ETR3 Antibody

What is ETR3 and what are its key functional characteristics?

ETR3 is an RNA-binding protein that regulates several post-transcriptional events including alternative splicing, mRNA translation, and stability . The protein contains three RNA recognition motifs (RRMs) - two at the N-terminus and one at the C-terminus - with a 210-amino acid divergent domain between them . ETR3 exhibits preferential binding to UG-rich sequences, particularly UG repeats and UGUU motifs, as well as CUG triplet repeats in the 3'-UTR of transcripts such as DMPK . It plays key roles in tissue-specific and developmentally regulated alternative splicing, with specific functions in exon inclusion/exclusion, such as activating TNNT2 exon 5 inclusion in embryonic but not adult skeletal muscle .

As a regulator of apoptosis, ETR3 modulates the cellular apoptosis program by regulating COX2-mediated prostaglandin E2 (PGE2) expression . Additionally, ETR3 may function as a specific regulator of miRNA biogenesis, as it binds to primary microRNA pri-MIR140 and, together with CELF1, negatively regulates the processing to mature miRNA .

How is the subcellular localization of ETR3 regulated?

ETR3 exhibits a complex subcellular distribution pattern controlled by multiple domains within the protein. When fused to GFP, ETR3 is predominantly nuclear . This localization is regulated by:

• A strong nuclear localization signal in the C-terminus that overlaps with the third RRM, capable of redirecting a normally cytoplasmic protein (pyruvate kinase) to the nucleus

• Nuclear localization and export activities within the divergent domain

• The nuclear export activity functioning through a CRM1-dependent pathway (sensitive to leptomycin B)

• Regions within the first two RRMs that contribute to cytoplasmic localization

This sophisticated regulation system allows ETR3 to shuttle between nuclear and cytoplasmic compartments, enabling it to participate in both nuclear RNA processing and cytoplasmic translation regulation. Immunofluorescence studies confirm this distribution pattern, showing both nuclear and cytoplasmic staining in cell lines such as HeLa and MCF7 .

What are the validated applications for ETR3 antibodies?

ETR3 antibodies have been validated for multiple research applications:

• Western blot (WB): The antibody detects a single band at the predicted size of 54 kDa in C6 and NIH 3T3 cell lysates at 1/5000 dilution

• Immunohistochemistry (IHC-P): Successful detection in paraffin-embedded human brain and rat lung tissues at 1/100 dilution following heat-mediated antigen retrieval with EDTA buffer pH 9

• Immunocytochemistry/Immunofluorescence (ICC/IF): Distinct nuclear and cytoplasmic staining in HeLa and MCF7 cells at 1/500 dilution

• Immunoprecipitation (IP): Effective pulldown of ETR3 from HeLa cell lysates at 1/50 dilution

• Flow cytometry (intracellular): Detection of ETR3 in permeabilized cells

These applications provide researchers with a comprehensive toolkit for studying ETR3 expression, localization, and interactions in various experimental systems.

How do the structural domains of ETR3 contribute to its RNA processing functions?

ETR3's domain architecture is intricately linked to its functional capabilities in RNA processing:

The two N-terminal RRMs and single C-terminal RRM are primarily responsible for RNA binding specificity . These domains recognize specific RNA motifs including UG-rich sequences, UG repeats, UGUU motifs, CUG triplet repeats, and AU-rich sequences . The RRMs allow ETR3 to bind to the muscle-specific splicing enhancer (MSE) intronic sites flanking the TNNT2 alternative exon 5, thereby regulating its inclusion during development .

The divergent domain, while less characterized for RNA binding, plays essential roles in protein-protein interactions and subcellular trafficking . This region contains both nuclear localization and export activities, allowing ETR3 to shuttle between compartments . The C-terminal region contains a strong nuclear localization signal that overlaps with the third RRM, ensuring nuclear accumulation for splicing regulation .

Functional studies have demonstrated that both the C-terminus and regions within the divergent domain are critical for ETR3's splicing activity . These domains likely facilitate interactions with the splicing machinery or other regulatory proteins to promote or repress exon inclusion.

What experimental approaches can distinguish ETR3 from other CELF family members?

Distinguishing ETR3 from other CELF family proteins requires careful experimental design:

• Antibody selection: Use highly specific monoclonal antibodies that recognize unique epitopes in ETR3. The Abcam recombinant monoclonal antibody [EPR13374-2] has been validated for specificity across multiple applications .

• Western blot analysis: ETR3 has an observed band size of 54 kDa , which can be distinguished from other CELF family members with different molecular weights.

