Recombinant Haemophilus influenzae Probable intracellular septation protein A (HI_0826)

What is Haemophilus influenzae Probable Intracellular Septation Protein A (HI_0826)?

HI_0826 is a bacterial protein from Haemophilus influenzae that functions as a probable intracellular septation protein. It is a member of the intracellular septation protein A family (IspZ) and is also known as YciB or Inner membrane-spanning protein YciB. The full-length protein consists of 185 amino acids and has characteristics of an integral membrane protein based on its amino acid sequence, which contains multiple hydrophobic regions consistent with transmembrane domains .

How does HI_0826 relate to other proteins in the YciB gene family?

HI_0826 belongs to the YciB gene family, named after the E. coli homolog. This protein family (PF03795 in Pfam and IPR005545 in INTERPRO) includes over 200 sequences, predominantly from bacteria with some fungal representatives. Genomic context analysis reveals that HI_0826 is typically found in an operon structure with HI_0827 (YciA, a hotdog fold acyl-CoA thioesterase) and HI_0828 (YciI), which has been characterized as having a ferredoxin-like α/β-fold with a histidine/aspartate centered catalytic site .

The conserved genomic organization across multiple bacterial species suggests functional relationships between these proteins, potentially in related biochemical pathways. Structural and sequence conservation patterns indicate that while YciB (HI_0826) is membrane-associated, it maintains functional interactions with the soluble proteins in its operon .

What are the optimal conditions for recombinant expression of HI_0826?

For recombinant expression of HI_0826, the following methodology has proven effective:

• Vector Selection: The gene encoding HI_0826 should be cloned into an expression vector containing an affinity tag (e.g., pET series vectors with His-tag).

• Host System: E. coli is the preferred expression system, particularly strains optimized for membrane protein expression such as C41(DE3) or C43(DE3).

• Growth Conditions:

  • Culture bacteria in LB medium supplemented with appropriate antibiotics

  • Grow at 37°C until OD600 reaches 0.6

  • Induce expression with IPTG (typically 0.4 mM final concentration)

  • After induction, lower temperature to 18-25°C for 4-16 hours to enhance proper folding

For selenomethionine-labeled protein production (for crystallographic studies), use a methionine auxotroph strain like E. coli B834(DE3) with defined media containing selenomethionine instead of methionine .

What purification strategy is most effective for obtaining high-purity HI_0826?

A multi-step purification protocol is recommended:

• Cell Lysis: Disrupt cells using sonication or high-pressure homogenization in a buffer containing 20-50 mM Tris-HCl (pH 7.5-8.0), 100-300 mM NaCl, and detergent (typically 1% DDM or LDAO for membrane proteins).

• Affinity Chromatography:

  • For His-tagged HI_0826, use Ni-NTA resin

  • Equilibrate column with buffer containing 20-50 mM imidazole

  • Wash with increasing imidazole concentrations (50-80 mM)

  • Elute with high imidazole (250-500 mM)

• Tag Removal (optional):

  • If the construct contains a protease cleavage site, treat with appropriate protease (e.g., thrombin)

  • Perform reverse Ni-NTA chromatography to separate cleaved protein

• Size Exclusion Chromatography:

  • Final polishing step using Superdex 200 or similar

  • Typical buffer: 20 mM HEPES (pH 7.0-7.5), 150 mM NaCl, 0.05% detergent

This protocol typically yields protein with >90% purity as assessed by SDS-PAGE .

How should recombinant HI_0826 be stored to maintain stability and activity?

For optimal stability, recombinant HI_0826 should be stored following these guidelines:

• Short-term Storage (1-2 weeks):

  • Store at 4°C in buffer containing 20 mM Tris/HEPES (pH 7.0-8.0), 100-150 mM NaCl

  • Avoid repeated freeze-thaw cycles

• Long-term Storage:

  • Aliquot purified protein

  • Flash-freeze in liquid nitrogen

  • Store at -20°C or preferably -80°C

  • Add 5-50% glycerol (final concentration) as cryoprotectant

  • Trehalose (6%) can be included in Tris/PBS-based buffer (pH 8.0) for lyophilization

• Reconstitution After Lyophilization:

  • Briefly centrifuge vial before opening

  • Reconstitute in deionized sterile water to 0.1-1.0 mg/mL concentration

  • For subsequent storage, add glycerol to 50% final concentration and aliquot for -20°C/-80°C storage

What methods are most effective for determining the structure of membrane proteins like HI_0826?

