NtR (Q9W288 / CG6698, *Drosophila melanogaster*) — Function-Assignment Review OpenScientist openscientist-autonomous 3 citations 3 artifacts 2026-09-21T05:43:44.661766 citations file

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NtR (Q9W288 / CG6698, Drosophila melanogaster) — Function-Assignment Review

Focus: Does NtR directly have the assigned functions — acetylcholine-gated channel complex (GO:0005892), contributes_to acetylcholine-gated cation-selective channel activity (GO:0022848), calcium ion transport (GO:0006816), cholinergic/synaptic signaling (GO:0007271)?

Hypothesis slug: ntr-cholinergic-calcium-and-synaptic-functions
Source: genes/DROME/NtR/NtR-ai-review.yaml (free-text)

Executive Judgment

Verdict: Partially supported / over-annotated at the specificity level (homology-only).

NtR is a bona fide member of the Cys-loop ligand-gated ion channel (LGIC) superfamily: it has a bona fide extracellular neurotransmitter-gated ion-channel ligand-binding domain (Pfam PF02931), one canonical Cys-loop signature (C337–x13–C351), and a four-transmembrane-helix C-terminal region (M1–M4) in the expected Cys-loop topology. At a general level, terms describing it as a ligand-gated / cation-permeable ion channel of the Cys-loop family localized to the plasma membrane are defensible.

However, the specific claims in the seed hypothesis — that the ligand is acetylcholine, that it is a calcium transporter, and that it functions in cholinergic synaptic transmission — are not supported by any primary experimental evidence. Every one of the 15 current GO annotations for Q9W288 is computational: the cholinergic/calcium/synaptic core is IBA (phylogenetic inference, ECO:0000318) propagated from the PANTHER subfamily, and the general channel terms are IEA from InterPro. There is no IDA/IMP/IPI/EXP annotation, no electrophysiology, no heterologous-expression assay, and no mutant phenotype for NtR.

Two independent lines of computational evidence actively undercut the ACh-specific inheritance:
1. NtR sits in its own PANTHER family (PTHR36695), not the canonical insect nAChR family — consistent with Matthews et al. 2018 (PMID:30429615, Extended Data 10d) placing the NTR branch outside canonical nicotinic AChRs.
2. NtR lacks the loop-C vicinal cysteines that define α-type ACh-binding subunits, and its transmembrane/pore region is divergent (it does not receive the canonical Neur_chan_memb pore fold; it gets a generic AcrB-like TM structural label). Cation-vs-anion selectivity and Ca²⁺ permeability therefore cannot be inferred from homology.

Bottom line for the curator: The Matthews phylogenetic placement means broad Cys-loop homology cannot be used to justify the ACh-, calcium-, and cholinergic-specific terms. These should be generalized, flagged as non-core, or down-qualified (retaining the contributes_to hedge where a subunit-level channel term is kept). The general LGIC/ion-channel/plasma-membrane terms may be retained. The hypothesis is not refuted (a synaptic role remains possible and is not disproven), but it is not established either.

Evidence Matrix

#	Citation	Evidence type	Supports/Refutes/Qualifies	Claim tested	Key finding	Context	Confidence & limitations
1	UniProt Q9W288; QuickGO (GO_REF:0000033, :0000002, :0000108)	Computational (database)	Qualifies	Evidence basis of the annotations	All 15 GO annotations are IBA or IEA; zero experimental codes	D. melanogaster gene product record	High confidence in the fact; means specificity is inference-only
2	Matthews et al. 2018, PMID:30429615 (ED Fig 10d; cited from seed)	Structural/evolutionary	Qualifies / competing	Is NtR a canonical nAChR?	NTR branch placed outside canonical nicotinic AChRs	Aedes/insect Cys-loop phylogeny	Figure not independently retrievable here; phylogeny undercuts ACh-inheritance
3	InterPro/Pfam for Q9W288 (PF02931; PANTHER PTHR36695)	Structural/evolutionary (computational)	Qualifies	Family placement	LBD confirms Cys-loop membership; but own PANTHER family PTHR36695, not nAChR family	Domain/orthology analysis	High confidence; supports "divergent, non-canonical"
4	This work — sequence analysis (Cys positions)	Structural/evolutionary (computational)	Qualifies / refutes α-subunit	ACh-binding α-subunit determinants	One Cys-loop (C337–C351); no loop-C vicinal cysteines → non-α subunit	585-aa sequence	Heuristic; α-subunit ACh contact residues absent
5	This work — Kyte-Doolittle hydropathy	Structural/evolutionary (computational)	Supports (general) / qualifies (specific)	Channel topology	Signal peptide + LBD + 4 TM (M1–M4) → canonical Cys-loop architecture	585-aa sequence	Supports general channel identity; pore selectivity unresolved
6	InterPro structural assignments (SSF63712 vs SSF82866; PF12248)	Structural/evolutionary (computational)	Qualifies	Is the pore a canonical channel pore?	LBD gets Cys-loop folds; TM region gets generic AcrB TM fold, not Neur_chan_memb pore	Domain analysis	Divergent pore → cation/Ca selectivity not inferable
7	Jones, Brown & Sattelle 2007, PMID:17216517	Review	Qualifies	Existence of divergent subunits	"each insect possesses at least one highly divergent nAChR subunit"	Insect nAChR gene families	Review-level; frames NtR as the expected divergent member
8	Dent 2006, PMID:16586016	Review/evolutionary	Qualifies	α vs non-α clades	Invertebrate nAChRs split into α-type and non-α clades; large divergent nAChR-like groups exist	Cross-species Cys-loop phylogeny	Review-level; supports non-α/divergent placement
9	FlyBase/mygene (gene 43935, FBgn0029147)	Database	Qualifies	Is there curated function?	No curated functional summary; only IBA/IEA GO	D. melanogaster	Absence of evidence, not evidence of absence

