VPS33A Antibody

What is VPS33A and what are its primary cellular functions?

VPS33A (Vacuolar protein sorting-associated protein 33A) plays a critical role in vesicle-mediated protein trafficking to lysosomal compartments. It functions in both endocytic membrane transport and autophagic pathways. As a core component of the HOPS (homotypic fusion and protein sorting) and CORVET (class C core vacuole/endosome tethering) complexes, VPS33A mediates tethering and docking events during SNARE-mediated membrane fusion. These complexes are proposed to be involved in the Rab5-to-Rab7 endosome conversion .

VPS33A is essential for:

• Fusion of endosomes and autophagosomes with lysosomes

• Regulation of late endocytic, phagocytic and autophagic traffic towards lysosomes

• SNARE complex assembly for membrane fusion events

The function of VPS33A in autophagosome-lysosome fusion involves syntaxin 17 (STX17) but not UV radiation resistance-associated gene protein (UVRAG) .

How is VPS33A involved in different protein complexes?

VPS33A is a critical component of two distinct hexameric protein complexes:

• HOPS Complex (Homotypic fusion and protein sorting):

  • Consists of six subunits: VPS11, VPS16, VPS18, VPS33A, VPS39, and VPS41

  • Recruited to Rab7 on late endosomal membranes

  • Regulates traffic toward lysosomes

• CORVET Complex (Class C core vacuole/endosome tethering):

  • Functions as a Rab5 effector

  • Mediates early endosome fusion in specific endosome subpopulations

VPS33A serves as the SM (Sec1/Munc18) protein within these complexes, providing template functionality for SNARE assembly. The recruitment of VPS33A to the HOPS complex via its interaction with VPS16 is crucial for endosome-lysosome fusion in mammalian cells .

What are the predicted and observed molecular weights for VPS33A?

When investigating VPS33A via Western blot, researchers should be aware of both predicted and observed molecular weights:

• Predicted molecular weight: 67.6-68 kDa

• Observed molecular weight: Typically between 66-68 kDa depending on experimental conditions

In a Western blot study using anti-VPS33A antibody (ab88254) at 1 μg/mL:

• VPS33A in human pancreas tissue lysate showed the predicted 68 kDa band

• VPS33A in HepG2 cell lysate demonstrated the predicted 68 kDa band

• In VPS33A-transfected 293T cell lines, a band was observed at 66 kDa versus the predicted 68 kDa

These slight variations highlight the importance of proper controls when interpreting Western blot results.

What are the optimal applications and dilutions for VPS33A antibodies?

VPS33A antibodies have been successfully validated for multiple applications. Based on the search results, here are the recommended applications and dilution ranges:

For optimal results in Western blotting, multiple sources recommend using antibody concentrations of approximately 1 μg/mL . The actual working concentration may vary based on sample type and experimental conditions, so optimization is recommended for each specific application .

When performing immunoprecipitation experiments, it's advisable to use 0.5-4.0 μg of antibody for 1.0-3.0 mg of total protein lysate to achieve sufficient pull-down efficiency .

How can I validate the specificity of VPS33A antibodies?

Validating antibody specificity is critical for reliable research results. For VPS33A antibodies, consider these validation approaches:

• Positive control testing: Use tissues/cell lines known to express VPS33A:

  • Validated positive samples include: human brain tissue, human cerebellum tissue, HepG2 cells, and NIH/3T3 cells

• Knockdown/knockout validation:

  • Perform siRNA knockdown of VPS33A and confirm reduction in signal

  • Use CRISPR/Cas9 knockout cells as definitive negative controls

• Overexpression systems:

  • Compare VPS33A-transfected versus non-transfected cell lines

  • Confirm increased signal in tagged-VPS33A expressing cells

• Cross-validation with multiple antibodies:

  • Use antibodies targeting different epitopes of VPS33A

  • Compare staining patterns across multiple detection methods

• Western blot validation:

  • Confirm single band at expected molecular weight (approximately 68 kDa)

  • Evaluate signal reduction in knockdown samples

For example, in one study, lysates from control and VPS33A-knockdown cells were analyzed by immunoblotting with specific VPS33A antibodies, with actin serving as a loading control to confirm knockdown efficiency of approximately 80% .

How can VPS33A antibodies be used to study protein-protein interactions within the HOPS complex?

