Recombinant Dictyostelium discoideum Peroxisomal membrane protein 11 homolog (pex11)

What is the evolutionary relationship of D. discoideum Pex11 to other Pex11 family proteins?

D. discoideum Pex11 is part of the highly conserved Pex11 protein family found across eukaryotes. Comparative genomic analyses reveal that:

• Pex11 is one of the most ancient and conserved peroxins, present in the last eukaryotic common ancestor .

• The Pex11 family has undergone independent paralogizations in different lineages, resulting in multiple paralogs in various organisms .

• Phylogenetic analysis distinguishes two main subfamilies within the Pex11 family:

  • One containing fungal PEX11 and vertebrate PEX11α/β

  • One containing fungal PEX11C and vertebrate PEX11γ

D. discoideum, as a slime mold, possesses a single Pex11 protein that likely represents the ancestral form before the expansion of this protein family in other lineages .

How does recombinant D. discoideum Pex11 protein differ from the native protein?

The recombinant D. discoideum Pex11 protein typically includes:

• A fusion tag (commonly His-tag) at the N-terminus to facilitate purification

• Expression in a heterologous system (usually E. coli)

• Potential post-translational modifications that may differ from the native protein

The key differences include:

When using recombinant protein for functional studies, researchers should consider how these differences might affect experimental outcomes .

What are the optimal conditions for expressing and purifying recombinant D. discoideum Pex11?

For optimal expression and purification of recombinant D. discoideum Pex11:

Expression Protocol:

• Transform expression plasmid (e.g., pQE60-Pex11) into E. coli M15 or BL21(DE3) cells

• Grow cells in LB medium at 37°C to an optical density of 1.0

• Induce with 1 mM IPTG

• Continue incubation at a reduced temperature (30°C) for 4-6 hours

Purification Strategy:

• Harvest cells by centrifugation (6000 rpm, 10 min, 4°C)

• Resuspend in denaturing lysis buffer containing 8M urea

  • For general applications: 50 mM Tris, 8M urea, pH 8.0

  • For Ni-NTA purification: 100 mM NaH₂PO₄, 10 mM Tris, 8M urea, pH 8.0

• Disrupt cells using French press or sonication

• Clear lysate by centrifugation (20,000g, 1h, 12°C)

• Purify using Ni-NTA affinity chromatography with gradient elution

• Further purify using ion-exchange chromatography (e.g., Mono-Q resin)

• Concentrate using appropriate molecular weight cutoff devices

For working with the purified protein, reconstitution into liposomes is often necessary to study its membrane-remodeling activities.

How can researchers effectively study Pex11 membrane remodeling activity in vitro?

To study Pex11 membrane remodeling activity in vitro, researchers can use the following methodological approach:

Materials Needed:

• Purified recombinant Pex11 N-terminal domain or synthetic peptides containing the amphipathic helix

• Synthetic liposomes with appropriate lipid composition

Liposome Preparation Protocol:

• Prepare small unilamellar vesicles (SUVs) that mimic peroxisomal membrane composition:

  • Use a mixture of PC/PE/PI/PS/CL to resemble peroxisomal membranes

  • For control experiments, prepare neutral liposomes (PC only or PC/PE)

Membrane Remodeling Assays:

• Liposome Binding Assay:

  • Incubate Pex11 protein/peptide with liposomes

  • Separate bound and unbound fractions by centrifugation

  • Analyze using Tricine SDS-PAGE and silver staining

• Tubulation Assay:

  • Incubate liposomes with increasing concentrations of Pex11 protein/peptide

  • Examine using electron microscopy for membrane tubulation

  • Quantify tubule formation, diameter, and morphology

• Turbidimetric Measurements:

  • Monitor changes in light scattering as an indicator of liposome remodeling

  • Calculate changes in transmittance against peptide concentration

Controls and Validation:

• Use mutant peptides with disrupted amphipathic helix structure (e.g., introducing proline residues)

• Use peptides with altered hydrophobic surface (e.g., I69E, I72E, F75E mutations)

• Compare wild-type and mutant peptides for their membrane binding and tubulation capacity

This experimental approach has successfully demonstrated that the N-terminal amphipathic helix of Pex11 can remodel membranes in vitro, especially those with negatively charged phospholipids resembling peroxisomal membranes .

