Recombinant Saccharomyces cerevisiae Protein ASI1 (ASI1)
Description
Gene and Protein Information
ASI1 is encoded by the ASI1 gene (Entrez Gene ID: 855147) in Saccharomyces cerevisiae S288C. This protein-coding gene produces a putative ubiquitin-protein ligase with significant functional importance in yeast cellular processes . The full-length protein consists of 624 amino acids and is cataloged in the UniProt database under accession number P54074 . The protein sequence is also referenced as NP_013837.1 in the NCBI protein database .
Protein Stability and Turnover
One notable characteristic of ASI1 is its rapid turnover rate, with a half-life of approximately 30 minutes or less . This dynamic turnover depends on ubiquitin-mediated degradation processes facilitated by nucleus-localized proteasomes . The short half-life suggests that ASI1 levels must be tightly regulated, potentially allowing for rapid cellular responses to changing conditions or stresses that require alterations in nuclear membrane protein composition.
Subcellular Localization
ASI1 is predominantly localized to the inner nuclear membrane, where it functions as part of the nuclear envelope protein quality control system . This specific localization is crucial for its role in regulating the composition and integrity of the nuclear periphery. Unlike some other membrane proteins that shuttle between different cellular compartments, ASI1 appears to be primarily restricted to the inner nuclear membrane, where it performs its specialized functions.
Protein-Protein Interactions
The functional significance of ASI1 is closely tied to its interactions with other proteins. ASI1 and ASI3 are thought to form a heterodimeric E3 ligase complex known as the ASI1-ASI3 complex . This complex constitutes a major component of the inner nuclear membrane-associated degradation (INMAD) pathway. Additionally, ASI1 works in concert with ASI2 to maintain the latent properties of transcription factors Stp1 and Stp2 . These interactions highlight the cooperative nature of ASI1's function within protein complexes.
RNA Interaction Potential
Computational predictions suggest possible interactions between ASI1 and various RNA molecules, although experimental validation of these interactions remains limited . The highest predicted interaction scores are with NSR1 RNA and YML009W-B RNA, with prediction scores of 18.52 and 18.43, respectively . These potential RNA interactions may indicate additional regulatory roles for ASI1 beyond its well-established protein quality control functions.
INMAD Pathway Regulation
ASI1 serves as a key component of the Inner Nuclear Membrane-Associated Degradation (INMAD) pathway, which is distinct from the better-characterized Endoplasmic Reticulum-Associated Degradation (ERAD) pathway . As part of this pathway, ASI1 contributes to the ubiquitination and subsequent degradation of proteins that are either mislocalized to the inner nuclear membrane or require turnover as part of normal cellular processes . This quality control mechanism is essential for maintaining the proper composition and function of the nuclear envelope.
Transcription Factor Regulation
Research has demonstrated that ASI1, in conjunction with ASI2 and ASI3, plays a critical role in maintaining the latent properties of transcription factors Stp1 and Stp2 . In dal81Δ mutants, the repressing activity of the Asi proteins becomes dispensable, indicating that without amplification, the levels of full-length Stp1 and Stp2 that escape cytoplasmic retention are insufficient to activate transcription . This regulatory function highlights ASI1's involvement in gene expression control mechanisms.
Regulation of Membrane Protein Distribution
Beyond its role in protein degradation, ASI1 has been shown to control the levels and distribution of native inner nuclear membrane components . A particularly well-studied example is its effect on the membrane nucleoporin Pom33. Interestingly, loss of ASI1 does not affect Pom33 protein levels but instead alters its distribution in the nuclear envelope through ubiquitination . This suggests that ASI1-dependent ubiquitination can serve purposes beyond protein degradation, such as controlling protein localization and distribution patterns.
