Taste receptor type 1 member 3 (T1R3), a class C (family 3) G protein-coupled receptor with an extracellular Venus-flytrap ligand-binding module, a cysteine-rich linker domain, and a seven-transmembrane signaling domain. Across vertebrates, TAS1R3 is the obligate shared dimerization partner for both type-1 taste receptors: it pairs with TAS1R2 to form the canonical sweet-taste receptor and with TAS1R1 to form the umami (amino-acid) taste receptor. Birds lack TAS1R2, so avian TAS1R3 normally partners only with TAS1R1 to form an umami/amino-acid receptor. In hummingbirds (family Trochilidae), however, the ancestral TAS1R1-TAS1R3 umami heterodimer was evolutionarily repurposed, through amino-acid substitutions concentrated in the TAS1R3 Venus-flytrap domain, to respond to carbohydrates instead of amino acids, restoring a sweet-taste-like sensory capability that lets hummingbirds detect nectar sugars. This receptor localizes to the plasma membrane of taste receptor cells, where ligand binding triggers G protein-coupled signal transduction. This particular UniProt record (A0A7L3N407) is an unreviewed, fragmentary (Flags: Fragment; non-terminal at both ends) sequence predicted from preliminary Bird 10,000 Genomes (B10K) whole-genome shotgun data for the hummingbird species Oreotrochilus melanogaster (black-breasted hillstar); no species-specific experimental characterization (expression, receptor pharmacology, or behavior) exists for this exact sequence. The functional picture above is inferred from homology/domain architecture (PANTHER family PTHR24061, TAS1R subfamily) and from the well-documented genus/family-wide sweet-taste transformation phenomenon in hummingbirds, which was established experimentally in a different hummingbird species, Calypte anna (Anna's hummingbird).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0001582 detection of chemical stimulus involved in sensory perception of sweet taste | IEA GO_REF:0000108 | ACCEPT | Summary: Logically inferred (inter-ontology link, GO_REF:0000108) from the co-annotated MF term GO:0033041 "sweet taste receptor activity". This is consistent with the hummingbird-specific repurposing of TAS1R1-TAS1R3 as a carbohydrate/sugar receptor documented in Anna's hummingbird, though it has not been directly tested for this species or accession. Reason: Derived deterministically from the sweet-taste-receptor-activity MF annotation via a standard inter-ontology logical mapping; given ACCEPT of the parent MF term below, this BP child is equally well-supported by the hummingbird-lineage evidence in PMID:25146290. Supporting Evidence: PMID:25146290 Hummingbird T1R1-T1R3 responded to several carbohydrates, including sucrose, fructose, and glucose. |
| GO:0004930 G protein-coupled receptor activity | IEA GO_REF:0000120 | ACCEPT | Summary: InterPro/PANTHER domain-based annotation reflecting the class C GPCR architecture of the protein: extracellular Venus-flytrap (ANF_lig-bd_rcpt-family), nine-cysteine domain (IPR011500), and 7TM GPCR_3 domain (IPR000337), together with five predicted transmembrane helices identified by Phobius in the UniProt record. Broad but accurate molecular activity classification for a class C GPCR family member; the more specific activity is separately captured by GO:0033041 below. Reason: The domain architecture (Venus-flytrap + cysteine-rich + 7TM) is diagnostic of family 3/class C GPCRs, and TAS1R3 orthologs across vertebrates function as GPCRs. Broad but accurate; coexists with, and does not conflict with, the more specific sweet-taste-receptor-activity annotation. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: UniProt subcellular-location and PANTHER-based annotation. Consistent with five Phobius-predicted transmembrane helices (residues 561-801) in the C-terminal 7TM domain and a signal peptide at the N-terminus, both features of a cell-surface GPCR. Reason: Class C GPCRs, including all characterized TAS1R family members, are integral plasma-membrane proteins that must be surface-trafficked to detect extracellular ligands and couple to heterotrimeric G proteins; this is a canonical, well-supported location for the family regardless of species-specific validation. Supporting Evidence: file:9AVES/TAS1R3/TAS1R3-deep-research-falcon.md because T1R3 is a seven-transmembrane receptor that detects extracellular nutrients |
| GO:0007186 G protein-coupled receptor signaling pathway | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO domain-based annotation of the broad GPCR signaling process, consistent with the receptor's class C GPCR domain architecture and with heterologous-cell calcium-signaling assays used to characterize hummingbird T1R1-T1R3 activity. Reason: Correct at the level of the broad parent process; ligand binding by class C GPCRs such as TAS1R1-TAS1R3 triggers canonical heterotrimeric G-protein-coupled signal transduction, as demonstrated functionally for hummingbird T1R1-T1R3 in a heterologous calcium-reporter assay. Supporting Evidence: PMID:25146290 Responses of bird T1Rs were measured in heterologous cells by means of calcium-sensitive photoprotein reporters. |
| GO:0016020 membrane | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro2GO domain-based annotation to the generic parent "membrane" term, redundant with but not contradicted by the more specific GO:0005886 "plasma membrane" annotation above. Reason: True (the protein has five predicted transmembrane helices), but redundant given the more specific, independently-supported plasma-membrane annotation above; uninformative on its own. Not core. Consistent with the equivalent generic-membrane calls for the other taste-receptor genes in this same PR. |
