Recombinant Macaca mulatta Taste receptor type 2 member 43 (TAS2R43)

What is the structure and function of Macaca mulatta TAS2R43?

TAS2R43 is a member of the G-protein coupled receptor T2R family involved in bitter taste perception. While specific information on the Macaca mulatta variant is limited, comparative analysis with human TAS2R43 indicates:

• It is likely a seven-transmembrane G protein-coupled receptor

• Functions in the detection of bitter compounds in oral and potentially extraoral tissues

• Signals through heterotrimeric G proteins to transduce bitter taste signals

The human TAS2R43 protein consists of 309 amino acids, and the Macaca mulatta version is expected to have similar structural characteristics. Like other TAS2Rs, it likely has:

• 7 transmembrane domains

• External N-terminus

• Cytoplasmic C-terminus

• Binding pocket for bitter compounds

What expression systems are most effective for producing recombinant Macaca mulatta TAS2R43?

Multiple expression systems have proven effective for TAS2R proteins from Macaca mulatta, each with distinct advantages:

• Cell-free expression systems: Commonly used for Macaca mulatta TAS2R43 production, providing yields with ≥85% purity as determined by SDS-PAGE

• E. coli expression systems: Successfully used for other Macaca mulatta TAS2R proteins (such as TAS2R39 and TAS2R42), typically producing proteins with >90% purity

• Mammalian cell expression: Human cell lines (particularly HEK293 variants) provide appropriate post-translational modifications and membrane insertion for functional studies

For structural studies, E. coli or cell-free systems are often preferred for higher yields, while functional studies typically employ mammalian expression systems that ensure proper folding and membrane localization.

How can researchers verify successful expression and membrane localization of recombinant TAS2R43?

Verifying proper expression and membrane localization is crucial before conducting functional assays. Based on methodology used for other TAS2R proteins:

• Immunocytochemistry: The most direct approach involves using epitope tags (commonly Rho or His tags) added to the N-terminus of TAS2R43 to detect membrane localization. Testing both permeabilized and unpermeabilized cells can distinguish between total expression and surface expression

• Western blotting: Using SDS-PAGE and immunoblotting with tag-specific antibodies to confirm protein expression and molecular weight

• Fluorescence microscopy: For GFP-tagged constructs, allowing direct visualization of receptor localization

Research has shown that insufficient cell surface expression may prevent successful deorphanization of some TAS2Rs, as observed in this comparative analysis of mouse taste receptors:

Table adapted from mouse TAS2R expression study

What functional assays are most appropriate for studying Macaca mulatta TAS2R43 activation?

The most commonly used functional assays for TAS2R activation include:

• Calcium flux assays: The gold standard utilizes FLP-In T-REX 293-Gα16gust44 cells with inducible TAS2R43 expression. Upon receptor activation, calcium-sensitive fluorescent dyes (e.g., Fluo-4 AM) detect intracellular calcium increases

• Inducible expression systems: Using tetracycline-controlled expression provides important experimental control:

  • Induction time optimization (typically 3-18 hours depending on the receptor)

  • Non-induced cells serve as negative controls (mock)

• Dose-response analysis: Testing compounds at concentrations ranging from 0.01 to 100 μM to determine:

  • Threshold concentrations

  • EC50 values

  • Maximum response amplitudes

Research indicates that Gα16gust44-based systems show higher sensitivity than Gα15-based assays for detecting low-efficacy activators of taste receptors .

How has TAS2R43 evolved in primates, and what factors drive its evolution?

Evolutionary analysis of TAS2R genes in primates reveals:

• Extensive variability: The total number of TAS2R genes in primates ranges from 27 to 51, with evidence of both gene losses and gains throughout primate evolution

• Diet-driven selection: Phylogenetically independent contrast analysis shows the number of intact TAS2R genes significantly correlates with feeding preferences

• Lineage-specific duplications: TAS2R genes cluster into 21 phylogenetic clades, including anthropoid-specific, Strepsirrhini-specific, and Cercopithecidae-specific duplications

• Selective gene losses: Substantial reductions (≥5 genes) occurred in several primate lineages, including those leading to Strepsirrhini

The evolution of TAS2R genes appears to reflect adaptations to different dietary niches, particularly regarding the detection of potentially toxic plant compounds.

What are the key amino acid residues that determine TAS2R43 ligand specificity and function?

While specific information on Macaca mulatta TAS2R43 is limited, research on human TAS2R43 identifies critical residues:

• Position 35: The W35S substitution significantly reduces receptor functionality. Tryptophan at position 35 is particularly important for recognition of certain bitter compounds like aloin

• Position 212: The H212R variant shows stronger association with caffeine perception and coffee liking, suggesting its importance in caffeine recognition

These findings highlight how specific amino acid positions can determine both receptor functionality and compound specificity. Comparative analysis between human and Macaca mulatta TAS2R43 sequences would be valuable for identifying conserved functional residues.

What is the relationship between TAS2R43 genetic variation and bitter taste perception?

