Recombinant Chaetomium globosum Vacuolar ATPase assembly integral membrane protein VMA21 (VMA21)
Description
Introduction to VMA21 Protein
The Vacuolar ATPase assembly integral membrane protein VMA21 (VMA21) from Chaetomium globosum is an integral membrane protein involved in the assembly of the vacuolar proton-translocating ATPase (V-ATPase) complex. C. globosum, a soil fungus (strain ATCC 6205 / CBS 148.51 / DSM 1962 / NBRC 6347 / NRRL 1970), produces this protein as part of its cellular machinery essential for vacuolar function . The recombinant form of this protein has been synthesized for research purposes, enabling detailed investigations into its characteristics and functions.
V-ATPase complexes are evolutionarily conserved across eukaryotes and play crucial roles in cellular processes by acidifying various intracellular compartments. The assembly of these complex molecular machines requires dedicated assembly factors, with VMA21 being one of the most significant components. While most research on VMA21 has been conducted in yeast models, the protein's importance extends to filamentous fungi like Chaetomium globosum, where it likely maintains similar critical functions.
Biochemical Properties
The recombinant C. globosum VMA21 protein is typically stored in a Tris-based buffer containing 50% glycerol, optimized for maintaining protein stability . This storage solution helps prevent protein degradation while preserving its native conformation for experimental applications. For extended storage, the protein can be maintained at -20°C or -80°C, though repeated freezing and thawing cycles are not recommended to preserve functional integrity .
Membrane Topology
Based on its amino acid sequence and knowledge from homologous proteins, C. globosum VMA21 is predicted to be an integral membrane protein with transmembrane domains that anchor it to the endoplasmic reticulum (ER) membrane. The hydrophobic regions in the sequence (particularly in the LLAFTLGMIVIPIGSYF and AGALAAIMANVVLIGYIFVA segments) likely form membrane-spanning helices .
Functional Domains
Drawing parallels from research on yeast VMA21, the C. globosum VMA21 likely contains essential functional domains:
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Transmembrane domains that anchor it to the ER membrane
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Interaction sites for binding to V-ATPase subunits
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A potential dilysine motif (KK) at the C-terminus (GKKDR), which may function as an ER retention signal
This dilysine motif is particularly significant as studies in yeast have shown that it plays a crucial role in retaining the protein in the ER, where it performs its assembly function . Mutation of these lysine residues in yeast results in mislocalization of the protein to the vacuole, demonstrating their importance for proper subcellular localization.
The V-ATPase Complex
The vacuolar H⁺-ATPase (V-ATPase) is a multisubunit molecular machine composed of two main sectors:
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V₁ sector: Peripheral membrane complex responsible for ATP hydrolysis
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V₀ sector: Integral membrane complex responsible for proton translocation
These sectors work together to pump protons across membranes, establishing pH gradients critical for various cellular processes .
VMA21's Role in Assembly
Based on studies of yeast VMA21, the C. globosum homolog likely functions as an assembly factor rather than a structural component of the mature V-ATPase complex. The protein appears to reside primarily in the endoplasmic reticulum, where it facilitates the assembly of the V₀ sector of the V-ATPase .
The critical nature of this function is demonstrated in yeast vma21 mutants, which fail to properly assemble the V-ATPase complex. In these mutants, the peripheral V₁ subunits accumulate in the cytosol while the 100-kDa integral membrane subunit undergoes rapid degradation . This suggests that without VMA21, the V₀ sector cannot properly assemble, preventing functional V-ATPase complex formation.
Proposed Assembly Mechanism
The following table summarizes the proposed steps in VMA21-mediated V-ATPase assembly:
| Assembly Stage | VMA21 Function | Outcome |
|---|---|---|
| Initial interaction | Binds to nascent V₀ subunits in ER | Stabilizes V₀ subunits |
| Complex formation | Facilitates correct folding and interaction of V₀ subunits | Assembled V₀ sector |
| Quality control | Ensures proper conformation of assembled V₀ | Prevention of defective complex export |
| Transport facilitation | Potentially assists in transport to Golgi apparatus | Progression toward vacuolar membrane |
| Recycling | Returns to ER via dilysine motif after assembly | Maintenance of VMA21 pool in ER |
This process appears to be highly regulated and essential for proper vacuolar function across fungal species .