VPS33A antibodies are valuable tools for investigating protein-protein interactions within the HOPS and CORVET complexes. Here are methodological approaches:

• Co-immunoprecipitation (Co-IP):

  • Immunoprecipitate VPS33A and probe for other HOPS components

  • Example protocol: Lysates from cells expressing HA-tagged VPS16 variants can be subjected to immunoprecipitation with anti-HA affinity matrix, followed by immunoblotting for endogenous VPS33A and VPS18 to assess complex formation

• Proximity ligation assays:

  • Detect in situ protein-protein interactions between VPS33A and other HOPS components

  • Particularly useful for visualizing interactions in specific cellular compartments

• GST pull-down assays:

  • Use purified GST-tagged VPS33A to pull down interaction partners

  • One study showed purified VPS33A-GST can pull down myc-VPS16 variants produced by in vitro transcription/translation

• Yeast two-hybrid screening:

  • Identify novel interaction partners or map interaction domains

  • Can be used to validate mutations that disrupt specific protein-protein interactions

• Structural analysis complementation:

  • Based on the crystal structure of human VPS33A, specific residues like K428, Y438, and I441 are critical for interaction with VPS16

  • Mutational analysis of these residues can determine their importance in complex formation

Studies have shown that mutations in key residues disrupt the VPS33A-VPS16 interaction. For example, mutations K428D, Y438D, or I441K in VPS33A prevent co-immunoprecipitation with wild-type HA-VPS16, confirming that these residues are required for the interaction with VPS16 in cells .

What is the role of VPS33A in lysosomal disease pathology and how can antibodies help investigate this?

VPS33A mutations have been linked to a rare lysosomal disease resembling mucopolysaccharidosis (MPS) with unusual systemic features. VPS33A antibodies are crucial for investigating disease mechanisms:

• Investigating VPS33A mutant protein stability:

  • The R498W mutation in VPS33A associated with MPS-like disease can be studied using antibodies to detect protein levels

  • Immunoblotting of patient-derived fibroblasts showed reduced abundance of full-length VPS33A and other HOPS/CORVET components

  • Treatment with proteasome inhibitors (e.g., MG-132, bortezomib) rescued mutant protein from degradation, suggesting destabilization as a disease mechanism

• Characterizing cellular phenotypes:

  • VPS33A antibodies can visualize endosomal/lysosomal compartments in patient cells

  • Patient fibroblasts show vacuolation with disordered endosomal/lysosomal compartments and abnormal endocytic trafficking of lactosylceramide

  • Immunofluorescence studies can reveal colocalization patterns with organelle markers

• Therapeutic screening:

  • Antibodies enable assessment of potential therapeutic interventions

  • For example, measuring VPS33A protein recovery after treatment with proteasome inhibitor bortezomib or eliglustat (a glucosylceramide synthesis inhibitor)

• Structure-function analysis:

  • Based on the 3D crystal structure of human VPS33A, the R498W mutation is predicted to destabilize protein folding

  • Immunoblotting confirmed reduced abundance of mutant VPS33A and other HOPS components

The pathological mechanism involves diminished intracellular abundance of intact VPS33A, leading to impaired endosome-lysosome and autophagosome-lysosome fusion. These defects result in accumulation of glycosaminoglycans (GAGs) and other substrates characteristic of mucopolysaccharidosis disorders .

How can VPS33A antibodies be used to investigate autophagy pathways?

VPS33A plays a critical role in autophagosome-lysosome fusion, making antibodies against this protein valuable tools for autophagy research:

• Autophagosome accumulation assays:

  • Immunostaining for endogenous LC3 (autophagosome marker) in cells with VPS33A depletion

  • Studies show significant accumulation of LC3-positive autophagosomes after depletion of VPS33A or VPS16

  • Quantification methods include:

    • Fluorescence signal measurement by confocal microscopy

    • Automated microscopy for high-throughput analysis

• Investigating VPS33A-dependent fusion machinery:

  • Co-immunoprecipitation with VPS33A antibodies can identify interactions with autophagy-related proteins

  • VPS33A's role in autophagosome-lysosome fusion involves STX17 but not UVRAG

• Differential roles of VPS33A versus VPS33B:

  • Comparative knockdown studies using antibodies against both proteins

  • Research shows VPS33A and VPS16 (but not VIPAR or VPS33B) are required for autophagosome-lysosome fusion

• Rescue experiments:

