Recombinant Proteus mirabilis Fumarate reductase subunit C (frdC)

What is the basic structure and function of fumarate reductase in Proteus mirabilis?

Fumarate reductase in Proteus mirabilis is a membrane-bound respiratory enzyme complex typically composed of four subunits (FrdABCD). The FrdC subunit serves as a membrane anchor protein that helps localize the complex to the cytoplasmic membrane. The fumarate reductase complex catalyzes the reduction of fumarate to succinate as part of anaerobic respiration, where fumarate serves as the terminal electron acceptor. This reaction is essentially the reverse of the reaction catalyzed by succinate dehydrogenase in aerobic conditions .

The FrdA subunit contains the catalytic site with a covalently bound FAD, while FrdB contains iron-sulfur clusters for electron transfer. FrdC and FrdD are hydrophobic membrane-anchoring subunits that interact with quinones in the membrane, facilitating electron transfer from the respiratory chain to the catalytic subunits . The complete complex is crucial for P. mirabilis to grow under anaerobic conditions, which are often encountered during infection.

How does frdC differ from other subunits of the fumarate reductase complex?

The frdC subunit differs from other components of the fumarate reductase complex primarily in its structure and function. Unlike FrdA and FrdB, which form the catalytic core of the enzyme, FrdC is a hydrophobic membrane protein that anchors the complex to the cytoplasmic membrane. FrdC contains transmembrane helices that span the membrane and interact with quinone molecules, facilitating electron transfer from the quinone pool to the iron-sulfur clusters in FrdB.

While FrdA is involved in fumarate binding and contains the catalytic site with a covalently bound FAD cofactor, and FrdB contains iron-sulfur clusters essential for electron transfer, FrdC functions as a structural component that ensures proper localization and orientation of the complex within the membrane . This membrane anchoring is critical for the enzyme's ability to participate in the electron transport chain during anaerobic respiration.

What is the genomic organization of the frd operon in Proteus mirabilis?

The fumarate reductase (frd) operon in Proteus mirabilis typically follows the organization pattern observed in other Enterobacteriaceae, consisting of four genes (frdA, frdB, frdC, and frdD) that encode the four subunits of the fumarate reductase complex. These genes are arranged in a single transcriptional unit that is regulated primarily by oxygen levels through the FNR (fumarate and nitrate reduction) regulatory system.

The operon is typically organized with frdA at the 5' end, followed by frdB, frdC, and frdD. Transcription of the operon is induced under anaerobic conditions and repressed in the presence of oxygen. Regulatory elements including a promoter region and FNR binding site are typically located upstream of the frdA gene. This organization ensures coordinated expression of all components required for a functional fumarate reductase complex.

What are the recommended methods for cloning and expressing recombinant P. mirabilis frdC?

For successful cloning and expression of recombinant Proteus mirabilis frdC, researchers should consider the following methodological approach:

Cloning Strategy:

• PCR amplification of the frdC gene from P. mirabilis genomic DNA using high-fidelity DNA polymerase and primers containing appropriate restriction sites

• Restriction digestion and ligation into an expression vector with a suitable promoter (e.g., T7 or tac)

• Addition of affinity tags (His6, GST, or FLAG) at either N- or C-terminus to facilitate purification

Expression Systems:

• Bacterial Expression: E. coli BL21(DE3) or C43(DE3) strains are recommended for membrane protein expression

• Cell-Free Expression: Consider for difficult-to-express membrane proteins

• Eukaryotic Expression: Yeast systems like Pichia pastoris may be suitable for complex membrane proteins

Expression Conditions:

• Use lower temperatures (16-25°C) to minimize inclusion body formation

• Consider induction with lower concentrations of IPTG (0.1-0.5 mM)

• Add membrane-stabilizing agents like glycerol (5-10%)

• Include specific detergents for membrane protein solubilization during extraction

Since FrdC is a membrane protein, special attention must be paid to extraction and purification conditions to maintain protein structure and function. Detergent screening is critical, with mild detergents like n-dodecyl-β-D-maltoside (DDM) or digitonin often proving effective for membrane protein isolation.

What techniques are most effective for analyzing the interaction between frdC and other components of the fumarate reductase complex?

Several advanced biophysical and biochemical techniques are particularly effective for analyzing interactions between frdC and other fumarate reductase complex components:

Co-immunoprecipitation (Co-IP):

• Use antibodies against one subunit to pull down the entire complex

• Western blotting with antibodies specific to each subunit can confirm interactions

• Particularly useful for analyzing native protein complexes from P. mirabilis

Surface Plasmon Resonance (SPR):

• Quantitatively measures binding kinetics and affinity between purified subunits

• Can determine association/dissociation rates between frdC and other subunits

• Requires immobilization of one component on a sensor chip

Cross-linking Mass Spectrometry:

• Chemical cross-linking followed by mass spectrometry analysis can identify interaction interfaces

• Zero-length cross-linkers like EDC or longer cross-linkers like DSS can map the spatial proximity of residues

• Helps generate structural models of the complex

Förster Resonance Energy Transfer (FRET):

• Label different subunits with fluorophore pairs

• Measure energy transfer as an indication of proximity (typically <10 nm)

• Can be performed in vitro with purified proteins or in vivo with tagged constructs

Bacterial Two-Hybrid Assays:

• Modified for membrane proteins by using split-ubiquitin or BACTH systems

• Allows screening of specific interactions between frdC and other subunits

• Results can be quantified through reporter gene expression

Cryo-Electron Microscopy:

• High-resolution structural analysis of the entire complex

• Particularly valuable for membrane protein complexes that are difficult to crystallize

• Can reveal conformational changes upon substrate binding

When analyzing these interactions, it's crucial to consider the membrane environment, as detergent micelles or lipid nanodiscs might be required to maintain the native conformation of frdC during analysis.

