Recombinant Danio rerio Probable G-protein coupled receptor 173 (gpr173)

What is GPR173 and what protein family does it belong to?

GPR173 (also known as Super conserved receptor expressed in brain 3 or SREB3) is a G-protein coupled receptor belonging to the Class A (Rhodopsin) orphan receptors family, specifically within the Class A orphans GPR173 subfamily . This receptor is characterized by its seven-transmembrane domain structure typical of GPCRs, with specific structural features that distinguish it from other GPCRs. In Danio rerio (zebrafish), GPR173 is encoded by the gpr173 gene (also known as sreb3) and consists of 387 amino acids in its full-length form .

What expression systems are available for recombinant Danio rerio GPR173 production?

Multiple expression systems have been developed for producing recombinant GPR173, each with distinct advantages depending on research needs:

E. coli-expressed Danio rerio GPR173 is available as a full-length protein (1-387 amino acids) with N-terminal His tag, typically supplied as a lyophilized powder with greater than 90% purity as determined by SDS-PAGE .

How can I validate the expression and purity of recombinant GPR173?

Validating recombinant GPR173 expression and purity requires complementary analytical techniques:

• SDS-PAGE analysis: Confirms protein size (expected MW of ~43 kDa plus tag) and purity greater than 90% .

• Western blotting: Using anti-His antibodies or specific anti-GPR173 antibodies to confirm identity. For His-tagged proteins, anti-tag ELISA can also be employed .

• Analytical SEC (HPLC): Important for assessing protein homogeneity and oligomeric state. This technique helps detect aggregation and confirms proper folding .

• Mass spectrometry: For precise molecular weight confirmation and identification of post-translational modifications.

• Functional assays: To confirm that the recombinant protein maintains biological activity. For GPCRs, this often involves ligand binding assays or downstream signaling evaluation.

For zebrafish GPR173 expressed in E. coli, validation typically includes SDS-PAGE showing greater than 90% purity, with additional confirmation via immunological techniques using tag-specific antibodies .

What are the optimal storage and reconstitution conditions for preserving GPR173 activity?

Maintaining optimal storage and reconstitution conditions is critical for preserving GPR173 structural integrity and activity:

Storage recommendations:

• Store lyophilized protein at -20°C/-80°C upon receipt

• Working aliquots may be stored at 4°C for up to one week

• Avoid repeated freeze-thaw cycles which can cause protein denaturation

• For long-term storage, aliquot reconstituted protein with 5-50% glycerol (final concentration) and store at -20°C/-80°C

Reconstitution protocol:

• Briefly centrifuge the vial before opening to bring contents to the bottom

• Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• Add glycerol to 50% final concentration for stability

• Create single-use aliquots to avoid freeze-thaw cycles

The protein is typically supplied in Tris/PBS-based buffer with 6% trehalose at pH 8.0, which helps maintain stability during lyophilization and reconstitution .

How does the sequence of Danio rerio GPR173 compare to mammalian orthologs?

Danio rerio GPR173 shares significant sequence homology with mammalian orthologs, particularly in the transmembrane regions and key functional motifs. When comparing the full-length 387-amino acid zebrafish sequence with human, mouse, and rat orthologs:

The highest conservation is observed in the transmembrane domains and motifs critical for G-protein coupling, suggesting functional conservation across species. This level of homology makes zebrafish GPR173 a valuable model for studying the fundamental biology of this receptor family .

What experimental applications is recombinant GPR173 suitable for?

Recombinant Danio rerio GPR173 can be utilized in multiple experimental applications:

• ELISA: For protein-protein interaction studies, antibody screening, and quantitative analysis .

• Western Blot (WB): For detection and quantification of GPR173 in tissue or cell lysates, often using anti-tag antibodies when working with recombinant protein .

• SDS-PAGE: For purity assessment and molecular weight confirmation .

• Analytical SEC (HPLC): For analyzing protein homogeneity, oligomeric state, and stability under different buffer conditions .

• Structural biology: Including X-ray crystallography or cryo-EM for determining 3D structure, particularly important as GPR173 is an orphan receptor.

• Ligand screening: Using techniques such as surface plasmon resonance (SPR) or differential scanning fluorimetry (DSF) to identify potential binding partners.

