Recombinant Mycobacterium ulcerans ATP synthase subunit b (atpF)

What is the unique structural organization of Mycobacterium ATP synthase subunits b and δ?

In mycobacteria, including M. ulcerans, the genes encoding the b-δ subunits of F-ATP synthase have a unique organization. The putative b subunit (atpF) and δ subunit (atpH) genes have fused to form a single gene (atpH) . This fusion creates a combined b-δ protein that functions as part of the peripheral stator stalk. Additionally, the atpF gene in mycobacteria encodes information for subunit b', which is a shorter b-type subunit lacking the C-terminus .

The mycobacterial ATP synthase complex has a composition of α₃:β₃:γ:δ:ε:a:b:b':c₉, which includes both the fused b-δ protein and the shorter b' protein . This structural arrangement is part of what makes mycobacterial F-ATP synthases unique compared to other prokaryotic ATP synthases.

How does the function of M. ulcerans ATP synthase differ from other bacterial ATP synthases?

Mycobacterial F₁F₀-ATP synthases, including that of M. ulcerans, are incapable of ATP-driven proton translocation due to their latent ATPase activity . This characteristic is crucial for mycobacteria as it prevents:

• Wasteful ATP hydrolysis

• Disruption of the proton motive force, which would be lethal to mycobacteria

This latent ATPase activity is a significant functional difference from many other bacterial ATP synthases that can readily hydrolyze ATP. The ATP hydrolysis activity of isolated M. smegmatis F-ATP synthase was measured at 0.4±0.1 μmol min⁻¹ (mg protein)⁻¹, which is slightly higher than the ATPase activity of about 0.04±0.01 μmol min⁻¹ (mg total protein)⁻¹ observed in other studies . These values indicate the highly regulated and suppressed nature of ATP hydrolysis in mycobacterial ATP synthases.

What are effective protocols for recombinant expression of M. ulcerans ATP synthase subunit b?

While the search results don't detail a specific protocol for M. ulcerans atpF, we can infer effective methods based on related recombinant protein expression studies:

• Expression system selection: E. coli is commonly used for expression of mycobacterial proteins, as demonstrated with the recombinant full-length M. ulcerans ATP synthase subunit a (atpB) .

• Construct design:

  • Include an N-terminal or C-terminal His-tag for purification

  • Use codon optimization for E. coli expression if necessary

  • Consider fusion partners to enhance solubility if needed

• Expression conditions:

  • Optimal temperature (typically 18-30°C)

  • Induction parameters (IPTG concentration, induction time)

  • Media composition (rich vs. minimal)

For the recombinant M. ulcerans ATP synthase subunit a (atpB), expression in E. coli with an N-terminal His-tag was successful , suggesting similar approaches may work for subunit b.

What purification challenges are specific to the fused b-δ subunit, and how can they be addressed?

The fused nature of the b-δ subunit presents specific purification challenges:

• Membrane association: As part of the stator stalk, the b portion interacts with membrane components, requiring careful solubilization strategies.

• Structural integrity: Maintaining the native conformation of the fused protein during purification is critical.

• Verification methods: MALDI mass spectrometry has been successfully used to identify the fused b-δ subunit (48 kDa band) in purified M. smegmatis F-ATP synthase .

Recommended approach:

• Use mild detergents for solubilization

• Include stabilizing agents in buffers

• Employ affinity chromatography (His-tag) followed by size exclusion chromatography

• Store in appropriate buffer conditions (e.g., Tris/PBS-based buffer with 6% Trehalose, pH 8.0)

• Add glycerol (5-50%) for long-term storage at -20°C/-80°C

• Avoid repeated freeze-thaw cycles

How can researchers assess the functional integrity of recombinant M. ulcerans ATP synthase subunit b?

Functional assessment of recombinant M. ulcerans ATP synthase subunit b requires multiple complementary approaches:

• Structural verification:

  • SDS-PAGE for molecular weight confirmation

  • Western blotting with specific antibodies

  • Mass spectrometry for sequence verification

• Complex assembly assays:

  • Co-purification with other ATP synthase subunits

  • Reconstitution experiments with purified components

  • Electron microscopy to visualize complex formation

• Functional assays:

  • ATP synthesis measurements in reconstituted systems

  • Limited ATP hydrolysis activity measurements (acknowledging the latent nature of mycobacterial ATPase activity)

  • Proton translocation assays using pH-sensitive dyes or electrodes

For intact ATP synthase complexes from mycobacteria, ATP hydrolysis activity can be measured using established enzymatic assays, with expected values around 0.4±0.1 μmol min⁻¹ (mg protein)⁻¹ based on M. smegmatis studies .

