Recombinant Mycobacterium avium ATP synthase subunit b (atpF)

What is the structure and function of Mycobacterium avium ATP synthase subunit b (atpF)?

Mycobacterium avium ATP synthase subunit b (atpF) is a 178 amino acid protein that serves as a critical component of the F₀ sector of ATP synthase . The protein functions as part of the stator arm in the ATP synthase complex, connecting the membrane-embedded F₀ sector to the catalytic F₁ sector.

The protein contains a single N-terminal membrane-embedded α-helix, which interacts with subunit a of the ATP synthase complex. Research indicates that two copies of subunit b are present in bacterial ATP synthases, with the N-terminal α-helices making different interactions with subunit a . Specifically, one b subunit interacts with transmembrane α-helices 1, 2, 3, and 4 of subunit a, while the other interacts with α-helices 5 and 6 and the loop between α-helices 3 and 4 .

Unlike earlier cross-linking studies suggesting proximity between the N-termini of the two b-subunits, high-resolution structural data reveals that the transmembrane α-helices of the b-subunits are positioned on opposite sides of subunit a . This structural arrangement is crucial for the proper assembly and function of the ATP synthase complex.

How does recombinant atpF protein expression differ from native expression?

Recombinant Mycobacterium avium ATP synthase subunit b is typically expressed in heterologous systems such as E. coli, with modifications that facilitate purification and downstream applications . When expressing recombinant atpF, researchers commonly add affinity tags such as histidine tags (His-tag) to the N-terminus to enable purification through affinity chromatography .

The expression conditions differ significantly from native conditions, as E. coli has a faster growth rate and different membrane composition compared to Mycobacterium avium. These differences can affect protein folding, post-translational modifications, and protein-protein interactions. To maximize functional protein yield, optimization of expression parameters is essential:

When expressing membrane proteins like atpF, researchers should consider additional factors such as detergent selection for solubilization and maintaining the integrity of the transmembrane domain.

How does atpF contribute to ATP synthase assembly and function in mycobacteria?

The ATP synthase b subunit (atpF) plays a crucial role in the assembly and function of the ATP synthase complex in mycobacteria. Based on structural and functional studies, atpF contributes through several mechanisms:

• Structural support: The b subunit forms a critical part of the peripheral stalk (stator arm) that prevents rotation of the F₁ catalytic domain during ATP synthesis . This stator function is essential for maintaining the catalytic efficiency of the complex.

• Assembly coordination: Evidence from yeast and mammalian systems suggests that ATP synthase assembly occurs in a modular fashion, with the b subunit being integrated after assembly of the c-ring and binding of F₁, but before the final incorporation of subunits a and A6L . This sequential assembly process ensures proper formation of the functional complex.

• Subunit interactions: The b subunit forms crucial interactions with both the membrane sector and the catalytic sector. The N-terminal membrane-embedded α-helix makes specific contacts with subunit a, while the cytoplasmic domain interacts with δ and α subunits of the F₁ sector .

• Dimerization domain: Residues 62-122 of the b subunit form a dimerization domain that is crucial for proper ATP synthase function . This region adopts an extremely elongated structure with characteristics consistent with an α-helical coiled-coil, having a frictional ratio of 1.60, a maximal dimension of 95 Å, and a radius of gyration of 27 Å .

Disruption of atpF function through mutations can severely impact ATP synthase assembly and activity, highlighting its essential role in energy metabolism in mycobacteria.

What role does atpF play in mycobacterial drug resistance mechanisms?

The ATP synthase complex, including subunit b (atpF), has emerged as an important target for antimycobacterial drugs, with implications for drug resistance mechanisms:

• Direct involvement in resistance: While most documented resistance mutations to drugs like bedaquiline (BDQ) occur in the c subunit (atpE) rather than in atpF, the b subunit's interactions with other components of the ATP synthase complex can indirectly influence drug binding and efficacy .

• Structural context of resistance: Bedaquiline resistance mutations have been identified in six distinct positions in the c subunit: Asp28→Gly, Asp28→Ala, Leu59→Val, Glu61→Asp, Ala63→Pro, and Ile66→Met . These mutations define a cleft located between adjacent c subunits in the C ring, which encompasses the proton-binding site (Glu61) . Given that atpF interacts with the c-ring through subunit a, alterations in atpF structure could potentially influence this binding site.