What are the challenges in purifying functional recombinant frdC and how can they be overcome?

Purification of functional recombinant frdC presents several significant challenges due to its nature as an integral membrane protein:

Major Challenges and Solutions:

• Low Expression Levels:

  • Challenge: Membrane proteins typically express at lower levels than soluble proteins

  • Solution: Use specialized expression strains (C41/C43), codon-optimized genes, and membrane protein-specific promoters

  • Consider fusion partners like MBP or SUMO that can enhance expression and solubility

• Protein Misfolding and Inclusion Body Formation:

  • Challenge: Membrane proteins often misfold when overexpressed

  • Solution: Express at lower temperatures (16-20°C) and use slower induction (low IPTG concentrations)

  • Consider refolding protocols using mild detergents if inclusion bodies form

• Membrane Extraction Efficiency:

  • Challenge: Efficient extraction of membrane-embedded proteins

  • Solution: Screen multiple detergents (DDM, LMNG, CHAPS, digitonin) for optimal extraction

  • Use gentle solubilization conditions with longer incubation times at 4°C

• Maintaining Stability During Purification:

  • Challenge: Membrane proteins often destabilize when removed from the lipid bilayer

  • Solution: Include lipids or lipid-like molecules (cholesterol hemisuccinate, specific phospholipids) in purification buffers

  • Use amphipols or nanodiscs for final stabilization of the purified protein

• Assessing Functionality:

  • Challenge: Confirming that purified frdC retains native structure and function

  • Solution: Develop activity assays specific to frdC's role in electron transfer

  • Use biophysical techniques (CD spectroscopy, fluorescence) to confirm proper folding

Purification Strategy Table:

For functional studies, reconstitution into proteoliposomes may be necessary to recreate the native membrane environment and allow proper interaction with other components of the fumarate reductase complex.

How does the fumarate reductase complex contribute to P. mirabilis pathogenicity and virulence?

The fumarate reductase complex contributes significantly to Proteus mirabilis pathogenicity and virulence through several mechanisms:

Anaerobic Survival and Colonization:
The fumarate reductase complex enables P. mirabilis to survive and replicate in oxygen-limited environments commonly encountered during infection, particularly in the urinary tract. By catalyzing the reduction of fumarate to succinate during anaerobic respiration, it provides an alternative electron acceptor pathway when oxygen is unavailable. This metabolic flexibility allows P. mirabilis to colonize deep tissue sites and establish persistent infections .

Biofilm Formation:
Fumarate reductase activity supports bacterial growth under the oxygen-limited conditions found within biofilms. The ability to form robust biofilms is a critical virulence factor for P. mirabilis, particularly in catheter-associated urinary tract infections. Biofilms provide protection against host immune responses and antibiotics, contributing to treatment resistance and persistent infection.

Flagellar Motility Regulation:
Research has demonstrated that the fumarate reductase complex interacts with the flagellar motor switch protein FliG, influencing bacterial motility. In particular, the FrdA subunit of the complex binds to FliG in the presence of fumarate, enhancing clockwise rotation of flagella . This modulation of flagellar rotation affects swimming behavior, potentially influencing the ability of P. mirabilis to swarm across surfaces and ascend the urinary tract during infection.

Metabolic Adaptation During Infection:
The versatility provided by the fumarate reductase complex allows P. mirabilis to utilize alternative carbon sources and electron acceptors available in the host environment. Similar to findings in other bacteria, the complex likely plays a crucial role in the metabolic flexibility that supports adaptation to changing host conditions during different stages of infection .

Resistance to Oxidative Stress:
The fumarate reductase complex may contribute to resistance against oxidative stress generated by host immune responses. By maintaining redox balance under anaerobic conditions, it helps bacteria survive the hostile environment created by the host inflammatory response.

These multifaceted contributions to pathogenicity highlight why the fumarate reductase complex, including the frdC subunit, represents a potential target for novel antimicrobial development strategies.

What is the role of frdC in the electron transport chain, and how does it compare between different bacterial species?

The frdC subunit plays a critical role in the electron transport chain (ETC) of many bacteria, including Proteus mirabilis, particularly under anaerobic conditions:

Role in Electron Transport:
FrdC functions as one of the membrane anchor subunits of the fumarate reductase complex, containing transmembrane helices that position the complex within the cytoplasmic membrane. Its primary function is to facilitate the interaction with and oxidation of menaquinol (reduced menaquinone) in the membrane, transferring electrons to the iron-sulfur clusters in FrdB and ultimately to FrdA for the reduction of fumarate to succinate. This process couples the oxidation of menaquinol to the generation of a proton motive force across the membrane, contributing to energy conservation during anaerobic respiration.

Comparative Analysis Across Bacterial Species:

Key Differences and Similarities:

These differences reflect evolutionary adaptations to specific environmental niches and metabolic requirements, while maintaining the core function of electron transfer between quinones and the catalytic subunits of the complex.

How might mutations in the frdC gene affect antibiotic resistance in P. mirabilis?

Mutations in the frdC gene can potentially influence antibiotic resistance in Proteus mirabilis through several distinct mechanisms:

Metabolic Adaptation:
FrdC mutations that modify fumarate reductase function could trigger metabolic adaptations that indirectly affect antibiotic susceptibility. Bacteria with altered central metabolism often exhibit changes in growth rate, biofilm formation capacity, and stress responses, all of which can influence antibiotic efficacy. Particularly, the ability to survive under anaerobic conditions might be compromised, potentially affecting persistence during antibiotic treatment.