Determining membrane protein structures like HI_0826 requires specialized approaches:

• X-ray Crystallography:

  • Detergent Screening: Test various detergents (DDM, LDAO, C8E4, etc.) for protein stability and crystal formation

  • Crystallization Methods: Vapor diffusion (hanging drop or sitting drop), lipidic cubic phase

  • Crystallization Conditions: Screen with commercial membrane protein crystallization kits, typically at 4-20°C

  • Data Collection: Use synchrotron radiation sources for high-resolution diffraction data

  • Phase Determination: Consider selenomethionine labeling for MAD/SAD phasing or heavy atom derivatives

• Cryo-electron Microscopy:

  • Sample Preparation: Reconstitution into nanodiscs or amphipols

  • Vitrification: Optimize blotting time and ice thickness

  • Data Collection: Use direct electron detectors and automated collection strategies

  • Image Processing: Apply motion correction, CTF estimation, particle picking, 2D/3D classification

• NMR Spectroscopy (for dynamics studies):

  • Isotope Labeling: Express protein with 15N, 13C labeling

  • Detergent Selection: Use deuterated detergents to reduce background signals

  • Experiment Selection: TROSY-based experiments designed for large membrane proteins

How can researchers analyze protein-protein interactions involving HI_0826 and its operon partners?

To analyze interactions between HI_0826 and its operon partners (such as HI_0827 and HI_0828), researchers can employ several complementary techniques:

• Co-immunoprecipitation (Co-IP):

  • Express HI_0826 with an affinity tag (His, FLAG, etc.)

  • Solubilize membrane fractions with mild detergents

  • Perform pull-down assays using tag-specific antibodies

  • Analyze co-precipitated proteins by Western blotting or mass spectrometry

• Bacterial Two-Hybrid System:

  • Clone HI_0826, HI_0827, and HI_0828 into bacterial two-hybrid vectors

  • Transform into reporter strain and analyze protein-protein interactions through reporter gene activation

  • Particularly useful for membrane proteins as it operates in a bacterial environment

• Surface Plasmon Resonance (SPR):

  • Immobilize purified HI_0826 on a sensor chip

  • Flow potential interacting partners over the surface

  • Measure binding kinetics and affinity constants

  • For membrane proteins, consider capturing in nanodiscs or supported lipid bilayers

• Crosslinking Mass Spectrometry:

  • Treat protein complexes with chemical crosslinkers

  • Digest and analyze by mass spectrometry

  • Identify crosslinked peptides to map interaction interfaces

• Fluorescence Resonance Energy Transfer (FRET):

  • Label proteins with fluorescent tags

  • Co-express in bacterial cells

  • Measure FRET signal to determine proximity and interaction

What assays can be used to determine the role of HI_0826 in bacterial cell division?

To investigate HI_0826's role in bacterial cell division, researchers should consider these methodological approaches:

• Gene Knockout and Complementation Studies:

  • Generate HI_0826 knockout strains

  • Analyze phenotypes (cell morphology, division rates, septum formation)

  • Perform complementation with wild-type and mutant variants

  • Use inducible promoters to control expression levels

• Fluorescence Microscopy:

  • Create fluorescent protein fusions (GFP-HI_0826)

  • Examine localization during different cell cycle stages

  • Co-localize with known division proteins (FtsZ, FtsA, etc.)

  • Use time-lapse microscopy to track protein dynamics during division

• Electron Microscopy:

  • Analyze septum formation in wild-type vs. knockout cells

  • Use immunogold labeling to precisely localize HI_0826

  • Examine ultrastructural changes in cell envelope and division sites

• Protein-Protein Interaction Mapping:

  • Identify division-related interaction partners through pull-down assays

  • Confirm interactions using bacterial two-hybrid or FRET approaches

  • Map interaction domains through truncation and site-directed mutagenesis

• Conditional Depletion Assays:

  • Place HI_0826 under control of an inducible promoter

  • Monitor cellular effects during protein depletion

  • Analyze changes in cell division timing and efficiency

How can researchers investigate the potential membrane topology of HI_0826?

