Recombinant Stenotrophomonas maltophilia ATP synthase subunit b (atpF)

What is Stenotrophomonas maltophilia ATP synthase subunit b (atpF)?

Stenotrophomonas maltophilia ATP synthase subunit b (atpF) is a crucial component of the F0F1-ATP synthase complex in this opportunistic pathogen. The protein is part of the membrane-embedded F0 portion of ATP synthase, serving as a stator stalk that connects the F1 catalytic domain to the F0 proton channel. In S. maltophilia, the atpF gene typically encodes a protein of approximately 150-170 amino acids with a molecular weight of around 17-19 kDa. As part of the ATP synthase complex, atpF plays an essential role in energy metabolism by participating in the conversion of the proton gradient across the bacterial membrane into ATP production.

S. maltophilia is known for its intrinsic resistance to multiple and broad-spectrum antibiotics, making it a significant clinical concern . The bacterium contains a variety of virulence factors and has remarkable adaptability to different environments . Studying components of its essential energy-generating machinery, such as ATP synthase, can provide insights into potential targets for novel antimicrobial strategies.

What is the function of ATP synthase subunit b in bacterial cells?

ATP synthase subunit b serves several critical functions in bacterial cells:

• Structural support: It forms a peripheral stalk connecting the membrane-embedded F0 sector with the catalytic F1 sector of the ATP synthase complex.

• Counter-rotation prevention: It acts as a stator that prevents unproductive rotation of the F1 sector during ATP synthesis, allowing rotational energy from proton translocation to be efficiently converted to ATP production.

• Assembly scaffold: The subunit plays a crucial role in the proper assembly of the entire ATP synthase complex.

• Energy coupling: It participates in coupling proton movement across the membrane to the conformational changes needed for ATP synthesis in the F1 sector.

In S. maltophilia, efficient ATP synthase function is particularly important due to the bacterium's metabolic versatility, which allows it to thrive in diverse environments ranging from hospital settings to plant rhizospheres . The ATP synthase complex contributes to the organism's adaptability and potentially to its virulence capabilities in opportunistic infections.

How is the atpF gene organized in the S. maltophilia genome?

In the S. maltophilia genome, the atpF gene is typically located within the atp operon, which encodes all subunits of the F0F1-ATP synthase. The organization follows a pattern similar to other gamma-proteobacteria, with these key features:

• Operon structure: The atp operon usually contains genes in the order atpIBEFHAGDC, where atpF represents the gene encoding subunit b.

• Genetic context: The atpF gene is flanked by atpE (encoding subunit c) upstream and atpH (encoding subunit delta) downstream in the typical arrangement.

• Promoter region: The atp operon is generally controlled by a single promoter located upstream of the first gene, although internal promoters may exist.

• Regulatory elements: Expression of the atp operon, including atpF, is subject to regulation by factors responding to the energy status and growth phase of the bacteria.

What expression systems are suitable for producing recombinant S. maltophilia atpF?

Several expression systems can be employed for producing recombinant S. maltophilia atpF, each with distinct advantages and challenges:

• Escherichia coli expression systems:

  • BL21(DE3): Most commonly used for cytoplasmic expression with T7 promoter-based vectors

  • C41(DE3) and C43(DE3): Specialized strains for membrane proteins that may be toxic

  • ArcticExpress: Provides cold-adapted chaperones for improved protein folding at lower temperatures

  • SHuffle: Engineered for improved disulfide bond formation in the cytoplasm

• Cell-free expression systems:

  • E. coli extract-based cell-free systems can be advantageous for potentially toxic membrane proteins

  • Allow direct manipulation of the reaction environment to optimize protein solubility

• Yeast expression systems:

  • Pichia pastoris: Useful for proteins requiring eukaryotic post-translational modifications

  • Saccharomyces cerevisiae: Well-established genetic tools and compatibility with membrane proteins

• Homologous expression in S. maltophilia:

  • Can be considered for functional studies requiring native protein interactions

  • Requires development of genetic manipulation tools specific for S. maltophilia

For most research applications, E. coli-based systems, particularly BL21(DE3) derivatives with pET vectors, offer the best balance of yield, ease of use, and cost-effectiveness. Fusion tags such as 6xHis, MBP, or SUMO can significantly improve solubility and facilitate purification.

What are the basic purification methods for recombinant atpF protein?

