Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005886 plasma membrane	IBA GO_REF:0000033	ACCEPT	Summary: The TAX-2/TAX-4 CNG channel acts at the plasma membrane of ciliated sensory neurons (including the ciliary membrane). Core cellular component. Supporting Evidence: PMID:23886944 identify sequences required for efficient ciliary targeting and localization of the TAX-2 CNGB and TAX-4 CNGA subunits.
GO:0005222 intracellularly cAMP-activated cation channel activity	IBA GO_REF:0000033	MARK AS OVER ANNOTATED	Summary: This IBA term is inherited from the PANTHER CNG family (which includes cAMP-responsive vertebrate CNGA subunits). The native C. elegans TAX-4/TAX-2 heteromer is highly selective for cGMP over cAMP, so cAMP-activated cation channel activity over-states cAMP as a physiological ligand for this channel; cGMP is the relevant activating ligand. Marked as over-annotated rather than removed because CNG channels retain weak cAMP responsiveness in vitro. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTHR45638 · Cyclic Nucleotide-Gated Cation Channel SUPPORTS SOURCE BUT NOT TARGET Family includes cAMP-responsive vertebrate CNGA subunits, but the C. elegans TAX-4/TAX-2 channel is cGMP-selective, so cAMP-activated activity should not propagate to tax-2. Supporting Evidence: PMID:10064800 it remains highly selective for cGMP over cAMP.
GO:0030553 cGMP binding	IBA GO_REF:0000033	ACCEPT	Summary: TAX-2 contains a C-terminal cyclic-nucleotide-binding domain (CNBD; PROSITE PS50042, residues 538-642) consistent with cyclic-nucleotide sensing by the CNG channel it forms with TAX-4. cGMP is the physiological ligand of the heteromer. Retained as a core-consistent function of the channel, though direct biochemical cGMP binding by the TAX-2 CNBD specifically (as opposed to the TAX-4 alpha subunit) has not been demonstrated (see knowledge gaps). Supporting Evidence: PMID:10064800 it remains highly selective for cGMP over cAMP.
GO:0098655 monoatomic cation transmembrane transport	IBA GO_REF:0000033	ACCEPT	Summary: The CNG channel conducts cations (Ca2+/Na+/K+) across the membrane; TAX-2 is part of that channel. Core transport process.
GO:0017071 intracellular cyclic nucleotide activated cation channel complex	IBA GO_REF:0000033	ACCEPT	Summary: TAX-2 is a subunit of the intracellular cyclic-nucleotide-activated cation channel complex (the TAX-4/TAX-2 heteromer). Core cellular component/complex. Supporting Evidence: PMID:10064800 Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed in HEK293 cells, but TAX-2 does not form a channel on its own.
GO:0005216 monoatomic ion channel activity	IEA GO_REF:0000002	ACCEPT	Summary: Consistent with TAX-2 being a subunit of an ion channel (Pfam PF00520 ion-transport/pore domain). Accurate but general parent term; the specific channel activity is captured by GO:0005223.
GO:0005221 intracellularly cyclic nucleotide-activated monoatomic cation channel activity	IEA GO_REF:0000002	ACCEPT	Summary: Correct at the level of the cyclic-nucleotide-gated channel TAX-2 helps form; the more specific cGMP-activated term (GO:0005223) is the precise function.
GO:0006811 monoatomic ion transport	IEA GO_REF:0000002	ACCEPT	Summary: General parent of the channel's cation-transport process. Accurate but broad.
GO:0016020 membrane	IEA GO_REF:0000120	ACCEPT	Summary: TAX-2 is a multi-pass membrane protein; correct but less informative than the plasma-membrane/ciliary-membrane localization.
GO:0055085 transmembrane transport	IEA GO_REF:0000002	ACCEPT	Summary: General parent of the channel's transmembrane cation-transport process. Accurate but broad.
GO:0005223 intracellularly cGMP-activated cation channel activity	IDA PMID:10064800 Functional reconstitution of a heteromeric cyclic nucleotide...	ACCEPT	Summary: Core molecular function. Functional reconstitution in HEK293 cells shows that TAX-2 co-assembles with the alpha subunit TAX-4 into a cGMP-gated cation channel; TAX-2 does not form a channel alone and acts as a modifying beta subunit that lowers the heteromer's cGMP sensitivity. The 'contributes_to' qualifier correctly reflects that TAX-2 participates in, rather than independently enables, this channel activity. Supporting Evidence: PMID:10064800 with TAX-4 as the alpha subunit and TAX-2 acting as a modifying beta subunit. PMID:10064800 The heteromeric TAX-4/TAX-2 channel is 25-fold less sensitive to cGMP than the TAX-4 channel, but it remains highly selective for cGMP over cAMP.
GO:0034703 cation channel complex	IDA PMID:10064800 Functional reconstitution of a heteromeric cyclic nucleotide...	ACCEPT	Summary: TAX-2 is part of the reconstituted heteromeric cation channel. Core cellular component/complex, directly supported. Supporting Evidence: PMID:10064800 Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed in HEK293 cells, but TAX-2 does not form a channel on its own.
GO:0010628 positive regulation of gene expression	IMP PMID:25303524 Chemosensation of bacterial secondary metabolites modulates ...	KEEP AS NON CORE	Summary: Downstream, activity-dependent output. In the ASJ chemosensory neuron the TAX-2/TAX-4 channel transduces detection of P. aeruginosa secondary metabolites, leading to induced expression of the DAF-7/TGF-beta neuromodulator. This is an indirect consequence of sensory channel activity, not a direct gene-regulatory function of TAX-2. Non-core. Supporting Evidence: PMID:25303524 induces expression of the neuromodulator DAF-7/TGF-β
GO:0007199 G protein-coupled receptor signaling pathway coupled to cGMP nucleotide second messenger	IMP PMID:20436480 C. elegans phototransduction requires a G protein-dependent ...	KEEP AS NON CORE	Summary: In ASJ phototransduction, a G-protein/cGMP cascade opens cGMP-sensitive CNG channels; light-evoked currents are abolished in tax-2 (and tax-4) mutants, placing the channel as the effector downstream of the GPCR-coupled cGMP pathway. Non-core downstream signaling context. Supporting Evidence: PMID:20436480 This dark current was apparently carried by CNG channels, as it can be blocked by the CNG channel-specific inhibitor L-cis-diltiazem and was absent in the CNG channel mutants tax-2 and tax-4
GO:0010754 negative regulation of receptor guanylyl cyclase signaling pathway	IMP PMID:23940325 In vivo genetic dissection of O2-evoked cGMP dynamics in a C...	KEEP AS NON CORE	Summary: In vivo cGMP/Ca2+ imaging of an O2-sensing neuron shows that cGMP-gated channels participate in negative-feedback loops that shape cGMP signaling dynamics. This reflects the channel's role within feedback regulation of the cGMP pathway rather than a direct enzymatic regulation of guanylyl cyclase. Non-core. Supporting Evidence: PMID:23940325 Elevated levels of cGMP and Ca(2+) stimulate competing negative feedback loops that shape cGMP dynamics.
GO:0070482 response to oxygen levels	IMP PMID:23940325 In vivo genetic dissection of O2-evoked cGMP dynamics in a C...	KEEP AS NON CORE	Summary: O2-evoked rises in cGMP drive a sustained Ca2+ response that depends on cGMP-gated ion channels (the TAX-2/TAX-4 channel). A downstream sensory response mediated by channel activity. Non-core. Supporting Evidence: PMID:23940325 Increased cGMP leads to a sustained Ca(2+) response in the neuron that depends on cGMP-gated ion channels.
GO:0040040 thermosensory behavior	IMP PMID:7630402 Neural regulation of thermotaxis in Caenorhabditis elegans.	KEEP AS NON CORE	Summary: tax-2 was isolated as a thermotaxis-abnormal (tax) mutant, and the TAX-2/TAX-4 channel is essential for thermosensation in the AFD thermosensory neuron. This is a downstream sensory behavior mediated by channel activity. Non-core. The cited 1995 thermotaxis paper is the WormBase original reference; its cached record is abstract-only and centers on the AFD/AIY/AIZ circuit, but the tax-2 thermosensory requirement is independently established (PMID:10064800, PMID:9486798); annotation retained (defer to curator full-text) rather than removed. Supporting Evidence: PMID:9486798 The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic nucleotide-gated channel that is required for chemosensation, thermosensation and normal axon outgrowth of some sensory neurons.
GO:0098655 monoatomic cation transmembrane transport	IDA PMID:10064800 Functional reconstitution of a heteromeric cyclic nucleotide...	ACCEPT	Summary: Directly supported by functional reconstitution: the heteromeric channel conducts cations across the membrane and differs from the TAX-4 homomer in divalent-cation block and single-channel properties. Core transport process. Supporting Evidence: PMID:10064800 The heteromeric channel and the TAX-4 homomeric channel differ in their blockage by divalent cations and in their single channel properties.
GO:0006935 chemotaxis	IMP PMID:9486798 A cyclic nucleotide-gated channel inhibits sensory axon outg...	KEEP AS NON CORE	Summary: tax-2 activity is required for normal chemotaxis to NaCl and odorants, a downstream sensory behavior mediated by the CNG channel. Non-core. Supporting Evidence: PMID:9486798 tax-2 activity is required in the adult stage for normal chemotaxis to NaCl and odorants.
GO:0030516 regulation of axon extension	IMP PMID:9486798 A cyclic nucleotide-gated channel inhibits sensory axon outg...	KEEP AS NON CORE	Summary: tax-2 (with tax-4) is required to maintain correct sensory axon structure; in mutants, axon outgrowth resumes inappropriately in late larval/adult stages. A distinct, activity-dependent role of the channel in maintaining axon morphology. Non-core. Supporting Evidence: PMID:9486798 These results indicate that tax-2 and tax-4 are required for the maintenance of correct axon structure
GO:0009454 aerotaxis	IMP PMID:15220933 Oxygen sensation and social feeding mediated by a C. elegans...	KEEP AS NON CORE	Summary: Avoidance of high oxygen (aerotaxis) requires the sensory cGMP-gated channel tax-2/tax-4 together with the soluble guanylate cyclase gcy-35. Downstream sensory behavior mediated by channel activity. Non-core. Supporting Evidence: PMID:15220933 Avoidance of high oxygen levels by C. elegans requires the sensory cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase homologue, gcy-35.
GO:0055093 response to hyperoxia	IMP PMID:15220933 Oxygen sensation and social feeding mediated by a C. elegans...	KEEP AS NON CORE	Summary: The tax-2/tax-4 channel mediates responses that allow avoidance of hyperoxic (high-O2) environments. Downstream sensory response. Non-core. Supporting Evidence: PMID:15220933 Avoidance of high oxygen levels by C. elegans requires the sensory cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase homologue, gcy-35.
GO:0045664 regulation of neuron differentiation	IMP PMID:14711416 The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channe...	KEEP AS NON CORE	Summary: An indirect, activity-dependent effect: loss of the TAX-4 channel (which TAX-2 co-forms) alters AFD thermosensory-neuron gene expression and morphology. The cited paper's abstract foregrounds TAX-4, but TAX-2 is its obligate heteromeric partner; this is a downstream consequence of channel activity rather than a direct differentiation function. Non-core. Supporting Evidence: PMID:14711416 the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons
GO:0045944 positive regulation of transcription by RNA polymerase II	IMP PMID:14711416 The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channe...	KEEP AS NON CORE	Summary: Indirect, activity-dependent regulation of AFD-specific gene expression by the CNG channel; TAX-2 has no direct transcriptional activity. Retained as a downstream effect (abstract foregrounds TAX-4, the obligate partner subunit). Non-core. Supporting Evidence: PMID:14711416 the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons
GO:0048812 neuron projection morphogenesis	IMP PMID:14711416 The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channe...	KEEP AS NON CORE	Summary: Indirect effect on AFD sensory-neuron morphology consequent on loss of CNG channel activity. Downstream/non-core; abstract foregrounds TAX-4, the obligate partner subunit of TAX-2. Supporting Evidence: PMID:14711416 the TAX-4 cyclic nucleotide-gated channel regulate gene expression, morphology, and functions of the AFD thermosensory neurons
GO:0097730 non-motile cilium	IDA PMID:23886944 Cell- and subunit-specific mechanisms of CNG channel ciliary...	NEW	Summary: Proposed new annotation. The TAX-2 CNGB subunit is targeted to and localized within the non-motile sensory cilia of ciliated neurons; sequences required for ciliary targeting/localization of TAX-2 (and TAX-4) were mapped, and the subunits are relatively immobile in specific ciliary subcompartments. Core cellular component not currently in the GOA set. Supporting Evidence: PMID:23886944 identify sequences required for efficient ciliary targeting and localization of the TAX-2 CNGB and TAX-4 CNGA subunits.
GO:0007606 sensory perception of chemical stimulus	IMP PMID:9486798 A cyclic nucleotide-gated channel inhibits sensory axon outg...	NEW	Summary: Proposed new annotation summarizing the channel's core sensory role: tax-2 is required for chemosensation (chemotaxis to NaCl and odorants), a form of sensory perception of chemical stimulus mediated by the CNG channel. Captures the core sensory-transduction process at an appropriate level of generality. Supporting Evidence: PMID:9486798 The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic nucleotide-gated channel that is required for chemosensation, thermosensation and normal axon outgrowth of some sensory neurons.