• Compensatory mutations: Studies in M. abscessus have shown that while overexpression of wild-type or mutant atpE variants does not significantly change bedaquiline MIC values, specific mutations in atpE (D29V and A64P) can confer high resistance . While direct evidence for compensatory mutations in atpF is limited, the interconnected nature of the ATP synthase complex suggests that changes in atpF could potentially compensate for drug-resistance mutations in other subunits.

• Potential for targeting: The unique structural features of mycobacterial ATP synthase, including specific residues in atpF, present opportunities for developing new antibiotics with selective activity against mycobacteria while minimizing effects on human ATP synthase .

How can site-directed mutagenesis of atpF be used to study ATP synthase structure-function relationships?

Site-directed mutagenesis of atpF provides a powerful approach for investigating structure-function relationships in mycobacterial ATP synthase:

• Membrane interaction studies: Mutations in the N-terminal membrane-spanning domain can reveal how atpF anchors to the membrane and interacts with subunit a. Given that the two copies of subunit b interact differently with subunit a, selective mutations can help distinguish their unique roles .

• Dimerization interface analysis: Mutations in the dimerization domain (residues 62-122) can elucidate the specific interactions required for b-subunit dimerization and its contribution to the stability of the peripheral stalk .

• Drug resistance mechanisms: Introducing mutations in atpF that alter its interaction with the c-ring or subunit a could potentially affect the binding of drugs like bedaquiline, providing insights into resistance mechanisms .

• Functional coupling: Mutations at the interface between atpF and the F₁ sector can reveal how energy is transmitted between the membrane-embedded proton channel and the catalytic sites.

Methodological approach for site-directed mutagenesis studies:

What are the optimal conditions for expressing and purifying recombinant atpF?

Expressing and purifying recombinant Mycobacterium avium ATP synthase subunit b (atpF) requires careful optimization due to its membrane association and structural complexity:

Expression System Recommendations:

• Host selection: E. coli BL21(DE3) or C41(DE3)/C43(DE3) strains are preferred for membrane protein expression . The latter strains are specifically engineered for toxic membrane proteins.

• Vector choice: pET series vectors with T7 promoter allow controlled expression. For atpF specifically, vectors that add an N-terminal His-tag facilitate purification while maintaining protein function .

• Expression conditions:

  • Induce at OD₆₀₀ of 0.6-0.8

  • Use lower IPTG concentrations (0.1-0.5 mM)

  • Incubate at 16-18°C for 16-20 hours to improve proper folding

  • Supplement with 1% glucose to reduce basal expression

Purification Protocol:

• Cell lysis: Sonication or French press in buffer containing 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, and protease inhibitors.

• Membrane isolation: Ultracentrifugation at 100,000 × g for 1 hour.

• Membrane solubilization: Use mild detergents like n-dodecyl-β-D-maltoside (DDM) at 1% concentration.

• Affinity chromatography: Ni-NTA resin with imidazole gradient elution (20-250 mM).

• Secondary purification: Size exclusion chromatography using Superdex 200 column.

• Storage: Store in buffer containing 20 mM Tris-HCl pH 8.0, 150 mM NaCl, 0.03% DDM, and 6% trehalose at -80°C .

Quality Control Metrics:

How can researchers validate the proper folding and function of recombinant atpF?

Validating proper folding and function of recombinant Mycobacterium avium ATP synthase subunit b (atpF) is critical for downstream applications. Several complementary approaches are recommended:

• Structural analysis:

  • Circular dichroism (CD) spectroscopy to assess secondary structure content (should show high α-helical content)

  • Thermal shift assays to evaluate protein stability

  • Limited proteolysis to probe for properly folded domains resistant to digestion

• Oligomerization assessment:

  • Size exclusion chromatography to confirm dimeric state

  • Analytical ultracentrifugation to determine frictional ratio (~1.60) and elongated structure

  • Small-angle X-ray scattering (SAXS) to verify maximal dimension (~95 Å) and radius of gyration (~27 Å)

