Recombinant Pan paniscus Taste receptor type 2 member 31 (TAS2R31)

What is TAS2R31 and what is its primary function in Pan paniscus?

TAS2R31 (Taste receptor type 2 member 31) is a G protein-coupled receptor (GPCR) encoded by the TAS2R31 gene that functions primarily as a bitter taste receptor. In Pan paniscus, as in other primates, this receptor is part of the broader TAS2R family responsible for detecting bitter compounds. The receptor is structurally similar to other sensory receptors including opsins and olfactory receptors .

TAS2R31 has been extensively documented to respond to saccharin in vitro, serving as a key mediator in bitter taste perception . The receptor's primary function involves initiating transduction cascades upon agonist binding, which ultimately generates neural signals interpreted as bitter taste . While the evolutionary distance between Pan paniscus and humans is small, subtle species-specific differences in receptor structure may affect ligand binding profiles and sensitivity thresholds.

How is recombinant Pan paniscus TAS2R31 protein typically produced for research applications?

Recombinant Pan paniscus TAS2R31 protein production generally follows protocols similar to those used for Pan troglodytes. The standard production method involves:

• Gene synthesis or cloning of the full-length TAS2R31 coding sequence (1-309 amino acids)

• Insertion into an expression vector with an N-terminal His-tag

• Expression in bacterial systems such as E. coli

• Protein purification via affinity chromatography

• Lyophilization for storage stability

For optimal experimental utility, the following storage and reconstitution methods are recommended:

Methodologically, researchers should centrifuge vials before opening and prepare working aliquots that can be stored at 4°C for up to one week to maintain protein integrity .

What are the key structural characteristics of TAS2R31 that influence its functional properties?

TAS2R31 is a transmembrane receptor with several key structural features that influence its function:

• Transmembrane domains: The protein contains multiple transmembrane regions that anchor it to the cell membrane of taste receptor cells and other tissues where it's expressed.

• Binding pocket: Contains specific residues that interact with bitter compounds. Notably, position 35 (R35W substitution) has been identified as functionally significant in TAS2R31, dramatically affecting receptor responses to compounds like saccharin and acesulfame potassium .

• Signal transduction regions: The cytoplasmic domains interact with G proteins to initiate downstream signaling events, triggering calcium ion influx and subsequent cell depolarization .

These structural features collectively determine receptor sensitivity, specificity, and signaling efficiency when exposed to bitter compounds.

How do polymorphisms in Pan paniscus TAS2R31 affect receptor function and experimental design considerations?

Polymorphisms in TAS2R31 significantly impact receptor function, necessitating careful experimental design. While human TAS2R31 polymorphisms have been well-characterized, Pan paniscus variations have received less attention but likely follow similar functional patterns.

The R35W substitution represents one of the most functionally significant polymorphisms identified in TAS2R31, with strong effects on receptor function and direct associations with taste responses to bitter off-tastes of saccharin and acesulfame potassium . Other substitutions, such as D45H, demonstrate more complex conditional effects, altering receptor function only when combined with specific alleles (e.g., the W35 allele) .

When designing experiments with Pan paniscus TAS2R31:

• Genotyping: Researchers should sequence the full TAS2R31 coding region from their Pan paniscus samples to identify polymorphisms before functional studies.

• Haplotype analysis: Consider examining complete haplotypes rather than individual SNPs, as functional effects often result from combined substitutions.

• Comparative approaches: Include human and Pan troglodytes TAS2R31 variants as references in functional assays to contextualize Pan paniscus-specific findings.

• Heterologous expression systems: When using cell-based assays, carefully select expression systems that minimize interference with receptor trafficking and signaling.

• Dose-response relationships: Polymorphisms may alter EC50 values rather than completely abolishing function, necessitating full dose-response curves rather than single-concentration experiments.

This polymorphic complexity underscores the need for comprehensive genetic characterization prior to functional studies to avoid confounding experimental results .

What methodological approaches can be used to study TAS2R31 activation in different cellular contexts?

Studying TAS2R31 activation requires sophisticated methodological approaches depending on the cellular context:

In Taste Receptor Cells (Primary Function):

• Calcium imaging: Measures intracellular calcium mobilization following receptor activation using fluorescent indicators (Fura-2, Fluo-4).

• Patch-clamp electrophysiology: Records membrane potential changes directly from individual cells.

