Recombinant Aeromonas salmonicida UPF0761 membrane protein ASA_4118 (ASA_4118)

What is the basic structure and function of ASA_4118 membrane protein?

ASA_4118 is a UPF0761 family membrane protein found in Aeromonas salmonicida, comprising 290 amino acids. The protein's amino acid sequence (MQQKFGGRLGHALSFLRHDGGHFAQFVWSRFQHDRLTVTAGYLAYVTLLSLVPMIAVVFGMMSAFPVFQTLKQAMEQFVYHNFVPTAGEMLKEYIDGFVANATNTTAVGIGALIVVALMLISAIDKNLNYIWRSTQGRPLAQAFAMYWMILTLGPVLIGGSIAISSYIFSLRLFGAESLFGIGYLLLRSLPFLFSVLTFLLVYTVVPNCKVRLVHAFIGALVAATLFELAKRGFAIYITNFPSYQAIYGALATIPVLFVWVYLSWLVVLLGAETTACLGEYEKPVSEELA) suggests multiple transmembrane domains . While the specific function remains under investigation, this membrane protein family is likely involved in transport processes, membrane integrity, or signaling pathways. Structural prediction models suggest that ASA_4118 spans the membrane multiple times, with both intracellular and extracellular domains that may interact with other cellular components or environmental factors. The protein's role within A. salmonicida may relate to virulence mechanisms, as the bacterium is known to cause furunculosis, a significant pathogen affecting fish in aquaculture settings .

What expression systems are suitable for producing recombinant ASA_4118?

Escherichia coli has been successfully used as an expression system for producing recombinant ASA_4118 with an N-terminal His tag . For optimal expression, researchers should consider using specialized E. coli strains designed for membrane protein expression, such as C41(DE3), C43(DE3), or Lemo21(DE3). The expression protocol should include careful optimization of induction conditions, including IPTG concentration (typically 0.1-0.5 mM), induction temperature (often lowered to 16-25°C to slow production and facilitate proper folding), and induction duration . Research has demonstrated that the growth phase at which cells are harvested is critical; cells should be grown under tightly-controlled conditions and harvested prior to glucose exhaustion, just before the diauxic shift, to maximize membrane protein yields . The choice of growth media can also significantly impact expression levels, with rich media formulations often providing better results for membrane proteins compared to minimal media.

How should ASA_4118 protein be stored to maintain stability?

For optimal stability, lyophilized ASA_4118 protein should be stored at -20°C or -80°C upon receipt . Working aliquots can be maintained at 4°C for up to one week to avoid repeated freeze-thaw cycles, which can compromise protein integrity . When reconstituting the protein, it is recommended to use deionized sterile water to achieve a concentration of 0.1-1.0 mg/mL. Adding glycerol to a final concentration of 5-50% (with 50% being standard practice) before aliquoting facilitates long-term storage at -20°C/-80°C . The storage buffer typically consists of a Tris/PBS-based solution with 6% Trehalose at pH 8.0, which helps maintain protein stability during freeze-thaw cycles . For experiments requiring active protein, it's essential to verify structural integrity after thawing using techniques such as circular dichroism or limited proteolysis to ensure that the protein has maintained its native conformation.

What challenges are associated with purifying ASA_4118, and how can they be addressed?

Purification of ASA_4118, like other membrane proteins, presents several challenges due to its hydrophobic nature and tendency to aggregate. A methodological approach begins with optimized cell lysis using a combination of mechanical disruption (sonication or French press) and detergents suitable for membrane proteins, such as n-dodecyl-β-D-maltoside (DDM), n-decyl-β-D-maltoside (DM), or lauryl maltose neopentyl glycol (LMNG) . For His-tagged ASA_4118, immobilized metal affinity chromatography (IMAC) using Ni-NTA or Co-NTA resins provides effective initial purification . Critical parameters include using detergent concentrations above the critical micelle concentration throughout purification to prevent protein aggregation and adding small amounts of lipids to stabilize the protein. Size exclusion chromatography as a final purification step helps remove aggregates and ensure monodispersity. Protein purity should be verified by SDS-PAGE (expecting >90% purity) , and functional integrity can be assessed through biophysical characterization techniques such as circular dichroism or fluorescence spectroscopy.