• Immunoprecipitation followed by mass spectrometry: Perform IP with the validated antibody (1/50 dilution for the Abcam antibody) followed by mass spectrometry to confirm the identity of the pulled-down protein.

• RNA binding profile analysis: ETR3 has characteristic binding preferences for UG-rich sequences, UG repeats, and UGUU motifs . RNA immunoprecipitation followed by sequencing (RIP-seq) can identify the specific RNA targets bound by ETR3.

• Subcellular localization patterns: ETR3 exhibits a specific nucleocytoplasmic distribution regulated by its unique domains . Immunofluorescence with careful co-localization studies can help distinguish it from other family members.

• Functional assays: ETR3 specifically activates TNNT2 exon 5 inclusion in embryonic but not adult skeletal muscle . Such functional tests can reveal ETR3-specific activities.

• Knockdown validation: siRNA targeting ETR3-specific sequences can confirm antibody specificity by demonstrating reduced signal in knockdown samples.

What are the optimal immunohistochemistry conditions for detecting ETR3 in fixed tissues?

For successful immunohistochemical detection of ETR3 in fixed tissues, the following optimized protocol has been validated:

• Tissue preparation: Paraffin embedding has been validated for human brain and rat lung tissues .

• Antigen retrieval: Heat-mediated antigen retrieval with EDTA buffer at pH 9 is critical and must be performed before commencing the staining protocol . This step is essential for unmasking the ETR3 epitopes that may be obscured during fixation.

• Antibody dilution and incubation: The optimal dilution for the Abcam ETR3 antibody [EPR13374-2] is 1/100 for IHC-P applications . Incubate sections with the primary antibody at this dilution for optimal signal-to-noise ratio.

• Detection system: Use a prediluted HRP Polymer for Rabbit IgG as the secondary detection reagent . This system provides amplified signal while maintaining low background.

• Counterstaining: Hematoxylin counterstaining provides nuclear context that helps interpret ETR3's subcellular distribution .

• Controls: Include both positive controls (human brain and rat lung tissues have been validated ) and negative controls (primary antibody omission) in each experiment.

• Visualization: Bright-field microscopy with appropriate magnification (40-100×) to observe both nuclear and cytoplasmic staining patterns characteristic of ETR3.

This protocol ensures specific and reproducible detection of ETR3 in tissue sections while minimizing background and non-specific signals.

How can ETR3-RNA interactions be studied using antibody-based approaches?

ETR3-RNA interactions can be comprehensively studied using several antibody-based techniques:

• RNA Immunoprecipitation (RIP): This technique uses the validated ETR3 antibody (recommended at 1/50 dilution based on IP protocols ) to pull down ETR3-RNA complexes. The associated RNAs can then be identified by RT-PCR, microarray, or sequencing. This approach has been valuable for confirming ETR3's binding to targets like TNNT2, apoB mRNA, and COX2 mRNA .

• Cross-linking Immunoprecipitation (CLIP): By incorporating UV cross-linking, CLIP provides higher resolution of direct binding sites. The ETR3 antibody can be used to immunoprecipitate cross-linked RNA-protein complexes, followed by partial RNA digestion and sequencing to identify binding motifs with nucleotide resolution.

• Photoactivatable Ribonucleoside-Enhanced CLIP (PAR-CLIP): This refined version of CLIP incorporates photoreactive nucleosides into RNAs, enhancing cross-linking efficiency and allowing precise mapping of ETR3 binding sites.

• RNA Electrophoretic Mobility Shift Assay (EMSA) with Supershift: Using labeled RNA containing UG-rich sequences, UG repeats, or UGUU motifs (known ETR3 binding preferences ), perform EMSA followed by addition of the ETR3 antibody to create a supershift that confirms ETR3-RNA interaction specificity.

• Immunofluorescence Combined with RNA FISH: Co-localization of ETR3 protein (detected by immunofluorescence using the antibody at 1/500 dilution ) with specific RNAs detected by fluorescence in situ hybridization can provide spatial information about these interactions within cells.

• In Vivo RNA Binding Assay: Transfect cells with tagged target RNAs, perform immunoprecipitation with the ETR3 antibody, and analyze associated RNAs to confirm binding in cellular contexts.

• RNA Pulldown Followed by Western Blot: Biotinylated RNA containing ETR3 binding motifs can be used to pull down interacting proteins, followed by Western blot detection using the ETR3 antibody (1/5000 dilution ) to confirm the interaction.

What roles does ETR3 play in the pathogenesis of RNA processing disorders?