Determining the membrane topology of HI_0826 requires specialized techniques:

• Computational Prediction:

  • Use transmembrane prediction algorithms (TMHMM, HMMTOP, Phobius)

  • Apply topology prediction tools specific for bacterial membrane proteins

  • Based on the amino acid sequence (MKQLLDFIPLILFFITYKLGGVREAAIVLVVATILQIVILKWKYGMVEKQQKIMASAVVFFGLLTAYFNEIRYLQWKVTIINGLFAIVLLVAQFQFKTPLIKKLLGKELQLPEKAWNTLNFGWAIFFIICMLVNIYISHNMSEEAWVDFKSFGIIGMTVIATIISGVYIYRYLPKDGSNSKDGEK), multiple transmembrane domains are predicted

• Reporter Fusion Approaches:

  • PhoA Fusion Method: Create fusions with alkaline phosphatase (active in periplasm)

  • GFP Fusion Method: Create fusions with GFP (fluorescent in cytoplasm)

  • Generate libraries of fusion proteins with truncations at different positions

  • Measure reporter activity to determine cytoplasmic vs. periplasmic localization

• Cysteine Scanning Mutagenesis:

  • Introduce cysteine residues at different positions

  • Treat intact cells with membrane-impermeable sulfhydryl reagents

  • Analyze accessibility patterns to determine exposed regions

• Protease Protection Assays:

  • Create membrane vesicles with defined orientation

  • Treat with proteases (e.g., trypsin, proteinase K)

  • Identify protected fragments using antibodies against different epitopes

• Cryo-EM or X-ray Crystallography:

  • Determine 3D structure in lipid environment

  • Map transmembrane helices and connecting loops

What approaches can determine if HI_0826 functions independently or requires interaction with other proteins in its operon?

To investigate functional relationships between HI_0826 and other proteins in its operon (HI_0827 and HI_0828), researchers can use these methodological approaches:

• Coordinated Gene Expression Analysis:

  • Measure transcript levels of all operon genes under various conditions

  • Use qRT-PCR or RNA-seq to quantify coordinated expression patterns

  • Identify conditions that specifically upregulate or downregulate the entire operon

• Synthetic Lethality Screening:

  • Generate single, double, and triple knockouts of operon genes

  • Analyze growth, morphology, and division defects

  • Identify combinations that produce synthetic phenotypes

• Biochemical Reconstitution:

  • Purify individual proteins and reconstitute in liposomes or nanodiscs

  • Measure activity with and without partner proteins

  • Determine if activity requires complex formation

• Protein Complex Isolation:

  • Use tandem affinity purification to isolate native complexes

  • Analyze composition by mass spectrometry

  • Determine stoichiometry and stability of complexes

• Yeast Three-Hybrid System:

  • Test if interactions between two proteins are mediated by the third

  • Can help determine if HI_0826 serves as a scaffold for other operon proteins

• In vivo Crosslinking:

  • Use formaldehyde or other crosslinkers in living cells

  • Identify native complexes through immunoprecipitation

  • Analyze interaction dependencies through selective gene knockouts

How can structural information about HI_0826 be used to design inhibitors for antimicrobial development?

Structure-based inhibitor design for HI_0826 would follow this methodological framework:

• Structural Characterization:

  • Determine high-resolution structure using X-ray crystallography or cryo-EM

  • Identify potential binding pockets through computational analysis

  • Focus on conserved regions that might be essential for function

• Virtual Screening Approach:

  • Prepare the structure for molecular docking (add hydrogens, optimize protonation states)

  • Select or create compound libraries for virtual screening

  • Perform high-throughput docking using software like AutoDock, Glide, or GOLD

  • Rank compounds based on predicted binding energy and interactions with key residues

• Fragment-Based Drug Design:

  • Screen fragment libraries using NMR, X-ray, or SPR techniques

  • Identify fragments that bind to different sites on HI_0826

  • Link or grow fragments to create higher-affinity compounds

• Structure-Activity Relationship Studies:

  • Synthesize promising candidates identified in silico

  • Test activity in biochemical and cellular assays

  • Iteratively modify compounds based on activity data

  • Use structural information to guide optimization

• Validation Studies:

  • Confirm binding mode through co-crystallization or NMR studies

  • Perform site-directed mutagenesis of predicted binding residues

  • Assess selectivity against human homologs or other bacterial proteins

  • Evaluate antimicrobial activity, cytotoxicity, and ADME properties

What experimental approaches can elucidate the molecular mechanism of HI_0826 in septation?