Purification of recombinant S. maltophilia atpF typically involves a multi-step process that depends on the expression system and fusion tags employed. Here's a methodological approach:

• Cell lysis:

  • For membrane-associated atpF: Gentle lysis using specialized detergents (e.g., n-dodecyl-β-D-maltoside (DDM), Triton X-100) to solubilize the protein

  • For cytoplasmic expression: Sonication, French press, or enzymatic lysis with lysozyme

• Initial purification based on affinity tags:

  • For His-tagged atpF: Immobilized metal affinity chromatography (IMAC) using Ni-NTA or Co-based resins

  • For MBP-tagged atpF: Amylose resin affinity chromatography

  • For GST-tagged atpF: Glutathione-Sepharose affinity chromatography

• Secondary purification steps:

  • Ion exchange chromatography (typically using Q Sepharose or SP Sepharose)

  • Size exclusion chromatography (Superdex 75 or 200) for final polishing and buffer exchange

• Tag removal (if necessary):

  • Proteolytic cleavage using specific proteases (TEV, PreScission, or SUMO protease)

  • Reverse affinity chromatography to remove the cleaved tag

• Assessment of purity and integrity:

  • SDS-PAGE with Coomassie staining (expect >95% purity)

  • Western blotting with anti-His antibodies or specific antibodies against atpF

  • Mass spectrometry to confirm protein identity and integrity

For membrane proteins like atpF, maintaining an appropriate detergent environment throughout purification is critical to preserve native conformation and prevent aggregation. The careful selection of detergents that mimic the lipid environment of S. maltophilia membranes can significantly improve protein stability and functional integrity during purification.

How does the structure of S. maltophilia atpF differ from other bacterial ATP synthase subunit b proteins?

The structure of S. maltophilia ATP synthase subunit b shares general architectural features with bacterial b subunits while exhibiting some distinct characteristics that may relate to the organism's unique ecological adaptations . Comparative structural analysis reveals:

• Domain organization:

  • N-terminal membrane-anchoring domain (approximately residues 1-25): More hydrophobic in S. maltophilia compared to E. coli, potentially reflecting adaptation to different membrane compositions

  • Central dimerization domain (approximately residues 26-120): Contains characteristic heptad repeats forming a right-handed coiled-coil structure

  • C-terminal F1-binding domain (approximately residues 121-160): Shows higher sequence conservation across species due to functional constraints in interactions with the δ and α subunits

• Sequence variations:

  • S. maltophilia atpF typically shows 40-60% sequence identity with atpF from other gamma-proteobacteria

  • Higher conservation in the C-terminal domain (70-80% identity) compared to the N-terminal domain (30-40% identity)

  • Unique residue substitutions in the dimerization domain that may affect stability and flexibility

• Structural implications:

  • Molecular dynamics simulations suggest that the S. maltophilia atpF dimer has altered flexibility properties compared to other bacterial species

  • These differences may reflect adaptation to different environmental stresses or metabolic requirements

  • The altered flexibility could impact how mechanical energy is transferred during ATP synthesis

The unique structural features of S. maltophilia atpF could contribute to the organism's metabolic versatility and survival in diverse environments, from clinical settings to plant-associated niches . Understanding these differences provides insights into species-specific ATP synthase regulation and potential targets for selective inhibition.

What are the challenges in expressing and purifying functional recombinant atpF?

Expressing and purifying functional recombinant S. maltophilia atpF presents several significant challenges that researchers must address:

• Membrane association challenges:

  • The N-terminal transmembrane domain makes atpF partially hydrophobic, leading to potential aggregation during expression

  • Proper incorporation into membranes or micelles is essential for maintaining native conformation

  • Solution: Optimization of detergent types and concentrations; consideration of nanodisc or liposome reconstitution

• Coiled-coil dimerization:

  • Native atpF functions as a homodimer with a parallel coiled-coil structure

  • Improper dimerization during recombinant expression can lead to non-functional protein

  • Solution: Co-expression with partner subunits; optimization of refolding conditions if expressed as inclusion bodies

• Proteolytic degradation:

  • The extended coiled-coil region of atpF can be susceptible to proteolytic cleavage

  • Solution: Inclusion of protease inhibitors; use of protease-deficient expression strains; optimization of purification speed

• Yield limitations:

  • Expression levels of membrane-associated proteins are often lower than soluble proteins

  • Solution: Use of strong promoters, codon optimization, and specialized expression strains

This data illustrates the importance of lowered temperature, reduced inducer concentration, and extended expression time for improving both yield and solubility of recombinant atpF. The co-expression with molecular chaperones can further enhance proper folding and solubility of this challenging membrane protein.

How can site-directed mutagenesis be used to study S. maltophilia atpF function?

Site-directed mutagenesis is a powerful approach for dissecting the structure-function relationships of S. maltophilia atpF. This technique can systematically probe how specific amino acid residues contribute to protein folding, dimerization, membrane integration, and interactions with other ATP synthase subunits.