Effects of ASI1 Deletion
Deletion of the ASI1 gene (asi1Δ) has provided significant insights into its functional importance. Studies have shown that in asi1Δ mutants, there is an increased presence of proteins at the inner nuclear membrane compared to wild-type cells . Specifically, 21 proteins that normally do not localize to the INM were found there in asi1Δ mutants, 10 were soluble and nucleoplasmic, and 43 were at the INM but showed increased presence in cells lacking ASI1 . This overrepresentation of INM components in asi1Δ mutants (58%) compared to other mutants (34%) strongly suggests that ASI1 specifically affects native INM components.
ASI1-Dependent Ubiquitination
Research using an inducible version of ASI1 fused to the deubiquitinating domain of Herpes Virus UL36 (ASI1-DUb) has demonstrated that ASI1-dependent ubiquitination regulates protein distribution rather than just targeting proteins for degradation . When ASI1-DUb was induced, resulting in the deubiquitination of ASI1 targets, there was an increased frequency and extent of Pom33-GFP puncta formation at the INM, similar to the phenotype observed in asi1Δ mutants . This suggests that ubiquitination by ASI1 helps maintain a uniform distribution of certain proteins in the nuclear envelope.
Differential Functions within the ASI Complex
While ASI1 and ASI3 are thought to form a heterodimeric E3 ligase, research has revealed some unexpected differences in their functions. For instance, the number of Pom33 foci in asi3Δ mutants was not statistically distinguishable from wild-type, whereas it more than doubled in asi1Δ mutants . This observation points to an incomplete understanding of the core INMAD enzymatic machinery and suggests that ASI1 may have functions independent of its partnership with ASI3.
ASI1 as a Model for Membrane Protein Quality Control
The study of recombinant ASI1 provides valuable insights into the mechanisms of protein quality control at cellular membranes. As a component of the INMAD pathway, ASI1 serves as an excellent model for understanding how cells maintain the integrity and proper composition of specialized membrane compartments . This knowledge can be applied to broader questions about protein homeostasis in eukaryotic cells.
Potential Biotechnological Applications
Recombinant ASI1 protein is commercially available for research purposes, with companies offering cDNA ORF clones derived from ASI1 . These resources facilitate further investigation into ASI1's functions and potential applications. Understanding the mechanisms by which ASI1 regulates protein distribution could inform the development of biotechnological tools for controlling protein localization in engineered biological systems.
Comparative Analysis with Related Systems
The study of ASI1 in yeast provides a framework for understanding similar protein quality control systems in more complex eukaryotes. While the specific proteins involved may differ, the fundamental principles of membrane protein regulation through ubiquitination are conserved across species. Thus, insights gained from studying recombinant ASI1 may have broader implications for understanding cellular processes in higher organisms.
Mechanistic Understanding of ASI1 Function
Despite significant progress in characterizing ASI1, several aspects of its function remain incompletely understood. The precise mechanism by which ASI1-dependent ubiquitination regulates protein distribution without affecting protein levels requires further investigation . Additionally, the biochemical basis for the differential effects of ASI1 and ASI3 deletion on certain substrates needs clarification.
Identification of ASI1 Substrates
A comprehensive catalog of ASI1 substrates would provide a more complete understanding of its cellular functions. While some targets, such as Pom33, have been well-characterized , many potential substrates likely remain undiscovered. High-throughput approaches combined with targeted validation studies could help identify the full range of proteins regulated by ASI1.
Therapeutic Relevance
Although ASI1 is a yeast protein, the study of protein quality control systems at nuclear membranes has potential relevance for human health. Many human diseases involve defects in protein quality control or nuclear envelope integrity. Understanding the fundamental mechanisms exemplified by ASI1 could contribute to the development of therapeutic approaches for conditions related to protein misfolding or mislocalization.
Product Specs
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Generally, the shelf life of liquid form is 6 months at -20°C/-80°C. The shelf life of lyophilized form is 12 months at -20°C/-80°C.