Core Functions

TAX-2 is the modulatory beta subunit of the sensory cyclic nucleotide-gated (CNG) cation channel. It cannot form a channel on its own; it co-assembles with the alpha (pore-forming) subunit TAX-4 into the native heteromeric channel that is directly opened by intracellular cGMP and conducts cations (Ca2+/Na+/K+) across the plasma/ciliary membrane of ciliated sensory neurons. As a beta subunit TAX-2 contributes ion-channel (pore) structure and tunes the channel: the TAX-4/TAX-2 heteromer is ~25-fold less cGMP-sensitive than the TAX-4 homomer and differs in divalent-cation block and single-channel behavior. This channel is the effector of cGMP-mediated sensory signal transduction.

Molecular Function:

monoatomic ion channel activity

Directly Involved In:

monoatomic cation transmembrane transport sensory perception of chemical stimulus

Cellular Locations:

plasma membrane non-motile cilium

In Complex:

intracellular cyclic nucleotide activated cation channel complex

Supporting Evidence:

PMID:10064800
Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed in HEK293 cells, but TAX-2 does not form a channel on its own.
PMID:10064800
with TAX-4 as the alpha subunit and TAX-2 acting as a modifying beta subunit.
PMID:23886944
identify sequences required for efficient ciliary targeting and localization of the TAX-2 CNGB and TAX-4 CNGA subunits.

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:10064800

Functional reconstitution of a heteromeric cyclic nucleotide-gated channel of Caenorhabditis elegans in cultured cells.

PMID:14711416

The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channel regulate thermosensory neuron gene expression and function in C. elegans.

PMID:15220933

Oxygen sensation and social feeding mediated by a C. elegans guanylate cyclase homologue.

PMID:20436480

C. elegans phototransduction requires a G protein-dependent cGMP pathway and a taste receptor homolog.

PMID:23886944

Cell- and subunit-specific mechanisms of CNG channel ciliary trafficking and localization in C. elegans.

PMID:23940325

In vivo genetic dissection of O2-evoked cGMP dynamics in a Caenorhabditis elegans gas sensor.

PMID:25303524

Chemosensation of bacterial secondary metabolites modulates neuroendocrine signaling and behavior of C. elegans.

PMID:7630402

Neural regulation of thermotaxis in Caenorhabditis elegans.

PMID:8893027

Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation and chemosensation in C. elegans.

PMID:9486798

A cyclic nucleotide-gated channel inhibits sensory axon outgrowth in larval and adult Caenorhabditis elegans: a distinct pathway for maintenance of sensory axon structure.

Suggested Experiments

Experiment: Reconstitute and solve the structure of the TAX-4/TAX-2 heteromer (cryo-EM) and perform side-by-side electrophysiology of TAX-4 homomer versus TAX-4/TAX-2 heteromer with targeted mutations in the TAX-2 CNBD and pore, to define how TAX-2 tunes cGMP sensitivity, ion selectivity and divalent-cation block.

Experiment: Quantify TAX-2 and TAX-4 subunit copy number/ratio in defined sensory cilia in vivo (calibrated fluorescence or single-molecule counting) and correlate with cell-specific cGMP dose-response to map homomeric versus heteromeric channels onto neurons.

Knowledge Gaps

What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).

Gap: How the TAX-2 beta subunit mechanistically modulates the channel — lowering cGMP sensitivity ~25-fold and changing divalent-cation block and single-channel behavior relative to the TAX-4 homomer — and whether this tuning is uniform across neurons, is not defined.

OPEN BIOLOGY RESIDUAL_SUBGAP

What is known: In heterologous cells the TAX-4/TAX-2 heteromer is 25-fold less cGMP-sensitive than the TAX-4 homomer yet stays cGMP-selective, and the two forms differ in divalent-cation block and single-channel properties; the structural/mechanistic basis of this modulation by TAX-2 is not established.

Significance: The beta subunit sets the channel's cGMP operating range and ion handling, so how TAX-2 tunes gating directly determines each sensory neuron's stimulus threshold and dynamic range.

What would resolve it: Structure/function of the TAX-4/TAX-2 heteromer (cryo-EM and mutagenesis of TAX-2 CNBD/pore residues) with side-by-side homomer-versus-heteromer electrophysiology.

Provenance (the field's own admissions):

"The heteromeric TAX-4/TAX-2 channel is 25-fold less sensitive to cGMP than the TAX-4 channel, but it remains highly selective for cGMP over cAMP." — PMID:10064800

Gap: Whether the TAX-2 cyclic-nucleotide-binding domain itself binds cGMP and whether that binding is functional (vs. a modulatory/non-canonical CNBD as in vertebrate CNGB beta subunits) is unknown.

OPEN BIOLOGY MF_DARK

What is known: TAX-2 has a predicted CNBD (PROSITE PS50042, residues 538-642) and carries an IBA cGMP-binding annotation, but direct cGMP binding by the TAX-2 CNBD (as opposed to the TAX-4 alpha subunit) has not been demonstrated biochemically.

Significance: Whether TAX-2's CNBD is a functional ligand sensor or a purely structural modulator changes how the heteromer integrates cGMP and how its sensitivity is set.

What would resolve it: Direct ligand-binding assays on isolated/mutated TAX-2 CNBD and CNBD-mutant channel electrophysiology.

Provenance (the field's own admissions):

"it remains highly selective for cGMP over cAMP." — PMID:10064800

Gap: The endogenous TAX-4:TAX-2 subunit stoichiometry and whether channel composition varies between neuron types or between ciliary subcompartments of a single neuron is inferred, not measured.

OPEN BIOLOGY RESIDUAL_SUBGAP

What is known: TAX-4 + TAX-2 co-assemble into a functional heteromer in HEK293 cells while TAX-2 alone cannot form a channel; in vivo, ciliary localization of the two subunits is interdependent and heterogeneous across the cilium, but the native stoichiometry is not quantified.

Significance: Subunit composition sets ligand sensitivity, ion handling and localization, so it determines how each sensory neuron converts a cGMP signal into a response.

What would resolve it: Quantitative in vivo subunit-copy-number measurement (e.g. calibrated fluorescence or single-molecule counting) per neuron and per ciliary subcompartment.

Provenance (the field's own admissions):

"CNG channel subunits can be localized to discrete ciliary compartments in individual sensory neurons in C. elegans, suggesting that channel composition is heterogeneous across the cilium." — PMID:23886944
"most of the native channels in C. elegans are likely to be hetero-oligomers of TAX-4 and TAX-2 subunits" — PMID:10064800

Gap: Whether TAX-2 has roles independent of TAX-4 — for example partnering other CNG alpha subunits, or non-conducting structural/trafficking roles — is unresolved.

OPEN BIOLOGY RESIDUAL_SUBGAP

What is known: TAX-2 forms no channel on its own, so an independent TAX-2 conductance is unlikely; but ciliary localization is subunit-interdependent and TAX-2 is expressed in cells where its full complement of partners and any TAX-4-independent contribution have not been mapped.

Significance: Distinguishing an obligate TAX-4 partner from a subunit with broader partnerships or scaffolding roles would clarify which sensory functions strictly require the canonical TAX-4/TAX-2 heteromer.

What would resolve it: Interaction proteomics and cell-specific epistasis (tax-2 vs tax-4 vs other CNG subunits) with functional imaging in defined neurons.

Provenance (the field's own admissions):

"Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed in HEK293 cells, but TAX-2 does not form a channel on its own." — PMID:10064800

Deep Research

Falcon

(tax-2-deep-research-falcon.md)

Comprehensive Research Report: *tax-2* (UniProt G5EEE2) in *Caenorhabditis elegans* Falcon Edison Scientific Literature 27 citations 2 artifacts 2026-07-04T17:22:13.093109

Edison artifact artifact-00 (text/markdown)

Edison artifact artifact-01 (text/markdown)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive Research Report: tax-2 (UniProt G5EEE2) in Caenorhabditis elegans

1. Gene Identity and Protein Overview

The tax-2 gene (locus F36F2.5, also designated CELE_F36F2.5) in Caenorhabditis elegans encodes the β (B-type) subunit of a cyclic nucleotide-gated (CNG) cation channel (mckemy2007temperaturesensingacross pages 3-4, ferkey2021chemosensorysignaltransduction pages 14-15). This assignment is fully consistent with the UniProt G5EEE2 entry, which lists domains for CNG_cation_channel (IPR050866), cNMP-binding domain (IPR000595), cNMP-binding domain superfamily (IPR018490), cNMP-binding domain signature (IPR018488), and ion transport domain (IPR005821). The gene name tax-2 derives from its original identification in screens for mutants with abnormal thermotaxis (thermotaxis abnormal), though the gene has subsequently been shown to play essential roles in multiple sensory modalities.