How can researchers optimize the expression conditions for maximum yield of correctly folded ASA_4118?

Optimizing expression conditions for ASA_4118 requires a systematic approach investigating multiple parameters. Start by testing different E. coli expression strains, particularly those designed for membrane proteins. Expression vector selection is also crucial; vectors with tightly controlled promoters (like pET or pBAD systems) allow fine regulation of expression levels. A full factorial experimental design should examine:

• Induction temperature: Test 16°C, 25°C, and 30°C

• IPTG concentration: Trial 0.1 mM, 0.5 mM, and 1.0 mM

• Growth media: Compare LB, TB, and 2xYT media formulations

• Harvest timing: Collect samples at early, mid, and late log phases

Research has demonstrated that for membrane proteins, the most rapid growth conditions are often not optimal for production, and the growth phase at which cells are harvested is critical . Expression levels should be monitored using Western blotting with anti-His antibodies, while correct folding can be assessed by extractability in mild detergents and through thermal stability assays. Incorporating small molecular chaperones or fusion partners like Mistic or SUMO may also enhance proper folding of this challenging membrane protein.

What is known about the evolutionary conservation of ASA_4118 across different bacterial species?

Evolutionary analysis of ASA_4118 reveals it belongs to the UPF0761 membrane protein family, with homologs present across various bacterial species, particularly within the Aeromonas genus and related gamma-proteobacteria. Genomic sequencing of multiple A. salmonicida strains has contributed to our understanding of the conservation patterns of this protein . Analysis of A. salmonicida genomes has revealed that gene clusters exclusive to psychrophilic clades (cold-adapted strains) are associated with outer membrane proteins, which may include ASA_4118 . This conservation pattern suggests functional importance in the organism's adaptation to specific environmental niches. Comparative genomics approaches have identified that certain membrane proteins show distinct patterns between psychrophilic and mesophilic strains of Aeromonas, highlighting their potential role in temperature adaptation mechanisms . The evolutionary conservation of transmembrane domains versus loop regions can provide insights into functionally important regions of the protein that may be targeted for future structural or functional studies.

How can multiscale molecular dynamics simulations be applied to study the structure-function relationship of ASA_4118?

Investigating ASA_4118 through multiscale molecular dynamics simulations requires a sophisticated computational approach combining atomistic (AT) and coarse-grained (CG) models. Begin by generating a homology model of ASA_4118 using servers like SWISS-MODEL or Phyre2, followed by refinement with experimental constraints if available. For membrane embedding, utilize the CHARMM-GUI Membrane Builder to place the protein in a lipid bilayer mimicking the A. salmonicida membrane composition.

The multiscale simulation protocol should follow these methodological steps:

• Initial CG simulations (using Martini force field) to achieve proper membrane equilibration and investigate large-scale conformational changes over microsecond timescales

• Conversion of selected CG frames to atomistic resolution using backmapping tools

• Atomistic simulations (300-500 ns) with CHARMM36 or AMBER lipid force fields to capture detailed molecular interactions

This approach aligns with current research combining computational methods to study membrane proteins at different scales . The integration of simulation with experimental validation provides the most robust framework for elucidating the molecular mechanisms of ASA_4118 function. Research has demonstrated that combining AT/CG protocols with cryo-EM maps can be particularly valuable for multi-domain proteins and protein complexes , a strategy that could be applied to ASA_4118 once preliminary structural data becomes available.

What role might ASA_4118 play in the virulence and antibiotic resistance mechanisms of A. salmonicida?