Conservation Across Fungi
While the search results provide limited direct comparative information, the functional importance of VMA21 suggests high conservation across fungal species. Studies in yeast have established the critical nature of this protein, and its presence in Chaetomium globosum indicates evolutionary conservation of this important assembly factor .
Functional Comparison with Yeast VMA21
Based on the available information, several parallels between C. globosum VMA21 and its yeast counterpart can be drawn:
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Both are small integral membrane proteins of approximately 8.5 kDa
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Both likely function in the endoplasmic reticulum
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Both appear to be essential for V-ATPase assembly
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Both possess hydrophobic regions consistent with transmembrane domains
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Both contain potential ER retention signals in their C-terminal regions
These similarities suggest a conserved role for VMA21 in fungal V-ATPase assembly, though specific functional differences may exist that have not yet been characterized.
Biotechnological Applications
The recombinant C. globosum VMA21 protein has potential applications in various research contexts:
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Structural studies of V-ATPase assembly mechanisms
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Development of antifungal agents targeting V-ATPase assembly
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Comparative studies of protein trafficking across fungal species
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Investigation of membrane protein assembly pathways
As a commercially available product, the recombinant protein enables researchers to conduct these investigations without the need to isolate the native protein .
Medical and Agricultural Significance
Understanding VMA21 function has broader implications:
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Potential antifungal targets: Disruption of V-ATPase assembly could inhibit fungal growth
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Agricultural applications: Controlling soil fungi through targeting conserved assembly factors
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Medical relevance: Insights into related human disorders involving V-ATPase dysfunction
Recent research has identified connections between VMA21 homologs and human disease, including a muscle-specific isoform implicated in X-linked myopathy with excessive autophagy , highlighting the broader significance of understanding this protein family.
Knowledge Gaps
Despite its importance, several aspects of C. globosum VMA21 remain underexplored:
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Detailed structural characterization through crystallography or cryo-EM
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Specific interaction partners in C. globosum
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Regulatory mechanisms controlling its expression and activity
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Species-specific functions that may differ from yeast models
These knowledge gaps represent opportunities for future research directions.
Future Research Directions
Promising avenues for further investigation include:
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Comparative functional studies between C. globosum VMA21 and homologs from other species
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Structure-function analyses to identify critical residues for assembly activity
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In vivo studies examining localization and trafficking
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Investigations into potential interactions with other assembly factors like Vma12p and Vma22p, which have been identified in yeast
Such studies would significantly advance our understanding of this critical assembly factor.
Product Specs
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Target Background
Q&A
What is the fundamental role of VMA21 in cellular physiology?
VMA21 functions as an essential assembly factor for the Vacuolar H+-ATPase complex (V-ATPase), which is a multisubunit protein complex required for acidification of intracellular compartments. Specifically, VMA21 facilitates the assembly of the V0 domain (the membrane-embedded sector) of the V-ATPase in the endoplasmic reticulum (ER). Without proper VMA21 function, V-ATPase assembly is compromised, leading to impaired lysosomal acidification and disruption of multiple cellular processes dependent on proper pH regulation .
How does VMA21 structure relate to its function in V-ATPase assembly?
VMA21 is an integral membrane protein that contains multiple transmembrane regions. The protein has specific domains that interact with V-ATPase subunits, particularly the V0 domain components. In humans, mutations affecting these interaction domains (such as p.Asn63Gly located in the luminal loop region) can impair binding with V-ATPase components and disrupt the assembly process. Research has shown that certain missense mutations reduce interaction with the assembly factor ATP6AP2 and V0 subunit ATP6V0C, indicating that specific structural elements are crucial for VMA21's assembly factor activity .
What are the optimal methods for expressing and purifying recombinant Chaetomium globosum VMA21?
