tax-4 (Q03611, CNG_CAEEL) research notes

Research journal for the C. elegans gene tax-4 (WormBase WBGene00006526; ZC84.2;
UniProt Q03611). Provenance is recorded inline as [PMID:xxxx "verbatim quote"].

Identity / one-line

TAX-4 is the alpha (principal, pore-forming) subunit of a cyclic nucleotide-gated
(CNG) cation channel. It is directly gated by intracellular cGMP (and, less
sensitively, cAMP), conducts Ca2+/Na+/K+ cations, and localizes to the sensory cilia
(and dendrites) of many ciliated sensory neurons, where it is the effector channel of
cGMP-based sensory transduction. TAX-4 forms functional homotetramers in vitro and
in structural studies, and in vivo partners with the modulatory beta subunit TAX-2
to form the native heteromeric sensory channel.

KNOWN (well supported)

Molecular function: cGMP-gated cation channel (alpha subunit)

• TAX-4 alone forms a functional, highly cGMP-sensitive channel when expressed in
  mammalian cells; TAX-2 alone does not.
  PMID:10064800
• TAX-4 = alpha subunit, TAX-2 = modifying beta subunit; the native channel is a
  hetero-oligomer.
  PMID:10064800
• The heteromeric channel is ~25-fold less cGMP-sensitive than the TAX-4 homomer but
  stays cGMP-selective; the two forms also differ in divalent-cation block and single-
  channel properties.
  PMID:10064800
• Original genetic/expression characterization: tax-4 mutants fail multiple sensory
  behaviors; TAX-4 is homologous to vertebrate CNG channels and functions as a CNG
  channel in cultured cells; cGMP is the intracellular messenger.
  PMID:8893027

Structure (all solved on the C. elegans TAX-4 protein)

• cryo-EM of the C. elegans CNG channel in the cGMP-bound open state; homotetramer with
  an unusual voltage-sensor-like domain (deficient voltage dependence); C-linker gating
  ring couples cyclic-nucleotide binding to the gate; selectivity filter lined by a
  functionally important glutamate + backbone carbonyls.
  PMID:28099415
  PMID:28099415
• Mechanism of ligand (cGMP) activation; open/closed states; central-gate residues
  (F403, V407) and CNBD contacts characterized by mutagenesis.
  PMID:32483338
• UniProt SUBUNIT: Homotetramer. {ECO:0000269|PubMed:28099415, ...:32483338, ...:35233102}.
  Note the IPI identical protein binding GOA annotations (PMID:28099415, PMID:32483338)
  reflect this homotetramer self-association.

Cellular localization

• Expressed at the sensory endings of thermosensory, gustatory, and olfactory neurons;
  TAX-4::GFP partly localized to sensory endings.
  PMID:8893027
• Localizes to (non-motile) sensory cilia; ciliary trafficking/localization of the
  TAX-4 CNGA and TAX-2 CNGB subunits is cell- and subunit-specific.
  PMID:23886944
• In AFD, the cGMP-gated channel is anchored within the proximal ciliary region by
  NPHP-2/inversin — the ciliary "inversin compartment".
  PMID:25335890
• Cilia membrane composition/morphogenesis context (TUB-1/Tubby); CNG channel is a
  ciliary signaling cargo. PMID:31259686 (localization/tissue-specificity reference in
  UniProt; SUBCELLULAR LOCATION Cell projection, cilium.)

Biological processes (cGMP sensory transduction as effector channel)

• Thermosensation / thermotaxis: tax-4 mutants fail temperature responses; TAX-4 is
  required for AFD thermosensitivity and isothermal tracking.
  PMID:8893027
  PMID:21315599
• AFD gene expression / neuron function: CMK-1 and TAX-4 regulate AFD gene
  expression, morphology and function; TAX-4 needed in larval stages to maintain adult
  gene expression; acts cell-autonomously.
  PMID:14711416
• O2 sensation / aerotaxis / hyperoxia avoidance: high-O2 avoidance requires the
  tax-2/tax-4 CNG channel + sGC gcy-35.
  PMID:15220933
• O2-evoked cGMP/Ca2+ dynamics: cGMP rise → sustained Ca2+ response that depends on
  cGMP-gated ion channels (negative-feedback architecture).
  PMID:23940325
• CO2 sensation / avoidance: CO2-evoked Ca2+ responses in AFD and BAG neurons
  require cGMP-gated ion channels; CO2 avoidance requires channels containing TAX-2/TAX-4.
  PMID:21435556
  PMID:21435556
• Phototransduction (ASJ): light → G-protein → cGMP → opening of cGMP-sensitive CNG
  channels in ASJ photoreceptor neuron.
  PMID:20436480
• Chemosensation / neuroendocrine signaling: chemosensory detection in ASJ of P.
  aeruginosa metabolites induces daf-7/TGF-β expression (UniProt: TAX-4 up-regulates daf-7
  transcription in ASI/ASJ). PMID:25303524 (positive regulation of gene expression).
• Sensory axon maintenance: tax-2/tax-4 required for maintenance of correct sensory
  axon structure (inappropriate late-larval axon outgrowth in mutants).
  PMID:9486798