What techniques are available for studying Pex11 localization and function in vivo?

Several complementary techniques are effective for studying Pex11 localization and function in vivo:

Fluorescence Microscopy Approaches:

• GFP-fusion protein expression:

  • Clone Pex11 with C-terminal or N-terminal GFP tag

  • Express in D. discoideum or heterologous systems

  • Co-express with peroxisomal matrix markers (e.g., DsRed-SKL)

  • Visualize using confocal or fluorescence microscopy

• High-content microscopy screening:

  • Generate mutant libraries expressing Pex11-GFP

  • Extract morphological features from images

  • Apply outlier detection algorithms to identify strains with altered Pex11 localization

  • Calculate "outlyingness scores" to quantify phenotypic changes

Genetic and Functional Analysis:

• Gene knockout studies:

  • Generate pex11 deletion strains

  • Analyze peroxisome morphology, number, and size

  • Examine mislocalization of peroxisomal matrix proteins

  • Assess degradation of peroxisomal membrane proteins

• Re-targeting assays:

  • Create fusion constructs with mitochondrial targeting sequences

  • Express in pex mutant backgrounds

  • Analyze protein localization and organelle morphology

• Functional complementation:

  • Express wild-type or mutant Pex11 in pex11 deletion backgrounds

  • Quantify rescue of peroxisome proliferation defects

  • Analyze formation of peroxisome extensions in dnm1 pex11 double mutants

These approaches have revealed that Pex11 localizes to the peroxisomal membrane where it functions in membrane remodeling and peroxisome proliferation .

How do mutations in the amphipathic helix region of Pex11 affect its function in peroxisome dynamics?

The amphipathic helix in the N-terminal region of Pex11 is critical for its function in peroxisome membrane remodeling. Systematic mutation analysis reveals:

Key Structural Elements of the Amphipathic Helix:

• Hydrophobic surface that interacts with membrane lipids

• Polar/positively charged residues that interact with lipid headgroups

• α-helical structure essential for membrane interaction

Effects of Different Mutations:

Mechanistic Insights:

• The amphipathic helix inserts into the outer leaflet of the peroxisomal membrane

• This insertion causes membrane asymmetry and drives membrane bending/curvature

• Mutations affecting either the hydrophobic surface or the helical structure abolish this membrane remodeling capacity

• Helix insertion preferentially occurs with negatively charged membranes resembling peroxisomal membrane composition

These findings demonstrate that maintaining both the amphipathic properties of the helix and its α-helical structure are crucial for Pex11's function in peroxisome membrane dynamics .

How does the function of D. discoideum Pex11 compare to its orthologs in other organisms?

Comparative functional analysis reveals both conserved and divergent aspects of Pex11 function across eukaryotic lineages:

Conserved Functions:

• Membrane remodeling: The membrane-remodeling capacity of the N-terminal amphipathic helix is conserved from yeast to humans

• Peroxisome proliferation: In most organisms, Pex11 regulates peroxisome number and size

• Dynamin recruitment: Pex11 interacts with dynamin-related proteins (DRPs) involved in membrane scission

Species-Specific Differences:

Evolutionary Context:

• D. discoideum Pex11 likely represents a more ancestral form of the protein

• The Pex11 family has undergone independent paralogizations in different lineages

• Despite this diversification, the core membrane-remodeling function is conserved

This comparative analysis suggests that while D. discoideum Pex11 shares the conserved membrane-remodeling function with its orthologs, species-specific adaptations have occurred throughout evolution .

What is the role of D. discoideum Pex11 in peroxisome-organelle interactions?