To determine the precise molecular mechanism of HI_0826 in bacterial septation, a multi-pronged approach is necessary:

• High-Resolution Imaging:

  • Super-Resolution Microscopy: Use techniques like PALM, STORM, or SIM to visualize HI_0826 localization at septation sites with nanometer precision

  • Cryo-Electron Tomography: Image whole cells to visualize HI_0826 in the context of the divisome complex

  • Time-Resolved Imaging: Track protein recruitment sequence during septation

• Site-Directed Mutagenesis:

  • Create amino acid substitutions at conserved residues

  • Assess effects on protein localization, function, and cell division

  • Create a structure-function map of critical regions

• In vitro Reconstitution:

  • Purify HI_0826 and reconstitute in liposomes

  • Measure effects on membrane properties (curvature, fluidity)

  • Add purified divisome components to assess interactions

• Force Measurements:

  • Use atomic force microscopy to measure mechanical properties of septa

  • Compare wild-type with HI_0826 mutants or knockouts

  • Determine if HI_0826 contributes to mechanical strength of division sites

• Metabolic Labeling:

  • Use click chemistry to label newly synthesized peptidoglycan

  • Determine if HI_0826 affects spatiotemporal patterns of cell wall synthesis

  • Combine with super-resolution microscopy for precise localization

• Crosslinking and Mass Spectrometry:

  • Use in vivo crosslinking to capture transient interactions

  • Identify crosslinked peptides by mass spectrometry

  • Map interaction interfaces at amino acid resolution

How does the cellular stoichiometry of proteins in the HI_0826 operon affect bacterial cell division?

Investigating the stoichiometric relationships in the HI_0826 operon requires quantitative approaches:

• Absolute Protein Quantification:

  • Use selected reaction monitoring (SRM) or parallel reaction monitoring (PRM) mass spectrometry

  • Employ isotope-labeled peptide standards for each operon protein

  • Measure absolute copy numbers per cell under various conditions

  • Create a table of protein stoichiometry:

  Protein	Copy Number (Rich Media)	Copy Number (Minimal Media)	Cell Cycle Variation
  HI_0826	[Value from experiments]	[Value from experiments]	[% change]
  HI_0827	[Value from experiments]	[Value from experiments]	[% change]
  HI_0828	[Value from experiments]	[Value from experiments]	[% change]

• Controlled Expression Systems:

  • Create strains with inducible promoters controlling each operon gene

  • Systematically vary expression levels and measure effects on division

  • Determine optimal and critical stoichiometries

  • Identify limiting components in the system

• Fluorescent Protein Tagging and Quantification:

  • Tag each operon protein with different fluorescent proteins

  • Quantify relative abundance through calibrated fluorescence microscopy

  • Track dynamic changes during the cell cycle

  • Correlate with cell division events

• Mathematical Modeling:

  • Develop computational models incorporating protein interactions and stoichiometry

  • Simulate effects of altered expression levels

  • Make predictions that can be tested experimentally

  • Refine models based on experimental outcomes

• Single-Cell Analysis:

  • Measure cell-to-cell variability in protein levels

  • Correlate with division timing and fidelity

  • Determine how stoichiometric noise affects robust division

How conserved is HI_0826 across bacterial species and what does this reveal about its function?

Analysis of HI_0826 conservation provides insights into its evolutionary importance and functional constraints:

• Sequence Conservation Analysis:

  • Perform comprehensive sequence alignment across diverse bacterial phyla

  • Identify universally conserved residues and domains

  • Map conservation onto predicted structural features

  • Calculate conservation scores for each amino acid position

• Phylogenetic Profiling:

  • Determine presence/absence patterns across bacterial species

  • Correlate with bacterial morphology, division mechanisms, and ecological niches

  • Identify co-evolving gene families

• Synteny Analysis:

  • Compare genomic context across species

  • Determine if operon structure (HI_0825-HI_0828) is conserved

  • Identify species where gene order or operon structure differs

• Selection Pressure Analysis:

  • Calculate dN/dS ratios to identify regions under purifying or positive selection

  • Determine if membrane-spanning regions face different selective pressures than loops

  • Identify potential sites of host-pathogen interaction

• Functional Complementation:

  • Test if HI_0826 homologs from diverse species can complement H. influenzae knockout

  • Identify species-specific functional differences

  • Determine minimal functional domains through chimeric proteins

What experimental approaches can differentiate the roles of HI_0826, HI_0827, and HI_0828 in bacterial physiology?