• Functional validation:

  • Reconstitution with other ATP synthase subunits to assess complex formation

  • ATP synthesis/hydrolysis assays using reconstituted complexes

  • Proton translocation assays in proteoliposomes

• Interaction studies:

  • Pull-down assays to verify binding to partner subunits (particularly subunit a and δ)

  • Surface plasmon resonance to quantify binding affinities

  • Cross-linking studies to map interaction interfaces

• Drug binding studies:

  • Isothermal titration calorimetry to measure binding of ATP synthase inhibitors

  • Competition assays with known ATP synthase ligands

What experimental approaches can be used to study atpF interactions with other ATP synthase subunits?

Understanding atpF interactions with other ATP synthase subunits requires a multi-faceted experimental approach:

• Co-immunoprecipitation (Co-IP):

  • Express His-tagged atpF and pull down interacting partners

  • Identify co-precipitated proteins by mass spectrometry

  • Verify specific interactions using antibodies against known ATP synthase subunits

• Crosslinking coupled with mass spectrometry:

  • Use chemical crosslinkers of varying lengths to capture protein-protein interactions

  • Digest crosslinked complexes and analyze by MS/MS

  • Map interaction interfaces at amino acid resolution

  • Note: Previous crosslinking studies suggested proximity between N-termini of b-subunits, but structural data indicates they are on opposite sides of subunit a

• Förster Resonance Energy Transfer (FRET):

  • Label atpF and potential partner subunits with fluorescent probes

  • Measure energy transfer as indication of proximity

  • Use in reconstituted systems or in live cells with fluorescent protein fusions

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS):

  • Compare deuterium uptake of atpF alone versus in complex with partner subunits

  • Identify protected regions that likely form interaction interfaces

• Mutagenesis coupled with functional assays:

  • Introduce mutations at predicted interaction sites

  • Assess effects on complex assembly and function

  • Particularly valuable for studying how atpF interacts with subunit a, where the N-terminal α-helix makes specific contacts

• Cryo-electron microscopy:

  • Obtain structures of the complete ATP synthase complex

  • Map the position of atpF relative to other subunits

  • Compare with existing bacterial ATP synthase structures

The integration of these approaches can provide a comprehensive understanding of how atpF contributes to ATP synthase structure and function through its interactions with other subunits.

How can researchers design experiments to study the role of atpF in drug resistance?

Designing experiments to study the role of atpF in drug resistance requires a systematic approach combining genetic, biochemical, and structural methodologies:

• Generation of resistant strains:

  • Select for spontaneous resistant mutants by culturing M. avium in increasing concentrations of ATP synthase inhibitors such as bedaquiline

  • Sequence atpF and other ATP synthase genes in resistant isolates

  • Alternatively, use directed evolution approaches to accelerate resistance development

• Genetic validation approaches:

  • Create isogenic strains with point mutations in atpF using homologous recombination techniques similar to those used for atpE mutations

  • Use complementation studies with wild-type and mutant atpF to confirm the role of specific mutations

  • Develop conditional knockdown systems to assess the impact of reduced atpF expression

• Biochemical and biophysical characterization:

  • Express and purify wild-type and mutant atpF proteins

  • Assess changes in protein stability, oligomerization, and interactions with other subunits

  • Measure binding affinity of drugs to reconstituted ATP synthase complexes containing wild-type or mutant atpF

  • Evaluate the impact on ATP synthesis and proton translocation activities

• Structural biology approaches:

  • Obtain high-resolution structures of ATP synthase complexes containing wild-type or mutant atpF

  • Use molecular docking simulations to predict how mutations might affect drug binding

  • Apply hydrogen-deuterium exchange mass spectrometry to map conformational changes induced by mutations

• Drug screening framework:

  • Establish a screening system using reconstituted ATP synthase with different atpF variants

  • Test compound libraries for differential inhibition of wild-type versus mutant enzymes

  • Develop structure-based design of new inhibitors that maintain activity against resistant variants

Experimental design considerations:

By integrating these approaches, researchers can establish the specific contribution of atpF to drug resistance mechanisms and potentially identify strategies to overcome resistance through rational drug design.