No primary functional assay, mutant phenotype, localization, or interaction study specific to NtR was found.

GO Curation Implications (leads — require curator verification)

GO ID	Term	Aspect	Current evidence	Lead action	Rationale
GO:0022848	ACh-gated cation-selective channel activity	MF	IBA	Generalize to e.g. GO:0022824 (transmitter-gated monoatomic cation channel) or GO:0005230, or keep with `contributes_to` + explicit low-confidence note	ACh ligand not assayed; subfamily outside canonical nAChRs
GO:0005892	acetylcholine-gated channel complex	CC	IBA	Generalize / flag	No evidence NtR co-assembles into an ACh receptor pentamer
GO:0006816	calcium ion transport	BP	IBA	Remove or generalize to GO:0034220 ion transmembrane transport	Ca²⁺ permeability inherited from α7-like members; NtR pore divergent, untested
GO:0007271	synaptic transmission, cholinergic	BP	IBA	Generalize / treat as non-core	Cholinergic identity unproven; no expression/synapse data
GO:0005230	extracellular ligand-gated ion channel activity	MF	IEA	Retain (general)	Supported by LBD homology; conservative and defensible
GO:0034220	monoatomic ion transmembrane transport	BP	IBA/IEA	Retain (general)	Defensible from 4-TM channel topology
GO:0042391	regulation of membrane potential	BP	IBA	Retain (general)	Reasonable for an ion channel
GO:0005886	plasma membrane	CC	IBA	Retain	Consistent with signal peptide + 4-TM membrane protein
GO:0045202 / 0098794 / 0060079 / 0099565	synapse / postsynapse / EPSP / postsynaptic transmission	CC/BP	IEA (GOC auto)	Treat as non-core / flag	Auto-propagated from the cholinergic terms; no localization data

Preserve the contributes_to qualifier wherever a channel-activity term is retained: NtR would at most be one subunit contributing to a multimeric channel's activity, so contributes_to is the correct hedge and should not be dropped.

Do not default to "protein binding" — the LBD-based ligand-gated-ion-channel and ion-transport terms are more informative and are homology-supported at the general level.

Mechanistic Scope

Immediate molecular function actually tested by evidence: membership in the Cys-loop LGIC superfamily via an extracellular ligand-binding domain and a 4-TM channel body. This supports a general "transmitter-gated ion channel subunit" identity.
Not established as direct activity: (i) the identity of the gating ligand (acetylcholine vs GABA/histamine/glutamate/protons/orphan), (ii) cation vs anion selectivity, (iii) calcium permeability, (iv) participation in a specific ACh-receptor complex.
Downstream / inferred-only: cholinergic synaptic transmission, EPSP generation, postsynaptic membrane-potential regulation — these are propagated from the (unproven) ACh assignment, not measured. They should be treated as pathway-context inferences, not core function.

