliced together using appropriate linkers

• The construct typically includes:

  • 2 CTL epitopes

  • 9 HTL epitopes

  • 6 linear B cell epitopes

  • 6 conformational B cell epitopes

Validation of Epitope-based Vaccines:

• In silico testing shows that well-designed multi-epitope vaccines are non-allergenic

• Molecular docking studies indicate strong interaction with TLR4

• Immune simulation results suggest these constructs can initiate both cellular and humoral immunity

Advantages of the Epitope-based Approach:

• Focuses immune response on the most immunogenic portions of VirB10

• Can combine epitopes from multiple antigens (e.g., VirB8 and VirB10)

• Potentially safer than whole protein or attenuated vaccines

• Can be designed to overcome antigenic variation between strains

The multi-epitope approach represents a promising strategy for developing effective vaccines against brucellosis, as it can induce broad immunity while minimizing potential adverse reactions.

How does VirB10 coordinate the assembly and function of the entire T4SS complex?

VirB10 plays a central coordinating role in T4SS assembly and function through its unique structure and dynamic properties:

Transenvelope Bridging Function:

• VirB10 spans from the inner membrane to the outer membrane, physically connecting different subassemblies of the T4SS

• It interacts with VirD4 at the inner membrane and the VirB7-VirB9 heterodimer at the outer membrane

• This forms a bridge between T4SS subassemblies at the two membranes, critical for machine function

Energy-Sensing Mechanism:

• VirB10 couples inner membrane ATP energy consumption to substrate transfer through the T4SS channel

• ATP utilization by VirD4 and VirB11 ATPases triggers a conformational change in VirB10

• This structural transition is required for stable complex formation with the VirB7-VirB9 heterodimer

• The energy-activated form of VirB10 enables substrate passage through the distal portion of the translocation channel

Sequential Assembly Process:
Three sequential reactions coordinate T4SS assembly:

• ATP utilization by VirD4 and VirB11

• VirB10 structural transition

• VirB10 complex formation with the outer membrane VirB7-VirB9 heterodimer

Core Complex Formation:

• VirB7, VirB9, and VirB10 form a "core complex" during T4SS biogenesis

• The C-terminal region of VirB10 plays a key role in regulating passage of secretion substrates across the outer membrane

• Recent cryo-EM structures reveal VirB10 as the crucial element that "glues" distinct sub-complexes together

Mismatch Symmetry Coordination:

• T4SS includes 16 copies of VirB10, but only 14 are used in the O-layer

• Potentially only 12 VirB10 are involved in the Arches

• Only five VirB10 proteins participate in VirB6 binding

• Only six VirB10 proteins make contacts with VirB4 ATPase

Regulation of Pilus Biogenesis:

• VirB10 regulates T4SS-mediated pilus biogenesis

• A trans-membrane region of VirB10 can obstruct the VirB2 pilin recruitment site on VirB6

• This provides a checkpoint mechanism for pilus assembly

• The regulation of pilus formation vs. secretion represents a key decision point in T4SS function

This multifaceted role of VirB10 in coordinating T4SS assembly and function makes it a critical component for understanding bacterial secretion mechanisms and developing targeted interventions against bacterial pathogens.