Methodological approach to atpF mutagenesis studies:

• Target selection for mutation:

  • Conserved residues identified through multi-species sequence alignment

  • Residues at predicted interfacial regions with other subunits

  • Residues in the transmembrane domain for membrane association studies

  • Residues in the coiled-coil region for dimerization studies

• Mutagenesis techniques:

  • QuikChange PCR-based mutagenesis for single mutations

  • Golden Gate assembly for multiple mutations

  • Gibson Assembly for more complex modifications or domain swapping

• Functional characterization of mutants:

  • Protein expression and stability assessment (Western blotting, thermal shift assays)

  • Dimerization analysis (native PAGE, chemical crosslinking, analytical ultracentrifugation)

  • Interaction studies with partner subunits (pull-down assays, surface plasmon resonance)

  • Membrane association (flotation assays, detergent resistance)

  • Complementation assays in ATP synthase-deficient strains

This data illustrates how mutations at the coiled-coil interface dramatically affect dimerization and function, while mutations at solvent-exposed positions have minimal impact. Such systematic mutational analysis can map critical residues for atpF function and provide insights into the molecular mechanism of ATP synthase assembly and activity.

What is the role of atpF in antibiotic resistance mechanisms of S. maltophilia?

The role of ATP synthase subunit b (atpF) in antibiotic resistance mechanisms of S. maltophilia involves both direct and indirect pathways that contribute to the organism's notorious multidrug resistance profile :

• Energy-dependent resistance mechanisms:

  • ATP synthase provides energy for multiple antibiotic efflux pumps (like SmeDEF and SmeABC systems)

  • Mutations affecting atpF function could modulate ATP production and thus impact efflux pump efficiency

  • Studies suggest that suboptimal ATP synthase function can trigger compensatory metabolic changes that affect antibiotic susceptibility

• Membrane potential and antibiotic uptake:

  • ATP synthase contributes to maintaining the proton motive force across the bacterial membrane

  • Alterations in atpF can affect membrane potential, which in turn affects uptake of aminoglycosides and other charged antibiotics

  • Data suggests that even subtle changes in ATP synthase composition can alter membrane energetics and antibiotic sensitivity

• Stress response and persistence:

  • ATP limitation from altered atpF function may trigger stringent response mechanisms

  • This can lead to persister cell formation, which shows enhanced antibiotic tolerance

  • Experimental evidence indicates correlations between ATP synthase expression levels and persistence rates

The complex relationships between ATP synthase function and antibiotic resistance highlight atpF as both a contributor to resistance mechanisms and a potential target for combination therapies aimed at overcoming the inherent multidrug resistance of S. maltophilia .

How can structural studies of recombinant atpF contribute to drug development against S. maltophilia?

Structural studies of recombinant S. maltophilia atpF can significantly advance drug development efforts against this multidrug-resistant pathogen by identifying potential binding sites for novel inhibitors and understanding the molecular mechanisms of ATP synthase function:

• Structure determination approaches:

  • X-ray crystallography of atpF in isolation or as part of subcomplexes

  • Cryo-electron microscopy of reconstituted ATP synthase complexes

  • Nuclear magnetic resonance (NMR) for dynamic studies of specific domains

  • Molecular dynamics simulations to identify transient binding pockets

• Drug target identification:

  • Interface regions between atpF and other subunits represent potential sites for disrupting assembly

  • The dimerization domain of atpF presents opportunities for designing coiled-coil disruptors

  • The membrane-binding domain could be targeted by compounds that disrupt membrane association

• Structure-based drug design strategies:

  • Virtual screening of compound libraries against identified binding pockets

  • Fragment-based approaches focusing on the unique structural features of S. maltophilia atpF

  • Peptide-based inhibitors designed to mimic natural binding partners

• Validation of structural insights:

  • Binding studies using surface plasmon resonance or isothermal titration calorimetry

  • Functional assays measuring ATP synthase activity in the presence of designed inhibitors

  • Cellular studies assessing impact on bacterial growth and virulence

The development of ATP synthase inhibitors based on structural studies of atpF could provide much-needed new treatment options for infections caused by multidrug-resistant S. maltophilia, which is intrinsically resistant to most known antibiotics .

How does atpF interact with other ATP synthase subunits in S. maltophilia?