For expressing recombinant VMA21, researchers should consider using eukaryotic expression systems due to VMA21's integral membrane nature and potential post-translational modifications. For purification, a two-step approach is recommended:
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Initial extraction using mild detergents (e.g., n-dodecyl-β-D-maltoside) to solubilize the membrane-bound protein without disrupting its structural integrity
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Affinity chromatography using either His-tag or antibody-based methods followed by size-exclusion chromatography to achieve high purity
When working with recombinant VMA21, researchers should incorporate quality control steps including western blotting and functional assays to verify the proper folding and activity of the purified protein. Expression in yeast systems may be particularly advantageous for functional studies given the established assays for V-ATPase activity in this model organism .
What experimental approaches can be used to assess VMA21's interaction with V-ATPase components?
Several complementary techniques can be employed to characterize VMA21's interactions:
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Co-immunoprecipitation (Co-IP): Using tagged versions of VMA21 to pull down associated V-ATPase subunits. Studies have demonstrated that immunoprecipitation of VMA21 can co-precipitate all five V0 subunits, confirming its role in V0 assembly .
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Yeast two-hybrid assays: While challenging for membrane proteins, modified membrane yeast two-hybrid systems can help identify direct binding partners.
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FRET-based approaches: For studying interactions in live cells, particularly to observe the dynamics of assembly in real-time.
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Cross-linking mass spectrometry: To identify precise interaction interfaces between VMA21 and its binding partners.
Studies have shown that VMA21 interactions with V0 subunits are mediated through the proteolipid subunit Vma11p, and this methodological approach has revealed that V0 assembly occurs in a defined sequence .
How can researchers assess the functional consequences of VMA21 mutations or deletions?
Multiple complementary approaches can be used to evaluate VMA21 dysfunction:
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Lysosomal acidification assays: Using pH-sensitive dyes such as LysoSensor or LysoTracker to measure changes in lysosomal pH. Patient fibroblasts with VMA21 mutations show reduced number and intensity of LysoSensor-positive and LysoTracker-positive punctae, indicating impaired acidification .
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V-ATPase assembly analysis: Western blotting to assess the steady-state levels of V0 and V1 subunits. VMA21 deficiency specifically affects V0 domain assembly while V1 subunit levels remain unchanged .
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Zinc tolerance assay in yeast: A functional assay that leverages the dependence of yeast on V-ATPase activity for survival in elevated zinc conditions. Expression of human VMA21 with patient-derived mutations fails to rescue yeast growth under these conditions, demonstrating functional impairment .
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Lipid droplet accumulation analysis: Microscopy-based assessment of lipid droplet formation in autolysosomes, which is a characteristic cellular phenotype of VMA21 deficiency .
| Assay Type | Readout | Interpretation |
|---|---|---|
| LysoSensor/LysoTracker | Fluorescence intensity | Inversely correlates with pH; reduced signal indicates impaired acidification |
| Western blot | V0 subunit levels | Reduced levels indicate impaired assembly |
| Zinc tolerance | Yeast growth | No/limited growth indicates V-ATPase dysfunction |
| Lipid droplet analysis | LD size and number | Increased LDs indicate impaired lipophagy |
What cellular processes are most sensitive to VMA21 dysfunction, and how can researchers monitor these effects?
Several cellular processes are particularly affected by VMA21 dysfunction:
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Autophagy: Monitor using LC3-II accumulation and p62/SQSTM1 levels via western blotting, or assess autophagosome-lysosome fusion using fluorescent reporters.
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Lipid metabolism: Analyze using lipidomics approaches and visualization of lipid droplets with Oil Red O or BODIPY staining.
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Protein glycosylation: Assess through specialized glycoprotein staining methods or mass spectrometry-based glycan profiling. Patients with VMA21 mutations show abnormal glycosylation patterns of hepatocyte-derived proteins .
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Cholesterol metabolism: Measure cellular cholesterol levels and assess activation of sterol response element-binding protein (SREBP) pathways. VMA21 deficiency triggers endoplasmic reticulum stress and sequestration of unesterified cholesterol in lysosomes, activating SREBP-mediated cholesterol synthesis .
What are the molecular mechanisms underlying tissue-specific pathologies in VMA21 deficiency disorders?
VMA21 deficiency causes remarkably tissue-specific pathologies despite its ubiquitous expression. The mechanisms behind this specificity represent an active research frontier:
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X-linked mutations in VMA21 can lead to autophagic myopathy (specifically affecting muscle).