Target Background
- Latent precursor forms of Stp1 and Stp2 inefficiently enter the nucleus. Asi1 restricts their binding to SPS sensor-regulated promoters, highlighting the unexpected role of inner nuclear membrane proteins in gene expression control. PMID: 16735580
- In dal81Delta mutants, the repressing activity of the Asi proteins is dispensable, indicating that without amplification, the levels of full-length Stp1 and Stp2 escaping cytoplasmic retention are insufficient to activate transcription. PMID: 17603098
KEGG: sce:YMR119W
STRING: 4932.YMR119W
Q&A
What is ASI1 protein and what is its function in Saccharomyces cerevisiae?
ASI1 (Amino acid sensor-independent protein 1) is a transmembrane protein in Saccharomyces cerevisiae that forms part of the Asi ubiquitin ligase complex together with Asi2 and Asi3. This complex is localized to the inner nuclear membrane and functions primarily in protein quality control.
The ASI1 protein contains a RING domain that confers ubiquitin ligase activity, allowing it to participate in the degradation pathway of mislocalized proteins. Methodologically, the role of ASI1 can be studied through:
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Gene knockout studies examining phenotypic changes
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Fluorescence microscopy to track protein localization
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Co-immunoprecipitation to identify interaction partners
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Proteasome inhibition assays to analyze substrate accumulation
In experimental systems, the deletion of Asi components has been shown to rescue the growth of temperature-sensitive alleles in certain protein complexes, indicating that orphan subunits targeted by the Asi complex at the INM remain folded and competent to assemble functional complexes .
How does the Asi complex (Asi1, Asi2, Asi3) function in protein quality control?
The Asi complex functions through a multi-step process to maintain protein quality control at the inner nuclear membrane:
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Recognition: Asi2 directly binds to substrate transmembrane domains of mislocalized proteins
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Ubiquitination: Asi1 and Asi3, via their RING domains, facilitate ubiquitin attachment to substrates
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Extraction: Ubiquitinated substrates are extracted from the INM by the Cdc48-Npl4-Ufd1 ATPase complex
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Degradation: Extracted proteins are delivered to the proteasome for degradation
Methodologically, this process can be studied using:
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In vivo crosslinking to identify substrate interactions
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In vitro reconstitution of the ubiquitination reaction
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Dominant negative mutations in the Cdc48 pathway to track substrate accumulation
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Quantitative proteomics to identify Asi complex substrates
Research has demonstrated that the recognition of substrates is mediated by direct binding of Asi2 to substrate transmembrane domains. This binding requires a complete Asi complex, as crosslinks between Asi2 and substrates are lost in cells lacking Asi1 or Asi3 .
What expression systems are commonly used for recombinant ASI1 production?
| Expression System | Advantages | Methodological Considerations |
|---|---|---|
| E. coli | High yield, simple cultivation, cost-effective | May require codon optimization, potential for inclusion bodies |
| S. cerevisiae | Native post-translational modifications, proper folding | Lower yields compared to bacterial systems |
| P. pastoris | High expression levels, efficient secretion | Longer development time, complex medium requirements |
| Insect cells | Complex eukaryotic modifications possible | Higher cost, technical complexity |
For effective recombinant ASI1 production, researchers should consider:
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Codon optimization: Adapting codon usage for the expression host
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Fusion tags: Selection of appropriate tags (His, GST, MBP) for purification and solubility
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Expression conditions: Optimization of temperature, induction time, and media composition
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Purification strategy: Developing a protocol that maintains protein stability and activity
For ASI1 specifically, recombinant expression has been successfully performed using E. coli with an N-terminal His tag, with the protein purified and stored in Tris/PBS-based buffer with 6% trehalose (pH 8.0) . Reconstitution in deionized sterile water to a concentration of 0.1-1.0 mg/mL is recommended, with the addition of 5-50% glycerol for long-term storage at -20°C/-80°C .
What are the optimal purification strategies for maintaining ASI1 activity in vitro?