TAX-2 partners with TAX-4 (the α subunit) to form the principal heteromeric CNG channel required for sensory transduction across a broad array of C. elegans sensory neurons (bargmann2006chemosensationinc. pages 10-12, ferkey2021chemosensorysignaltransduction pages 14-15). The TAX-2/TAX-4 channel is structurally most similar to vertebrate rod photoreceptor CNG channels (mckemy2007temperaturesensingacross pages 3-4).

2. Molecular Function and Channel Biophysics

2.1 Primary Function

TAX-2 functions as the obligate β subunit of a heteromeric cGMP-gated nonselective cation channel. The channel is directly gated by intracellular cyclic GMP (cGMP), converting changes in intracellular cGMP concentration—produced or consumed by upstream guanylyl cyclases and phosphodiesterases—into ion flux across the plasma membrane, thereby depolarizing sensory neurons and permitting calcium entry (mckemy2007temperaturesensingacross pages 3-4, li2024inhibitionofa pages 6-7, bargmann2006chemosensationinc. pages 10-12).

2.2 Subunit Composition and Biophysical Properties

Heterologous expression studies (in HEK293 cells) have established the following key biophysical properties:

TAX-4 homomeric channels can form functional channels independently, exhibiting high cGMP sensitivity (K_D ≈ 4 × 10⁻⁷ M) with poor sensitivity to cAMP (mckemy2007temperaturesensingacross pages 3-4).
TAX-2 cannot form functional channels on its own (mckemy2007temperaturesensingacross pages 3-4).
TAX-2/TAX-4 heteromeric channels exhibit approximately 25-fold reduced cGMP sensitivity compared to TAX-4 homomers while maintaining selectivity for cGMP over cAMP (mckemy2007temperaturesensingacross pages 3-4). They are highly permeable to calcium but less sensitive to divalent ion block than TAX-4 homomers (mckemy2007temperaturesensingacross pages 3-4). TAX-4 homomeric channels stay open approximately seven times longer than heteromeric TAX-4/TAX-2 channels, with heteromeric channels having approximately 10-fold lower cGMP affinity (ferkey2021chemosensorysignaltransduction pages 14-15).

These properties indicate that TAX-2 modulates channel kinetics and sensitivity, likely tuning the dynamic range of sensory responses in vivo. The following table summarizes the biophysical comparison:

Property	TAX-4 Homomeric Channel	TAX-2/TAX-4 Heteromeric Channel
cGMP sensitivity (KD)	High sensitivity; KD ≈ 4 × 10⁻⁷ M for cGMP (mckemy2007temperaturesensingacross pages 3-4)	~25-fold lower cGMP sensitivity than TAX-4 alone; still cGMP-gated (mckemy2007temperaturesensingacross pages 3-4)
cAMP sensitivity	Poor sensitivity to cAMP; strongly prefers cGMP (mckemy2007temperaturesensingacross pages 3-4)	Retains selectivity for cGMP over cAMP (mckemy2007temperaturesensingacross pages 3-4)
Calcium permeability	Calcium permeable (mckemy2007temperaturesensingacross pages 3-4)	Highly calcium permeable; supports Ca²⁺ influx in sensory transduction (mckemy2007temperaturesensingacross pages 3-4)
Divalent ion block sensitivity	More sensitive to divalent ion block (mckemy2007temperaturesensingacross pages 3-4)	Less sensitive to divalent ion block than TAX-4 alone (mckemy2007temperaturesensingacross pages 3-4)
Ability to form functional channels independently	Yes; TAX-4 alone can form a functional channel in heterologous expression systems (mckemy2007temperaturesensingacross pages 3-4)	Yes as a co-expressed heteromer; requires both TAX-2 and TAX-4 for this channel composition (mckemy2007temperaturesensingacross pages 3-4, ferkey2021chemosensorysignaltransduction pages 14-15)
Open time	Longer open time; about 7-fold longer than heteromeric TAX-2/TAX-4 channels (ferkey2021chemosensorysignaltransduction pages 14-15)	Shorter open time than TAX-4 homomers (ferkey2021chemosensorysignaltransduction pages 14-15)
Structural similarity	CNG channel α-like subunit; channel complex is structurally similar to rod photoreceptor CNG channels (mckemy2007temperaturesensingacross pages 3-4)	Heteromeric CNG channel most similar to rod photoreceptor channels; TAX-2 is the β-subunit partner (mckemy2007temperaturesensingacross pages 3-4)

Table: This table compares the key biophysical features of TAX-4 homomeric channels and TAX-2/TAX-4 heteromeric channels in C. elegans. It is useful for distinguishing the specific functional contribution of TAX-2 as the β-subunit in the sensory CNG channel complex.

2.3 Alternative Subunit Partnerships

Recent work has revealed that TAX-2 can function either with TAX-4 or with alternative α subunits such as CNG-3. In AFD thermosensory neurons, TAX-2 is required for tactile-dependent locomotion modulation, but TAX-4 is dispensable for this process, suggesting that TAX-2 can pair with CNG-3 or other α subunits to form functionally distinct channel complexes depending on the sensory context (rosero2026afdthermosensoryneurons pages 13-14).

3. Expression Pattern and Subcellular Localization

3.1 Neuronal Expression

TAX-2 is expressed in a broad set of sensory neurons in the amphid and other sensory organs of C. elegans. These include AWC (olfactory neurons for volatile attractants), ASE (gustatory neurons for water-soluble chemicals and salts), AWB (olfactory avoidance neurons for volatile repellents), AFD (thermosensory neurons), BAG (CO₂-sensing neurons), ASG, ASI, ASJ, ASK (chemosensory and neuroendocrine neurons), and ADE neurons (ferkey2021chemosensorysignaltransduction pages 14-15, mckemy2007temperaturesensingacross pages 3-4, rosero2026afdthermosensoryneurons pages 14-15). TAX-2/TAX-4 channels are also expressed in body cavity neurons AQR, PQR, and URX, which function as oxygen sensors exposed to pseudocoelomic fluid (menini2009theneurobiologyof pages 36-38).

3.2 Subcellular Localization

TAX-2 protein is localized to sensory cilia and dendritic structures of sensory neurons—the precise sites where environmental stimuli are transduced into electrical signals (mckemy2007temperaturesensingacross pages 3-4, bargmann2006chemosensationinc. pages 10-12). The channel is primarily found in cilia rather than in axonal compartments (bargmann2006chemosensationinc. pages 10-12). The specific subdomain localization within cilia is neuron-specific and functionally important (ferkey2021chemosensorysignaltransduction pages 14-15). Notably, tax-2 and tax-4 mutant cilia retain normal structural morphology, normal dye uptake, and normal intraflagellar transport, demonstrating that the channel functions downstream of ciliary structure rather than being required for cilia formation per se (li2024inhibitionofa pages 7-8).

The following table summarizes TAX-2 expression across neuron types and the corresponding sensory modalities:

Neuron	Sensory Modality	Upstream Guanylyl Cyclase(s)	Biological Process	Key Evidence/References
AWC	Olfaction; volatile attractants	ODR-1, DAF-11	Odor-evoked cGMP signaling gates TAX-2/TAX-4 to drive chemotaxis; also contributes to activity-dependent gene expression and odor plasticity	tax-2 expressed in AWC; TAX-2/TAX-4 act downstream of GPCR/G protein signaling with ODR-1 and DAF-11 in AWC olfaction (bargmann2006chemosensationinc. pages 10-12, ferkey2021chemosensorysignaltransduction pages 14-15)
ASE	Gustation; water-soluble chemicals, salt, alkalinity	GCY-14 for alkalinity; other ASE rGCs not fully specified here	Salt chemotaxis and pH/alkalinity responses require TAX-2/TAX-4-mediated cGMP signaling	tax-2 expressed in ASE; TAX-2/TAX-4 required for chemotaxis to water-soluble compounds and calcium responses to NaCl/pH changes (bargmann2006chemosensationinc. pages 10-12, ferkey2021chemosensorysignaltransduction pages 14-15, bargmann2006chemosensationinc. pages 1-3)
AWB	Olfactory avoidance; volatile repellents	Not clearly resolved in the retrieved evidence; cGMP pathway upstream likely includes GPCR-regulated GC signaling	Avoidance of volatile repellents and modulation of odor responses	tax-2 expressed in AWB; TAX-2/TAX-4 required for AWB-mediated avoidance and localize to sensory cilia; PDE-5 also modulates AWB olfactory signaling (bargmann2006chemosensationinc. pages 10-12, ferkey2021chemosensorysignaltransduction pages 14-15, galande2025rolesofcyclic pages 12-13)
AFD	Thermosensation	GCY-8, GCY-18, GCY-23	Thermotaxis and thermotransduction through cGMP-triggered opening of TAX-2/TAX-4, causing Ca2+ fluctuations	tax-2 expressed in AFD; TAX-2/TAX-4 localized to cilia/dendrites; AFD thermosensation uses GCY-8/18/23 upstream of the channel (mckemy2007temperaturesensingacross pages 3-4, galande2025rolesofcyclic pages 15-16)
BAG	CO2 sensing; also implicated in O2-related circuit functions	GCY-9	CO2-evoked sensory signaling and behavioral plasticity	tax-2 expressed in BAG; recent pathway summaries place GCY-9 upstream of TAX-2/TAX-4 in BAG CO2 signaling (rosero2026afdthermosensoryneurons pages 14-15, ferkey2021chemosensorysignaltransduction pages 14-15, galande2025rolesofcyclic pages 15-16)
AQR/PQR/URX	Oxygen sensing / aerotaxis	GCY-35, GCY-36	Body-cavity neuron O2 sensing; cGMP opens TAX-2/TAX-4 to control aerotaxis	TAX-2/TAX-4 function downstream of soluble guanylyl cyclases GCY-35/36 in AQR/PQR/URX oxygen sensing (menini2009theneurobiologyof pages 36-38, galande2025rolesofcyclic pages 7-9)
ASG	Chemosensory / sensory integration	Not specified in retrieved evidence	Likely participates in TAX-2/TAX-4-dependent sensory signaling; specific pathway assignment remains limited in the retrieved literature	tax-2 expression reported in ASG, but neuron-specific upstream GC and process were not defined in the retrieved evidence (ferkey2021chemosensorysignaltransduction pages 14-15)
ASI	Neuroendocrine chemosensory signaling	DAF-11 implicated in dauer/pheromone-related cGMP signaling	Dauer regulation, sensory integration, and regulation of daf-7 expression	tax-2 expressed in ASI; TAX-2/TAX-4 contribute to dauer-related cGMP signaling and regulation of sensory gene expression/neuroendocrine outputs (ferkey2021chemosensorysignaltransduction pages 19-20, ferkey2021chemosensorysignaltransduction pages 14-15)
ASJ	Sensory/neuroendocrine signaling; thermo/photo-related cGMP pathways in broader literature	Not clearly specified in retrieved tax-2-centered evidence	Regulation of daf-7 expression and sensory-state signaling; recent PDE literature implicates ASJ cGMP dynamics	tax-2 expression reported in ASJ; channel complex regulates gene expression including daf-7 in ASJ/ASI, but specific upstream GC was not resolved in the retrieved evidence (ferkey2021chemosensorysignaltransduction pages 14-15, galande2025rolesofcyclic pages 9-10)
ASK	Chemosensory / pheromone-related signaling	DAF-11 implicated in pheromone/dauer context	Dauer pheromone signaling and sensory integration	tax-2 expressed in ASK; DAF-11 and G proteins act in pheromone signaling contexts where TAX-2/TAX-4 function downstream in dauer-related pathways (ferkey2021chemosensorysignaltransduction pages 19-20, ferkey2021chemosensorysignaltransduction pages 14-15)
ADE	Sensory neuron; modality not well defined here	Not specified in retrieved evidence	tax-2 expression indicates possible CNG-dependent sensory signaling, but a precise TAX-2-specific function in ADE was not resolved in the retrieved sources	ADE listed among tax-2-expressing neurons; specific upstream GC and function not defined in retrieved evidence (rosero2026afdthermosensoryneurons pages 14-15, ferkey2021chemosensorysignaltransduction pages 14-15)

