Recombinant Bartonella bacilliformis Beta- (1-->2)glucan export ATP-binding/permease protein NdvA

What is the molecular function of NdvA in Bartonella bacilliformis?

NdvA in B. bacilliformis likely functions as an ATP-binding transport protein involved in the export of cyclic β-(1-->2)glucan. Based on homology with similar proteins like the ndvA gene product in Rhizobium meliloti, this 67,100-dalton protein contains ATP-binding domains characteristic of bacterial export proteins . The protein is essential for the production and extracellular transport of β-(1-->2)glucan, which contributes to bacterial pathogenicity and host interaction. Similar to its homolog in R. meliloti, NdvA in B. bacilliformis likely facilitates the transport of synthesized β-(1-->2)glucan across the cell membrane, as evidenced by functional studies demonstrating absence of extracellular β-(1-->2)glucan in ndvA mutants despite retention of active synthesis intermediates .

How does NdvA structurally compare to other bacterial transport proteins?

NdvA belongs to the ATP-binding cassette (ABC) transporter superfamily. Sequence analysis reveals that NdvA shares significant homology with several bacterial ATP-binding transport proteins, particularly with Escherichia coli HlyB (hemolysin export protein) and the multidrug resistance (mdr) gene product in mammalian cells . The protein contains characteristic Walker A and Walker B motifs involved in ATP binding and hydrolysis. In R. meliloti, NdvA is encoded by a single large open reading frame that produces a 616 amino acid residue protein . The protein likely consists of transmembrane domains that form a channel across the membrane and nucleotide-binding domains that power the transport process through ATP hydrolysis.

What are optimal techniques for expressing and purifying recombinant NdvA?

Successful expression and purification of recombinant NdvA requires specialized approaches for membrane proteins. Based on established protocols for similar ATP-binding transporters, researchers should consider:

• Expression system selection: E. coli BL21(DE3) strains with pET or pBAD vectors containing rare codon supplementation are recommended for initial trials. Alternative systems include Pichia pastoris for eukaryotic post-translational modifications.

• Expression optimization:

  • Temperature: Lower temperatures (16-25°C) typically yield better folding

  • Inducer concentration: 0.1-0.5 mM IPTG for pET systems

  • Expression time: 4-16 hours depending on temperature

• Membrane extraction and solubilization:

  • Cell disruption by sonication or pressure-based methods

  • Membrane isolation through differential centrifugation

  • Solubilization with mild detergents (DDM, LMNG, or LDAO at 1-2%)

• Purification scheme:

  • IMAC (immobilized metal affinity chromatography) using His6-tagged constructs

  • Size exclusion chromatography for homogeneity assessment

  • Optional ion exchange chromatography for higher purity

Western blot analysis using specific antibodies against NdvA or epitope tags should be employed to track expression and purification efficiency. For functional studies, reconstitution into proteoliposomes is recommended to restore native-like membrane environment .

How can researchers effectively measure NdvA-mediated β-(1-->2)glucan export activity?

Quantifying NdvA transport activity requires a combination of biochemical and biophysical approaches:

• ATP hydrolysis assays: Measure ATPase activity using colorimetric phosphate detection methods (malachite green assay) or coupled-enzyme assays (pyruvate kinase/lactate dehydrogenase system).

• Transport assays:

  • Reconstitute purified NdvA into proteoliposomes

  • Load vesicles with radiolabeled or fluorescently labeled β-(1-->2)glucan substrates

  • Monitor substrate efflux over time under various conditions (ATP concentrations, temperature, pH)

• In vivo assays:

  • Isolate periplasmic and extracellular fractions from bacterial cultures

  • Separate β-(1-->2)glucan using anion-exchange chromatography followed by gel filtration

  • Quantify β-(1-->2)glucan using methods such as phenol-sulfuric acid assay or HPLC analysis

Based on methodologies detailed in search result , researchers should examine both cellular and supernatant fractions for β-(1-->2)glucan content, as demonstrated in R. meliloti studies where ndvA mutants showed absence of extracellular β-(1-->2)glucan despite the presence of the 235,000-Da protein intermediate involved in synthesis .

What role does NdvA play in Bartonella pathogenesis and host interaction?

While direct evidence for NdvA's role in B. bacilliformis pathogenesis is limited, insights can be drawn from research on related systems. In R. meliloti, ndvA mutants exhibit reduced motility and abnormal nodule formation on host plants , suggesting that in pathogenic Bartonella, NdvA-dependent export of β-(1-->2)glucan may similarly affect host-pathogen interactions.

Several potential mechanisms for NdvA's contribution to pathogenesis include:

• Host immune modulation: Exported β-(1-->2)glucan may interact with host pattern recognition receptors to modify immune responses.

• Biofilm formation: β-(1-->2)glucan could contribute to bacterial adherence and biofilm development within the host.

• Vascular interaction: Given that B. bacilliformis causes vasoproliferation during Carrion's disease , the exported β-(1-->2)glucan might potentially modulate endothelial cell responses, possibly interacting with the BafA-mediated proangiogenic activity that promotes endothelial cell proliferation .

• Stress resistance: The polysaccharide export system may enhance bacterial survival under osmotic stress or antimicrobial pressure within host environments.