Recent studies have revealed that Pex11 plays important roles in mediating peroxisome interactions with other cellular organelles:

Peroxisome-Mitochondria Interactions:

• Genome-wide localization studies in yeast identified Pex11 interactions with the ERMES (ER-Mitochondria Encounter Structure) complex

• In pex3 and pex19 mutants of S. cerevisiae, Pex11 localizes to mitochondria rather than peroxisomes, suggesting potential functional connections

• The mitochondrial localization of Pex11 in these mutants doesn't align with current models of PMP sorting, indicating unexplored functions

Peroxisome-ER Interactions:

• In H. polymorpha pex3 cells, Pex11 localizes to the ER where it is unstable

• This suggests potential roles in ER-to-peroxisome trafficking pathways

Quantitative Analysis of Pex11 Localization in Mutant Strains:
Analysis of pairwise distances of Pex11-GFP localization patterns in ERMES complex mutants revealed:

These findings suggest that specific components of the ERMES complex (particularly Mdm10 and Mdm12) influence Pex11 localization and potentially peroxisome-mitochondria interactions .

The role of D. discoideum Pex11 specifically in organelle interactions remains to be fully explored, but based on the conservation of Pex11 function across species, it likely participates in similar interactions in this organism .

How does D. discoideum Pex11 contribute to sterol biosynthesis and metabolism?

The role of peroxisomes in sterol biosynthesis is particularly interesting in D. discoideum, as this organism shows unusual compartmentalization of sterol biosynthetic enzymes:

Peroxisomal Localization of Sterol Biosynthesis Enzymes:

• In D. discoideum, several key enzymes in the sterol biosynthesis pathway localize to peroxisomes:

  • Squalene synthase

  • Squalene epoxidase

  • Oxidosqualene cyclase

  • Cycloartenol-C-24-methyltransferase

• This peroxisomal localization is unusual, as these enzymes are typically found in the endoplasmic reticulum in other organisms

Role of Pex11 in Supporting Sterol Metabolism:

• Pex11 is essential for proper peroxisome biogenesis and proliferation

• As peroxisomes house key sterol biosynthesis enzymes in D. discoideum, Pex11 likely plays an indirect but crucial role in sterol metabolism

• Pex11-mediated peroxisome membrane dynamics may facilitate the import of these enzymes and their substrates

PTS1 Targeting of Sterol Biosynthesis Enzymes:

• Several sterol biosynthesis enzymes in D. discoideum contain a Peroxisomal Targeting Signal 1 (PTS1) at their C-terminus:

  • Oxidosqualene synthase and cycloartenol-C-24-methyltransferase require their PTS1 for peroxisomal entry

  • Squalene synthase is largely peroxisomal even when its PTS1 is absent, suggesting additional targeting mechanisms

This unique compartmentalization of sterol biosynthesis in D. discoideum makes it an interesting model for studying the relationship between peroxisome dynamics (regulated by Pex11) and sterol metabolism, though more direct studies of Pex11's role in this process are needed .

How can recombinant D. discoideum Pex11 be used to study membrane dynamics in vitro?

Recombinant D. discoideum Pex11 is a valuable tool for studying fundamental aspects of membrane dynamics through the following experimental approaches:

Liposome-Based Systems:

• Tubulation Assays:

  • Incubate purified Pex11 or its N-terminal domain with liposomes

  • Visualize membrane tubulation using electron microscopy

  • Quantify tubule formation, diameter, and morphology changes

  • This approach has successfully demonstrated that the N-terminal domain of Pex11 proteins can tubulate liposomes in vitro

• Lipid Composition Effects:

  • Prepare liposomes with varying phospholipid compositions:

    • Negatively charged liposomes (PC/PE/PI/PS/CL)

    • Neutral liposomes (PC or PC/PE)

  • Compare Pex11 binding and membrane remodeling activity

  • This can reveal how lipid composition influences Pex11 function

Structure-Function Analysis:

• Mutagenesis Studies:

  • Generate mutant versions of the protein with alterations in the amphipathic helix

  • Compare membrane binding and remodeling activities

  • Correlate in vitro activities with in vivo peroxisome proliferation effects

• Domain Mapping:

  • Express and purify different domains of Pex11

  • Test their individual activities in membrane binding and remodeling

  • Identify minimal functional domains required for specific activities

The findings from such in vitro studies can provide fundamental insights into how membrane-remodeling proteins like Pex11 function at the molecular level .

What experimental systems are most suitable for investigating D. discoideum Pex11 in the context of peroxisome-organelle interactions?