What methods can detect the interaction between recombinant subunit b and other components of the ATP synthase complex?

To study protein-protein interactions involving the recombinant subunit b:

• Co-immunoprecipitation with antibodies against other ATP synthase subunits

• Pull-down assays using His-tagged recombinant subunit b to identify binding partners

• Surface plasmon resonance (SPR) to measure binding kinetics with other subunits

• Crosslinking studies to capture transient interactions within the complex

• Structural studies:

  • Single particle analysis by electron microscopy has been successfully used to determine the projected 2D and 3D structure of intact M. smegmatis F₁F₀ ATP synthase

  • This approach revealed the arrangement of subunits including the stator components (b' and fused b-δ)

How does the unique b-δ fusion in mycobacterial ATP synthase contribute to the regulation of enzyme activity?

The fused b-δ structure in mycobacterial ATP synthases appears to play a role in the unique regulatory properties of these enzymes:

• Structure-function relationship:

  • The 111 amino acid extended δ subunit portion of the fusion protein likely influences interactions with the F₁ sector

  • These interactions may contribute to the latent ATPase activity observed in mycobacterial ATP synthases

• Regulatory mechanisms:

  • While the C-terminal extension of nucleotide-binding subunit α has been shown to be a major contributor to ATPase suppression , the unique b-δ fusion may also participate in this regulatory network

  • The stator architecture formed by b-δ and b' likely influences the coupling between the F₁ and F₀ sectors

• Evolutionary significance:

  • The gene fusion appears to be conserved across mycobacteria, suggesting functional importance

  • This unique arrangement may represent an adaptation to the specific bioenergetic requirements of mycobacteria

Experimental approaches to further explore this include site-directed mutagenesis of specific residues within the fusion protein and comparative studies with modified versions where the fusion is disrupted.

What role might M. ulcerans ATP synthase play in antibiotic resistance mechanisms?

While the search results don't directly address ATP synthase's role in antibiotic resistance in M. ulcerans, several inferences can be made:

• Essential nature: Since mycobacterial F-ATP synthase is essential for growth , it represents a potential vulnerability that could be exploited for antibiotic development.

• Energy-dependent resistance mechanisms: M. ulcerans, like M. tuberculosis, may possess efflux pumps that require ATP for function. Efflux pump inhibitors have been shown to enhance the killing of intracellular multidrug-resistant M. tuberculosis , suggesting that energy-dependent processes contribute to resistance.

• Unique structural features: The distinctive characteristics of mycobacterial ATP synthase, including the fused b-δ subunit, may influence how antibiotics targeting energy metabolism affect M. ulcerans.

A comprehensive study would involve:

• Testing ATP synthase inhibitors against susceptible and resistant M. ulcerans strains

• Monitoring ATP synthase expression during antibiotic exposure

• Investigating whether mutations in ATP synthase components correlate with resistance phenotypes

What genetic tools are available for manipulating atpF expression in M. ulcerans for research purposes?

Based on successful genetic manipulation of M. ulcerans described in the search results:

• Promoter options:

  • MuG13 promoter - successfully amplified from both M. ulcerans ATCC 19423 and M. ulcerans Cu001

  • MOP (mycobacterial optimized promoter)

  • hsp60 promoter - functional in mycobacteria

• Vector systems:

  • Plasmids like p60LUX have been successfully used in M. ulcerans

  • Both integrative plasmids (pTY60H, pTY60K) and replicative plasmids can be employed

• Transformation methods:

  • Direct transformation of M. ulcerans is possible

  • Alternatively, use M. smegmatis as an intermediate host when direct transformation is challenging

• Verification methods:

  • PCR with specific primer pairs

  • Sequencing of amplified products

  • Reporter systems like luxAB for functional studies

What are effective approaches for studying atpF function using recombinant expression in heterologous hosts?