To distinguish the specific roles of each operon component, researchers should employ these methodological approaches:

• Individual and Combinatorial Gene Knockouts:

  • Generate single, double, and triple knockout strains

  • Characterize phenotypes comprehensively (growth, morphology, division, stress responses)

  • Perform complementation studies to confirm phenotype specificity

  • Create a phenotypic matrix:

  Genotype	Growth Rate	Cell Morphology	Division Defects	Membrane Integrity	Stress Response
  Wild-type	[Baseline]	[Baseline]	[Baseline]	[Baseline]	[Baseline]
  ΔHI_0826	[% change]	[Observations]	[Observations]	[% change]	[% change]
  ΔHI_0827	[% change]	[Observations]	[Observations]	[% change]	[% change]
  ΔHI_0828	[% change]	[Observations]	[Observations]	[% change]	[% change]
  ΔHI_0826 ΔHI_0827	[% change]	[Observations]	[Observations]	[% change]	[% change]
  ΔHI_0826 ΔHI_0828	[% change]	[Observations]	[Observations]	[% change]	[% change]
  ΔHI_0827 ΔHI_0828	[% change]	[Observations]	[Observations]	[% change]	[% change]
  Triple KO	[% change]	[Observations]	[Observations]	[% change]	[% change]

• Transcriptional Profiling:

  • Perform RNA-seq on single gene knockout strains

  • Identify genes with altered expression in each knockout

  • Compare transcriptional signatures to infer pathway involvement

• Metabolomic Analysis:

  • Measure metabolite profiles in wild-type and knockout strains

  • Identify metabolic pathways affected by each gene

  • Look for metabolite accumulation or depletion patterns

• Protein Localization Studies:

  • Create fluorescent protein fusions for each operon protein

  • Determine subcellular localization patterns

  • Assess if localization of one protein depends on others

• Biochemical Activity Assays:

  • Develop specific assays for predicted functions

  • For HI_0826: Measure membrane remodeling or lipid interactions

  • For HI_0827: Assess thioesterase activity with various substrates

  • For HI_0828: Test catalytic activities based on the ferredoxin-like fold

How do structural homologs of HI_0826 in other bacterial systems inform potential functions and mechanisms?

Comparative structural analysis provides mechanistic insights:

• Structural Homology Detection:

  • Perform fold recognition and threading analysis

  • Identify distant structural homologs not detectable by sequence alone

  • Use tools like Dali, VAST, or DeepFold to find structural relatives

• Comparative Active Site Analysis:

  • Identify conserved catalytic residues or binding pockets

  • Compare with enzymes of known function

  • Generate hypotheses about potential substrates or binding partners

• Molecular Dynamics Simulations:

  • Simulate HI_0826 behavior in membrane environments

  • Compare dynamics with structural homologs

  • Identify conserved dynamic properties suggesting functional mechanisms

• Protein-Protein Docking:

  • Model interactions with division machinery components

  • Compare with interaction interfaces of structural homologs

  • Predict key interface residues for experimental validation

• Evolutionary Structure-Function Analysis:

  • Map sequence conservation onto structural models

  • Identify structurally conserved regions despite sequence divergence

  • Determine if functional sites are maintained across homologs

• Chimeric Protein Engineering:

  • Create fusion proteins combining domains from HI_0826 and structural homologs

  • Test functionality in appropriate assays

  • Determine if functional modules are interchangeable

What are the major challenges in expressing and purifying membrane proteins like HI_0826, and how can they be addressed?