To investigate D. discoideum Pex11's role in peroxisome-organelle interactions, researchers can employ several complementary experimental systems:

Live Cell Imaging Systems:

• Dual-Color Fluorescence Microscopy:

  • Express Pex11-GFP alongside markers for other organelles:

    • Mitochondria (e.g., MitoTracker or mitochondrial-targeted RFP)

    • ER (e.g., ER-targeted RFP or mCherry)

    • Lipid droplets (e.g., BODIPY staining)

  • Perform time-lapse imaging to capture dynamic interactions

  • Quantify co-localization and contact site frequency

• Super-Resolution Microscopy:

  • Use techniques like STED, PALM, or STORM to visualize peroxisome-organelle contact sites

  • Achieve nanometer-resolution of membrane contacts not visible with conventional microscopy

Biochemical Approaches:

• Proximity Labeling:

  • Express Pex11 fused to proximity labeling enzymes (BioID or APEX2)

  • Identify proteins in proximity to Pex11 by mass spectrometry

  • Discover new interaction partners at organelle contact sites

• Subcellular Fractionation:

  • Isolate peroxisome-associated membrane fractions

  • Analyze co-purifying proteins from other organelles

  • Identify potential tethering complexes

Genetic Screens:

• ERMES Complex Mutants:

  • Generate mutants in ERMES complex components in D. discoideum

  • Analyze Pex11 localization and peroxisome-mitochondria interactions

  • Compare with findings from yeast studies showing Pex11 interactions with ERMES

• Synthetic Genetic Arrays:

  • Create double mutants combining pex11 deletion with mutations in genes involved in organelle contacts

  • Identify genetic interactions suggesting functional relationships

These experimental systems can provide complementary insights into how D. discoideum Pex11 mediates peroxisome interactions with other organelles, potentially revealing conserved and divergent aspects compared to other organisms .

How can knowledge of Pex11 structure-function relationships be applied to engineer peroxisome dynamics?

Understanding the structure-function relationships of Pex11 presents opportunities for engineering peroxisome dynamics for both research and biotechnological applications:

Engineered Peroxisome Proliferation:

• Inducible Pex11 Expression Systems:

  • Develop inducible promoters controlling Pex11 expression

  • Enable temporal control of peroxisome proliferation

  • Applications in studying peroxisome inheritance and distribution

• Enhanced Pex11 Variants:

  • Design Pex11 proteins with optimized amphipathic helices

  • Create gain-of-function mutations that increase membrane bending capacity

  • Generate variants with bulkier hydrophobic residues that produce smaller, more numerous peroxisomes

Targeted Peroxisome Manipulations:

• Synthetic Organelle Tethering:

  • Create chimeric proteins combining Pex11 with domains from other organelle-specific proteins

  • Engineer novel peroxisome-organelle contact sites

  • Study the functional consequences of altered organelle interactions

• Optogenetic Control of Peroxisome Dynamics:

  • Fuse Pex11 domains to light-responsive protein domains

  • Enable light-controlled activation of peroxisome proliferation

  • Provide spatial and temporal precision in manipulating peroxisome dynamics

Applications in Biotechnology:

• Enhanced Peroxisomal Metabolism:

  • Increase peroxisome abundance through Pex11 engineering

  • Potentially enhance metabolic pathways localized to peroxisomes

  • Applications in biofuel production or biodegradation of environmental pollutants

• Disease Modeling:

  • Engineer Pex11 variants mimicking human disease mutations

  • Study molecular mechanisms of peroxisomal disorders

  • Test potential therapeutic approaches targeting Pex11 function

These applications build on the fundamental understanding that the membrane-remodeling capacity of Pex11 depends on its conserved amphipathic helix, and that manipulating this domain can directly affect peroxisome proliferation and dynamics .

What are the unexplored aspects of D. discoideum Pex11 function that warrant further investigation?