| GO:0033041 sweet taste receptor activity | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter/PANTHER (PTHR24061 family, node PTN009078551) propagation of "sweet taste receptor activity". GO:0033041 is defined generically as "Combining with soluble sweet compounds to initiate a change in cell activity" (QuickGO), which is not restricted to the canonical TAS1R2-TAS1R3 mechanism. This matches the hummingbird-specific finding that the TAS1R1-TAS1R3 heterodimer (the only sweet-capable dimer available to birds, which lack TAS1R2) was evolutionarily repurposed to bind carbohydrates and trigger cellular signaling. This is the central, novel finding motivating this gene review and is treated as this hummingbird's core annotated molecular activity, even though it has not been directly assayed for Oreotrochilus melanogaster specifically. Reason: The GO:0033041 definition does not require the canonical T1R2-partner mechanism, only that the receptor combine with soluble sweet compounds to trigger a cellular response - which is exactly what was demonstrated for hummingbird TAS1R1-TAS1R3 (Calypte anna). Given the well-documented, phylogenetically widespread nature of this hummingbird-lineage transformation (which arose 42-72 million years ago, predating diversification of extant hummingbirds) and the concordant PANTHER/TreeGrafter family-level call, this is accepted as the core function by homology, while `description` and `knowledge_gaps` flag that direct experimental confirmation for this exact species/accession is lacking. Supporting Evidence: PMID:25146290 Responses were not observed when T1R1 or T1R3 alone was used, suggesting that hummingbird T1R1-T1R3 functions as an obligate heterodimer. PMID:25146290 Hummingbird T1R1-T1R3 responded to several carbohydrates, including sucrose, fructose, and glucose. |
| GO:0050909 sensory perception of taste | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA machine-learning rule annotation to the broad parent taste perception process, encompassing both the sweet- and umami-taste child terms also annotated below. Reason: Correct broad parent term; both sweet-taste and umami-taste perception are children of "sensory perception of taste," so this generic annotation is consistent with either or both of the more specific annotations being correct, but it is uninformative given that the more specific GO:0050916 (below) is independently held. Not core. Consistent with the equivalent generic-taste-perception calls for the other taste-receptor genes in this same PR. |
| GO:0050916 sensory perception of sweet taste | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter/PANTHER propagation of the organismal-level sweet-taste perception process, paralleling the GO:0033041 molecular-activity annotation. Supported by the same hummingbird-lineage evidence: receptor pharmacology tracked behavioral sugar preference in both captive and wild hummingbirds. Reason: This is the headline, well-supported functional claim for the TAS1R1-TAS1R3 heterodimer in the hummingbird lineage generally, directly paralleling and supporting the ACCEPT decision for GO:0033041. Treated as a core, not merely peripheral, biological process for this gene, with the caveat (noted in `description` and `knowledge_gaps`) that it is established for the genus/family, not directly for this specific accession or species. Supporting Evidence: PMID:25146290 We conclude that the molecular recognition properties of hummingbird T1R1-T1R3, together with those of other gustatory receptors, instruct taste behavior in both captive and wild hummingbirds. |
| GO:0050917 sensory perception of umami taste | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: TreeGrafter/PANTHER propagation reflecting the ancestral, family-wide function of the TAS1R1-TAS1R3 heterodimer (umami/amino-acid taste), which remains the function of this dimer in most non-hummingbird birds and other vertebrates that lack TAS1R2. In the hummingbird lineage specifically, this ancestral umami function was largely superseded by the derived sweet-taste function: hummingbird T1R1-T1R3 failed to detect carbohydrate-preferring amino acids the way chicken/swift T1R1-T1R3 does, though weak, low-affinity responses to some amino acids were retained. Reason: The PANTHER/TreeGrafter node correctly reflects that umami/amino-acid detection is the ancestral, family-wide function of the TAS1R1-TAS1R3 heterodimer (and remains the primary function in most other birds, which retain only this dimer). However, for the hummingbird lineage specifically, PMID:25146290 shows this function was largely transformed into sweet-taste detection, with only weak, low-affinity amino-acid responses retained rather than robust umami perception. Rather than REMOVE (the ancestral capacity is not entirely absent, and no experimental data exist to state it is fully lost in this particular species/accession) or ACCEPT as a co-equal core function (which would overstate the retained amino-acid sensitivity relative to the well-documented, dominant sweet-taste function), KEEP_AS_NON_CORE is used to retain this annotation as a non-core, likely-retained function while GO:0033041 / GO:0050916 (sweet taste) are treated as the core, derived function of this gene in this lineage. Supporting Evidence: PMID:25146290 cells expressing chicken or swift T1R1-T1R3 failed to detect carbohydrates at any concentration tested and instead recognized alanine and serine PMID:25146290 Low-affinity responses were observed to some amino acids, as with the human sweet receptor, which recognizes carbohydrates as well as proteins, dipeptides, and amino acids |
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Download this section (compressed HTML)Q: Does TAS1R3 from Oreotrochilus melanogaster, expressed together with its TAS1R1 ortholog in a heterologous system, reproduce the carbohydrate-selective receptor pharmacology (sucrose/fructose/glucose responsiveness) documented for Calypte anna T1R1-T1R3?