• FLIPR-based assays: High-throughput screening of multiple compounds using fluorescence plate readers.

In Extraoral Tissues (Airways, Gut):

• Tissue-specific reporter systems: Custom constructs with tissue-specific promoters driving expression of TAS2R31 coupled to luminescent or fluorescent reporters.

• Ex vivo tissue preparations: Organ bath studies measuring smooth muscle tension in airway tissue expressing TAS2R31.

• Immunohistochemistry combined with functional assays: Co-localization of TAS2R31 with functional readouts in tissue slices.

Molecular Interaction Studies:

• Bioluminescence resonance energy transfer (BRET): Measures protein-protein interactions between TAS2R31 and signaling partners.

• Surface plasmon resonance (SPR): Quantifies direct binding of ligands to purified receptor.

• Molecular dynamics simulations: Computationally predicts binding pocket interactions with potential ligands.

Researchers should carefully select methodologies based on their specific research questions, as each approach offers different advantages in sensitivity, throughput, and physiological relevance .

How does TAS2R31 contribute to extraoral functions beyond taste perception in primates?

TAS2R31 has significant extraoral functions beyond its canonical role in taste perception, with important implications for comparative primate physiology:

Respiratory System Functions:

TAS2R31 expression in airway smooth muscle contributes to bronchodilation through a distinct molecular mechanism. When activated, these receptors trigger increased intracellular calcium, which subsequently opens potassium channels, hyperpolarizing the membrane and causing smooth muscle relaxation . This mechanism may represent an evolutionary adaptation for detecting and responding to inhaled irritants or pathogens.

Gastrointestinal Roles:

In intestinal tissue, TAS2R activation triggers:

• Release of antimicrobial peptides (α-defensin 5, REG3A)

• Regulation of other innate immune factors including mucins and chemokines

• Effects on bacterial growth, particularly E. coli

A comprehensive RNA-Seq analysis revealed that TAS2R activation (with denatonium benzoate) induces:

• NRF2-mediated nutrient stress responses

• Unfolded protein responses

Metabolic and Immune Regulation:

TAS2Rs in gut tissues influence:

• Endocrine responses to bitter compounds

• Gastric emptying rates

• Potential monitoring of quorum sensing in intestinal flora

• Detection of parasites emitting TAS2R agonists

When designing studies examining these extraoral functions in Pan paniscus, researchers should consider tissue-specific expression patterns and signaling pathways that may differ from those in taste cells. Comparative studies with human and Pan troglodytes tissues may reveal species-specific adaptations in these non-gustatory roles .

What are the best experimental systems for assessing Pan paniscus TAS2R31 function in vitro?

Selecting appropriate experimental systems for assessing Pan paniscus TAS2R31 function requires careful consideration of multiple factors:

Heterologous Expression Systems:

Functional Assay Selection:

• Calcium mobilization assays: Most widely used for TAS2R functional characterization, utilizing calcium-sensitive dyes (Fura-2, Fluo-4) to measure receptor activation. These provide real-time, quantitative measurements of receptor function.

• Inositol phosphate (IP) accumulation: Measures downstream signaling events following receptor activation, providing complementary data to calcium assays.

• MAPK phosphorylation: Evaluates activation of mitogen-activated protein kinase pathways, relevant for extraoral TAS2R31 functions.

• Receptor internalization assays: Utilizes fluorescently tagged receptors to monitor trafficking following activation, providing insights into desensitization mechanisms.

Optimized Protocol Elements:

• Co-expression of appropriate G-proteins (typically Gα16gust44) to enhance coupling efficiency

• Inclusion of positive controls (known agonists like saccharin) and negative controls

• Dose-response analysis across physiologically relevant concentration ranges

• Maximization of signal-to-noise ratio through optimization of cell density and expression levels

The most robust experimental approach combines multiple assay systems to provide convergent evidence of receptor function, particularly when characterizing novel ligands or receptor variants .

How can researchers address challenges in expressing and characterizing membrane proteins like TAS2R31?