Several unexplored aspects of D. discoideum Pex11 function represent promising areas for future research:

Developmental Regulation:

• As a slime mold, D. discoideum undergoes complex developmental transitions from unicellular to multicellular forms

• How Pex11 expression and peroxisome dynamics change during development remains largely uncharacterized

• The potential roles of peroxisomes in D. discoideum development and differentiation merit investigation

Metabolic Functions:

• The role of Pex11-regulated peroxisome dynamics in D. discoideum metabolism, particularly in:

  • Sterol biosynthesis (uniquely compartmentalized in peroxisomes in this organism)

  • Fatty acid metabolism and energy homeostasis

  • Adaptation to changing nutrient conditions

Peroxisome-Organelle Communication:

• While Pex11-mediated peroxisome-organelle interactions have been studied in yeast and mammals, they remain unexplored in D. discoideum

• The potential role of D. discoideum Pex11 in organizing peroxisome-mitochondria or peroxisome-ER contact sites

• Comparative analysis with findings from other organisms to identify conserved mechanisms

Signal Transduction:

• How external signals regulate Pex11 activity and peroxisome proliferation in D. discoideum

• Potential post-translational modifications of Pex11 that modulate its function

• Integration of peroxisome dynamics with cellular signaling networks

These research directions could expand our understanding of Pex11 function beyond its established role in membrane remodeling and reveal unique aspects of peroxisome biology in this evolutionarily informative organism.

How could advances in structural biology techniques enhance our understanding of D. discoideum Pex11?

Advanced structural biology techniques offer promising approaches to resolve unanswered questions about D. discoideum Pex11 structure and function:

Cryo-Electron Microscopy (Cryo-EM):

• Determine high-resolution structures of Pex11 in membrane environments

• Visualize conformational changes during membrane interaction and remodeling

• Understand how Pex11 oligomerization contributes to membrane curvature generation

• Reveal the three-dimensional organization of Pex11 at sites of peroxisome constriction

Molecular Dynamics Simulations:

• Model interactions between the amphipathic helix and lipid membranes

• Predict how mutations affect helix-membrane interactions

• Simulate the membrane deformation process at the atomic level

• Guide the design of optimized Pex11 variants with enhanced membrane-remodeling capacity

Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS):

• Map regions of Pex11 that interact with membranes or other proteins

• Identify conformational changes upon membrane binding

• Detect structural differences between wild-type and mutant Pex11 proteins

NMR Spectroscopy:

• Determine the solution structure of the N-terminal domain containing the amphipathic helix

• Study dynamics of membrane interaction in real-time

• Characterize lipid-binding properties of different Pex11 domains

Cross-linking Mass Spectrometry:

• Identify Pex11 interaction partners in vivo

• Map the topology of Pex11 oligomers

• Elucidate the organization of Pex11 in protein complexes involved in peroxisome division

These structural biology approaches would provide unprecedented insights into how D. discoideum Pex11 functions at the molecular level, complementing the functional studies that have established its role in peroxisome membrane dynamics .

How might studies of D. discoideum Pex11 contribute to understanding human peroxisomal disorders?

D. discoideum Pex11 studies can provide valuable insights into human peroxisomal disorders through several research pathways:

Evolutionary Conservation and Disease Mechanisms:

• The conserved nature of Pex11 structure and function between D. discoideum and humans enables comparative studies

• Mutations identified in human PEX11β associated with neurological disorders can be modeled in D. discoideum Pex11

• The simpler genetic background of D. discoideum facilitates mechanistic studies of how Pex11 mutations affect peroxisome function

Disease-Relevant Processes:

• Peroxisome Dynamics: Impaired peroxisome division is observed in several peroxisomal disorders

• Sterol Metabolism: D. discoideum's unique peroxisomal compartmentalization of sterol biosynthesis enzymes provides a model for studying how peroxisomal metabolic pathways are affected in disease states

• Organelle Interactions: Peroxisome-mitochondria communication disruption is implicated in various disorders

Therapeutic Development Platforms:

• D. discoideum can serve as a rapid screening system for compounds that modulate Pex11 function

• Structure-function studies of D. discoideum Pex11 can guide the design of peptides or small molecules targeting human PEX11 proteins

• Genetic suppressor screens in D. discoideum pex11 mutants might identify pathways that could be targeted therapeutically

Specific Research Applications:

• Generate D. discoideum strains expressing human PEX11 variants associated with disease

• Study the effects of these variants on peroxisome morphology, division, and metabolic functions

• Identify genetic or pharmacological interventions that rescue mutant phenotypes