Table: This table summarizes where TAX-2 is expressed in C. elegans and links each neuron type to its sensory modality, likely upstream guanylyl cyclases, and major biological roles. It is useful for mapping TAX-2 onto specific cGMP-dependent sensory circuits while distinguishing well-supported assignments from areas where evidence remains limited.

4. Signaling Pathways

TAX-2/TAX-4 channels serve as the critical downstream effector in multiple cGMP-dependent sensory transduction cascades. The general signaling logic is conserved across modalities: an environmental stimulus activates guanylyl cyclases (either receptor-type or soluble), elevating intracellular cGMP, which directly gates TAX-2/TAX-4 channels to allow cation influx and neuronal depolarization.

4.1 Chemosensation (Olfaction and Gustation)

In AWC olfactory neurons, odorants are detected by G protein-coupled receptors (GPCRs) that activate the Gα protein ODR-3. ODR-3 regulates cGMP levels by activating receptor guanylyl cyclases ODR-1 and DAF-11 (which likely function as heterodimers), and/or modulating phosphodiesterases. The resulting increase in cGMP directly opens TAX-2/TAX-4 channels to depolarize the neuron (bargmann2006chemosensationinc. pages 10-12). In ASE gustatory neurons, TAX-2/TAX-4 channels are required for calcium flux responses to NaCl concentration changes and pH stimuli (ferkey2021chemosensorysignaltransduction pages 14-15). tax-2 mutants are defective in AWC-mediated chemotaxis to volatile odors, ASE-mediated chemotaxis to water-soluble attractants, and AWB-mediated avoidance of volatile repellents (bargmann2006chemosensationinc. pages 10-12).

4.2 Thermosensation

In AFD thermosensory neurons, temperature-responsive receptor guanylyl cyclases GCY-8, GCY-18, and GCY-23 produce cGMP in response to warming, which gates TAX-2/TAX-4 channels and promotes intracellular Ca²⁺ fluctuations necessary for thermotaxis behavior (mckemy2007temperaturesensingacross pages 3-4). tax-2 mutants show thermotactic deficits similar to those observed after AFD neuron ablation (mckemy2007temperaturesensingacross pages 3-4).

4.3 Oxygen Sensing (Aerotaxis)

In body cavity neurons AQR, PQR, and URX, dissolved oxygen activates soluble guanylyl cyclases GCY-35 and GCY-36, producing cGMP that opens TAX-2/TAX-4 channels to mediate aerotaxis behavior (menini2009theneurobiologyof pages 36-38, galande2025rolesofcyclic pages 7-9).

4.4 CO₂ Sensing

In BAG neurons, the soluble guanylyl cyclase GCY-9 produces cGMP in response to CO₂, activating TAX-2/TAX-4 channels for CO₂-evoked behavioral responses (galande2025rolesofcyclic pages 15-16).

4.5 Negative Feedback by Phosphodiesterases

Cyclic nucleotide phosphodiesterases PDE-1, PDE-2, and PDE-5 hydrolyze cGMP to provide negative feedback regulation of TAX-2/TAX-4 channel activity. In PQR oxygen-sensing neurons, PDE-1 restores cGMP to baseline after oxygen stimulation (galande2025rolesofcyclic pages 7-9). In AFD thermosensory neurons, PDE-1 and PDE-5 regulate cGMP in neuronal receptive endings, while PDE-2 is required for recovery and adaptation phases of thermotransduction (galande2025rolesofcyclic pages 15-16, galande2025rolesofcyclic pages 7-9, galande2025rolesofcyclic pages 12-13). In AWB olfactory neurons, PDE-5 modulates responses to low concentrations of odorants (galande2025rolesofcyclic pages 12-13). Complete disruption of PDE activity (quadruple pde-1:pde-2:pde-3:pde-5 mutants) results in elevated cGMP levels and loss of chemotactic responses to alkaline pH, phenocopying TAX-2/TAX-4 channel defects (galande2025rolesofcyclic pages 13-15).

5. Downstream Physiological Roles

5.1 Dauer Formation

tax-2 mutants exhibit a weak dauer-constitutive (Daf-c) phenotype, particularly at high temperatures, indicating a role in the cGMP signaling branch of dauer formation (fielenbach2008c.elegansdauer pages 3-5, bargmann2006chemosensationinc. pages 10-12, fielenbach2008c.elegansdauer pages 2-3). This phenotype is less severe than that of daf-11 guanylyl cyclase mutants, which produce potent Daf-c phenotypes, placing TAX-2/TAX-4 downstream of DAF-11 in the dauer signaling pathway (fielenbach2008c.elegansdauer pages 2-3). The cGMP pathway intersects with insulin/IGF-1-like signaling (IIS) and TGF-β signaling to control the dauer decision (fielenbach2008c.elegansdauer pages 3-5).

5.2 Longevity

Loss of TAX-2/TAX-4 channel function extends C. elegans lifespan. Mutations in tax-2 extend lifespan particularly at low temperatures (lin2017longevitycontrolby pages 1-2). The tax-4(p678) mutation extends mean lifespan from approximately 19 to 30 days (li2024inhibitionofa pages 6-7). Mechanistically, suppressed CNG channel activity on sensory cilia non-cell-autonomously activates the unfolded protein response of the endoplasmic reticulum (UPR^ER) in intestinal cells through the IRE-1–XBP-1 signaling pathway, promoting resistance to age-dependent protein aggregation and extending lifespan (li2024inhibitionofa pages 6-7, li2024inhibitionofa pages 1-2). This establishes a sensory neuron-to-intestine signaling axis mediated by TAX-2/TAX-4 channel activity.

5.3 Activity-Dependent Gene Expression

TAX-2/TAX-4 channels regulate activity-dependent gene expression in sensory neurons. In AWC neurons, channel activity controls expression of the receptor gene str-2, which is important for odor discrimination and olfactory neuron identity specification (ferkey2021chemosensorysignaltransduction pages 14-15, bargmann2006chemosensationinc. pages 10-12). In ASI and ASJ neurons, the channels regulate expression of daf-7 (TGF-β), a key neuroendocrine signal for dauer and reproductive growth decisions (ferkey2021chemosensorysignaltransduction pages 14-15).

5.4 Tactile-Dependent Behavioral Modulation

A recent study has uncovered an unexpected role for TAX-2 in the AFD thermosensory neuron beyond its canonical function in temperature detection: TAX-2 mediates tactile-dependent locomotion modulation. Intriguingly, TAX-4 is dispensable for this process, and TAX-2 may function with alternative α subunits such as CNG-3 in this context (rosero2026afdthermosensoryneurons pages 13-14). This modulation requires electrical synapses (innexins) between AFD and the AIB interneuron, indicating integration of TAX-2-dependent signals into broader circuit-level processing.

6. Genetic Alleles and Mutant Phenotypes

Several tax-2 alleles have been characterized: tax-2(p694) is the canonical loss-of-function allele used in most studies of CNG channel function and sensory signaling (li2024inhibitionofa pages 7-8); tax-2(p691) and tax-2(ot25) are additional alleles (hill2024molecularandneuronal pages 84-89). The engineered phosphomutant tax-2(oy191[S727A]) has been investigated in the context of rapid temperature adaptation in AFD neurons, where it does not affect rapid adaptation responses (hill2024molecularandneuronal pages 84-89). This suggests that phosphorylation of Ser727 may regulate other aspects of TAX-2 function or contribute to slower forms of sensory plasticity.

7. Evolutionary Context

TAX-2 is structurally most similar to β subunits of CNG channels expressed in vertebrate rod photoreceptors (mckemy2007temperaturesensingacross pages 3-4). The TAX-2/TAX-4 heteromeric channel shares homology with human CNG channels (li2024inhibitionofa pages 6-7). The conservation of cGMP-gated channel-dependent sensory transduction from nematodes to vertebrates underscores the ancient evolutionary origin of this signaling mechanism. Orthologs of tax-2 and tax-4 in parasitic nematodes such as Brugia malayi can partially rescue sensory defects in C. elegans mutants, confirming functional conservation across nematode species (rosero2026afdthermosensoryneurons pages 13-14).

8. Summary

tax-2 encodes the β subunit of the principal cGMP-gated cation channel in C. elegans sensory neurons. Together with the α subunit TAX-4, it forms a heteromeric channel localized to sensory cilia that serves as the critical downstream effector in multiple cGMP-dependent signal transduction pathways. These pathways mediate chemosensation, thermosensation, oxygen sensing, and CO₂ sensing. The channel is gated by cGMP produced by specific guanylyl cyclases (ODR-1/DAF-11 for chemosensation; GCY-8/18/23 for thermosensation; GCY-35/36 for O₂; GCY-9 for CO₂), with signal termination provided by phosphodiesterases PDE-1, PDE-2, and PDE-5. Beyond acute sensory transduction, TAX-2/TAX-4 channel activity influences activity-dependent gene expression, dauer developmental decisions, and organismal lifespan through non-cell-autonomous activation of the UPR^ER in intestinal cells. TAX-2 can also partner with alternative α subunits such as CNG-3 for context-specific functions, revealing a previously unappreciated combinatorial logic in CNG channel function in this organism.