Efflux Pump Expression:
Disruptions in energy metabolism resulting from frdC mutations might affect the expression or function of energy-dependent efflux pumps. These pumps actively export antibiotics from bacterial cells and are major contributors to multidrug resistance in P. mirabilis . Changes in proton motive force generation due to altered fumarate reductase function could impact the efficiency of these efflux systems.

Stress Response Modulation:
The bacterial stress response is intimately connected with metabolic state. Mutations in frdC that alter cellular energy production might modify stress response pathways, potentially enhancing resistance mechanisms such as the production of antibiotic-modifying enzymes or formation of persister cells that can survive antibiotic exposure.

Horizontal Gene Transfer Dynamics:
Interestingly, metabolic perturbations have been linked to altered rates of horizontal gene transfer, which is a primary mechanism for acquiring antibiotic resistance genes. The integrons that facilitate transfer of resistance genes in P. mirabilis might be indirectly affected by metabolic changes resulting from frdC mutations.

While direct evidence specifically linking frdC mutations to antibiotic resistance in P. mirabilis is limited in the provided search results, the interconnection between metabolic functions and resistance mechanisms suggests this is an important area for further investigation, particularly given the rising concern about multidrug-resistant P. mirabilis strains in clinical settings .

Can the fumarate reductase complex be targeted for novel antimicrobial development against P. mirabilis?

The fumarate reductase complex represents a promising target for novel antimicrobial development against Proteus mirabilis for several compelling reasons:

Essential Metabolic Function:
The fumarate reductase complex plays a critical role in anaerobic respiration, allowing P. mirabilis to survive in oxygen-limited environments encountered during infection. Inhibition of this complex could potentially prevent bacterial growth under anaerobic conditions, compromising colonization and persistence in host tissues. In Campylobacter jejuni, research has demonstrated that disruption of the frdA gene significantly impairs bacterial growth and reduces colonization capacity in animal models , suggesting similar effects might be observed in P. mirabilis.

Structural Uniqueness:
The structure of bacterial fumarate reductase differs significantly from mammalian succinate dehydrogenase, despite catalyzing similar reactions. These structural differences, particularly in the membrane-anchoring subunits like frdC, provide opportunities for selective targeting that could minimize effects on host enzymes. The quinone-binding site in frdC represents a particularly attractive target for inhibitor design.

Virulence Attenuation:
Beyond growth inhibition, targeting the fumarate reductase complex could attenuate virulence by interfering with multiple pathogenic mechanisms. The complex's involvement in flagellar rotation modulation suggests that inhibitors might impact motility and swarming behavior, which are critical virulence factors for P. mirabilis in urinary tract infections.

Resistance Considerations:
As a metabolic enzyme not directly targeted by current antibiotics, inhibitors of fumarate reductase would represent a novel class of antimicrobials. This novelty could be advantageous against multidrug-resistant P. mirabilis strains that show resistance to conventional antibiotics like ampicillin, amoxicillin, fluoroquinolones, and certain cephalosporins .

Potential Approaches for Inhibitor Development:

• Structure-based drug design: Using crystal structures of fumarate reductase complexes to design specific inhibitors that bind to catalytic sites or subunit interfaces

• Natural product screening: Evaluating compounds from natural sources for selective inhibition of fumarate reductase activity

• Fragment-based approaches: Identifying small molecular fragments that bind to specific pockets within the complex and optimizing them for improved potency and selectivity

• Peptidomimetic inhibitors: Developing peptide-based molecules that interfere with assembly of the complex or its interaction with other cellular components

While targeting the fumarate reductase complex offers considerable promise, challenges remain, including ensuring sufficient compound penetration into bacterial cells and addressing potential redundancy in metabolic pathways. Nevertheless, the increasing prevalence of antibiotic-resistant P. mirabilis strains underscores the importance of exploring such novel targets for antimicrobial development.

What methods are most effective for studying the role of frdC in P. mirabilis biofilm formation?

Studying the role of frdC in Proteus mirabilis biofilm formation requires a multifaceted approach combining genetic manipulation, advanced imaging, and quantitative analysis. The following methodologies are particularly effective:

Genetic Approaches:

• Targeted Gene Deletion and Complementation:

  • Create a clean deletion of the frdC gene using allelic exchange techniques

  • Complement the deletion with wild-type and mutant versions of frdC

  • Analyze changes in biofilm formation to establish cause-effect relationships

  • Include controls with deletions of other frd subunits to distinguish subunit-specific effects

• Site-Directed Mutagenesis:

  • Introduce specific mutations in functional domains of frdC

  • Focus on residues involved in quinone binding or protein-protein interactions

  • Analyze the impact of these mutations on both enzyme activity and biofilm formation

Biofilm Analysis Techniques:

• Static Biofilm Assays:

  • Crystal violet staining for quantitative biomass assessment

  • Resazurin (alamarBlue) assay for metabolic activity within biofilms

  • Congo red binding for assessment of extracellular matrix components

• Flow Cell Systems:

  • Real-time observation of biofilm development under controlled flow conditions

  • Closer mimicry of urinary tract environment with artificial urine medium

  • Ability to analyze biofilm architecture and resistance to shear forces

• Advanced Microscopy:

  • Confocal laser scanning microscopy with fluorescent reporters to visualize biofilm structure

  • Electron microscopy for high-resolution analysis of matrix components and bacterial interactions

  • Super-resolution microscopy to localize FrdC within biofilm cells

Biochemical and Metabolic Analysis:

• Metabolomics:

  • Compare metabolite profiles between wild-type and frdC mutant biofilms

  • Focus on TCA cycle intermediates and anaerobic respiration products

  • Identify adaptive metabolic shifts in response to frdC mutation

• Redox Analysis:

  • Measure redox potential within biofilms using microelectrodes

  • Analyze NAD+/NADH ratios to assess impact on cellular energetics

  • Determine oxygen gradients within biofilms using oxygen-sensitive probes

Experimental Design Considerations:

By combining these approaches, researchers can comprehensively assess how frdC contributes to P. mirabilis biofilm formation, potentially identifying new strategies to prevent catheter-associated urinary tract infections and other biofilm-related diseases caused by this pathogen.