Membrane protein expression and purification presents specific challenges that require specialized approaches:

• Expression Challenges and Solutions:

  • Challenge: Protein toxicity to expression host
    Solution: Use tightly controlled inducible systems (e.g., pBAD), lower induction temperatures (16-20°C), or specialized E. coli strains (C41/C43)

  • Challenge: Protein misfolding and aggregation
    Solution: Co-express with chaperones (GroEL/GroES), use fusion partners (MBP, SUMO), or optimize translation rates with rare codon-optimized strains

  • Challenge: Low expression levels
    Solution: Optimize codon usage, use strong promoters with tight regulation, screen multiple constructs with varying N/C termini

• Solubilization and Purification Challenges:

  • Challenge: Inefficient extraction from membranes
    Solution: Screen multiple detergents (DDM, LDAO, LMNG, etc.) at various concentrations, optimize temperature and time for solubilization

  • Challenge: Detergent-induced destabilization
    Solution: Add lipids during purification, use mild detergents, consider nanodiscs or amphipols for final stages

  • Challenge: Co-purification of lipids or contaminants
    Solution: Include additional purification steps (ion exchange, hydroxyapatite), use stringent washing conditions during affinity steps

• Stability Assessment Techniques:

  • Thermal shift assays modified for membrane proteins

  • Size exclusion chromatography to monitor aggregation

  • Dynamic light scattering to assess homogeneity

  • Circular dichroism to confirm secondary structure retention

How can researchers overcome difficulties in analyzing protein-protein interactions involving membrane proteins like HI_0826?

Membrane protein interaction analysis requires specialized approaches:

• Detergent Interference in Biochemical Assays:

  • Challenge: Detergents interfere with many interaction assays
    Solution: Use detergent-resistant techniques (MST, BLI), detergent-free systems (nanodiscs, SMALPs), or in-cell approaches (FRET, PLA)

  • Challenge: Artificial interactions induced by detergent micelles
    Solution: Validate interactions using multiple detergents, perform competition assays, or use lipid bilayer systems

• Capturing Transient Interactions:

  • Challenge: Membrane protein interactions may be dynamic or weak
    Solution: Use in vivo crosslinking (formaldehyde, DSP), proximity labeling (BioID, APEX), or zero-length crosslinkers (EDC)

  • Challenge: Determining interaction specificity
    Solution: Include appropriate controls, perform mutagenesis of predicted interface residues, use competition assays

• Reconstitution Systems:

  • Challenge: Creating physiologically relevant environments
    Solution: Use E. coli polar lipid extracts for liposomes, incorporate native lipids, or control lipid composition to match natural environment

  • Challenge: Correct orientation in liposomes
    Solution: Use fluorescent tags to determine orientation, employ protease protection assays, or create asymmetric liposomes

• Quantitative Analysis Methods:

  • Microscale thermophoresis (MST) in detergent solutions

  • Single-molecule FRET for dynamic interactions

  • Bio-layer interferometry (BLI) with captured proteins

  • Isothermal titration calorimetry (ITC) optimized for membrane proteins

What approaches can resolve discrepancies between predicted and experimental data for HI_0826 function?

When facing contradictions between predicted and experimental results, consider these methodological approaches:

• Revisiting Experimental Design:

  • Challenge: Expression tag interference with function
    Solution: Test multiple tag positions and types, include untagged controls, create tag-removal constructs

  • Challenge: Non-physiological conditions affecting results
    Solution: Test function under various conditions (pH, salt, temperature), use growth media mimicking infection environment

• Reconciling Structural Predictions:

  • Challenge: Transmembrane topology disagreement
    Solution: Apply multiple experimental approaches (PhoA/LacZ fusions, cysteine accessibility, protease protection), integrate results

  • Challenge: Dynamic structural changes not captured in static models
    Solution: Use EPR spectroscopy, HDX-MS, or DEER to detect conformational changes

• Functional Redundancy Analysis:

  • Challenge: Compensatory mechanisms masking knockout phenotypes
    Solution: Create conditional knockouts, use chemical genetics, or analyze synthetic lethality patterns

  • Challenge: Species-specific differences in function
    Solution: Perform comparative analysis across multiple bacterial species, test heterologous complementation

• Analytical Framework for Resolving Discrepancies:

  • Create comprehensive table of predictions vs. observations

  • Systematically test hypotheses explaining discrepancies

  • Develop new models incorporating all experimental data

  • Design critical experiments that can definitively distinguish between competing models