The potential role of ASA_4118 in A. salmonicida virulence and antibiotic resistance requires sophisticated investigative approaches. While direct evidence for ASA_4118's involvement is limited, its membrane localization suggests potential contributions to bacterial pathogenicity. Genomic analyses indicate that A. salmonicida contains numerous virulence factors, including outer membrane proteins that facilitate host interaction and survival mechanisms . As a membrane protein, ASA_4118 may participate in bacterial adaptation to environmental conditions during infection processes.

Recent research has identified over 100 antibiotic resistance genes in Aeromonas species, with A. salmonicida exhibiting species-specific resistance patterns . Analysis of antibiotic resistance factors in Chilean strains of A. salmonicida has revealed resistance against various drug classes, highlighting the clinical importance of understanding membrane protein contributions to these mechanisms . The characterization of novel plasmids in A. salmonicida has emphasized their role in virulence and antibiotic resistance , suggesting that investigating whether ASA_4118 interacts with plasmid-encoded factors could provide valuable insights into controlling furunculosis and managing antibiotic resistance in aquaculture settings.

How can cryo-EM and advanced structural biology techniques be optimized for determining the high-resolution structure of ASA_4118?

Determining the high-resolution structure of ASA_4118 using cryo-EM requires addressing the challenges associated with membrane protein structural biology. Initial stages should focus on protein engineering to enhance stability and homogeneity, including systematic screening of detergents and nanodiscs for optimal extraction and stabilization. Construct optimization through targeted mutations of flexible regions or introduction of conformational stabilizing modifications can significantly improve structural homogeneity.

For cryo-EM sample preparation, researchers should implement optimization of grid preparation parameters and incorporation of detergent alternatives such as amphipols, SMALPs, or nanodiscs that better preserve native lipid environments. Data collection and processing should utilize energy filters to enhance contrast, motion correction algorithms, and 3D variability analysis to identify and classify conformational states.

Research has demonstrated that combining CG/AT molecular dynamics with medium-resolution cryo-EM maps can be particularly valuable for large multi-domain proteins , a strategy that could be applied to ASA_4118. This integrated approach, combining computational methods with experimental structural data, has proven successful for challenging membrane proteins and could provide crucial insights into ASA_4118's structure-function relationships.

How do the genomic characteristics of ASA_4118 relate to A. salmonicida pathogenicity in different fish species?

The genomic context of ASA_4118 within A. salmonicida provides valuable insights into its potential role in pathogenicity across different fish hosts. Genomic sequencing of multiple A. salmonicida isolates, including Chilean strains from Atlantic salmon infections, has revealed important patterns in gene conservation and virulence factor distribution . The pan-genome analysis of A. salmonicida has identified gene families that contribute to understanding the psychrophilic and mesophilic clades, which may influence host range and virulence mechanisms .

Phylogenomic analysis has differentiated between typical and atypical psychrophilic isolates of A. salmonicida, providing a framework for understanding strain-specific virulence profiles . Research has shown that diverse insertion sequences and restriction-modification patterns highlight genomic structural differences between strains with varying virulence potential . Virulence factor predictions have emphasized exotoxin disparities between psychrophilic and mesophilic strains, which may relate to host specificity and pathogenicity mechanisms .

What are the common pitfalls in purifying ASA_4118, and how can researchers troubleshoot these issues?

Purification of membrane proteins like ASA_4118 frequently encounters specific challenges that require systematic troubleshooting. The most common issues and their methodological solutions include:

• Poor extraction efficiency:

  • Systematically screen detergents beyond standard options

  • Test extraction at various temperatures (4°C, 25°C)

  • Optimize detergent:protein ratios using small-scale extractions

  • Consider alternative solubilization methods like SMA copolymers

• Protein aggregation during purification:

  • Maintain detergent concentrations well above critical micelle concentration throughout all purification steps