Membrane proteins like TAS2R31 present significant technical challenges for researchers. Here are methodological approaches to address common issues:

Expression Challenges and Solutions:

• Poor surface expression:

  • Utilize N-terminal epitope tags rather than C-terminal tags

  • Include rhodopsin-derived N-terminal signal sequences

  • Optimize codon usage for expression system

  • Consider using inducible expression systems to reduce toxicity

• Protein misfolding:

  • Incorporate chemical chaperones in culture media (e.g., DMSO, glycerol)

  • Lower incubation temperature during expression (28-30°C)

  • Co-express with molecular chaperones

• Receptor degradation:

  • Add proteasome inhibitors during expression

  • Create fusion constructs with stable protein domains

  • Optimize detergent selection for membrane extraction

Characterization Strategies:

• Functional verification:

  • Employ multiple orthogonal assays (calcium imaging, BRET, receptor internalization)

  • Include well-characterized control receptors in parallel experiments

  • Validate with known agonists before testing novel compounds

• Structural analysis:

  • Consider nanobody stabilization approaches

  • Utilize cryo-electron microscopy for structural determination

  • Apply molecular dynamics simulations to predict conformational changes

• Binding studies:

  • Implement thermostabilized receptor variants for ligand binding assays

  • Use fragment-based screening approaches for identifying binding sites

  • Develop fluorescently labeled ligands for direct binding measurements

These methodological refinements can substantially improve the quality and reliability of data obtained from recombinant TAS2R31 studies, enabling more accurate characterization of this challenging membrane protein .

What are the implications of TAS2R31 variations for evolutionary and comparative studies in hominids?

TAS2R31 variations provide unique insights into primate evolution and adaptation:

Evolutionary Significance:

The TAS2R31 gene harbors extensive polymorphism across hominids, including numerous nonsynonymous variants that affect receptor function . This genetic diversity suggests that TAS2R31 has been subject to diverse selective pressures throughout primate evolution, potentially related to:

• Dietary adaptations: Different bitter compound profiles in available food sources

• Toxin avoidance mechanisms: Species-specific sensitivity to environmental toxins

• Extraoral functions: Variations in immune and respiratory responses

Comparative Analysis Framework:

When conducting evolutionary studies of Pan paniscus TAS2R31:

• Sequence comparison methodology:

  • Align full coding sequences from Pan paniscus, Pan troglodytes, Homo sapiens, and other primates

  • Calculate dN/dS ratios to identify regions under positive or purifying selection

  • Perform ancestral sequence reconstruction to trace evolutionary trajectory

• Functional divergence assessment:

  • Test receptor responses to a standardized panel of bitter compounds

  • Compare EC50 values and maximal responses across species variants

  • Identify species-specific ligands that may reflect ecological adaptations

• Population genetics analysis:

  • Evaluate allele frequencies within and between populations

  • Test for signatures of selective sweeps or balancing selection

  • Compare haplotype structures across hominid species

Evolutionary Hypotheses Table:

These evolutionary perspectives can provide critical context for understanding the functional significance of observed variations in Pan paniscus TAS2R31, connecting molecular mechanisms to adaptive outcomes .

What experimental controls should be implemented when studying recombinant Pan paniscus TAS2R31?

Robust experimental controls are essential for generating reliable data when working with recombinant Pan paniscus TAS2R31:

Positive Controls:

• Known agonists: Include saccharin and acesulfame potassium as established TAS2R31 activators

• Species-matched controls: Include well-characterized Pan paniscus receptors from the same family

• Expression verification: Implement anti-tag antibody detection to confirm protein expression

Negative Controls:

• Non-transfected cells: Essential for establishing baseline responses

• Inactive receptor mutants: Generate site-directed mutants with altered binding sites

• Non-ligand compounds: Include structurally related compounds known not to activate TAS2R31

System Validation Controls:

• Signal transduction verification: Include positive controls for calcium flux (e.g., ATP, ionomycin)

• Cell viability assessment: Monitor potential cytotoxicity of test compounds

• Receptor specificity confirmation: Test compounds against related TAS2R family members

Quality Control Parameters:

Implementing these methodological controls ensures that observed effects can be confidently attributed to Pan paniscus TAS2R31 activity rather than experimental artifacts or system-specific phenomena .

How can researchers integrate computational and experimental approaches when studying TAS2R31?