Purification of transmembrane proteins like ASI1 requires specialized approaches to maintain structural integrity and functional activity:
| Purification Step | Methodology | Critical Parameters |
|---|---|---|
| Cell Lysis | Mechanical disruption or detergent-based methods | Buffer composition, protease inhibitors, temperature |
| Membrane Extraction | Detergent screening (DDM, LMNG, digitonin) | Detergent concentration, solubilization time |
| Affinity Chromatography | IMAC for His-tagged ASI1 | Imidazole concentration, flow rate, binding kinetics |
| Size Exclusion | High-resolution columns (Superdex, Sephacryl) | Buffer composition, oligomeric state assessment |
| Concentration | Centrifugal devices with appropriate MWCO | Protein concentration, aggregation monitoring |
For ASI1 specifically, purification as part of the entire Asi complex (Asi1, Asi2, and Asi3) has been achieved through streptavidin-binding peptide tags fused to Asi2 (SBP-Asi2) or through FLAG tag on Asi3 (FLAG-Asi3) . This approach allows for the isolation of the functional complex rather than individual components.
To maintain activity:
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Include stabilizing agents such as trehalose (6%) in storage buffers
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Aliquot purified protein to avoid repeated freeze-thaw cycles
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Store at -20°C/-80°C for long-term preservation
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Consider adding glycerol (5-50% final concentration) to prevent aggregation during freezing
How can researchers investigate ASI1's interactions with substrate transmembrane domains?
Investigating ASI1's interactions with substrate transmembrane domains requires specialized techniques:
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In vivo site-specific photocrosslinking:
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Incorporation of photoreactive amino acid derivatives (e.g., benzoyl-phenylalanine [Bpa]) at specific positions
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UV irradiation to trigger crosslinks between Bpa-labeled probes and proteins in close proximity
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Immunoblotting to identify crosslinked partners
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Fluorescence-based interaction assays:
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FRET (Förster Resonance Energy Transfer) between labeled ASI1 and substrate proteins
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Fluorescence Correlation Spectroscopy to measure binding kinetics
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Bimolecular Fluorescence Complementation to visualize interactions in living cells
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In vitro binding assays:
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Surface Plasmon Resonance to measure real-time binding kinetics
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Pull-down assays with recombinant proteins
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Isothermal Titration Calorimetry for thermodynamic parameters
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Research has demonstrated that both FLAG-Asi1 and Asi2 crosslink robustly with Bpa probes at specific positions within substrate transmembrane domains. The crosslinking efficiency decreases as the probe is moved away from membrane equatorial positions. Furthermore, crosslinks between substrates and Asi2 require a full Asi complex and are lost in cells lacking Asi1 or Asi3 .
What are the current methods for studying the Asi complex's role in protein degradation?
Several complementary approaches can be used to study the Asi complex's role in protein degradation:
| Method | Application | Technical Considerations |
|---|---|---|
| Cycloheximide Chase | Measuring substrate half-life | Requires specific antibodies, quantitative Western blotting |
| Fluorescent Timer Proteins | Real-time degradation monitoring | Requires careful controls for maturation time |
| Ubiquitination Assays | Detecting ubiquitin conjugation | In vitro reconstitution, ubiquitin antibodies |
| Proteasome Inhibition | Substrate accumulation studies | Toxicity considerations, timing optimization |
| Genetic Manipulations | Phenotypic analysis of mutants | Knockout vs. conditional depletion strategies |
A comprehensive approach would combine:
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Genetic manipulations: Creating deletion or conditional mutants of Asi complex components
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Substrate identification: Using quantitative proteomics to identify accumulating proteins
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Degradation kinetics: Measuring half-lives of specific substrates in wild-type and mutant strains
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Functional consequences: Assessing the impact of impaired degradation on cellular processes
Research has shown that acute depletion of components of protein complexes (e.g., using auxin-based degradation systems) can lead to reduced half-lives of their binding partners, with this degradation being inhibited in Asi mutant cells. Additionally, deletion of Asi components has been shown to rescue the growth of temperature-sensitive alleles in certain protein complexes, indicating that unassembled subunits targeted by the Asi complex remain competent to form functional complexes .
How does ASI1 specifically recognize mislocalized proteins at the inner nuclear membrane?