The ATP synthase subunit b (atpF) in S. maltophilia forms critical interactions with multiple components of the ATP synthase complex, serving as a central element of the stator stalk. Understanding these interactions is essential for comprehending the assembly, stability, and function of the entire complex:

• Interaction with membrane components (F0 sector):

  • atpF forms a homodimer through parallel coiled-coil interactions

  • The N-terminal hydrophobic domain (residues 1-25) anchors into the membrane

  • Interacts with subunit a and potentially with the c-ring through specific residues

  • Forms the peripheral stalk starting from the membrane

• Interaction with the F1 sector:

  • The C-terminal domain (approximately residues 121-160) makes specific contacts with:

    • δ subunit: Primary interaction partner at the top of the peripheral stalk

    • α subunit: Secondary interactions that help position the stator relative to the catalytic subunits

  • These interactions prevent co-rotation of F1 with the central stalk during ATP synthesis

• Methods to study these interactions:

  • Crosslinking studies with bifunctional reagents

  • Co-immunoprecipitation with antibodies against specific subunits

  • FRET analysis with fluorescently labeled subunits

  • Bacterial two-hybrid or yeast two-hybrid screening

  • Cryo-EM structural analysis of the entire complex or subcomplexes

*Interaction strength scale: + weak, ++ moderate, +++ strong, ++++ essential

These interactions collectively ensure that the mechanical energy from proton translocation through F0 is efficiently converted to chemical energy in the form of ATP at the catalytic sites of F1.

What methods can be used to study atpF phosphorylation and other post-translational modifications?

Post-translational modifications (PTMs) of S. maltophilia atpF, including phosphorylation, may play important roles in regulating ATP synthase assembly, activity, and stability. Investigating these modifications requires a comprehensive analytical approach:

• Identification of PTM sites:

  • Mass spectrometry-based proteomics:

    • Bottom-up approach: Enzymatic digestion followed by LC-MS/MS

    • Top-down approach: Analysis of intact protein

    • Middle-down approach: Limited proteolysis generating larger peptides

  • Enrichment strategies:

    • Phosphopeptide enrichment using TiO2, IMAC, or phospho-specific antibodies

    • Other modifications may require specific enrichment techniques (e.g., immunoprecipitation)

• Functional characterization of PTMs:

  • Site-directed mutagenesis:

    • Phosphomimetic mutations (Ser/Thr to Asp/Glu)

    • Non-phosphorylatable mutations (Ser/Thr to Ala)

  • In vitro modification:

    • Incubation with purified kinases/phosphatases

    • Chemical modification to mimic PTMs

  • Activity assays comparing modified and unmodified proteins

• Temporal and condition-dependent PTM analysis:

  • Time-course experiments during bacterial growth

  • Stress conditions (antibiotic exposure, oxidative stress, nutrient limitation)

  • Host-pathogen interaction scenarios

*Abundance scale: + barely detectable, ++ low, +++ moderate, ++++ high, +++++ very high

• Advanced techniques for dynamic PTM studies:

  • Pulse-chase labeling with stable isotopes

  • Quantitative proteomics using iTRAQ, TMT, or SILAC

  • Protein-protein interaction changes using crosslinking mass spectrometry

  • Structural impacts using hydrogen-deuterium exchange mass spectrometry

These methodological approaches can reveal how S. maltophilia modulates ATP synthase function through post-translational modifications of atpF in response to changing environmental conditions or during different phases of infection.

How can recombinant atpF be used to study S. maltophilia interactions with host cells?

Recent research indicates that S. maltophilia can replicate within amoeba vacuoles, suggesting a potential intracellular lifestyle that might extend to interaction with mammalian host cells . Recombinant atpF can be utilized as a tool to study these interactions:

• Protein-based interaction studies:

  • Identification of potential host cell receptors that recognize atpF

  • Investigation of atpF immunogenicity and host immune responses

  • Examination of possible moonlighting functions of atpF outside the ATP synthase complex

• Cellular localization studies:

  • Fluorescently tagged recombinant atpF to track localization during host cell infection

  • Immuno-electron microscopy to determine precise subcellular localization

  • Protein interaction screening to identify host factors that bind atpF

• Infection model systems:

  • Amoeba models (Acanthamoeba castellanii) to study intracellular replication mechanisms

  • Mammalian cell culture models to assess potential intracellular survival strategies

  • Animal models to evaluate the role of atpF in virulence and host adaptation

• Immunological studies:

  • Evaluation of atpF as a potential vaccine antigen

  • Analysis of host antibody responses to atpF during infection

  • Investigation of atpF-mediated immune evasion mechanisms

These approaches can provide valuable insights into the role of ATP synthase components in S. maltophilia pathogenesis and host-pathogen interactions, potentially revealing new therapeutic strategies against this increasingly important opportunistic pathogen.