Integrating computational and experimental approaches creates powerful synergies in TAS2R31 research:

Computational Methods and Their Experimental Applications:

• Homology Modeling and Molecular Docking:

  • Generate 3D structural models of Pan paniscus TAS2R31 based on related GPCRs

  • Predict binding sites for candidate compounds

  • Experimental validation: Test predicted high-affinity ligands in cellular assays

  • Iterative refinement: Use experimental binding data to improve computational models

• Molecular Dynamics Simulations:

  • Simulate receptor conformational changes upon ligand binding

  • Predict effects of amino acid substitutions on protein stability

  • Experimental application: Guide site-directed mutagenesis experiments to target functionally critical residues

  • Validation approach: Compare simulation-predicted stability changes with thermal stability assays

• Machine Learning for Ligand Prediction:

  • Train models on known TAS2R ligand datasets to predict new compounds

  • Identify structural features associated with receptor activation

  • Experimental implementation: Screen computationally prioritized compounds

  • Feedback loop: Update models with new experimental data

What are the key considerations for translating in vitro findings about TAS2R31 to in vivo physiological relevance?

Translating in vitro findings about TAS2R31 to physiologically relevant in vivo contexts presents significant challenges that researchers must address methodically:

Translation Challenges and Strategies:

• Concentration Discrepancies:

  • Challenge: In vitro systems often use concentrations exceeding physiological levels

  • Solution: Establish dose-response relationships spanning physiological ranges

  • Validation approach: Measure actual compound concentrations in relevant tissues (e.g., taste buds, airways, intestine)

• Receptor Expression Differences:

  • Challenge: Heterologous systems typically overexpress receptors compared to native tissues

  • Solution: Develop controlled expression systems matching physiological levels

  • Measurement technique: Quantitative PCR and western blotting to compare expression levels

• Signaling Environment Complexity:

  • Challenge: In vivo systems contain complete signaling cascades not present in simplified models

  • Solution: Reconstitute key signaling components in vitro

  • Validation: Compare calcium signals in cell lines vs. primary taste cells or tissue slices

• Tissue-Specific Effects:

  • Challenge: TAS2R31 functions differently in taste buds versus extraoral tissues

  • Solution: Study receptor in tissue-specific contexts

  • Methodological approach: Develop tissue-specific conditional expression models

Transitional Research Pipeline:

For Pan paniscus TAS2R31 specifically, this translational challenge is compounded by ethical and practical limitations in conducting in vivo studies. Researchers may need to rely more heavily on comparative approaches, using insights from human and other primate models, alongside carefully designed ex vivo systems from available tissue samples .

How can TAS2R31 research contribute to understanding evolutionary adaptations in primate sensory systems?

TAS2R31 research provides a valuable window into primate sensory evolution through several methodological approaches:

Comparative Genomics Approaches:

Pan paniscus TAS2R31 can be analyzed within the broader context of primate evolution by:

• Phylogenetic analysis: Constructing gene trees to trace TAS2R31 evolution across primates, identifying:

  • Gene duplication events

  • Lineage-specific selection pressures

  • Rates of evolutionary change

• Selection signature detection: Applying statistical tests (dN/dS ratios, McDonald-Kreitman tests) to identify:

  • Positively selected sites potentially involved in adaptation

  • Conserved regions essential for basic receptor function

  • Relaxed selection in specific lineages

• Haplotype diversity assessment: Comparing polymorphism patterns between Pan paniscus, Pan troglodytes, and humans to detect:

  • Species-specific selective sweeps

  • Shared ancestral polymorphisms

  • Evidence of convergent evolution

Functional Evolutionary Analysis:

To link genetic changes to adaptive phenotypes:

• Ancestral sequence reconstruction: Recreating and testing ancestral TAS2R31 variants to track functional changes over evolutionary time

• Ecological correlation studies: Connecting receptor variations to dietary specializations across primate species

• Positive selection hotspot identification: Focusing on regions with accelerated evolution for functional testing

The extensive polymorphism in TAS2R31 across populations suggests this receptor has been subject to diverse selective pressures throughout primate evolution, potentially related to:

• Species-specific food preference adaptations

• Geographic variation in available bitter compounds

• Dual roles in taste perception and extraoral functions

What are the implications of TAS2R31 extraoral expression for drug development and disease research?