References

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(li2024inhibitionofa pages 6-7): Dongdong Li, Di Chen, Wei Li, and Guangshuo Ou. Inhibition of a cyclic nucleotide-gated channel on neuronal cilia activates unfolded protein response in intestinal cells to promote longevity. Proceedings of the National Academy of Sciences of the United States of America, Jun 2024. URL: https://doi.org/10.1073/pnas.2321228121, doi:10.1073/pnas.2321228121. This article has 3 citations and is from a highest quality peer-reviewed journal.
(rosero2026afdthermosensoryneurons pages 13-14): Manuel Rosero and Jihong Bai. Afd thermosensory neurons mediate tactile-dependent locomotion modulation in c. elegans. Apr 2026. URL: https://doi.org/10.7554/elife.106496, doi:10.7554/elife.106496. This article has 0 citations.
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(li2024inhibitionofa pages 7-8): Dongdong Li, Di Chen, Wei Li, and Guangshuo Ou. Inhibition of a cyclic nucleotide-gated channel on neuronal cilia activates unfolded protein response in intestinal cells to promote longevity. Proceedings of the National Academy of Sciences of the United States of America, Jun 2024. URL: https://doi.org/10.1073/pnas.2321228121, doi:10.1073/pnas.2321228121. This article has 3 citations and is from a highest quality peer-reviewed journal.
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(galande2025rolesofcyclic pages 12-13): Kranti K. Galande and Rick H. Cote. Roles of cyclic nucleotide phosphodiesterases in signal transduction pathways in the nematode caenorhabditis elegans. Cells, 14:1174, Jul 2025. URL: https://doi.org/10.3390/cells14151174, doi:10.3390/cells14151174. This article has 4 citations.
(galande2025rolesofcyclic pages 15-16): Kranti K. Galande and Rick H. Cote. Roles of cyclic nucleotide phosphodiesterases in signal transduction pathways in the nematode caenorhabditis elegans. Cells, 14:1174, Jul 2025. URL: https://doi.org/10.3390/cells14151174, doi:10.3390/cells14151174. This article has 4 citations.
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(ferkey2021chemosensorysignaltransduction pages 19-20): Denise M Ferkey, Piali Sengupta, and Noelle D L’Etoile. Chemosensory signal transduction in caenorhabditis elegans. Genetics, Mar 2021. URL: https://doi.org/10.1093/genetics/iyab004, doi:10.1093/genetics/iyab004. This article has 162 citations and is from a domain leading peer-reviewed journal.
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(fielenbach2008c.elegansdauer pages 3-5): Nicole Fielenbach and Adam Antebi. C. elegans dauer formation and the molecular basis of plasticity. Genes & development, 22 16:2149-65, Aug 2008. URL: https://doi.org/10.1101/gad.1701508, doi:10.1101/gad.1701508. This article has 741 citations and is from a highest quality peer-reviewed journal.
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(li2024inhibitionofa pages 1-2): Dongdong Li, Di Chen, Wei Li, and Guangshuo Ou. Inhibition of a cyclic nucleotide-gated channel on neuronal cilia activates unfolded protein response in intestinal cells to promote longevity. Proceedings of the National Academy of Sciences of the United States of America, Jun 2024. URL: https://doi.org/10.1073/pnas.2321228121, doi:10.1073/pnas.2321228121. This article has 3 citations and is from a highest quality peer-reviewed journal.
(hill2024molecularandneuronal pages 84-89): Tyler J Hill. Molecular and neuronal mechanisms underlying rapid experience-dependent plasticity in a single thermosensory neuron. Text, Jan 2024. URL: https://doi.org/10.48617/etd.1088, doi:10.48617/etd.1088. This article has 0 citations and is from a peer-reviewed journal.

Artifacts

Citations

mckemy2007temperaturesensingacross pages 3-4
ferkey2021chemosensorysignaltransduction pages 14-15
rosero2026afdthermosensoryneurons pages 13-14
menini2009theneurobiologyof pages 36-38
li2024inhibitionofa pages 7-8
galande2025rolesofcyclic pages 15-16
galande2025rolesofcyclic pages 7-9
galande2025rolesofcyclic pages 12-13
galande2025rolesofcyclic pages 13-15
lin2017longevitycontrolby pages 1-2
li2024inhibitionofa pages 6-7
hill2024molecularandneuronal pages 84-89
rosero2026afdthermosensoryneurons pages 14-15
ferkey2021chemosensorysignaltransduction pages 19-20
galande2025rolesofcyclic pages 9-10
li2024inhibitionofa pages 1-2
S727A
https://doi.org/10.1007/s00424-006-0199-6,
https://doi.org/10.1093/genetics/iyab004,
https://doi.org/10.1895/wormbook.1.123.1,
https://doi.org/10.1073/pnas.2321228121,
https://doi.org/10.7554/elife.106496,
https://doi.org/10.1201/9781420071993,
https://doi.org/10.3390/cells14151174,
https://doi.org/10.1101/gad.1701508,
https://doi.org/10.1016/j.tma.2017.07.001,
https://doi.org/10.48617/etd.1088,

📚 Additional Documentation

Notes

(tax-2-notes.md)

tax-2 (C. elegans) — Gene Review Notes

UniProt: G5EEE2 (G5EEE2_CAEEL) | WormBase: WBGene00006525 / F36F2.5 | 800 aa
Gene name origin: tax = taxis abnormal / thermotaxis-abnormal (isolated in chemotaxis/thermotaxis screens).

One-line summary

TAX-2 is the modulatory beta subunit of the sensory cyclic-nucleotide-gated (CNG) cation
channel of C. elegans. It partners with the alpha (pore-forming) subunit TAX-4 to form the
native heteromeric cGMP-gated channel in ciliated sensory neurons. TAX-2 cannot form a channel
on its own; it tunes the channel's ligand sensitivity and is required for its correct ciliary
localization.

Protein architecture (from UniProt G5EEE2)

Ion-transport (pore) domain: Pfam PF00520 (Ion_trans); TM helices predicted ~220-242, ~254-274 (Phobius).
Cyclic-nucleotide-binding domain (CNBD): Pfam PF00027; PROSITE PS50042 at residues 538-642.
InterPro family IPR050866 CNG_cation_channel; PANTHER PTHR45638 (Cyclic Nucleotide-Gated Cation Channel),
subfamily PTHR45638:SF1 "CYCLIC NUCLEOTIDE-GATED ION CHANNEL SUBUNIT B" — i.e. classified as a CNGB-type (beta) subunit.
TCDB 1.A.1.5.19 (voltage-gated ion channel VIC superfamily).
Long disordered N- and C-terminal regions (21-75; 733-800, acidic).

KNOWN (well-supported)

Molecular function / complex — CORE

TAX-4 (alpha) and TAX-2 (beta) form a heteromeric CNG channel when co-expressed in HEK293 cells;
TAX-2 does not form a channel on its own. Native channels are likely TAX-4/TAX-2 hetero-oligomers,
"with TAX-4 as the alpha subunit and TAX-2 acting as a modifying beta subunit."
PMID:10064800
PMID:10064800
TAX-2 is "most closely related to the beta subunits of the rod phototransduction channels."
PMID:10064800
The heteromer tunes ligand sensitivity: "The heteromeric TAX-4/TAX-2 channel is 25-fold less sensitive
to cGMP than the TAX-4 channel, but it remains highly selective for cGMP over cAMP." The heteromer and TAX-4
homomer also differ in divalent-cation block and single-channel properties.
PMID:10064800
Original genetics: "The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic nucleotide-gated
channel that is required for chemosensation, thermosensation and normal axon outgrowth of some sensory neurons."
PMID:9486798

Localization — CORE

Channel acts at the plasma membrane / sensory cilia of ciliated neurons. Ciliary targeting is
subunit- and cell-specific; sequences required for ciliary targeting/localization of the TAX-2 CNGB and
TAX-4 CNGA subunits have been mapped, and ciliary localization of the two subunits is interdependent.
PMID:23886944
PMID:23886944
The alpha subunit TAX-4::GFP is "partly localized at the sensory endings of these neurons."
PMID:8893027 (alpha subunit; supports plasma-membrane/sensory-ending localization of the channel)

Sensory physiology / behavior (channel-dependent) — NON-CORE (downstream of channel activity)

Chemotaxis: "tax-2 activity is required in the adult stage for normal chemotaxis to NaCl and odorants."
PMID:9486798
Sensory axon maintenance: "tax-2 and tax-4 are required for the maintenance of correct axon structure";
a ts tax-2 allele shows tax-2 activity is required in the adult to preserve axon morphology.
PMID:9486798
Oxygen sensing / aerotaxis / hyperoxia: "Avoidance of high oxygen levels by C. elegans requires the sensory
cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase homologue, gcy-35."
PMID:15220933
O2-evoked cGMP/Ca2+ dynamics & feedback: "Increased cGMP leads to a sustained Ca(2+) response in the neuron
that depends on cGMP-gated ion channels"; "Elevated levels of cGMP and Ca(2+) stimulate competing negative
feedback loops that shape cGMP dynamics."
PMID:23940325
PMID:23940325
Phototransduction (G-protein/cGMP pathway): light-evoked dark current in ASJ "was absent in the CNG channel
mutants tax-2 and tax-4"; LITE-1-dependent photocurrent "also required the GC DAF-11 and the CNG channel TAX-2 and TAX-4."
PMID:20436480
PMID:20436480
Thermosensation/thermotaxis: tax-2/tax-4 essential for thermosensation (PMID:10064800; PMID:9486798).
PMID:7630402 (Mori & Ohshima 1995, the WormBase original ref for thermosensory behavior) is the thermotaxis
neural-circuit paper; full text not in cache and the abstract centers on AFD/AIY/AIZ, but tax-2 was isolated
as a thermotaxis-abnormal (tax) mutant and its thermosensory requirement is independently established.
Chemosensory control of gene expression / neuroendocrine signaling: ASJ chemosensation of P. aeruginosa
metabolites "induces expression of the neuromodulator DAF-7/TGF-β"; TAX-2/TAX-4 is the ASJ chemosensory channel upstream.
PMID:25303524
Thermosensory-neuron gene expression / morphology: CMK-1 and TAX-4 (the channel TAX-2 co-forms) "regulate
gene expression, morphology, and functions of the AFD thermosensory neurons"; TAX-4 activity is required during
larval stages to maintain adult gene expression. (Abstract foregrounds TAX-4; TAX-2 is the obligate partner subunit.)
PMID:14711416

NOT known / uncertain (knowledge gaps)

Mechanism of beta-subunit modulation. How TAX-2 lowers cGMP sensitivity ~25-fold and alters divalent-cation
block / single-channel behavior of the heteromer, and whether it does so uniformly across neurons, is not
mechanistically defined. PMID:10064800
Does the TAX-2 CNBD bind cGMP? TAX-2 has a CNBD (PS50042, 538-642) and carries an IBA cGMP-binding annotation,
but direct biochemical cGMP binding by the TAX-2 CNBD (vs. a modulatory/non-canonical CNBD as in vertebrate CNGB)
has not been demonstrated. Beta-subunit CNBDs are frequently ligand-non-functional or purely modulatory.
In vivo stoichiometry & composition. The endogenous TAX-4:TAX-2 ratio and whether composition varies between
neurons or ciliary subcompartments is inferred, not measured; ciliary localization is subunit-interdependent
and heterogeneous across the cilium. [PMID:23886944, PMID:10064800]
tax-2 roles independent of tax-4. Because TAX-2 forms no channel alone, an independent conductance is unlikely,
but whether TAX-2 has tax-4-independent roles (e.g. partnering other alpha subunits such as CNG-1/CNG-3, or
non-conducting/structural/trafficking roles) is unresolved. [PMID:10064800, PMID:23886944]

Ligand-selectivity note (affects cAMP annotation)

The native worm heteromer "remains highly selective for cGMP over cAMP" PMID:10064800. The GOA IBA term
GO:0005222 (intracellularly cAMP-activated cation channel activity) is inherited from the PANTHER CNG family
(which contains cAMP-responsive vertebrate CNGA2) and over-states cAMP as a physiological ligand for the worm
TAX-2/TAX-4 channel → marked as over-annotated.