Methodological approaches to investigate these hypotheses would include creating ndvA knockout mutants and examining their phenotypes in relevant infection models, including ability to invade host cells, intracellular survival, and induction of vasoproliferative responses.

How do mutations in the ATP-binding domain affect NdvA function and bacterial phenotype?

Site-directed mutagenesis studies targeting the ATP-binding domain would provide valuable insights into NdvA function. Researchers should focus on:

• Conserved motifs: Introducing mutations in the Walker A (GxxxxGKT/S) and Walker B (hhhhDE, where h is hydrophobic) motifs, which are critical for ATP binding and hydrolysis.

• Experimental approaches:

  • In vitro ATPase activity assays with purified mutant proteins

  • β-(1-->2)glucan export measurements in bacterial cells expressing mutant NdvA

  • Bacterial phenotype characterization (growth, morphology, stress resistance)

• Expected outcomes:
  Mutations in the ATP-binding domain would likely disrupt energy coupling necessary for transport, resulting in phenotypes similar to ndvA deletion mutants. In R. meliloti, such mutants retain the ability to synthesize β-(1-->2)glucan intermediates but cannot export the completed molecule .

What are the key technical challenges in studying Bartonella NdvA protein structure?

Structural characterization of NdvA presents significant challenges common to membrane transport proteins:

• Expression and purification obstacles:

  • Low expression yields in heterologous systems

  • Protein instability outside the native membrane environment

  • Detergent selection affecting protein stability and functionality

• Crystallization difficulties:

  • Limited polar surface area for crystal contacts

  • Conformational heterogeneity due to multiple functional states

  • Detergent micelle interference with crystal formation

• Methodological solutions:

  • Explore fusion partners (T4 lysozyme, BRIL) to increase polar surface area

  • Utilize nanodiscs or amphipols as alternatives to detergents

  • Apply single-particle cryo-electron microscopy to bypass crystallization requirements

  • Employ hydrogen-deuterium exchange mass spectrometry to probe conformational dynamics

• Functional validation: Correlate structural findings with transport activity measured through radioactive substrate uptake or fluorescence-based assays in reconstituted systems.

How might comparative genomics of NdvA across Bartonella species inform therapeutic targeting?

Comparative analysis of NdvA across Bartonella species could reveal conserved features essential for function as well as species-specific adaptations:

• Bioinformatic approach:

  • Sequence alignment of NdvA orthologs from multiple Bartonella species (B. bacilliformis, B. henselae, B. quintana)

  • Identification of conserved domains essential for ATP binding and transport function

  • Evolutionary analysis to identify residues under selective pressure

• Structure-based drug design:

  • Homology modeling based on known ABC transporter structures

  • Identification of potential inhibitor binding pockets

  • Virtual screening of compound libraries against predicted structures

• Experimental validation:

  • Functional complementation assays to test ortholog interchangeability

  • Transport inhibition studies with candidate compounds

  • Assessment of inhibitor specificity using purified proteins and cellular assays

Given Bartonella's role in diseases like trench fever and bacillary angiomatosis , targeting the conserved features of NdvA could lead to broad-spectrum anti-Bartonella therapeutics. Additionally, understanding how NdvA interacts with host systems during infection, similar to how B. bacilliformis BafA interacts with VEGFR2 , could identify potential points for therapeutic intervention.

How does NdvA compare functionally with other bacterial protein export systems like the T4SS?

While NdvA functions as an ABC transporter for β-(1-->2)glucan export, Bartonella species also employ type IV secretion systems (T4SS) for protein effector export during infection. Key differences include:

• Substrate specificity:

  • NdvA primarily exports polysaccharides (β-(1-->2)glucan)

  • T4SS translocates effector proteins like Bartonella effector proteins (Beps)

• Structural complexity:

  • NdvA likely functions as a simpler system with fewer components

  • T4SS comprises multiple proteins forming a complex machinery spanning both membranes

• Energy requirements:

  • NdvA utilizes ATP hydrolysis directly through its ATP-binding domain

  • T4SS uses a combination of ATP hydrolysis and proton motive force

• Biological functions:

  • NdvA-exported β-(1-->2)glucan likely modifies host-pathogen interface

  • T4SS-exported Beps directly modulate host cellular functions, such as BepC which triggers actin stress fiber formation

Methodologically, researchers studying these systems should consider comparative approaches to determine how these parallel export mechanisms contribute to Bartonella pathogenesis and whether there is any functional interplay between them.

What insights can be gained from studying the 235 kDa protein intermediate in β-(1-->2)glucan synthesis?

The 235 kDa protein intermediate identified in R. meliloti as part of the β-(1-->2)glucan synthesis pathway provides valuable insights into NdvA function:

• Relationship to NdvA function:

  • Studies show this intermediate remains present and active in ndvA mutants

  • The protein can be labeled with UDP-[14C]glucose, indicating it participates in the synthesis pathway

  • NdvA appears to function downstream of this intermediate in the export process

• Experimental approaches:

  • Protein identification through mass spectrometry

  • Analysis of protein-glucan interactions through affinity purification

  • Investigation of protein modifications during the synthesis-export cycle

• Research applications:

  • Using the 235 kDa intermediate as a marker for β-(1-->2)glucan synthesis activity

  • Developing assays to differentiate synthesis defects from export defects

  • Identifying potential interaction sites between the synthesis machinery and NdvA export system