The discovery of TAS2R31 expression in extraoral tissues has significant implications for therapeutic development and disease research:

Airway Applications:

TAS2R31 activation in airway smooth muscle leads to bronchodilation through calcium-dependent mechanisms and potassium channel activation . This physiological response suggests:

• Therapeutic potential: TAS2R31 agonists could represent novel bronchodilators for asthma and COPD treatment

• Research approaches:

  • Screening selective TAS2R31 agonists with limited taste effects

  • Testing species-specific responses in primate airway models

  • Investigating signaling pathway differences between taste and airway tissues

Gastrointestinal Functions:

TAS2R activation in intestinal tissues triggers:

• Antimicrobial peptide release: α-defensin 5 and REG3A production

• Innate immune modulation: Regulation of mucins and chemokines affecting bacterial growth

• Metabolic signaling: Endocrine responses and gastric emptying regulation

Disease Research Applications:

TAS2R31 variations have been linked to:

• Respiratory infections: TAS2R polymorphisms associate with susceptibility to respiratory pathogens

• Metabolic regulation: Variations associate with glucose regulation

• Immune response modulation: TAS2R activation triggers NRF2-mediated stress responses

Methodological Framework for Drug Development:

These extraoral functions position TAS2R31 as a promising target for treating conditions involving innate immune dysregulation, respiratory diseases, and potentially metabolic disorders .

How might emerging technologies advance our understanding of TAS2R31 structure and function?

Emerging technologies offer unprecedented opportunities to advance TAS2R31 research:

Cryo-Electron Microscopy (Cryo-EM) Applications:

Recent advances in cryo-EM resolution now enable structural determination of challenging membrane proteins like GPCRs:

• Methodological advantages:

  • Does not require protein crystallization

  • Can capture multiple conformational states

  • Allows visualization of protein-ligand complexes

• TAS2R31-specific applications:

  • Determine high-resolution structures of Pan paniscus TAS2R31

  • Compare agonist-bound versus unbound conformations

  • Identify structural differences between primate TAS2R31 variants

CRISPR-Cas9 Genome Editing:

Precise genetic manipulation enables:

• Functional genomics approaches:

  • Generate isogenic cell lines with specific TAS2R31 variants

  • Create humanized or "bonobo-ized" receptor variants to study species differences

  • Establish knock-in/knockout models for physiological studies

• Regulatory element analysis:

  • Identify and modify TAS2R31 expression control elements

  • Study tissue-specific regulation mechanisms

  • Investigate epigenetic regulation patterns

Single-Cell Technologies:

These methods reveal cell-type specific patterns:

• Single-cell RNA sequencing (scRNA-seq):

  • Map TAS2R31 expression across diverse cell populations

  • Identify co-expressed signaling components

  • Discover novel cell types expressing TAS2R31

• Single-cell proteomics:

  • Quantify receptor protein levels at single-cell resolution

  • Correlate expression with functional responses

  • Map post-translational modifications

Organoid Technologies:

Three-dimensional tissue models enable:

• Physiologically relevant testing:

  • Generate taste bud organoids expressing TAS2R31

  • Create airway and intestinal organoids for extraoral function studies

  • Develop species-specific organoids to compare Pan paniscus versus human responses

• Disease modeling:

  • Study receptor function in patient-derived organoids

  • Model genetic variation effects in controlled environments

  • Test compound responses in complex tissue architectures

These technologies collectively promise to transform our understanding of TAS2R31 from isolated receptor studies to integrated physiological contexts, particularly valuable for comparative studies between human and non-human primate variants .

What are the current data gaps and future research priorities for Pan paniscus TAS2R31 studies?

Despite significant advances in TAS2R research, several critical knowledge gaps remain specific to Pan paniscus TAS2R31:

Current Data Gaps:

• Sequence and Polymorphism Characterization:

  • Limited genomic data on Pan paniscus TAS2R31 variants compared to humans

  • Insufficient population-level sequence data to assess diversity

  • Incomplete understanding of species-specific polymorphisms

• Functional Characterization:

  • Few direct functional studies of Pan paniscus TAS2R31

  • Limited knowledge of species-specific ligand responsiveness

  • Unclear extraoral expression patterns compared to humans and chimpanzees

• Evolutionary Context:

  • Incomplete understanding of selective pressures on Pan paniscus TAS2R31

  • Limited data on dietary correlates with receptor variations

  • Insufficient comparative data across closely related species

Research Priority Framework:

Implementation Strategy:

Addressing these gaps requires collaborative approaches combining:

• Field work to collect genomic samples from diverse Pan paniscus populations

• Comparative experimental approaches using standardized methodologies across species

• Interdisciplinary integration of molecular, ecological, and evolutionary perspectives

Prioritizing these research areas will establish a comprehensive understanding of Pan paniscus TAS2R31, bridging current knowledge gaps and providing valuable comparative insights into primate sensory evolution .