Annotation plan (25 GOA annotations)

CORE channel MF/CC/transport (ACCEPT): plasma membrane (IBA), cGMP binding (IBA), monoatomic cation
transmembrane transport (IBA+IDA), intracellular cyclic-nucleotide-activated cation channel complex (IBA),
monoatomic ion channel activity (IEA), intracellularly cyclic-nucleotide-activated monoatomic cation channel
activity (IEA), monoatomic ion transport (IEA), transmembrane transport (IEA), membrane (IEA),
intracellularly cGMP-activated cation channel activity (IDA, contributes_to), cation channel complex (IDA, part_of).
OVER-ANNOTATED: intracellularly cAMP-activated cation channel activity (IBA) — heteromer is cGMP-selective.
NON-CORE (KEEP): positive regulation of gene expression, GPCR signaling coupled to cGMP, negative regulation of
receptor guanylyl cyclase signaling, response to oxygen levels, thermosensory behavior, chemotaxis, regulation of
axon extension, aerotaxis, response to hyperoxia, regulation of neuron differentiation, positive regulation of
transcription by RNA Pol II, neuron projection morphogenesis. All are downstream sensory outputs of channel activity.

Cross-reference

tax-4 (alpha subunit) review: PR #1716 (branch claude/worm-tax-4-review). Channel-complex modeling kept consistent:
in_complex = GO:0017071; native channel is TAX-4/TAX-2 heteromer. tax-2 modeled with contributes_to_molecular_function
to reflect its beta/modulatory (non-pore-forming-alone) role.

