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.
| 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 Β· 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-Ξ² |
| 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. |
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Download this section (compressed HTML)Q: Does the TAX-2 CNBD bind cGMP, and is that binding required for the beta subunit's ~25-fold reduction of channel cGMP sensitivity?
Q: What is the endogenous TAX-4:TAX-2 stoichiometry in native sensory cilia, and does it differ between neuron types or ciliary subcompartments?
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.
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: How the TAX-2 beta subunit mechanistically modulates the channel β lowering cGMP sensitivity ~25-fold and changing divalent-cation block and single-channel behavior relative to the TAX-4 homomer β 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):
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):
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):
Gap: Whether TAX-2 has roles independent of TAX-4 β for example partnering other CNG alpha subunits, or non-conducting structural/trafficking roles β 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):
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