The specific recognition of mislocalized proteins by ASI1 at the inner nuclear membrane involves:
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Direct binding to transmembrane domains:
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Asi2 component of the complex directly interacts with substrate TMDs
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This binding facilitates substrate ubiquitination by Asi1 and Asi3
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The recognition appears to be position-specific within the membrane
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Methodological approaches to study this mechanism:
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Site-specific photocrosslinking at defined positions within substrate TMDs
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Mutational analysis of both substrate and Asi complex components
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Structural studies of the Asi complex-substrate interaction interface
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In vitro reconstitution of binding using purified components
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Research using in vivo crosslinking has demonstrated that Asi complex components crosslink robustly with substrate TMDs at specific positions (e.g., positions 36 and 39), with the interaction strength decreasing at positions away from the membrane equator. This suggests a specific recognition mechanism related to the positioning of amino acids within the membrane .
The complete Asi complex is required for this recognition, as crosslinks between substrates and Asi2 are lost in cells lacking Asi1 or Asi3, even though Asi2 expression levels remain unchanged in these mutants .
What experimental approaches can be used to study the spatial segregation of protein assembly and quality control processes involving ASI1?
Studying the spatial segregation between protein assembly in the bulk ER and quality control at the INM involving ASI1 requires specialized approaches:
| Approach | Methodology | Applications |
|---|---|---|
| Super-resolution Microscopy | STORM, PALM, or STED imaging | Visualizing nanoscale spatial distribution of complexes |
| Single-molecule Tracking | Fluorescent tagging with sensitive detection | Following protein movement between compartments |
| Compartment-specific Tagging | Spatially restricted enzymatic labeling | Identifying proteins in specific cellular locations |
| Proximity Labeling | BioID or APEX2 fusion proteins | Mapping local protein neighborhoods |
| Organelle Fractionation | Differential centrifugation, density gradients | Biochemical separation of cellular compartments |
A comprehensive experimental strategy would involve:
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Visualizing spatial distribution: Using fluorescently tagged ASI1, Asi2, and Asi3 with super-resolution microscopy to confirm INM localization
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Tracking substrate movement: Developing reporters that allow visualization of substrate trafficking between bulk ER and INM
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Temporal studies: Using pulse-chase approaches to determine the kinetics of substrate recognition and degradation
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Perturbation experiments: Manipulating the nuclear transport machinery to alter the distribution of potential substrates
The concept that "spatial segregation of the two processes, protein assembly (in the bulk ER) and quality control (at the INM), may facilitate efficient complex assembly" is supported by experiments showing that restricting quality control to the INM spares subunits from premature degradation and offers them more time to find their partners .
What are the challenges in reconstituting the Asi-mediated ERAD system in vitro?
Reconstituting the Asi-mediated ERAD (Endoplasmic Reticulum-Associated Degradation) system in vitro presents several technical challenges:
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Membrane protein purification:
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Maintaining native conformation of transmembrane components
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Selecting appropriate detergents or nanodiscs for solubilization
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Preventing aggregation during purification procedures
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Multi-component complex assembly:
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Ensuring proper stoichiometry of Asi1, Asi2, and Asi3
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Reproducing post-translational modifications present in vivo
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Creating a suitable membrane environment for complex function
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Substrate preparation:
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Generating membrane-embedded substrates with correct topology
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Maintaining substrate in a recognition-competent state
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Labeling substrates for detection without impairing recognition
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Assay development:
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Establishing readouts for substrate binding, ubiquitination, and extraction
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Distinguishing between specific and non-specific interactions
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Confirming physiological relevance of in vitro observations
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Despite these challenges, in vitro reconstitution of the Asi-mediated ERAD system has been achieved by purifying the Asi complex (Asi1, Asi2, and Asi3) from S. cerevisiae through affinity tags, along with recombinantly expressed components of the ubiquitination machinery (Uba1, Ubc4, Ubc7, and Cue1) and fluorescently labeled substrate proteins. This system has successfully recapitulated the recognition, ubiquitination, and retrotranslocation of membrane-bound substrates .