📄 View Raw YAML

id: G5EEE2
gene_symbol: tax-2
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:6239
  label: Caenorhabditis elegans
description: >-
  tax-2 encodes the modulatory beta subunit of a cyclic nucleotide-gated (CNG)
  cation channel, the C. elegans ortholog of vertebrate CNGB subunits. TAX-2
  cannot form a functional channel on its own; in vivo it co-assembles with the
  alpha (principal, pore-forming) subunit TAX-4 to build the native sensory CNG
  channel, a heteromer that is opened directly by intracellular cGMP and conducts
  Ca2+, Na+ and K+ across the membrane. As a beta subunit TAX-2 tunes rather than
  solely forms the pore: the TAX-4/TAX-2 heteromer is markedly (about 25-fold)
  less cGMP-sensitive than the TAX-4 homomer while remaining highly selective for
  cGMP over cAMP, and the two forms differ in divalent-cation block and
  single-channel behavior. TAX-2 harbors a pore/ion-transport domain and a
  C-terminal cyclic-nucleotide-binding domain, and is targeted, together with
  TAX-4, to the plasma membrane and to specific subcompartments of the non-motile
  sensory cilia of ciliated neurons, where their ciliary localization is mutually
  interdependent. As part of the effector channel of cGMP-mediated sensory signal
  transduction, TAX-2 is required for a broad range of sensory modalities,
  including chemosensation and chemotaxis, thermosensation, oxygen sensing and
  aerotaxis, and light responses; channel activity also feeds into downstream
  outputs such as sensory-neuron gene expression and the maintenance of sensory
  axon structure.
existing_annotations:
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      The TAX-2/TAX-4 CNG channel acts at the plasma membrane of ciliated sensory
      neurons (including the ciliary membrane). Core cellular component.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:23886944
      supporting_text: >-
        identify sequences required for efficient ciliary targeting and
        localization of the TAX-2 CNGB and TAX-4 CNGA subunits.
- term:
    id: GO:0005222
    label: intracellularly cAMP-activated cation channel activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      This IBA term is inherited from the PANTHER CNG family (which includes
      cAMP-responsive vertebrate CNGA subunits). The native C. elegans TAX-4/TAX-2
      heteromer is highly selective for cGMP over cAMP, so cAMP-activated cation
      channel activity over-states cAMP as a physiological ligand for this channel;
      cGMP is the relevant activating ligand. Marked as over-annotated rather than
      removed because CNG channels retain weak cAMP responsiveness in vitro.
    action: MARK_AS_OVER_ANNOTATED
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        it remains highly selective for cGMP over cAMP.
    propagation_review:
      root_cause: TERM_SCOPING_PROBLEM
      failure_modes:
      - FUNCTIONAL_DIVERGENCE
      source_entities:
      - source_id: PANTHER:PTHR45638
        source_label: Cyclic Nucleotide-Gated Cation Channel
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: >-
          Family includes cAMP-responsive vertebrate CNGA subunits, but the
          C. elegans TAX-4/TAX-2 channel is cGMP-selective, so cAMP-activated
          activity should not propagate to tax-2.
- term:
    id: GO:0030553
    label: cGMP binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      TAX-2 contains a C-terminal cyclic-nucleotide-binding domain (CNBD;
      PROSITE PS50042, residues 538-642) consistent with cyclic-nucleotide sensing
      by the CNG channel it forms with TAX-4. cGMP is the physiological ligand of
      the heteromer. Retained as a core-consistent function of the channel, though
      direct biochemical cGMP binding by the TAX-2 CNBD specifically (as opposed to
      the TAX-4 alpha subunit) has not been demonstrated (see knowledge gaps).
    action: ACCEPT
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        it remains highly selective for cGMP over cAMP.
- term:
    id: GO:0098655
    label: monoatomic cation transmembrane transport
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      The CNG channel conducts cations (Ca2+/Na+/K+) across the membrane; TAX-2 is
      part of that channel. Core transport process.
    action: ACCEPT
- term:
    id: GO:0017071
    label: intracellular cyclic nucleotide activated cation channel complex
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: part_of
  review:
    summary: >-
      TAX-2 is a subunit of the intracellular cyclic-nucleotide-activated cation
      channel complex (the TAX-4/TAX-2 heteromer). Core cellular component/complex.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed
        in HEK293 cells, but TAX-2 does not form a channel on its own.
- term:
    id: GO:0005216
    label: monoatomic ion channel activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Consistent with TAX-2 being a subunit of an ion channel (Pfam PF00520
      ion-transport/pore domain). Accurate but general parent term; the specific
      channel activity is captured by GO:0005223.
    action: ACCEPT
- term:
    id: GO:0005221
    label: intracellularly cyclic nucleotide-activated monoatomic cation channel activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      Correct at the level of the cyclic-nucleotide-gated channel TAX-2 helps form;
      the more specific cGMP-activated term (GO:0005223) is the precise function.
    action: ACCEPT
- term:
    id: GO:0006811
    label: monoatomic ion transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      General parent of the channel's cation-transport process. Accurate but broad.
    action: ACCEPT
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: >-
      TAX-2 is a multi-pass membrane protein; correct but less informative than the
      plasma-membrane/ciliary-membrane localization.
    action: ACCEPT
- term:
    id: GO:0055085
    label: transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      General parent of the channel's transmembrane cation-transport process.
      Accurate but broad.
    action: ACCEPT
- term:
    id: GO:0005223
    label: intracellularly cGMP-activated cation channel activity
  evidence_type: IDA
  original_reference_id: PMID:10064800
  qualifier: contributes_to
  review:
    summary: >-
      Core molecular function. Functional reconstitution in HEK293 cells shows that
      TAX-2 co-assembles with the alpha subunit TAX-4 into a cGMP-gated cation
      channel; TAX-2 does not form a channel alone and acts as a modifying beta
      subunit that lowers the heteromer's cGMP sensitivity. The 'contributes_to'
      qualifier correctly reflects that TAX-2 participates in, rather than
      independently enables, this channel activity.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        with TAX-4 as the alpha subunit and TAX-2 acting as a modifying beta
        subunit.
    - reference_id: PMID:10064800
      supporting_text: >-
        The heteromeric TAX-4/TAX-2 channel is 25-fold less sensitive to cGMP than
        the TAX-4 channel, but it remains highly selective for cGMP over cAMP.
- term:
    id: GO:0034703
    label: cation channel complex
  evidence_type: IDA
  original_reference_id: PMID:10064800
  qualifier: part_of
  review:
    summary: >-
      TAX-2 is part of the reconstituted heteromeric cation channel. Core cellular
      component/complex, directly supported.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed
        in HEK293 cells, but TAX-2 does not form a channel on its own.
- term:
    id: GO:0010628
    label: positive regulation of gene expression
  evidence_type: IMP
  original_reference_id: PMID:25303524
  qualifier: involved_in
  review:
    summary: >-
      Downstream, activity-dependent output. In the ASJ chemosensory neuron the
      TAX-2/TAX-4 channel transduces detection of P. aeruginosa secondary
      metabolites, leading to induced expression of the DAF-7/TGF-beta
      neuromodulator. This is an indirect consequence of sensory channel activity,
      not a direct gene-regulatory function of TAX-2. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:25303524
      supporting_text: >-
        induces expression of the neuromodulator DAF-7/TGF-β
- term:
    id: GO:0007199
    label: G protein-coupled receptor signaling pathway coupled to cGMP nucleotide
      second messenger
  evidence_type: IMP
  original_reference_id: PMID:20436480
  qualifier: involved_in
  review:
    summary: >-
      In ASJ phototransduction, a G-protein/cGMP cascade opens cGMP-sensitive CNG
      channels; light-evoked currents are abolished in tax-2 (and tax-4) mutants,
      placing the channel as the effector downstream of the GPCR-coupled cGMP
      pathway. Non-core downstream signaling context.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:20436480
      supporting_text: >-
        This dark current was apparently carried by CNG channels, as it can be
        blocked by the CNG channel-specific inhibitor L-cis-diltiazem and was absent
        in the CNG channel mutants tax-2 and tax-4
- term:
    id: GO:0010754
    label: negative regulation of receptor guanylyl cyclase signaling pathway
  evidence_type: IMP
  original_reference_id: PMID:23940325
  qualifier: involved_in
  review:
    summary: >-
      In vivo cGMP/Ca2+ imaging of an O2-sensing neuron shows that cGMP-gated
      channels participate in negative-feedback loops that shape cGMP signaling
      dynamics. This reflects the channel's role within feedback regulation of the
      cGMP pathway rather than a direct enzymatic regulation of guanylyl cyclase.
      Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:23940325
      supporting_text: >-
        Elevated levels of cGMP and Ca(2+) stimulate competing negative feedback
        loops that shape cGMP dynamics.
- term:
    id: GO:0070482
    label: response to oxygen levels
  evidence_type: IMP
  original_reference_id: PMID:23940325
  qualifier: involved_in
  review:
    summary: >-
      O2-evoked rises in cGMP drive a sustained Ca2+ response that depends on
      cGMP-gated ion channels (the TAX-2/TAX-4 channel). A downstream sensory
      response mediated by channel activity. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:23940325
      supporting_text: >-
        Increased cGMP leads to a sustained Ca(2+) response in the neuron that
        depends on cGMP-gated ion channels.
- term:
    id: GO:0040040
    label: thermosensory behavior
  evidence_type: IMP
  original_reference_id: PMID:7630402
  qualifier: involved_in
  review:
    summary: >-
      tax-2 was isolated as a thermotaxis-abnormal (tax) mutant, and the TAX-2/TAX-4
      channel is essential for thermosensation in the AFD thermosensory neuron. This
      is a downstream sensory behavior mediated by channel activity. Non-core. The
      cited 1995 thermotaxis paper is the WormBase original reference; its cached
      record is abstract-only and centers on the AFD/AIY/AIZ circuit, but the tax-2
      thermosensory requirement is independently established (PMID:10064800,
      PMID:9486798); annotation retained (defer to curator full-text) rather than
      removed.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:9486798
      supporting_text: >-
        The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic
        nucleotide-gated channel that is required for chemosensation, thermosensation
        and normal axon outgrowth of some sensory neurons.
- term:
    id: GO:0098655
    label: monoatomic cation transmembrane transport
  evidence_type: IDA
  original_reference_id: PMID:10064800
  qualifier: involved_in
  review:
    summary: >-
      Directly supported by functional reconstitution: the heteromeric channel
      conducts cations across the membrane and differs from the TAX-4 homomer in
      divalent-cation block and single-channel properties. Core transport process.
    action: ACCEPT
    supported_by:
    - reference_id: PMID:10064800
      supporting_text: >-
        The heteromeric channel and the TAX-4 homomeric channel differ in their
        blockage by divalent cations and in their single channel properties.
- term:
    id: GO:0006935
    label: chemotaxis
  evidence_type: IMP
  original_reference_id: PMID:9486798
  qualifier: involved_in
  review:
    summary: >-
      tax-2 activity is required for normal chemotaxis to NaCl and odorants, a
      downstream sensory behavior mediated by the CNG channel. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:9486798
      supporting_text: >-
        tax-2 activity is required in the adult stage for normal chemotaxis to NaCl
        and odorants.
- term:
    id: GO:0030516
    label: regulation of axon extension
  evidence_type: IMP
  original_reference_id: PMID:9486798
  qualifier: involved_in
  review:
    summary: >-
      tax-2 (with tax-4) is required to maintain correct sensory axon structure;
      in mutants, axon outgrowth resumes inappropriately in late larval/adult
      stages. A distinct, activity-dependent role of the channel in maintaining
      axon morphology. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:9486798
      supporting_text: >-
        These results indicate that tax-2 and tax-4 are required for the maintenance
        of correct axon structure
- term:
    id: GO:0009454
    label: aerotaxis
  evidence_type: IMP
  original_reference_id: PMID:15220933
  qualifier: involved_in
  review:
    summary: >-
      Avoidance of high oxygen (aerotaxis) requires the sensory cGMP-gated channel
      tax-2/tax-4 together with the soluble guanylate cyclase gcy-35. Downstream
      sensory behavior mediated by channel activity. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:15220933
      supporting_text: >-
        Avoidance of high oxygen levels by C. elegans requires the sensory
        cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase
        homologue, gcy-35.
- term:
    id: GO:0055093
    label: response to hyperoxia
  evidence_type: IMP
  original_reference_id: PMID:15220933
  qualifier: involved_in
  review:
    summary: >-
      The tax-2/tax-4 channel mediates responses that allow avoidance of hyperoxic
      (high-O2) environments. Downstream sensory response. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:15220933
      supporting_text: >-
        Avoidance of high oxygen levels by C. elegans requires the sensory
        cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase
        homologue, gcy-35.
- term:
    id: GO:0045664
    label: regulation of neuron differentiation
  evidence_type: IMP
  original_reference_id: PMID:14711416
  qualifier: involved_in
  review:
    summary: >-
      An indirect, activity-dependent effect: loss of the TAX-4 channel (which
      TAX-2 co-forms) alters AFD thermosensory-neuron gene expression and
      morphology. The cited paper's abstract foregrounds TAX-4, but TAX-2 is its
      obligate heteromeric partner; this is a downstream consequence of channel
      activity rather than a direct differentiation function. Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:14711416
      supporting_text: >-
        the TAX-4 cyclic nucleotide-gated channel regulate gene expression,
        morphology, and functions of the AFD thermosensory neurons
- term:
    id: GO:0045944
    label: positive regulation of transcription by RNA polymerase II
  evidence_type: IMP
  original_reference_id: PMID:14711416
  qualifier: involved_in
  review:
    summary: >-
      Indirect, activity-dependent regulation of AFD-specific gene expression by
      the CNG channel; TAX-2 has no direct transcriptional activity. Retained as a
      downstream effect (abstract foregrounds TAX-4, the obligate partner subunit).
      Non-core.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:14711416
      supporting_text: >-
        the TAX-4 cyclic nucleotide-gated channel regulate gene expression,
        morphology, and functions of the AFD thermosensory neurons
- term:
    id: GO:0048812
    label: neuron projection morphogenesis
  evidence_type: IMP
  original_reference_id: PMID:14711416
  qualifier: involved_in
  review:
    summary: >-
      Indirect effect on AFD sensory-neuron morphology consequent on loss of CNG
      channel activity. Downstream/non-core; abstract foregrounds TAX-4, the
      obligate partner subunit of TAX-2.
    action: KEEP_AS_NON_CORE
    supported_by:
    - reference_id: PMID:14711416
      supporting_text: >-
        the TAX-4 cyclic nucleotide-gated channel regulate gene expression,
        morphology, and functions of the AFD thermosensory neurons
- term:
    id: GO:0097730
    label: non-motile cilium
  evidence_type: IDA
  original_reference_id: PMID:23886944
  qualifier: located_in
  review:
    summary: >-
      Proposed new annotation. The TAX-2 CNGB subunit is targeted to and localized
      within the non-motile sensory cilia of ciliated neurons; sequences required
      for ciliary targeting/localization of TAX-2 (and TAX-4) were mapped, and the
      subunits are relatively immobile in specific ciliary subcompartments. Core
      cellular component not currently in the GOA set.
    action: NEW
    supported_by:
    - reference_id: PMID:23886944
      supporting_text: >-
        identify sequences required for efficient ciliary targeting and
        localization of the TAX-2 CNGB and TAX-4 CNGA subunits.
- term:
    id: GO:0007606
    label: sensory perception of chemical stimulus
  evidence_type: IMP
  original_reference_id: PMID:9486798
  qualifier: involved_in
  review:
    summary: >-
      Proposed new annotation summarizing the channel's core sensory role: tax-2 is
      required for chemosensation (chemotaxis to NaCl and odorants), a form of
      sensory perception of chemical stimulus mediated by the CNG channel. Captures
      the core sensory-transduction process at an appropriate level of generality.
    action: NEW
    supported_by:
    - reference_id: PMID:9486798
      supporting_text: >-
        The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic
        nucleotide-gated channel that is required for chemosensation, thermosensation
        and normal axon outgrowth of some sensory neurons.
core_functions:
- description: >-
    TAX-2 is the modulatory beta subunit of the sensory cyclic nucleotide-gated
    (CNG) cation channel. It cannot form a channel on its own; it co-assembles with
    the alpha (pore-forming) subunit TAX-4 into the native heteromeric channel that
    is directly opened by intracellular cGMP and conducts cations (Ca2+/Na+/K+)
    across the plasma/ciliary membrane of ciliated sensory neurons. As a beta
    subunit TAX-2 contributes ion-channel (pore) structure and tunes the channel:
    the TAX-4/TAX-2 heteromer is ~25-fold less cGMP-sensitive than the TAX-4
    homomer and differs in divalent-cation block and single-channel behavior. This
    channel is the effector of cGMP-mediated sensory signal transduction.
  molecular_function:
    id: GO:0005216
    label: monoatomic ion channel activity
  contributes_to_molecular_function:
    id: GO:0005223
    label: intracellularly cGMP-activated cation channel activity
  directly_involved_in:
  - id: GO:0098655
    label: monoatomic cation transmembrane transport
  - id: GO:0007606
    label: sensory perception of chemical stimulus
  locations:
  - id: GO:0005886
    label: plasma membrane
  - id: GO:0097730
    label: non-motile cilium
  in_complex:
    id: GO:0017071
    label: intracellular cyclic nucleotide activated cation channel complex
  supported_by:
  - reference_id: PMID:10064800
    supporting_text: >-
      Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed
      in HEK293 cells, but TAX-2 does not form a channel on its own.
  - reference_id: PMID:10064800
    supporting_text: >-
      with TAX-4 as the alpha subunit and TAX-2 acting as a modifying beta subunit.
  - reference_id: PMID:23886944
    supporting_text: >-
      identify sequences required for efficient ciliary targeting and localization
      of the TAX-2 CNGB and TAX-4 CNGA subunits.
knowledge_gaps:
- gap_statement: >-
    How the TAX-2 beta subunit mechanistically modulates the channel — lowering
    cGMP sensitivity ~25-fold and changing divalent-cation block and single-channel
    behavior relative to the TAX-4 homomer — and whether this tuning is uniform
    across neurons, is not defined.
  boundary: >-
    In heterologous cells the TAX-4/TAX-2 heteromer is 25-fold less cGMP-sensitive
    than the TAX-4 homomer yet stays cGMP-selective, and the two forms differ in
    divalent-cation block and single-channel properties; the structural/mechanistic
    basis of this modulation by TAX-2 is not established.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    The beta subunit sets the channel's cGMP operating range and ion handling, so
    how TAX-2 tunes gating directly determines each sensory neuron's stimulus
    threshold and dynamic range.
  resolution: >-
    Structure/function of the TAX-4/TAX-2 heteromer (cryo-EM and mutagenesis of
    TAX-2 CNBD/pore residues) with side-by-side homomer-versus-heteromer
    electrophysiology.
  provenance:
  - reference_id: PMID:10064800
    supporting_text: >-
      The heteromeric TAX-4/TAX-2 channel is 25-fold less sensitive to cGMP than the
      TAX-4 channel, but it remains highly selective for cGMP over cAMP.
- gap_statement: >-
    Whether the TAX-2 cyclic-nucleotide-binding domain itself binds cGMP and
    whether that binding is functional (vs. a modulatory/non-canonical CNBD as in
    vertebrate CNGB beta subunits) is unknown.
  boundary: >-
    TAX-2 has a predicted CNBD (PROSITE PS50042, residues 538-642) and carries an
    IBA cGMP-binding annotation, but direct cGMP binding by the TAX-2 CNBD (as
    opposed to the TAX-4 alpha subunit) has not been demonstrated biochemically.
  gap_kind:
  - BIOLOGY
  dark_aspect: MF_DARK
  status: OPEN
  significance: >-
    Whether TAX-2's CNBD is a functional ligand sensor or a purely structural
    modulator changes how the heteromer integrates cGMP and how its sensitivity is
    set.
  resolution: >-
    Direct ligand-binding assays on isolated/mutated TAX-2 CNBD and CNBD-mutant
    channel electrophysiology.
  provenance:
  - reference_id: PMID:10064800
    supporting_text: >-
      it remains highly selective for cGMP over cAMP.
- gap_statement: >-
    The endogenous TAX-4:TAX-2 subunit stoichiometry and whether channel
    composition varies between neuron types or between ciliary subcompartments of a
    single neuron is inferred, not measured.
  boundary: >-
    TAX-4 + TAX-2 co-assemble into a functional heteromer in HEK293 cells while
    TAX-2 alone cannot form a channel; in vivo, ciliary localization of the two
    subunits is interdependent and heterogeneous across the cilium, but the native
    stoichiometry is not quantified.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    Subunit composition sets ligand sensitivity, ion handling and localization, so
    it determines how each sensory neuron converts a cGMP signal into a response.
  resolution: >-
    Quantitative in vivo subunit-copy-number measurement (e.g. calibrated
    fluorescence or single-molecule counting) per neuron and per ciliary
    subcompartment.
  provenance:
  - reference_id: PMID:23886944
    supporting_text: >-
      CNG channel subunits can be localized to discrete ciliary compartments in
      individual sensory neurons in C. elegans, suggesting that channel composition
      is heterogeneous across the cilium.
  - reference_id: PMID:10064800
    supporting_text: >-
      most of the native channels in C. elegans are likely to be hetero-oligomers of
      TAX-4 and TAX-2 subunits
- gap_statement: >-
    Whether TAX-2 has roles independent of TAX-4 — for example partnering other CNG
    alpha subunits, or non-conducting structural/trafficking roles — is unresolved.
  boundary: >-
    TAX-2 forms no channel on its own, so an independent TAX-2 conductance is
    unlikely; but ciliary localization is subunit-interdependent and TAX-2 is
    expressed in cells where its full complement of partners and any TAX-4-independent
    contribution have not been mapped.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    Distinguishing an obligate TAX-4 partner from a subunit with broader partnerships
    or scaffolding roles would clarify which sensory functions strictly require the
    canonical TAX-4/TAX-2 heteromer.
  resolution: >-
    Interaction proteomics and cell-specific epistasis (tax-2 vs tax-4 vs other CNG
    subunits) with functional imaging in defined neurons.
  provenance:
  - reference_id: PMID:10064800
    supporting_text: >-
      Here we show that TAX-4 and TAX-2 form a heteromeric channel when expressed in
      HEK293 cells, but TAX-2 does not form a channel on its own.
suggested_questions:
- question: >-
    Does the TAX-2 CNBD bind cGMP, and is that binding required for the beta
    subunit's ~25-fold reduction of channel cGMP sensitivity?
- question: >-
    What is the endogenous TAX-4:TAX-2 stoichiometry in native sensory cilia, and
    does it differ between neuron types or ciliary subcompartments?
suggested_experiments:
- description: >-
    Reconstitute and solve the structure of the TAX-4/TAX-2 heteromer (cryo-EM) and
    perform side-by-side electrophysiology of TAX-4 homomer versus TAX-4/TAX-2
    heteromer with targeted mutations in the TAX-2 CNBD and pore, to define how TAX-2
    tunes cGMP sensitivity, ion selectivity and divalent-cation block.
- description: >-
    Quantify TAX-2 and TAX-4 subunit copy number/ratio in defined sensory cilia in
    vivo (calibrated fluorescence or single-molecule counting) and correlate with
    cell-specific cGMP dose-response to map homomeric versus heteromeric channels
    onto neurons.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:10064800
  title: Functional reconstitution of a heteromeric cyclic nucleotide-gated channel
    of Caenorhabditis elegans in cultured cells.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified. The definitive TAX-2 paper: TAX-4 and TAX-2 form a heteromeric
      cGMP-gated channel in HEK293 cells, TAX-2 cannot form a channel alone and acts
      as a modifying beta subunit, and the heteromer is ~25-fold less cGMP-sensitive
      than the TAX-4 homomer but stays highly selective for cGMP over cAMP. Grounds the
      core molecular function/complex, the beta-subunit modulation, and the cAMP
      over-annotation call.
- id: PMID:14711416
  title: The CMK-1 CaMKI and the TAX-4 Cyclic nucleotide-gated channel regulate thermosensory
    neuron gene expression and function in C. elegans.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Assays the AFD CNG channel; loss of channel activity alters AFD-specific gene
      expression, morphology and function. Abstract foregrounds the TAX-4 alpha
      subunit, but TAX-2 is its obligate heteromeric partner; supports the indirect,
      activity-dependent gene-expression / neuron-differentiation / morphology
      non-core annotations. Cached abstract-only.
- id: PMID:15220933
  title: Oxygen sensation and social feeding mediated by a C. elegans guanylate cyclase
    homologue.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Explicitly names the sensory cGMP-gated channel tax-2/tax-4 as required for
      high-O2 avoidance, together with the soluble guanylate cyclase gcy-35. Solid
      support for the aerotaxis and response-to-hyperoxia non-core annotations.
      Cached abstract-only.
- id: PMID:20436480
  title: C. elegans phototransduction requires a G protein-dependent cGMP pathway
    and a taste receptor homolog.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Full text (PMC) shows ASJ light-evoked currents are abolished in tax-2 and
      tax-4 mutants and that the LITE-1-dependent photocurrent requires the CNG
      channel TAX-2 and TAX-4 downstream of a G-protein/cGMP cascade. Supports the
      GPCR-cGMP-second-messenger non-core annotation.
- id: PMID:23886944
  title: Cell- and subunit-specific mechanisms of CNG channel ciliary trafficking
    and localization in C. elegans.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Directly studies the TAX-2 CNGB (and TAX-4 CNGA) subunit: maps sequences
      required for ciliary targeting/localization, shows subunit-interdependent and
      heterogeneous ciliary localization, and that TAX-2/TAX-4 are relatively immobile
      in cilia subcompartments. Grounds the plasma-membrane/ciliary localization and
      the in vivo composition/stoichiometry knowledge gap.
- id: PMID:23940325
  title: In vivo genetic dissection of O2-evoked cGMP dynamics in a Caenorhabditis
    elegans gas sensor.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      In vivo cGMP/Ca2+ imaging showing cGMP-gated channels are required for the
      sustained O2-evoked Ca2+ response and participate in negative-feedback loops.
      Supports the response-to-oxygen and negative-regulation-of-rGC-signaling non-core
      annotations. Cached abstract-only.
- id: PMID:25303524
  title: Chemosensation of bacterial secondary metabolites modulates neuroendocrine
    signaling and behavior of C. elegans.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      ASJ chemosensory detection of P. aeruginosa metabolites induces DAF-7/TGF-beta
      expression; the TAX-2/TAX-4 channel is the ASJ chemosensory effector upstream.
      Supports the positive-regulation-of-gene-expression non-core annotation. Cached
      abstract-only (does not name tax-2 in the abstract; retained per curator full text).
- id: PMID:7630402
  title: Neural regulation of thermotaxis in Caenorhabditis elegans.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      WormBase original reference for the thermosensory-behavior IMP. The 1995 paper
      dissects the AFD/AIY/AIZ thermotaxis circuit; its cached record is abstract-only
      and does not name tax-2, but tax-2 was isolated as a thermotaxis-abnormal mutant
      and its thermosensory requirement is independently established (PMID:10064800,
      PMID:9486798). Annotation retained (defer to curator full text) rather than removed.
- id: PMID:8893027
  title: Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation
    and chemosensation in C. elegans.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Foundational tax-4 paper; TAX-4::GFP localizes to sensory endings of the neurons.
      Used here as corroboration for the channel's plasma-membrane/sensory-ending
      localization (alpha subunit; TAX-2 is the obligate heteromeric partner). Cached
      abstract-only.
- id: PMID:9486798
  title: 'A cyclic nucleotide-gated channel inhibits sensory axon outgrowth in larval
    and adult Caenorhabditis elegans: a distinct pathway for maintenance of sensory
    axon structure.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Directly assays tax-2: a temperature-sensitive tax-2 allele shows tax-2 activity
      is required in the adult for chemotaxis to NaCl/odorants and for maintenance of
      correct sensory axon structure. Grounds the chemotaxis and regulation-of-axon-
      extension non-core annotations and the tax-2/tax-4 channel-subunit framing.
      Cached abstract-only.