How does the expression of frdC vary under different environmental conditions and what are the implications for in vivo studies?

The expression of frdC in Proteus mirabilis exhibits significant variation across different environmental conditions, which has important implications for designing relevant in vivo studies:

Oxygen Availability-Dependent Regulation:

The expression of the fumarate reductase complex, including frdC, is primarily regulated by oxygen concentration. Under anaerobic conditions, expression is strongly induced through the FNR (fumarate and nitrate reduction) regulatory system, which acts as a transcriptional activator for the frd operon when oxygen is absent. As oxygen levels increase, expression progressively decreases, with minimal expression under fully aerobic conditions. This regulation ensures that fumarate reductase is produced when needed for anaerobic respiration.

pH-Dependent Expression:

P. mirabilis encounters varying pH environments during infection, particularly in the urinary tract where pH can range from acidic to alkaline. Research suggests that frdC expression may be influenced by environmental pH, with potential upregulation under alkaline conditions that reflect the urease activity characteristic of P. mirabilis infections. This pH-dependent regulation may contribute to the bacterium's ability to thrive in the alkalinized environment it creates during urinary tract infections.

Nutrient Availability Effects:

The availability of specific carbon sources and electron acceptors significantly impacts frdC expression. In Campylobacter jejuni, the fumarate reductase complex is essential for metabolism of specific substrates like glutamate and proline . Similar substrate-dependent regulation likely occurs in P. mirabilis, with implications for metabolism during infection where specific amino acids may be the primary carbon sources available.

Growth Phase Variation:

Expression of frdC varies throughout the bacterial growth cycle, with highest expression typically occurring during late logarithmic and early stationary phases when oxygen becomes limiting in culture. This temporal regulation ensures efficient energy production as bacteria transition from aerobic to anaerobic metabolism.

Implications for In Vivo Study Design:

Practical Considerations for Research:

• Animal Models: When using animal models to study P. mirabilis infections, tissue oxygen levels should be considered, as artificially oxygenated environments may not reflect natural frdC expression patterns.

• Ex Vivo Systems: Tissue explant models that maintain natural oxygen gradients may provide more relevant insights into frdC expression than traditional culture systems.

• Sampling Techniques: In vivo sampling should be designed to preserve the environmental conditions at the infection site, as rapid exposure to oxygen could alter gene expression profiles.

• Reporter Systems: Developing fluorescent or luminescent reporters linked to the frdC promoter could allow real-time monitoring of expression in vivo.

Understanding these environmental influences on frdC expression is essential for designing experiments that accurately reflect the conditions encountered during infection, ultimately leading to more reliable and clinically relevant findings.

How might findings regarding P. mirabilis frdC contribute to our broader understanding of bacterial metabolism and pathogenesis?

Research on Proteus mirabilis frdC has the potential to significantly expand our understanding of bacterial metabolism and pathogenesis in several key areas:

Metabolic Adaptation During Infection:
P. mirabilis encounters diverse microenvironments during infection, from oxygen-rich bladder surfaces to oxygen-limited biofilms. Studying frdC regulation and function could reveal fundamental principles about how pathogenic bacteria adapt their metabolism to changing conditions during infection progression. These insights may extend to other pathogens that similarly navigate oxygen gradients during infection, informing broader theories about metabolic adaptation as a virulence strategy.

Metabolic Integration with Virulence Programs:
The connection between central metabolism and virulence factor expression remains an important frontier in bacterial pathogenesis research. P. mirabilis offers an excellent model system for studying this relationship, as the bacterium coordinates metabolic shifts with expression of motility, urease activity, and biofilm formation. The fumarate reductase complex, with frdC as a key component, sits at the intersection of metabolism and virulence, potentially serving as a metabolic sensor that influences virulence factor expression. Findings in this area could establish widely applicable principles about metabolism-virulence integration.

Evolution of Respiratory Flexibility:
Analysis of the P. mirabilis fumarate reductase complex could provide insights into the evolution of respiratory flexibility in bacteria. The observation that in some species like Campylobacter jejuni, the fumarate reductase functions as the sole succinate dehydrogenase raises interesting questions about the evolutionary history of these enzymes and how functional versatility emerges. Comparative studies of frdC across bacterial species could illuminate evolutionary paths toward metabolic flexibility and host adaptation.

Membrane Protein Dynamics and Bacterial Physiology:
As a membrane-anchoring protein, frdC contributes to the organization of respiratory complexes within the bacterial membrane. Research on its interactions with other membrane components could advance our understanding of bacterial membrane organization and how it influences cellular physiology. These findings would have implications beyond P. mirabilis, potentially informing studies of membrane protein dynamics in diverse bacterial species.

Host-Microbe Metabolic Interactions:
Studying how frdC-dependent metabolism influences host responses could reveal new aspects of host-microbe metabolic cross-talk. For instance, metabolites produced through fumarate reductase activity might modulate host immune responses or influence the composition of the local microbiome during infection. These insights would contribute to the emerging field of immunometabolism in infectious diseases.