Q: Does the full-length (non-fragmentary) Oreotrochilus melanogaster TAS1R3 sequence carry the same or homologous amino-acid substitutions in the Venus-flytrap domain that were shown to confer sugar responsiveness in Calypte anna T1R3 (e.g. the 19 substitutions in the 109-residue region identified by Baldwin et al. 2014)?
Q: Should GO:0033041 sweet taste receptor activity be annotated to a T1R3 subunit at all, given that sweet (here carbohydrate) sensing is a property of the T1R heterodimer and that hummingbirds lack TAS1R2, so the sugar receptor is the repurposed T1R1-T1R3 pair? Would a complex-level annotation (contributes_to, or the receptor complex term) state the function more exactly? A blinded OpenScientist run on this row judged the term partially supported on these grounds.
Experiment: Clone and heterologously co-express full-length TAS1R1 and TAS1R3 from Oreotrochilus melanogaster oral/tongue tissue, and measure calcium-reporter responses to sugars, sugar alcohols, and amino acids (as in PMID:25146290) to directly test whether the sweet-taste repurposing generalizes to this species.
Hypothesis: O. melanogaster TAS1R1-TAS1R3 responds to carbohydrates (sucrose, fructose, glucose) rather than to amino acids, mirroring Calypte anna T1R1-T1R3.
Experiment: Complete the genome assembly/gene model at this locus (the current B10K entry is a WGS-derived fragment) and confirm the full coding sequence, signal peptide, and transmembrane topology.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: No experimental data (expression, receptor pharmacology, or behavior) exist for TAS1R3 in Oreotrochilus melanogaster specifically, or for this exact UniProt accession (A0A7L3N407); functional inference is drawn entirely from a different hummingbird species (Calypte anna, and secondarily Archilochus colubris) in PMID:25146290, combined with homology/domain architecture (PANTHER family PTHR24061).
BIOLOGY MF_DARK
What is known: The genus/family-wide phenomenon of TAS1R1-TAS1R3 sweet-taste repurposing in hummingbirds is firmly established for Calypte anna and Archilochus colubris; the presence of the class C GPCR domain architecture in this O. melanogaster sequence is firmly established from the deposited (fragmentary) sequence itself.
Significance: Determines whether the sweet-taste-receptor annotation transferred here by homology actually holds for this species, or whether hillstar- specific sequence divergence alters ligand selectivity.
What would resolve it: Heterologous co-expression of full-length O. melanogaster TAS1R1 and TAS1R3 with a calcium-reporter ligand-response assay, as performed for Calypte anna in PMID:25146290.
Provenance (the field's own admissions):
Gap: Whether the deposited fragmentary sequence (Flags: Fragment; non-terminal at both position 1 and 832) captures the complete, functionally critical regions of the receptor - including the full Venus-flytrap domain and the hummingbird-lineage substitutions shown to confer sugar responsiveness in Calypte anna - is unknown.
CURATION MF_DARK
What is known: The sequence as deposited includes the full 7TM GPCR_3 domain (residues 559-823) and the C-terminal region; it is the N-terminal extent and completeness of the Venus-flytrap module that is uncertain given the fragment flag.
Significance: A gene model derived from preliminary WGS assembly may be missing exons or contain assembly errors that would confound any downstream experimental or comparative-genomic use of this specific record.
What would resolve it: Completion of the O. melanogaster genome assembly/gene model at this locus and confirmation of the full coding sequence.
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