tax-2

Gene Description

Existing Annotations Review

Core Functions

References

Suggested Questions for Experts

Suggested Experiments

Knowledge Gaps

Deep Research

Falcon

Comprehensive Research Report: tax-2 (UniProt G5EEE2) in Caenorhabditis elegans

1. Gene Identity and Protein Overview

2. Molecular Function and Channel Biophysics

2.1 Primary Function

2.2 Subunit Composition and Biophysical Properties

2.3 Alternative Subunit Partnerships

3. Expression Pattern and Subcellular Localization

3.1 Neuronal Expression

3.2 Subcellular Localization

4. Signaling Pathways

4.1 Chemosensation (Olfaction and Gustation)

4.2 Thermosensation

4.3 Oxygen Sensing (Aerotaxis)

4.4 CO₂ Sensing

4.5 Negative Feedback by Phosphodiesterases

5. Downstream Physiological Roles

5.1 Dauer Formation

5.2 Longevity

5.3 Activity-Dependent Gene Expression

5.4 Tactile-Dependent Behavioral Modulation

6. Genetic Alleles and Mutant Phenotypes

7. Evolutionary Context

8. Summary

Artifacts

Citations

📚 Additional Documentation

Notes

tax-2 (C. elegans) — Gene Review Notes

One-line summary

Protein architecture (from UniProt G5EEE2)

KNOWN (well-supported)

Molecular function / complex — CORE

Localization — CORE

Sensory physiology / behavior (channel-dependent) — NON-CORE (downstream of channel activity)

NOT known / uncertain (knowledge gaps)

Ligand-selectivity note (affects cAMP annotation)

Annotation plan (25 GOA annotations)

Cross-reference

📄 View Raw YAML