Novel Approaches to Antimicrobial Development:
The rising prevalence of multidrug-resistant P. mirabilis makes this pathogen an important model for exploring metabolism-targeted antimicrobial strategies. Discoveries about frdC structure, function, and regulation could inspire new approaches to antimicrobial development that target metabolic vulnerabilities rather than traditional targets like cell wall synthesis or protein translation. Such metabolic-focused strategies might have applications against a wide range of drug-resistant pathogens.

By advancing knowledge in these interconnected areas, research on P. mirabilis frdC contributes valuable pieces to the complex puzzle of bacterial pathogenesis, potentially opening new avenues for both fundamental microbiology research and applied therapeutic development.

What computational tools and databases are most valuable for analyzing frdC sequence, structure, and function?

Researchers studying Proteus mirabilis frdC can leverage a comprehensive suite of computational tools and databases to analyze sequence, structure, and function. Here are the most valuable resources organized by research application:

Sequence Analysis and Annotation:

• NCBI Protein Database and BLAST

  • Essential for identifying frdC sequences across bacterial species

  • Provides annotated sequences and genomic context information

  • BLAST allows identification of homologs and comparative analysis

• UniProt/SwissProt

  • Offers manually curated protein information with functional annotations

  • Provides sequence features, post-translational modifications, and domain organization

  • Links to experimental evidence supporting functional annotations

• Ensembl Bacteria

  • Provides genomic context, regulatory elements, and comparative genomics

  • Useful for analyzing operon structure and gene neighborhood of frdC

• HMMER

  • Employs profile hidden Markov models for sensitive sequence analysis

  • Particularly valuable for detecting distant homologs of frdC

Structural Analysis Tools:

Functional Analysis Resources:

• KEGG (Kyoto Encyclopedia of Genes and Genomes)

  • Maps metabolic pathways involving fumarate reductase

  • Allows cross-species comparison of fumarate metabolism

  • Provides context for understanding FrdC function in cellular metabolism

• InterPro / Pfam

  • Domain and family identification tools

  • Identifies conserved functional domains in FrdC

  • Provides evolutionary context through domain architecture analysis

• ConSurf Server

  • Maps sequence conservation onto protein structures

  • Identifies functionally important residues based on evolutionary conservation

  • Particularly useful for identifying potential quinone-binding sites

• APBS (Adaptive Poisson-Boltzmann Solver)

  • Calculates electrostatic properties of protein structures

  • Important for analyzing quinone interactions, which are often electrostatically driven

  • Helpful in understanding the membrane environment of FrdC

Comparative Genomics and Evolution:

• OrthoDB / OMA Browser

  • Identifies orthologous genes across species

  • Useful for evolutionary analysis of frdC

• MEGA (Molecular Evolutionary Genetics Analysis)

  • Software package for phylogenetic analysis

  • Enables evolutionary rate analysis and selection pressure assessment

• STRING Database

  • Provides protein-protein interaction networks

  • Identifies functional partners of FrdC beyond the fumarate reductase complex

Specialized Tools for Membrane Proteins:

• CHARMM-GUI Membrane Builder

  • Creates simulation systems for membrane proteins

  • Useful for molecular dynamics simulations of FrdC in lipid environments

• PPM Server

  • Positions protein structures in membranes

  • Valuable for understanding FrdC orientation and lipid interactions

• MemProtMD Database

  • Collection of membrane protein simulations

  • Provides context for membrane protein behavior in lipid bilayers

Data Integration Platforms:

• Cytoscape

  • Network visualization and analysis software

  • Integrates multiple data types (genomics, proteomics, interactions)

  • Useful for understanding FrdC in the context of cellular networks

• Galaxy Platform

  • Web-based platform for computational bioanalysis

  • Enables creation of reproducible analysis workflows

  • Particularly valuable for researchers with limited programming experience

These computational resources collectively provide a comprehensive toolkit for analyzing P. mirabilis frdC from sequence to structure to function, enabling researchers to generate hypotheses and guide experimental design efficiently.

What is the recommended protocol for expressing and purifying recombinant P. mirabilis frdC for structural studies?

Below is a comprehensive protocol for expressing and purifying recombinant Proteus mirabilis frdC optimized for structural studies. This protocol addresses the specific challenges of membrane protein purification and incorporates strategies to maximize yield and stability.

Protocol: Expression and Purification of Recombinant P. mirabilis FrdC for Structural Studies

Cloning and Vector Construction

Materials:

• Genomic DNA from Proteus mirabilis

• pET28a(+) vector (for bacterial expression) or pPICZ-B (for yeast expression)

• Restriction enzymes (NdeI and XhoI for pET28a)

• T4 DNA ligase

• High-fidelity DNA polymerase

• Chemically competent E. coli DH5α (for cloning)

Procedure:

• PCR amplify the frdC gene from P. mirabilis genomic DNA using primers containing appropriate restriction sites.

• Add a C-terminal 8×His-tag with a TEV protease cleavage site.

• Consider codon optimization for the expression host.

• Digest both PCR product and vector with appropriate restriction enzymes.

• Ligate insert into vector and transform into E. coli DH5α.

• Confirm construct by sequencing.

Expression Optimization

Expression Systems:

• Bacterial System: E. coli C43(DE3) or Lemo21(DE3) (specialized for membrane proteins)

• Alternative System: Pichia pastoris for complex membrane proteins requiring eukaryotic machinery

Bacterial Expression Protocol:

• Transform expression construct into E. coli C43(DE3).