  • VPS33A antibodies can validate expression of wild-type or mutant proteins in rescue experiments

  • For example, stable expression of VPS33A in knockdown cells can be confirmed by immunoblotting

• Autophagic flux measurement:

  • Monitoring changes in LC3-I to LC3-II conversion in the presence/absence of VPS33A

  • Combining with lysosomal inhibitors to assess blockage points in autophagy

In one study, researchers showed that recruitment of VPS33A to the HOPS complex via its interaction with VPS16 is crucial not only for endosome-lysosome fusion but also for autophagosome-lysosome fusion. This was demonstrated by quantifying LC3-positive structures in cells depleted of VPS33A compared to control cells treated with non-targeting siRNA .

What are common technical challenges when working with VPS33A antibodies and how can they be addressed?

Working with VPS33A antibodies may present several technical challenges. Here are common issues and recommended solutions:

• Specificity concerns:

  • Problem: Cross-reactivity with related proteins like VPS33B

  • Solution: Use antibodies raised against unique epitopes of VPS33A; validate specificity using knockdown/knockout controls; perform peptide competition assays

• Variable detection sensitivity:

  • Problem: Inconsistent detection across different cell types or tissues

  • Solution: Optimize antibody concentration for each sample type; adjust extraction methods to enhance VPS33A solubilization; use fresh samples as VPS33A may degrade during storage

• Immunoprecipitation efficiency:

  • Problem: Low pull-down efficiency in co-IP experiments

  • Solution: Increase antibody amount (0.5-4.0 μg for 1-3 mg of total protein); use mild lysis buffers to preserve protein-protein interactions; crosslink antibodies to beads for cleaner results

• Background in immunofluorescence:

  • Problem: High background signal in IF applications

  • Solution: Extend blocking time; use alternative blocking agents; optimize fixation methods; increase washing steps; dilute primary antibody (1:50-1:400)

• Detecting mutant VPS33A variants:

  • Problem: Reduced detection of disease-associated mutants (e.g., R498W)

  • Solution: Use antibodies targeting N-terminal epitopes that are less affected by C-terminal mutations; consider using proteasome inhibitors to stabilize mutant protein before detection

• Distinguishing between HOPS/CORVET complex-associated versus free VPS33A:

  • Problem: Difficulty determining if detected VPS33A is complex-bound

  • Solution: Use size exclusion chromatography before immunoblotting; perform sequential immunoprecipitation with antibodies against other complex components

For successful detection in Western blots, researchers have found that using antibody concentrations of approximately 1 μg/mL with standard SDS-PAGE and transfer conditions works well for detecting the ~68 kDa VPS33A protein in most mammalian samples .

How do mutations in key residues affect VPS33A detection and function?

Mutations in VPS33A can impact both protein detection and function in experimental systems:

• Key functional residues and their effects:

  Mutation	Location	Functional Impact	Detection Considerations
  K428D	VPS16 binding domain	Disrupts VPS16 interaction	Detectable but not in HOPS complex
  Y438D	VPS16 binding domain	Disrupts VPS16 interaction	Detectable but not in HOPS complex
  I441K	VPS16 binding domain	Disrupts VPS16 interaction	Detectable but not in HOPS complex
  R498W	Protein folding domain	Destabilizes protein structure	Reduced detection due to increased degradation

• R498W disease mutation effects:

  • The R498W mutation leads to reduced abundance of full-length VPS33A and other HOPS components

  • Based on the crystal structure, this mutation destabilizes VPS33A folding

  • Treatment with proteasome inhibitors can rescue the mutant protein from degradation

• Detection strategies for mutant proteins:

  • When studying VPS33A mutants, consider:

    • Using epitope tags (HA, Myc) for reliable detection

    • Employing proteasome inhibitors before lysis to prevent degradation

    • Comparing antibodies targeting different epitopes to ensure detection

• Functional assessment using antibodies:

  • Co-immunoprecipitation experiments can determine if mutations disrupt specific protein interactions

  • For example, while WT Vps33A-Myc co-immunoprecipitates with WT HA-Vps16, mutant forms (K428D, Y438D, or I441K) fail to do so

  • Antibodies can help validate if mutants are incorporated into the HOPS/CORVET complexes

• Disease-relevant experimental designs:

  • For studying MPS-like disease mechanisms, antibodies can monitor VPS33A levels in patient-derived fibroblasts

  • Comparing antibody detection before and after treatment with proteasome inhibitors can reveal potential therapeutic approaches

Understanding how mutations affect VPS33A detection is critical for accurate interpretation of experimental results, particularly when studying disease-associated variants or conducting structure-function analyses.