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Other VMA21 variants cause liver disorders characterized by steatosis, mild cholestasis, chronic elevation of aminotransferases, and elevated LDL cholesterol.
Research suggests these tissue-specific effects may result from:
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Differential expression levels of compensatory assembly factors in various tissues.
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Tissue-specific demands for lysosomal function (e.g., hepatocytes with high metabolic activity).
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Different thresholds for cellular dysfunction across tissues.
Investigating these mechanisms requires tissue-specific conditional knockout models and comprehensive metabolic profiling of affected tissues. Researchers should compare VMA21-deficient models with those lacking other V-ATPase assembly factors to identify commonalities and differences in tissue-specific manifestations .
How does VMA21 dysfunction affect the interplay between autophagy and lipid metabolism?
VMA21 deficiency creates a unique window into the relationship between autophagy and lipid metabolism. The key findings from patient studies reveal:
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Lysosomal acidification and degradation of phagocytosed materials are impaired.
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This impairment leads to lipid droplet accumulation in autolysosomes.
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VMA21 deficiency triggers ER stress and sequestration of unesterified cholesterol in lysosomes.
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These changes activate sterol response element-binding protein-mediated cholesterol synthesis pathways.
These observations suggest a mechanistic link where impaired autophagy (specifically lipophagy) due to V-ATPase dysfunction leads to abnormal lipid accumulation. This creates a feedforward loop of cholesterol sequestration and increased synthesis.
Researchers investigating this interplay should employ dual-labeling approaches to simultaneously monitor autophagic flux and lipid dynamics in live cells. Pharmacological modulation of either pathway can provide insights into their interdependence in the context of VMA21 dysfunction .
How do VMA21 homologs from different species compare in their assembly factor functions?
Comparative analysis of VMA21 across species provides valuable insights into conserved functional domains:
This cross-species complementation approach provides a powerful tool for functional validation of VMA21 variants. Researchers can utilize this yeast system to rapidly screen mutations for functional consequences before proceeding to more complex mammalian models .
What is the current understanding of the relationship between Chaetomium globosum VMA21 and human VMA21 homologs?
While the search results don't provide specific information about Chaetomium globosum VMA21 in relation to human homologs, researchers investigating this relationship should consider:
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Sequence alignment and structural modeling to identify conserved domains.
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Heterologous expression studies to assess functional complementation.
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Biochemical characterization of species-specific interaction partners.
Understanding these relationships could provide insights into the evolution of V-ATPase assembly mechanisms and potentially reveal novel assembly factor interactions that might be leveraged for therapeutic development.
What are the key technical challenges in studying recombinant integral membrane proteins like VMA21, and how can they be overcome?
Working with integral membrane proteins presents several challenges:
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Expression and purification: Due to their hydrophobic nature, membrane proteins often aggregate or misfold during recombinant expression.
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Solution: Use specialized expression systems (insect cells, yeast) with appropriate detergents for solubilization.
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Functional assessment: Traditional enzymatic assays may not be applicable.
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Structural analysis: Membrane proteins are challenging subjects for structural biology.
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Solution: Consider new approaches like cryo-EM of reconstituted protein in nanodiscs or lipid bilayers.
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Interaction studies: Membrane environment can be crucial for proper interactions.
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Solution: Use membrane-based two-hybrid systems or in situ proximity labeling approaches.
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How can researchers effectively analyze contradictory data in VMA21 functional studies?
When faced with contradictory results in VMA21 research, consider:
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Cell type-specific effects: Different cell types may have varying dependency on VMA21 function or express different compensatory factors.
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Approach: Compare results across multiple cell types, including patient-derived cells where available.
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Methodological differences: Variations in experimental conditions can significantly impact outcomes.
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Approach: Standardize key parameters across experiments, particularly detergent types and concentrations when working with membrane proteins.
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Genetic background effects: Particularly in model organisms, background mutations may influence phenotypic outcomes.
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Approach: Use isogenic controls and validate findings in multiple genetic backgrounds.
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Hypomorphic versus null mutations: Different mutations may retain varying degrees of residual function.