  • Add glycerol (10-20%) to all buffers to enhance stability

  • Incorporate lipids into purification buffers

  • Perform all procedures at 4°C and include protease inhibitors

• Low binding to affinity resin:

  • Verify tag accessibility through Western blotting with anti-His antibodies

  • Test different metal ions (Ni²⁺, Co²⁺) for IMAC purification

  • Optimize imidazole concentrations in binding and washing buffers

Research has shown that membrane protein production is recognized as the primary bottleneck in structural genomics programs, with researchers often relying on trial-and-error approaches . A systematic approach addressing these challenges has been shown to significantly improve membrane protein purification outcomes, particularly when combined with tightly-controlled growth conditions and harvesting cells at optimal time points .

How can researchers integrate experimental and computational approaches to characterize ASA_4118 function?

Integrating experimental and computational approaches provides a powerful framework for characterizing ASA_4118 function. A comprehensive research strategy should include:

• Sequence-based computational predictions:

  • Transmembrane topology prediction using multiple algorithms (TMHMM, Phobius, TOPCONS)

  • Identification of conserved motifs and functional domains through comparison with characterized proteins

  • Prediction of post-translational modifications and protein-protein interaction sites

• Structural modeling and validation:

  • Homology modeling based on related membrane proteins with known structures

  • Molecular dynamics simulations to refine models and predict conformational flexibility

  • Experimental validation using site-directed mutagenesis of predicted functional residues

• Functional characterization:

  • Development of robust functional assays based on predicted activities

  • Creation of reporter systems to monitor protein activity in vivo

  • Phenotypic analysis of knockout or overexpression strains

Recent research has demonstrated the value of combining multiscale computational approaches with experimental validation for membrane proteins . The integration of atomic-level and coarse-grained simulations can provide insights into protein behavior at different time and length scales, while experimental data serves to validate and refine computational models. This iterative approach between computation and experiment has proven particularly valuable for challenging membrane proteins like ASA_4118.

What considerations are important when designing site-directed mutagenesis experiments for ASA_4118?

Designing effective site-directed mutagenesis experiments for ASA_4118 requires careful consideration of multiple factors to ensure meaningful results:

• Target selection based on informed predictions:

  • Identify conserved residues through multiple sequence alignment with homologous proteins

  • Focus on predicted functional domains or motifs (e.g., potential transport channels, binding sites)

  • Consider membrane topology when selecting residues (transmembrane vs. loop regions)

  • Prioritize residues unique to psychrophilic strains that may relate to environmental adaptation

• Mutation design principles:

  • For functional studies: Consider conservative substitutions that maintain charge/size but alter specific properties

  • For stability studies: Target residues at protein interfaces or lipid-facing positions

  • For topology validation: Introduce reporter tags or glycosylation sites at predicted loop regions

  • Design complementary mutations that test the same hypothesis through different approaches

• Experimental validation approaches:

  • Expression level verification using Western blotting with anti-His antibodies

  • Proper folding assessment through extraction efficiency in mild detergents

  • Functional impact evaluation using activity assays or phenotypic assessments

  • Structural changes characterization using biophysical techniques (CD, fluorescence spectroscopy)

The methodical approach to mutagenesis should consider the challenges associated with membrane protein expression and ensure that observed effects are due to the specific mutation rather than indirect effects on protein folding or stability. Research has shown that growth conditions significantly impact membrane protein yields , suggesting that mutant proteins should be expressed and analyzed under tightly controlled conditions to obtain reliable comparisons between variants.

How can structural information about ASA_4118 contribute to understanding A. salmonicida pathogenesis?