• Antibody production: As an immunogen for generating specific antibodies against GPR173.

• Pull-down assays: Using the His tag to identify potential interacting partners from cell lysates.

The appropriate application depends on the experimental goals and the specific expression system used to produce the recombinant protein .

What strategies can help optimize the functional expression of membrane proteins like GPR173?

Optimizing the functional expression of membrane proteins like GPR173 requires addressing several challenges:

• Expression system selection:

  • For functional studies, mammalian expression systems like HEK-293 cells provide proper folding and post-translational modifications

  • For structural studies requiring higher yields, E. coli or cell-free protein synthesis may be preferred

• Construct design optimization:

  • Consider truncating flexible regions while preserving functional domains

  • Add fusion partners to improve stability and expression (e.g., SUMO, MBP)

  • Optimize codon usage for the expression host

• Solubilization strategies:

  • Screen detergents or lipid nanodisc formulations for optimal extraction

  • Consider bicelles or nanodiscs for maintaining native-like membrane environment

  • Test different detergent concentrations and combinations

• Purification enhancement:

  • Implement two-step purification strategies (e.g., IMAC followed by size exclusion)

  • Optimize buffer conditions including pH, salt concentration, and stabilizing additives

  • Consider adding ligands or antagonists during purification to stabilize conformation

• Stability improvement:

  • Add stabilizing agents such as cholesterol or specific lipids

  • Screen thermal stabilizing mutations

  • Include glycerol (5-50%) or trehalose (6%) in storage buffers

These approaches should be systematically tested to determine optimal conditions for your specific experimental requirements.

What analytical methods can be used to assess GPR173 functional activity?

Assessing the functional activity of an orphan receptor like GPR173 requires creative approaches in the absence of known ligands:

• G-protein coupling assays:

  • BRET/FRET-based methods to detect conformational changes

  • [³⁵S]GTPγS binding assays to measure G-protein activation

  • Calcium mobilization assays if Gq-coupling is suspected

• Signaling pathway analysis:

  • cAMP accumulation assays for Gs/Gi coupling assessment

  • IP3 production for Gq pathway activation

  • ERK1/2 phosphorylation for MAPK pathway engagement

  • β-arrestin recruitment assays for internalization

• Ligand discovery approaches:

  • Reverse pharmacology screening using tissue extracts

  • Computational prediction of potential ligands based on structural homology

  • Yeast-based reporter systems for high-throughput screening

• Conformational stability assessment:

  • Thermal shift assays to detect ligand-induced stabilization

  • Limited proteolysis to identify protected conformations

  • Hydrogen-deuterium exchange mass spectrometry

• Cell-based functional readouts:

  • Internalization assays using fluorescently-tagged receptors

  • Transcriptional reporter assays linked to potential downstream pathways

  • Phenotypic screens in zebrafish models with genetic manipulations of gpr173

These methods can be combined in a systematic approach to characterize GPR173 function even in the absence of identified ligands.

What are common challenges in working with recombinant GPCRs like GPR173?

Researchers face several technical challenges when working with recombinant GPCRs:

• Low expression yields: GPCRs often express at lower levels than soluble proteins.

  • Solution: Optimize codons, use specialized expression vectors, or explore insect cell or mammalian expression systems that may improve yields .

• Protein instability: GPCRs can be unstable outside their native membrane environment.

  • Solution: Include stabilizing agents like glycerol (5-50%) and trehalose (6%) in buffers; avoid repeated freeze-thaw cycles; store working aliquots at 4°C for short durations .

• Improper folding: Incorrect folding can occur, particularly in prokaryotic expression systems.

  • Solution: Consider expression in HEK-293 cells or cell-free protein synthesis systems that better support GPCR folding .

• Aggregation issues: GPCRs may aggregate during purification or storage.

  • Solution: Optimize detergent types and concentrations; monitor protein homogeneity using analytical SEC (HPLC) .

• Functional validation: As an orphan receptor, confirming functional activity of GPR173 is challenging.

  • Solution: Use indirect assays such as conformational stability tests or G-protein coupling assays.

• Post-translational modification differences: Expression system may not recapitulate natural PTMs.

  • Solution: Choose mammalian expression systems for studies where PTMs are critical.