Conflicts and Alternatives

Phylogenetic placement conflict: Matthews et al. (PMID:30429615) ED10d and the independent PANTHER family assignment (PTHR36695, not the nAChR family) both place NtR outside canonical nAChRs. IBA propagation of ACh-specific terms across a branch that the tree itself separates from nAChRs is internally inconsistent and is the central reason to down-qualify.
Non-α subunit: absence of loop-C vicinal cysteines means NtR cannot supply the principal (+) ACh-binding face even if it were in an ACh receptor — it could only ever be a structural/complementary subunit.
Divergent pore: the TM region does not match the canonical channel-pore fold, so cation selectivity and Ca²⁺ transport (α7-like inheritance) are unsupported; NtR could be an anion channel or an orphan/atypical channel.
Paralog/carry-over risk: the ACh/calcium/synapse terms have the hallmark of homology carry-over from well-studied nAChR α-subunits onto a divergent "orphan" member — the classic over-annotation pattern the review is meant to catch.
Alternative identity: NtR resembles the "highly divergent" insect Cys-loop subunits (cf. pHCl, CG-series orphans) that are often not ACh-gated; several are pH-, GABA-, or ligand-unknown channels.

Knowledge Gaps

Gap	What was checked	Why it matters	What would resolve it
Gating ligand identity	Domains, phylogeny, sequence — no assay found	ACh-specific terms depend on it	Heterologous expression + agonist screen (ACh, GABA, Glu, His, protons)
Ion selectivity (cation vs anion; Ca²⁺)	Pore-fold divergence noted; M2 not confidently classifiable	Justifies "cation-selective" and "calcium transport"	Reversal-potential / ion-substitution electrophysiology
Subunit partners / complex	No interaction data	Justifies "acetylcholine-gated channel complex"	Co-IP / proteomics; co-expression functional reconstitution
Neuronal/synaptic expression	No curated expression summary retrieved	Justifies synaptic BP/CC terms	scRNA-seq brain atlas, reporter/antibody localization
Matthews ED10d specifics	Cited from seed; figure not retrievable here	Anchor for the "outside nAChR" claim	Curator to inspect PMID:30429615 ED Fig 10d and its PAINT nodes

Discriminating Tests

Two-electrode voltage clamp / patch clamp on NtR expressed in Xenopus oocytes or S2 cells, screening ACh vs GABA/Glu/histamine/protons — distinguishes ACh-gated from alternative-ligand or orphan channel.
Ion-substitution reversal-potential experiments and Ca²⁺-imaging under agonist — tests cation selectivity and calcium permeability directly.
Co-assembly assay (tagged NtR + canonical Dα/Dβ nAChR subunits) — tests membership in an ACh receptor complex.
Single-cell brain/VNC expression atlas query and endogenous tagging — tests neuronal/synaptic localization.
Explicit inspection of the PANTHER PTHR36695 tree / PAINT annotations — confirm which experimentally characterized node (if any) is the source of the IBA ACh terms; if no experimentally validated ACh-gated member exists in the subfamily, the IBA propagation is unsupported.

Curation Leads (require curator verification)

Action change: Down-qualify or generalize the ACh-specific MF (GO:0022848), CC (GO:0005892), and BP (GO:0006816 calcium; GO:0007271 cholinergic) terms from confident to homology-only/non-core; retain general channel terms (GO:0005230, GO:0034220, GO:0042391, GO:0005886). Keep contributes_to on any retained channel-activity term.
Candidate reference to verify — PMID:30429615 (Matthews et al. 2018), Extended Data Fig 10d: verify that the NTR branch is drawn outside the canonical nicotinic AChR clade. (Snippet must be verified by the curator against the figure; abstract text was not programmatically retrievable in this run.)
Candidate reference — PMID:17216517 (Jones, Brown & Sattelle 2007): snippet to verify — "each insect possesses at least one highly divergent nAChR subunit."
Candidate reference — PMID:16586016 (Dent 2006): snippet to verify — "There are two clades of invertebrate nAChRs, one of alpha-type subunits and one of non-alpha subunits..."
Candidate replacement GO terms: GO:0005230 (extracellular ligand-gated monoatomic ion channel activity) or GO:0022824 (transmitter-gated monoatomic cation channel activity) in place of the ACh-specific GO:0022848, pending assay.
Suggested questions for curator: (1) Does the PTHR36695 subfamily contain any experimentally characterized ACh-gated member that legitimizes the IBA? (2) Is there any transcriptomic evidence of neuronal expression? (3) Should the calcium-transport term be removed entirely given the divergent, untested pore?
Suggested experiments: heterologous electrophysiology with a multi-ligand panel; ion-selectivity measurement; co-assembly test (as above).

Limitations of this review

No primary functional literature exists for NtR; conclusions rest on sequence/domain/phylogeny analysis plus database evidence codes.
The Matthews ED10d figure and the PANTHER PAINT nodes were not programmatically retrievable in this run; the "outside canonical nAChR" placement is corroborated independently by the distinct PANTHER family (PTHR36695) but the exact figure should be curator-verified.
TM/pore selectivity classification (cation vs anion) was not confidently resolvable from sequence alone.