• Inoculate 10 mL LB medium containing kanamycin (50 μg/mL) and grow overnight at 37°C.

• Dilute overnight culture 1:100 into fresh medium (consider using terrific broth for higher yields).

• Grow at 37°C until OD600 reaches 0.6-0.8.

• Reduce temperature to 18°C and induce with 0.1-0.3 mM IPTG.

• Continue expression for 16-20 hours at 18°C.

Optimization Parameters:

• Test multiple expression strains (C41, C43, Lemo21, Rosetta)

• Test induction OD600 (0.4, 0.6, 0.8, 1.0)

• Test IPTG concentrations (0.1, 0.3, 0.5, 1.0 mM)

• Test expression temperatures (15°C, 18°C, 25°C, 30°C)

• Test addition of membrane-stabilizing agents (5-10% glycerol, 1% glucose)

Membrane Preparation

Materials:

• Cell lysis buffer: 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 5% glycerol, 1 mM EDTA, protease inhibitor cocktail

• Membrane resuspension buffer: 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 10% glycerol

Procedure:

• Harvest cells by centrifugation (6,000 × g, 15 min, 4°C).

• Resuspend in lysis buffer (5 mL per gram of wet cell weight).

• Lyse cells using one of the following methods:

  • French press (1,000-1,500 psi, 2-3 passes)

  • Sonication (10-15 cycles: 15 sec on, 45 sec off) on ice

  • Cell disruptor (25-30 kpsi)

• Remove unbroken cells and debris by centrifugation (10,000 × g, 20 min, 4°C).

• Collect membranes by ultracentrifugation (150,000 × g, 1.5 h, 4°C).

• Wash membrane pellet by resuspending in membrane buffer and repeating ultracentrifugation.

• Resuspend washed membranes in membrane buffer to ~10 mg/mL protein concentration.

• Flash-freeze in liquid nitrogen and store at -80°C if not used immediately.

Detergent Screening

Prior to large-scale purification, perform small-scale detergent screening to identify optimal solubilization conditions:

Detergents to Test:

• n-Dodecyl-β-D-maltoside (DDM): 1-2%

• n-Decyl-β-D-maltoside (DM): 1-2%

• Lauryl maltose neopentyl glycol (LMNG): 0.5-1%

• Digitonin: 1-2%

• Glyco-diosgenin (GDN): 0.5-1%

• CHAPS: 1-2%

Procedure:

• Aliquot washed membranes (1 mg total protein per test condition).

• Add detergent to final concentration listed above.

• Incubate with gentle rotation for 2 h at 4°C.

• Centrifuge at 100,000 × g for 30 min at 4°C.

• Analyze supernatant and pellet by SDS-PAGE and Western blot using anti-His antibody.

• Select detergent providing highest yield with minimal aggregation.

Large-Scale Purification

Materials:

• Solubilization buffer: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, selected detergent, 5 mM imidazole, protease inhibitors

• Wash buffer: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, 0.05% selected detergent, 20 mM imidazole

• Elution buffer: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, 0.05% selected detergent, 300 mM imidazole

• SEC buffer: 20 mM HEPES pH 7.5, 150 mM NaCl, 5% glycerol, 0.03% selected detergent

Procedure:

• Solubilization:

  • Add selected detergent to membrane suspension at the optimal concentration.

  • Incubate with gentle rotation for 2-3 h at 4°C.

  • Centrifuge at 100,000 × g for 45 min at 4°C to remove insoluble material.

• Affinity Chromatography:

  • Apply solubilized material to Ni-NTA resin (0.5-1 mL per 10 mg total membrane protein).

  • Incubate with gentle rotation for 1-2 h at 4°C.

  • Pack into a column and collect flow-through.

  • Wash with 10 column volumes of wash buffer.

  • Elute with 5-10 column volumes of elution buffer, collecting 0.5-1.0 mL fractions.

  • Analyze fractions by SDS-PAGE.

• Tag Cleavage (optional):

  • Add TEV protease (1:20 w/w ratio to FrdC).

  • Dialyze overnight against 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 5% glycerol, 0.03% detergent.

  • Pass through Ni-NTA column to remove uncleaved protein and His-tagged TEV.

• Size Exclusion Chromatography:

  • Concentrate pooled fractions to 2-5 mL using 50 kDa MWCO concentrator.

  • Apply to Superdex 200 16/600 column pre-equilibrated with SEC buffer.

  • Collect 0.5 mL fractions and analyze by SDS-PAGE.

  • Pool monodisperse peak fractions.

Stabilization for Structural Studies

Methods for Stabilizing FrdC:

• Reconstitution into Nanodiscs:

  • Prepare MSP1D1 scaffold protein and lipids (POPC:POPG, 3:1).

  • Mix purified FrdC, MSP1D1, and lipids at appropriate molar ratios (typically 1:2:120).

  • Remove detergent using Bio-Beads SM-2 over 4-6 h at 4°C.

  • Purify nanodiscs by size exclusion chromatography.

• Reconstitution into Amphipols:

  • Mix purified FrdC with amphipol A8-35 at 1:3 (w/w) ratio.

  • Incubate for 4 h at 4°C.

  • Remove detergent using Bio-Beads SM-2.

  • Purify by size exclusion chromatography.

• Lipid Cubic Phase for Crystallization:

  • Mix concentrated FrdC with monoolein at a ratio of 2:3 (v/v) using coupled syringes.

  • Set up crystallization trials in 96-well plates using LCP dispensing robot.

  • Incubate at 20°C and monitor crystal growth.