Understanding these challenges and implementing appropriate solutions can significantly improve experimental outcomes when working with recombinant GPR173.

How can I develop a deorphanization strategy for GPR173?

Developing a systematic deorphanization strategy for GPR173 requires a multi-faceted approach:

• Bioinformatic analysis:

  • Phylogenetic comparison with related GPCRs whose ligands are known

  • Structural modeling to predict potential ligand binding pockets

  • Gene expression correlation analysis to identify co-expressed genes that might encode ligands

• Tissue-based screening:

  • Prepare tissue extracts from brain regions where GPR173 is highly expressed

  • Fractionate extracts and test for receptor activation

  • Focus on zebrafish brain regions for compatibility with the Danio rerio receptor

• Candidate-based approaches:

  • Test peptides expressed in regions matching receptor expression

  • Screen small molecule libraries based on structural predictions

  • Evaluate lipids and other signaling molecules that activate related orphan receptors

• Functional readout optimization:

  • Develop multiple assay formats (calcium mobilization, cAMP, β-arrestin, etc.)

  • Create chimeric G-proteins to enhance coupling efficiency

  • Establish reporter cell lines with amplified signal detection systems

• Cross-species comparison:

  • Test whether ligands of related receptors in other species might activate GPR173

  • Evaluate conservation of binding pocket residues across species

  • Consider evolutionary conserved signaling pathways

This comprehensive approach increases the likelihood of identifying the endogenous ligand(s) for GPR173 and understanding its physiological function.

What are emerging approaches for studying orphan GPCRs like GPR173?

Several cutting-edge approaches are advancing our understanding of orphan GPCRs:

• Structural biology breakthroughs:

  • Cryo-EM techniques allow structure determination with smaller protein quantities

  • Computational approaches like AlphaFold2 can predict structures with increasing accuracy

  • Fragment-based screening using crystallography to identify binding pockets

• Single-cell technologies:

  • Single-cell RNA-seq to identify co-expression patterns with potential ligands

  • Single-cell proteomics to map receptor expression in specific cell populations

  • Spatial transcriptomics to visualize receptor expression in tissue context

• CRISPR-based functional genomics:

  • Genome-wide screens to identify genes affecting receptor signaling

  • Precise genetic manipulation in zebrafish models to study gpr173 function in vivo

  • Base editing to introduce specific mutations for structure-function analysis

• Advanced biosensors:

  • BRET/FRET-based conformational sensors with improved sensitivity

  • Engineered nanobodies as conformational biosensors

  • Development of GPR173-specific biosensors for real-time activation monitoring

• Multi-omics integration:

  • Combining proteomics, metabolomics, and transcriptomics to identify signaling networks

  • Systems biology approaches to place GPR173 in broader signaling contexts

  • Machine learning to predict ligands from multi-dimensional data

These emerging approaches offer new opportunities to understand GPR173 biology beyond traditional deorphanization strategies.

How can comparative studies between zebrafish and mammalian GPR173 advance our understanding?

Comparative studies between zebrafish and mammalian GPR173 provide valuable insights:

• Evolutionary conservation analysis:

  • Identifying highly conserved residues suggests functional importance

  • Diversified regions may indicate species-specific adaptations

  • Molecular clock analysis to understand evolutionary pressure on receptor function

• Developmental biology advantages:

  • Zebrafish embryo transparency enables real-time imaging of receptor expression

  • Rapid development allows studying GPR173 function across developmental stages

  • Easy genetic manipulation facilitates functional studies in vivo

• Physiological role exploration:

  • Comparison of expression patterns between species suggests conserved functions

  • Behavioral studies in zebrafish models with gpr173 mutations

  • Pharmacological cross-reactivity testing between species variants

• Translational research opportunities:

  • Zebrafish models for high-throughput drug screening targeting GPR173

  • Validation of findings in mammalian systems to establish clinical relevance

  • Development of transgenic reporter lines for pathway visualization

• Data integration framework:

  • Correlation of structural differences with functional divergence

  • Mapping sequence variations to phenotypic differences across species

  • Building predictive models for ligand specificity based on interspecies comparison

These comparative approaches can accelerate understanding of GPR173 biology and potentially identify conserved ligands and signaling pathways.