ETR3's functions in RNA processing suggest significant implications in several disorders:

• Myotonic Dystrophy: ETR3 binds to (CUG)n triplet repeats in the 3'-UTR of transcripts such as DMPK . In myotonic dystrophy type 1, CUG repeat expansions in DMPK may sequester ETR3 and other RNA-binding proteins, leading to dysregulated splicing of multiple targets. Understanding ETR3's interactions with these expanded repeats can provide insights into disease mechanisms.

• Neurodevelopmental Disorders: ETR3 promotes inclusion of exonS 21 and exclusion of exon 5 of the NMDA receptor R1 pre-mRNA . Dysregulation of this splicing event could affect NMDA receptor function, potentially contributing to neurological disorders characterized by excitotoxicity or synaptic dysfunction.

• Muscular Disorders: ETR3 activates TNNT2 exon 5 inclusion in embryonic, but not adult, skeletal muscle and binds to muscle-specific splicing enhancer (MSE) sites . Alterations in ETR3 function could disrupt the developmentally regulated splicing program in muscle, contributing to congenital myopathies.

• Cancer Progression: ETR3 increases COX2 mRNA stability and inhibits its translation in epithelial cells after radiation injury . It also modulates the cellular apoptosis program through regulation of COX2-mediated prostaglandin E2 expression . These functions suggest ETR3 could influence cancer progression through effects on inflammation and apoptosis pathways.

• Heart Development and Disease: ETR3 was originally cloned from chicken heart , suggesting important cardiac functions. Its splicing regulatory activities could influence cardiac-specific isoforms of various proteins, potentially contributing to cardiomyopathies when dysregulated.

• RNA Editing Disorders: ETR3 is involved in apoB RNA editing activity . Disruption of this function could affect lipid metabolism and contribute to metabolic disorders.

Research approaches for studying ETR3 in these disorders include analysis of expression and localization in patient samples, identification of aberrant splicing events correlating with ETR3 dysfunction, and development of animal models with altered ETR3 expression or function.

What are the validated protocols for ETR3 protein detection by Western blot?

For optimal Western blot detection of ETR3, the following validated protocol is recommended:

• Sample preparation:

  • Use cell lysates from validated positive control lines such as C6, NIH 3T3, or HeLa cells

  • Load approximately 10 μg of total protein per lane

  • Include protease inhibitors in lysis buffers to prevent degradation

• Gel electrophoresis and transfer:

  • Use 10-12% polyacrylamide gels for optimal resolution around the 54 kDa range

  • Transfer to PVDF or nitrocellulose membrane using standard protocols

• Blocking and antibody incubation:

  • Block membrane with 5% non-fat dry milk or BSA in TBST

  • Primary antibody: Use the validated Abcam ETR3 antibody at 1/5000 dilution

  • Secondary antibody: Apply Goat Anti-Rabbit IgG (H+L) Peroxidase conjugate at 1/1000 dilution

• Detection and analysis:

  • Look for a single specific band at the expected molecular weight of 54 kDa

  • For quantitative analysis, normalize to appropriate loading controls

• Controls and validation:

  • Positive controls: C6 and NIH 3T3 cell lysates have been validated

  • Negative control: Consider using lysate from a cell line with ETR3 knockdown

  • Size verification: Include molecular weight markers spanning the 50-60 kDa range

This protocol consistently produces specific detection of ETR3 with minimal background, enabling reliable protein expression analysis across different experimental conditions.

How should immunofluorescence protocols be optimized for ETR3 subcellular localization studies?

To accurately visualize ETR3's complex subcellular distribution, the following optimized immunofluorescence protocol has been validated:

• Cell preparation and fixation:

  • Culture cells on glass coverslips to 70-80% confluence

  • Fix with 4% paraformaldehyde to preserve cellular architecture

  • Permeabilize with 0.1% Triton X-100 to allow antibody access to intracellular ETR3

• Blocking and antibody incubation:

  • Block with 3-5% BSA or normal serum in PBS to reduce non-specific binding

  • Primary antibody: Apply the Abcam ETR3 antibody at 1/500 dilution

  • Secondary antibody: Use Goat anti-rabbit IgG with fluorescent conjugate (e.g., Alexa Fluor® 555) at 1/200 dilution

  • Counterstain nuclei with DAPI to provide context for assessing nuclear/cytoplasmic distribution

• Cell types and positive controls:

  • HeLa and MCF7 cells have been validated for ETR3 immunofluorescence detection

  • These cell lines show characteristic nuclear and cytoplasmic ETR3 staining patterns

• Imaging considerations:

  • Use confocal microscopy for detailed subcellular localization analysis

  • Capture z-stacks to fully assess three-dimensional distribution

  • Maintain consistent exposure settings between experimental and control samples

• Controls and validation:

  • Negative control: PBS only instead of primary antibody

  • Specificity control: ETR3 knockdown cells to confirm antibody specificity

  • Co-localization markers: Consider including markers for specific subcellular compartments

• Advanced approaches:

  • Time-lapse imaging of live cells expressing fluorescently-tagged ETR3 to study dynamic trafficking

  • Fluorescence recovery after photobleaching (FRAP) to assess mobility between compartments

  • Treatment with leptomycin B to block CRM1-dependent nuclear export and observe changes in localization

This protocol enables detailed characterization of ETR3's nucleocytoplasmic distribution and can reveal changes in localization under different experimental conditions.

What is the optimal approach for ETR3 immunoprecipitation from complex samples?

For successful immunoprecipitation of ETR3 and its associated complexes, the following validated protocol is recommended:

• Sample preparation:

  • Use fresh cell lysates prepared under non-denaturing conditions

  • HeLa cells have been validated as a positive control system for ETR3 IP

  • Include protease and phosphatase inhibitors in lysis buffers

  • For RNA-protein complex studies, add RNase inhibitors

• Pre-clearing:

  • Pre-clear lysates with protein A/G beads to reduce non-specific binding

  • Rotate for 1 hour at 4°C, then remove beads by centrifugation

• Antibody binding:

  • Use the Abcam ETR3 antibody at 1/50 dilution for IP applications

  • Incubate cleared lysates with antibody overnight at 4°C with gentle rotation

  • Add fresh protein A/G beads and continue incubation for 1-2 hours

• Washing and elution:

  • Perform stringent washing steps (typically 4-5 washes) to remove non-specific binding

  • Elute bound proteins using SDS sample buffer for Western blot analysis

  • For protein-interaction studies, consider native elution with peptide competition

• Detection and analysis:

  • For Western blot detection after IP, use Anti-Rabbit IgG (HRP) specific to the non-reduced form of IgG at 1/1500 dilution

  • The predicted and observed band size for ETR3 is 54 kDa

• Controls and validation:

  • Negative control: Use PBS instead of primary antibody or cell lysate

  • Isotype control: Use non-specific rabbit IgG to assess background

  • Input sample: Include pre-IP lysate (5-10%) to confirm target protein presence

• Specialized applications:

  • RNA immunoprecipitation (RIP): Include RNase inhibitors and extract RNA from complexes

  • Co-IP for protein interactions: Probe membranes for known or suspected interaction partners

  • Mass spectrometry: Analyze eluted proteins to identify novel interaction partners

This protocol enables robust isolation of ETR3 and its associated complexes, supporting diverse experimental applications from protein interaction studies to RNA binding analyses.

How can ETR3 antibodies be validated in knockdown/knockout experimental systems?

Comprehensive validation of ETR3 antibodies in knockdown/knockout systems ensures experimental rigor and reproducibility:

• Western blot validation:

  • Compare protein levels between control and knockdown/knockout samples

  • Use the validated antibody dilution (1/5000) and look for reduced/absent signal at 54 kDa

  • Quantify band intensity normalized to loading controls

  • For knockdown, expect dose-dependent reduction; for knockout, expect complete absence

• Immunofluorescence validation:

  • Apply the antibody at 1/500 dilution as validated for cell staining

  • Compare staining patterns between control and knockdown/knockout cells

  • Assess both intensity and subcellular distribution changes

  • Capture images with identical exposure settings for valid comparisons

• Immunohistochemistry validation:

  • For tissue-specific knockouts, perform IHC using validated conditions

  • Use heat-mediated antigen retrieval with EDTA buffer pH 9

  • Apply antibody at 1/100 dilution as validated for IHC-P

  • Compare staining patterns and intensities with appropriate controls

• Flow cytometry validation:

  • Perform intracellular staining to quantitatively assess ETR3 expression

  • Compare fluorescence intensity distributions between control and experimental populations

• Immunoprecipitation validation:

  • Perform IP followed by Western blot using the validated IP protocol (1/50 antibody dilution)

  • Compare IP efficiency between control and experimental samples

  • Assess both ETR3 levels and co-immunoprecipitating partners

• Controls and experimental design:

  • Include multiple knockdown constructs targeting different regions of ETR3

  • For transient knockdown, establish a time course to determine optimal validation timing

  • Include rescue experiments reintroducing ETR3 to knockout models

  • Consider potential compensatory upregulation of related CELF family proteins