Quality Control Assessments

Before proceeding to structural studies, assess protein quality using:

• Analytical SEC:

  • Check for monodispersity and absence of aggregation.

• Dynamic Light Scattering:

  • Measure particle size distribution and polydispersity.

• Thermostability Assays:

  • Perform CPM assay or nanoDSF to assess protein stability.

• Negative Stain EM:

  • Check particle homogeneity and integrity.

• Mass Spectrometry:

  • Confirm protein identity and assess post-translational modifications.

• Functional Assay:

  • Verify that purified FrdC retains native conformation through activity or binding assays.

This comprehensive protocol provides a roadmap for obtaining pure, stable, and homogeneous P. mirabilis FrdC suitable for structural studies such as X-ray crystallography, cryo-electron microscopy, or NMR spectroscopy. The exact conditions may require optimization for the specific construct and downstream applications.

What are the most effective methods for analyzing the interaction between P. mirabilis frdC and quinones?

Analyzing the interaction between Proteus mirabilis frdC and quinones requires specialized techniques that address the hydrophobic nature of both the membrane protein and its lipophilic quinone substrates. Below are the most effective methods for investigating these interactions:

Biochemical and Biophysical Methods:

Enzyme Kinetic Assays

Principle: Measures the rate of quinone reduction/oxidation catalyzed by the fumarate reductase complex containing frdC.

Protocol:

• Spectrophotometric Assays:

  • Monitor quinone reduction at appropriate wavelength (275-290 nm for ubiquinone, 245-260 nm for menaquinone)

  • Use purified fumarate reductase complex or membrane preparations

  • Measure initial rates at varying quinone concentrations (0.1-500 μM)

  • Determine Km and Vmax values for different quinone substrates

Analysis:

• Compare kinetic parameters between wild-type and mutant frdC proteins

• Analyze substrate specificity by testing different quinone types:

  • Ubiquinone (UQ)

  • Menaquinone (MK)

  • Demethylmenaquinone (DMK)

  • Synthetic quinone analogs

Isothermal Titration Calorimetry (ITC)

Principle: Directly measures thermodynamic parameters of quinone binding to frdC.

Protocol:

• Purify frdC in detergent micelles or nanodiscs

• Prepare quinone solutions in the same buffer

• Titrate quinones (0.1-1 mM) into protein solution (10-50 μM)

• Monitor heat changes during binding

Analysis:

• Determine binding affinity (Kd), enthalpy (ΔH), entropy (ΔS), and stoichiometry

• Compare binding parameters for different quinone types

• Evaluate the effect of mutations in frdC on binding properties

Microscale Thermophoresis (MST)

Principle: Measures changes in the thermophoretic mobility of fluorescently labeled protein upon ligand binding.

Protocol:

• Label purified frdC with fluorescent dye (preferably site-specifically)

• Prepare serial dilutions of quinones (nM to μM range)

• Mix labeled protein with quinone dilutions

• Measure thermophoretic movement using specialized instrumentation

Analysis:

• Determine binding affinities under near-native conditions

• Requires minimal protein amounts compared to ITC

• Suitable for detergent-solubilized membrane proteins

Structural and Computational Methods:

Site-Directed Mutagenesis and Functional Analysis

Protocol:

• Identify potential quinone-binding residues based on homology models or structural predictions

• Generate single or multiple point mutations in the frdC gene

• Express and purify mutant proteins

• Assess quinone binding and enzyme activity as described above

Analysis:

• Identify residues critical for quinone binding and electron transfer

• Create a functional map of the quinone-binding site

Photoaffinity Labeling

Principle: Uses photoreactive quinone analogs that form covalent bonds with proximal protein residues upon UV irradiation.

Protocol:

• Synthesize or obtain photoactivatable quinone analogs

• Incubate purified frdC with the analog

• UV-irradiate to trigger cross-linking

• Digest protein and analyze labeled peptides by mass spectrometry

Analysis:

• Directly identifies residues in the quinone-binding pocket

• Provides distance constraints for molecular modeling

• Confirms the binding site location in the native protein

Molecular Docking and Molecular Dynamics Simulations

Protocol:

• Generate a structural model of P. mirabilis frdC based on homology or AI-prediction methods

• Prepare quinone structures with appropriate protonation states

• Perform molecular docking to identify potential binding modes

• Validate and refine binding poses using molecular dynamics simulations in explicit membrane environments

Analysis:

• Predicts binding modes and key protein-quinone interactions

• Estimates binding energy and stability of different quinones

• Identifies potential water molecules or lipids involved in binding

Advanced Spectroscopic Methods:

Electron Paramagnetic Resonance (EPR) Spectroscopy

Principle: Detects unpaired electrons in semiquinone intermediates during electron transfer.

Protocol:

• Prepare membrane preparations or purified fumarate reductase complex

• Add quinones and substrates to generate steady-state radical species

• Rapidly freeze samples in liquid nitrogen

• Measure EPR spectra at appropriate microwave frequencies

Analysis:

• Detects and characterizes quinone radical intermediates

• Provides information on the electronic environment of the quinone-binding site

• Can distinguish between different semiquinone species

Solid-State Nuclear Magnetic Resonance (ssNMR)

Protocol:

• Prepare isotopically labeled frdC (13C, 15N)

• Reconstitute into lipid bilayers or nanodiscs

• Add 13C-labeled quinones

• Perform multidimensional NMR experiments

Analysis:

• Provides atomic-level information on protein-quinone interactions

• Can detect conformational changes upon quinone binding

• Works with membrane-embedded proteins in their native-like environment

Comparative Analysis Table:

For comprehensive analysis of P. mirabilis frdC-quinone interactions, a combination of these methods would provide complementary information. Starting with enzyme kinetics and computational modeling to establish basic parameters, followed by site-directed mutagenesis to validate key residues, and finally more advanced spectroscopic techniques to characterize the details of the interaction would represent an effective research strategy.