Structural characterization of ASA_4118 would provide foundational knowledge for understanding A. salmonicida pathogenesis in several key ways:

• Identification of functional domains:

  • Mapping the topology and functional domains could reveal regions exposed on the bacterial surface that may interact with host factors

  • Structural information would facilitate the identification of potential transport pathways or signaling mechanisms

  • Comparison with virulence-associated membrane proteins could highlight common structural features

• Integration with genomic data:

  • Structural insights would complement existing genomic analysis of A. salmonicida strains

  • Correlation between structural features and genomic variations across psychrophilic and mesophilic strains could explain host specificity

  • Mapping of insertion sequences and restriction-modification systems onto the protein structure might reveal impacts on protein function

• Understanding environmental adaptation:

  • Structural elements unique to psychrophilic strains might explain cold adaptation mechanisms

  • Membrane protein structures often reveal adaptations to specific lipid environments relevant to pathogen survival

Research has shown that A. salmonicida contains diverse virulence factors and exhibits various antibiotic resistance mechanisms . Structural characterization of membrane proteins like ASA_4118 would provide crucial insights into how these mechanisms operate at the molecular level, potentially revealing new targets for intervention strategies against furunculosis in aquaculture settings.

What novel experimental techniques could advance our understanding of ASA_4118 function?

Advancing our understanding of ASA_4118 function requires innovative experimental approaches that address the challenges of membrane protein characterization:

• Advanced microscopy techniques:

  • Single-molecule fluorescence resonance energy transfer (smFRET) to capture conformational changes during function

  • High-speed atomic force microscopy (HS-AFM) to visualize dynamic structural changes in native-like environments

  • Correlative light and electron microscopy (CLEM) to link cellular localization with structural features

• Membrane mimetic systems:

  • Nanodiscs incorporating native lipids from A. salmonicida membranes

  • Droplet interface bilayers for functional electrophysiological measurements

  • Microfluidic systems for controlled reconstitution and functional assays

• Genetic approaches:

  • CRISPR-Cas9 genome editing to create precise chromosomal mutations in A. salmonicida

  • Conditional depletion systems to study protein function in vivo

  • Synthetic genetic arrays to identify genetic interactions and functional pathways

• Integrated structural biology:

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to map dynamic regions and ligand binding sites

  • Electron paramagnetic resonance (EPR) spectroscopy to measure distances between specific residues

  • Cross-linking mass spectrometry to identify interaction partners and binding interfaces

Research has demonstrated that combining multiple experimental approaches provides the most comprehensive characterization of membrane proteins . Particularly promising are techniques that combine computational methods with experimental validation, such as integrating molecular dynamics simulations with cryo-EM data or EPR distance measurements . These integrated approaches are likely to yield the most significant advances in understanding ASA_4118 function.

How might advances in protein engineering be applied to enhance the study of ASA_4118?

Protein engineering approaches offer powerful tools to overcome the inherent challenges in studying membrane proteins like ASA_4118:

• Stability enhancement strategies:

  • Consensus-based design using evolutionary information to identify stabilizing mutations

  • Introduction of disulfide bonds to rigidify flexible regions

  • Thermostabilization through systematic alanine scanning and combination of beneficial mutations

  • Fusion with stability-enhancing domains that maintain native function

• Functional probes and reporters:

  • Site-specific incorporation of fluorescent amino acids for spectroscopic studies

  • Introduction of environmentally sensitive probes at key functional sites

  • Creation of split reporters to monitor conformational changes or protein-protein interactions

  • Engineering of metal binding sites for paramagnetic NMR studies

• Production optimization:

  • Design of optimized signal sequences for improved membrane targeting

  • Creation of chimeric constructs with well-expressed homologs

  • Development of inducible expression systems with fine-tuned control

  • Engineering of tags and fusion partners that enhance expression while maintaining function

Research has demonstrated that tightly-controlled growth conditions and precise harvest timing significantly impact membrane protein yields . Combining these optimized expression conditions with protein engineering approaches could dramatically improve the quantity and quality of ASA_4118 available for structural and functional studies. Additionally, the application of computational design tools, informed by evolutionary analysis of psychrophilic and mesophilic Aeromonas strains , could guide rational engineering strategies specifically tailored to this challenging membrane protein.