NOT known / open questions (candidate knowledge_gaps)

1. In vivo subunit stoichiometry of the native TAX-4/TAX-2 channel. In vitro/structural
   work shows TAX-4 forms homotetramers PMID:28099415, and TAX-4+TAX-2 form a heteromer
   in HEK293 PMID:10064800, but the exact TAX-4:TAX-2 subunit ratio and arrangement of
   the native sensory channel in worm cilia is not established.
2. Homomeric vs heteromeric gating in vivo. The homomeric TAX-4 channel is ~25× more
   cGMP-sensitive than the TAX-4/TAX-2 heteromer PMID:10064800; which form operates in
   which neuron, and how the beta subunit tunes ligand sensitivity/selectivity in the
   physiological setting, is unresolved. Subunit composition may even vary across ciliary
   subcompartments within one cell PMID:23886944.
3. Ligand selectivity in vivo (cGMP vs cAMP). TAX-4 is cGMP-selective but also cAMP-
   responsive (GO:0005222); whether cAMP-gating is ever physiologically used, and what sets
   in-vivo ligand tuning, is unclear.
4. Ion selectivity / Ca2+ vs monovalent flux in vivo. The channel conducts Ca2+, Na+,
   K+ (UniProt CATALYTIC ACTIVITY / Rhea); the relative in-vivo permeation and how Ca2+
   entry vs depolarization is apportioned across the many neuron types is not quantified.
5. Mechanism of the non-channel/"regulatory" roles (AFD gene-expression maintenance
   PMID:14711416; daf-7 induction PMID:25303524; axon-structure maintenance
   PMID:9486798) — presumably all downstream of channel-mediated Ca2+ signaling, but the
   causal chain from channel activity to transcriptional/morphological output is not fully
   defined.

Annotation-review orientation (core vs non-core)

• CORE molecular function: cGMP-gated cation channel activity (GO:0005223,
  intracellularly cGMP-activated cation channel activity), cAMP-activated variant
  (GO:0005222), cGMP binding (GO:0030553); monoatomic cation transmembrane transport
  (GO:0098655).
• CORE cellular component: plasma membrane (GO:0005886); non-motile cilium
  (GO:0097730); cation channel complex / CNG channel complex (GO:0034703, GO:0017071);
  dendrite (GO:0030425); ciliary inversin compartment (GO:0097543).
• CORE-ish process: sensory perception of chemical stimulus (GO:0007606); detection of
  chemical stimulus involved in sensory perception (GO:0050907); the specific modalities
  (thermosensory behavior, thermotaxis, aerotaxis, response to hyperoxia/oxygen, detection
  of CO2, phototransduction, chemotaxis, olfactory behavior) are downstream sensory
  outputs — KEEP but as NON-CORE consequences of the one channel activity.
• NON-CORE / pleiotropic consequences: regulation of axon extension (GO:0030516),
  neuron projection morphogenesis (GO:0048812), regulation of neuron differentiation
  (GO:0045664), positive regulation of transcription/gene expression (GO:0045944,
  GO:0010628), positive regulation of growth rate (GO:0040010), calcium-mediated signaling
  (GO:0019722), negative regulation of receptor guanylyl cyclase signaling (GO:0010754).

IEA over-annotations to flag (from InterPro/ARBA electronic mapping)

• GO:0005249 voltage-gated potassium channel activity (IEA:InterPro) — misleading.
  TAX-4 belongs to the CNG family within the voltage-gated-channel superfamily but has an
  unusual voltage-sensor-like domain accounting for deficient voltage dependence
  PMID:28099415 and is a non-selective cation channel, not a K+-selective, voltage-
  gated K+ channel. Family-level InterPro (EAG/ELK/ERG signature) drags in this term.
• GO:0006813 / GO:0071805 potassium ion transport / transmembrane transport (IEA) and
  GO:0006811 monoatomic ion transport (IEA) — K+-specific process terms are an
  over-annotation of the same non-selective-cation reality; the channel does conduct K+ but
  is not a dedicated K+ transporter. General monoatomic cation transport (GO:0098655) is
  the accurate term.