For studying M. ulcerans atpF function in heterologous hosts:

• E. coli expression systems:

  • Demonstrated success with His-tagged recombinant M. ulcerans ATP synthase subunits

  • Suitable for structural studies and basic functional characterization

  • May require codon optimization for efficient expression

• Mycobacterial expression hosts:

  • M. smegmatis serves as a useful surrogate host for mycobacterial proteins

  • More similar cellular environment compared to E. coli

  • Successful transformation of M. smegmatis with ligation products has been demonstrated

• Functional complementation:

  • Expression of M. ulcerans atpF in ATP synthase-deficient strains of model organisms

  • Assessment of growth rescue and ATP synthase activity restoration

  • Comparative studies with modified versions of the protein

• Reporter systems:

  • Luciferase reporters have been successfully used in recombinant M. ulcerans strains

  • Enable monitoring of gene expression and protein function in vivo

How can structural information about M. ulcerans ATP synthase subunit b inform drug development efforts?

The unique structural features of mycobacterial ATP synthase make it an attractive target for selective antimicrobial development:

• Targetable unique features:

  • The fused b-δ subunit structure

  • Interfaces between the peripheral stalk and other components

  • Regulatory sites specific to mycobacterial ATP synthases

• Structural determination approaches:

  • Electron microscopy and single particle analysis have been successfully used for intact mycobacterial F₁F₀ ATP synthase

  • X-ray crystallography of individual components or subcomplexes

  • NMR studies of specific domains or interactions

• Drug development strategies:

  • Structure-based virtual screening against binding pockets

  • Fragment-based drug discovery

  • Peptidomimetic approach targeting protein-protein interfaces

• Rationale for targeting:

  • Mycobacterial F-ATP synthase is essential for growth, unlike in other prokaryotes

  • Dissipation of the proton motive force is lethal to mycobacteria

  • The unique structural features may allow for selective targeting

What comparative structural analyses between M. ulcerans and other mycobacterial ATP synthases have been conducted?

The search results indicate several comparative structural analyses:

• M. ulcerans vs. M. marinum:

  • 98% nucleotide sequence identity and genome-wide synteny

  • Small differences observed in the G13 promoter sequences between M. ulcerans strain ATCC 19423, the genome-sequenced strain M. ulcerans Agy99, and M. marinum

  • These differences are outside the functional region, suggesting the promoter is relatively conserved

• M. smegmatis as a structural model:

  • The first purification protocol and structural characterization of intact M. smegmatis F₁F₀ ATP synthase has been reported

  • This provides a valuable model for understanding M. ulcerans ATP synthase structure

• Common mycobacterial features:

  • The fusion of b-δ subunits appears to be conserved across mycobacteria

  • The latent ATPase activity is a shared characteristic of mycobacterial ATP synthases

  • The essential nature of ATP synthase for growth is consistent across mycobacterial species

Further comparative studies could involve detailed sequence analyses of ATP synthase components across mycobacterial species, particularly focusing on the b-δ fusion protein and its interactions within the complex.

What emerging technologies could enhance our understanding of M. ulcerans ATP synthase function and regulation?

Several cutting-edge technologies hold promise for advancing our understanding of M. ulcerans ATP synthase:

How might studying M. ulcerans ATP synthase contribute to developing new treatments for Buruli ulcer?

M. ulcerans causes Buruli ulcer , and targeting its ATP synthase could lead to novel therapeutic approaches:

• Target validation:

  • The essential nature of mycobacterial F-ATP synthase for growth makes it a promising drug target

  • The unique structural features, including the b-δ fusion, offer potential for selective targeting

• Drug development strategies:

  • Small molecules targeting the unique interfaces in the ATP synthase complex

  • Peptide-based inhibitors designed to disrupt specific interactions

  • Allosteric modulators affecting the regulation of ATP synthesis/hydrolysis

• Combination therapy approaches:

  • ATP synthase inhibitors could be combined with existing antibiotics

  • Efflux pump inhibitors enhance killing of intracellular multidrug-resistant mycobacteria , suggesting potential synergy with ATP synthase inhibitors

• Experimental models:

  • Recombinant bioluminescent M. ulcerans strains provide tools for rapid assessment of antibacterial activity

  • These models could be used to evaluate ATP synthase-targeting compounds

• Translational research:

  • Understanding the bioenergetics of M. ulcerans during infection

  • Investigating how ATP synthase function relates to virulence and persistence

  • Developing biomarkers based on ATP synthase activity for monitoring treatment efficacy