What resources are available for training graduate students on experimental approaches for studying membrane proteins like frdC?

Graduate students beginning research on membrane proteins like Proteus mirabilis frdC can access a variety of specialized training resources. Below is a comprehensive guide to educational materials and training opportunities:

Academic Courses and Workshops:

• Cold Spring Harbor Laboratory Courses

  • "Membrane Protein Structure, Function, and Dynamics" course

  • Hands-on training in expression, purification, and structural characterization

  • Two-week intensive format with distinguished faculty

  • Website: https://meetings.cshl.edu/courses.aspx

• EMBO Practical Courses

  • "Membrane Protein Expression, Purification and Crystallization" workshop

  • "Advanced methods in macromolecular crystallization" with membrane protein sections

  • European-based courses with excellent technical training

  • Website: https://www.embo.org/events/

• Gordon Research Conferences and Seminars

  • "Membrane Protein Folding" conference

  • "Bacterial Cell Surfaces" conference with sections on membrane proteins

  • Opportunities for students to present posters and network with experts

  • Website: https://www.grc.org/

• University of Toronto Membrane Protein Centre Workshops

  • Annual workshops on membrane protein methodologies

  • Focus on expression systems, detergent screening, and functional assays

  • Offers hands-on training opportunities

Online Educational Resources:

• Coursera and edX Courses

  • "Membrane Proteins: Structure and Function" (Coursera)

  • "Structural Biology: Membrane Proteins" (edX)

  • Self-paced learning with video lectures and assessments

• iBiology Courses and Lectures

  • Video lectures by leading membrane protein researchers

  • "Membrane Protein Structural Biology" course series

  • Free access with detailed protocols and case studies

  • Website: https://www.ibiology.org/

• Protein Data Bank Educational Resources

  • Tutorials on membrane protein visualization and analysis

  • "Molecule of the Month" features on membrane proteins

  • Interactive tools for structural analysis

  • Website: https://www.rcsb.org/

Protocol Resources and Method Collections:

• Current Protocols in Protein Science

  • Detailed protocols for membrane protein purification

  • Step-by-step guides with troubleshooting advice

  • Regularly updated with new methodologies

• Methods in Enzymology Volumes

  • Volume 557: "Membrane Proteins – Engineering, Purification and Crystallization"

  • Volume 556: "Analytical Methods for Studying and Monitoring Membrane Proteins"

  • Comprehensive protocols with theoretical background

• Springer Protocols

  • "Membrane Protein Structure and Function Characterization" collection

  • Detailed methods with materials lists and expected outcomes

• JoVE (Journal of Visualized Experiments)

  • Video protocols for membrane protein techniques

  • Visual demonstration of complex procedures

  • Website: https://www.jove.com/

Laboratory Skills Training Programs:

• New England Biolabs Membrane Protein Expression and Purification Workshop

  • Hands-on training with various expression systems

  • Detergent screening and purification strategies

  • Available as periodic workshops at research institutions

• Thermo Fisher Scientific Virtual Membrane Protein Lab

  • Online simulation of membrane protein experiments

  • Interactive troubleshooting modules

  • Complementary to hands-on training

• Diamond Light Source Membrane Protein Laboratory

  • Training in membrane protein crystallization

  • Access to specialized equipment and expertise

  • Collaborative opportunities for structural studies

  • Website: https://www.diamond.ac.uk/

Specialized Resources for Bacterial Respiratory Complexes:

• IMPC (International Membrane Protein Conference) Workshops

  • Focus sessions on bacterial respiratory complexes

  • Networking with specialists in fumarate reductase research

• Bacterial Electron Transfer Processes and Their Regulation (BETPR) Conference

  • Specialized sessions on respiratory complexes

  • Opportunity to present research on frdC and related proteins

Textbooks and Reference Materials:

• "Membrane Proteins: Folding, Association, and Design" (Ghirlanda and Senes, 2014)

  • Comprehensive coverage of membrane protein biochemistry

  • Methods sections with practical advice

• "Structural Biology of Membrane Proteins" (Pebay-Peyroula, 2007)

  • Focus on structural determination methods

  • Case studies of successfully solved structures

• "Bacterial Membranes: Structural and Molecular Biology" (Remaut and Fronzes, 2013)

  • Specific focus on bacterial membrane proteins

  • Relevant to respiratory complex studies

Online Communities and Resources:

• Research Gate Membrane Protein Interest Group

  • Forum for technique discussions and troubleshooting

  • Network of researchers working on similar systems

• MPDB (Membrane Proteins of Known 3D Structure Database)

  • Curated collection of membrane protein structures

  • Educational resources and visualization tools

  • Website: https://blanco.biomol.uci.edu/mpstruc/

• Membrane Protein Network (MemProNet)

  • Collaborative network of membrane protein researchers

  • Resources for training and education

  • Workshops and webinars on current techniques

Funding Opportunities for Training:

• NIH F31/F32 Fellowship Programs

  • Predoctoral and postdoctoral fellowships

  • Can include specialized training in membrane protein techniques

• NSF Graduate Research Fellowship Program

  • Support for graduate students pursuing membrane protein research

  • Includes educational component

• EMBO Short-Term Fellowships

  • Funding for short visits to labs with membrane protein expertise

  • Opportunities to learn specialized techniques