Human studies provide insights into how genetic variation affects TAS2R43 function:

• Functional variants: Two amino acid changes (W35S and H212R) cause largely diminished protein functionality in human TAS2R43

• Phenotypic associations: Specific SNPs in human TAS2R43 are associated with:

  • Coffee liking (explaining 0.32% of variance)

  • Differences in caffeine bitter perception

  • Response to sulfonyl amide sweeteners' bitter aftertaste

• Compound specificity: Different variants show differential responses to specific compounds:

  • W35R is important for aloin recognition

  • H212R appears more strongly associated with caffeine perception

These findings suggest that genetic variation in TAS2R43 contributes to individual differences in bitter taste perception, which may influence dietary preferences.

What is known about TAS2R43 expression patterns beyond taste cells?

While traditionally considered taste-specific, TAS2R receptors show extraoral expression:

• Airway epithelial cells: In humans, TAS2R43 activation by bitter compounds increases intracellular calcium and stimulates ciliary beat frequency, potentially acting as chemosensory receptors to detect and eliminate noxious agents

• Gastrointestinal tract: TAS2R43 is expressed in the GI tract, where it may contribute to nutrient sensing and digestive functions

• Tissue-specific regulation: Research on other TAS2Rs suggests that gene regulation in taste papillae differs from that in other tissues, which may also apply to TAS2R43

This extraoral expression suggests broader physiological roles beyond taste perception, potentially including defensive mechanisms against toxic compounds.

How does TAS2R43 interact with downstream signaling components?

TAS2R43, like other TAS2Rs, couples to specific signaling components:

• G protein coupling: Signals through gustducin (GNAT3) and potentially other G protein alpha subunits

• Second messenger pathways: Activates PLCB2 (phospholipase C beta-2), leading to production of inositol trisphosphate (IP3)

• Ion channel activation: Signals through the calcium-regulated cation channel TRPM5, leading to membrane depolarization

The protein interaction network for human TAS2R43 includes:

• GNAT3: Guanine nucleotide-binding protein G(t) subunit alpha-3 (score: 0.938)

• PLCB2: 1-phosphatidylinositol 4,5-bisphosphate phosphodiesterase beta-2 (score: 0.867)

• TAS1R3: Taste receptor type 1 member 3 (lower confidence interaction)

How does copy number variation affect TAS2R genes, and what are the implications for TAS2R43?

Studies of copy number variation (CNV) in human TAS2R genes reveal:

• High-frequency deletion alleles: Two TAS2R loci (TAS2R43 and TAS2R45) harbor high-frequency deletion alleles resulting in copy number variation

• Deletion characteristics:

  • The TAS2R43 deletion allele (43Δ) is 37.8kb in length

  • It spans the complete coding region (~1kb) along with extensive flanking sequence

  • Global frequency of 43Δ is approximately 0.33

• Evolutionary insights:

  • Comparisons with chimpanzee genomes indicate the deletions evolved recently through unequal recombination

  • Geographic variation points to an African origin for the deletions

• Structural impact: These deletions result in the complete absence of functional protein when present in homozygous form, creating natural "knockout" individuals

While specific CNV data for Macaca mulatta TAS2R43 isn't provided, these human findings suggest the importance of investigating CNV in non-human primate studies.

What are the challenges in determining the 3D structure of TAS2R43?

Determining the three-dimensional structure of TAS2R proteins presents significant challenges:

• Technical limitations: As a membrane protein, TAS2R43 is difficult to crystallize for X-ray crystallography. The search results note that "the 3D structure of it hasn't been determined"

• Structural prediction: Secondary structure prediction using tools like TOPCONS can identify:

  • Transmembrane regions (seven predicted for human TAS2R43)

  • Glycosylation sites (positions 161 and 176 in human TAS2R43)

  • Other functionally important regions

• Protein stability: Recombinant TAS2R proteins require careful handling:

  • Storage recommendations include -20°C/-80°C with aliquoting to avoid freeze-thaw cycles

  • Addition of 5-50% glycerol for long-term storage

  • Reconstitution in appropriate buffers to maintain stability

Future structural studies may benefit from advances in cryo-electron microscopy, which has successfully determined structures of other challenging GPCRs.

What is the agonist profile of TAS2R43 compared to other TAS2R family members?

Understanding the agonist profile of TAS2R43 relative to other TAS2Rs provides insight into bitter taste coding:

• Overlapping activation patterns: Analysis of TAS2R activation profiles shows that while TAS2R43 responds to a range of agonists, most are also agonists for other TAS2Rs

• Receptor specificity: Despite overlapping profiles, each TAS2R (including TAS2R43) is activated by a unique subset of compounds

• Tuning breadth variation: TAS2R receptors vary greatly in their breadth of tuning, ranging from very broadly tuned "generalist" receptors to narrowly tuned "specialist" receptors

• Known activators: In humans, TAS2R43 is activated by:

  • Sulfonyl amide sweeteners (saccharin and acesulfame K)

  • Caffeine (along with TAS2R7, TAS2R10, TAS2R14, and TAS2R46)

This combinatorial activation pattern allows discrimination between thousands of bitter compounds with a limited number of receptors.