How can VPS33A antibodies be utilized to investigate novel therapeutic approaches for lysosomal disorders?

VPS33A antibodies are instrumental in developing and evaluating therapeutic strategies for VPS33A-associated lysosomal disorders:

• Proteasome inhibitor therapy assessment:

  • VPS33A antibodies can monitor protein levels before and after treatment with proteasome inhibitors

  • Studies showed that bortezomib, a clinically approved proteasome inhibitor, rescued mutant VPS33A (R498W) from degradation

  • Immunoblotting confirmed increased VPS33A protein levels after treatment

• Substrate reduction therapy evaluation:

  • VPS33A antibodies can assess the effects of eliglustat (a glucosylceramide synthesis inhibitor) on cellular phenotypes

  • Research demonstrated that eliglustat partially corrected impaired lactosylceramide trafficking in patient-derived fibroblasts

• Gene therapy vector validation:

  • Antibodies allow confirmation of successful gene transfer and expression levels

  • Detection of wild-type VPS33A expression in mutant cells after viral transduction

• Pharmacological chaperone screening:

  • VPS33A antibodies can identify compounds that stabilize mutant proteins

  • Detection of increased protein levels and improved subcellular localization

• Monitoring therapeutic outcomes:

  • Tracking changes in VPS33A-dependent pathways:

    • Endosome-lysosome fusion efficiency

    • Autophagosome clearance

    • Glycosaminoglycan accumulation reduction

In a key study, researchers found that treating patient-derived fibroblasts with bortezomib partially corrected the impaired lactosylceramide trafficking defect, suggesting a therapeutic avenue for this fatal orphan disease. Importantly, both proteasome inhibition and substrate reduction approaches showed promise, highlighting multiple potential intervention strategies .

What are the methodological considerations for using VPS33A antibodies in advanced imaging techniques?

When employing VPS33A antibodies for advanced imaging applications, researchers should consider these methodological aspects:

• Super-resolution microscopy optimization:

  • For techniques like STED, STORM, or PALM:

    • Use highly specific primary antibodies with minimal cross-reactivity

    • Select smaller fluorophore-conjugated secondary antibodies

    • Consider directly conjugated primary antibodies to reduce distance to target

    • Optimize fixation to preserve membrane structures where VPS33A functions

• Live-cell imaging approaches:

  • For dynamic studies of VPS33A:

    • Use anti-GFP/RFP antibodies with VPS33A fusion constructs

    • Validate that tagged constructs maintain native interactions

    • Employ nanobodies for reduced size and better penetration

    • Consider inducible expression systems to control protein levels

• Correlative light and electron microscopy (CLEM):

  • For ultrastructural localization:

    • Use gold-conjugated secondary antibodies for EM detection

    • Apply pre-embedding labeling for better epitope preservation

    • Validate specificity with knockout controls

    • Consider proximity-labeling approaches (APEX2, HRP) fused to VPS33A

• Multi-color co-localization studies:

  • For examining VPS33A within HOPS/CORVET complexes:

    • Select antibodies raised in different species to avoid cross-reactivity

    • Perform sequential staining with thorough washing

    • Include appropriate controls for fluorophore bleed-through

    • Consider spectral unmixing for closely emitting fluorophores

• Intracellular trafficking studies:

  • For tracking VPS33A dynamics:

    • Combine with organelle markers (Rab5, Rab7, LAMP1)

    • Use pulse-chase approaches with endocytic cargo

    • Employ photoactivatable or photoconvertible fusion proteins

    • Consider FRAP (Fluorescence Recovery After Photobleaching) to measure mobility

• Sample preparation optimization:

  • Different fixation methods may affect epitope accessibility:

    • Paraformaldehyde (4%) works well for most applications

    • Methanol fixation may improve access to certain epitopes

    • Avoid glutaraldehyde if possible as it can increase autofluorescence

    • Test different permeabilization agents (Triton X-100, saponin, digitonin)

When imaging VPS33A in the context of lysosomal diseases, researchers have successfully used immunofluorescence to demonstrate vacuolation and disordered endosomal/lysosomal compartments in patient fibroblasts, providing insights into disease pathology .