id: E7FFT2
gene_symbol: tmc2a
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:7955
  label: Danio rerio
description: tmc2a (transmembrane channel-like 2a) is one of two zebrafish co-orthologs of vertebrate TMC2, the
  paralog tmc2b being the other; both lie on chromosome 5, tmc2b next to tmc1. Tmc2a is a multi-pass membrane protein
  of the TMC family and a pore-forming subunit of the hair-cell mechanoelectrical transduction (MET) channel, a
  non-selective cation channel with high calcium permeability that opens when tip-link tension deflects the hair
  bundle. Its cytoplasmic N-terminus binds the cytoplasmic domain of the tip-link protein Pcdh15a. tmc2a is expressed
  only in sensory hair cells, from 1 dpf in the inner ear, where it is enriched in central (striolar) hair cells
  and in the upper layer of the cristae, and at low levels in a subset of lateral-line hair cells that respond to
  posterior-to-anterior deflection. Tmc2a is the main Tmc subunit for hearing through the saccule, is required for
  mechanotransduction in upper-layer crista hair cells and for high-frequency utricular sensitivity, and together
  with Tmc2b carries all lateral-line transduction; in neuromasts it enlarges the responses of the hair cells that
  express it.
existing_annotations:
- term:
    id: GO:0005245
    label: voltage-gated calcium channel activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  supporting_entities:
  - MGI:MGI:2151017
  - PANTHER:PTN002761430
  review:
    summary: Voltage-gated calcium channel activity, propagated from the PANTHER TMC2 node (mouse Tmc2 donor).
    action: MODIFY
    reason: The ion-channel part is right but the gating stimulus is wrong. TMC channels are opened by tension in
      the tip link that accompanies hair-bundle deflection, not by membrane voltage, and they conduct cations non-selectively
      with high calcium permeability. The same row was modified to the mechanically gated cation channel term in
      the human TMC1 review; the same argument applies to every member of this node. In zebrafish the Tmc2a N-terminus
      acts in a complex with the tip-link protein Pcdh15a, and loss of Tmc2a reduces mechanically evoked hair-cell
      responses.
    proposed_replacement_terms:
    - id: GO:0140135
      label: mechanosensitive monoatomic cation channel activity
    supported_by:
    - &id001
      reference_id: PMID:25114259
      supporting_text: Overexpression of the Tmc2a N terminus results in mislocalization of Pcdh15a within hair
        bundles, together with a significant decrease in mechanosensitive responses, suggesting that a Pcdh15a–Tmc
        complex is critical for mechanotransduction.
    - &id002
      reference_id: PMID:39773557
      supporting_text: Significant progress in our understanding of MET has been recently made by identifying the
        transmembrane channel-like proteins 1 and 2 (TMC1 and TMC2) as the pore-forming subunits of the MET channel
        (Kawashima et al., 2011; Pan et al., 2018).
    propagation_review:
      root_cause: TERM_SCOPING_PROBLEM
      failure_modes:
      - GRANULARITY_MISMATCH
      source_entities:
      - source_id: MGI:MGI:2151017
        source_label: Tmc2 (mouse)
        source_status: SOURCE_BAD
        comment: QuickGO shows mouse Tmc2 carries GO:0005245 by IMP/IGI from PMID:23871232, a study of calcium permeability
          and single-channel currents of the mechanically gated hair-cell channel, not of voltage gating (as traced
          in the human TMC1 review).
      - source_id: PANTHER:PTN002761430
        source_status: SUPPORTS_SOURCE_BUT_NOT_TARGET
        comment: The node correctly groups vertebrate TMC2 orthologs including both zebrafish copies; only the gating
          mechanism in the chosen term is wrong.
- term:
    id: GO:0005262
    label: calcium channel activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  supporting_entities:
  - UniProtKB:Q8R4P4
  review:
    summary: Calcium channel activity transferred by UniProt curators from mouse Tmc2 (Q8R4P4).
    action: ACCEPT
    reason: The MET channel formed by TMC subunits is a non-selective cation channel that is highly permeable to
      calcium, so calcium conduction is part of the activity. In zebrafish, neuromast hair bundles that express
      Tmc2a in addition to Tmc2b give larger calcium signals than bundles with Tmc2b alone. The term does not capture
      the mechanical gating, which is covered by GO:0008381 and the proposed GO:0140135.
    supported_by:
    - &id005
      reference_id: PMID:36905930
      supporting_text: This idea is supported by our results, as we show that the calcium signals in hair bundles
        expressing both Tmc2b and Tmc2a are larger compared to those expressing just Tmc2b (Figure 6).
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25114259
  qualifier: enables
  supporting_entities:
  - UniProtKB:Q5ICW6
  review:
    summary: Tmc2a N-terminus binds the cytoplasmic domains (CD1 and CD3 isoforms) of the tip-link protein Pcdh15a
      in yeast two-hybrid and co-immunoprecipitation assays.
    action: MODIFY
    reason: The interaction is well supported (yeast two-hybrid, co-IP in HEK293T cells, and a dominant-negative
      effect of the Tmc2a N-terminus on Pcdh15a localization in hair bundles), but 'protein binding' is uninformative.
      The partner is protocadherin 15a, a cadherin-superfamily tip-link protein, so the more informative binding
      term is cadherin binding. The functional consequence, coupling of the channel to the tip link, is captured
      by the channel activity and mechanosensory process terms.
    proposed_replacement_terms:
    - id: GO:0045296
      label: cadherin binding
    supported_by:
    - reference_id: PMID:25114259
      supporting_text: Tmc2a interacted with both CD1 and CD3 isoforms of Pcdh15a, demonstrating that binding is
        not restricted to a particular Pcdh15a isoform.
    - *id001
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  supporting_entities:
  - MGI:MGI:2151016
  - PANTHER:PTN001151822
  - WB:WBGene00020490
  - ZFIN:ZDB-GENE-060526-280
  review:
    summary: Plasma membrane location propagated across the TMC family.
    action: ACCEPT
    reason: TMC proteins are multi-pass membrane proteins that act in the plasma membrane of hair-cell stereocilia.
      Correct but general; the specific location is the stereocilium tip membrane.
    supported_by:
    - *id001
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IDA
  original_reference_id: PMID:25114259
  qualifier: located_in
  review:
    summary: Plasma membrane location curated from Maeda et al. 2014.
    action: ACCEPT
    reason: The location is correct for a multi-pass channel subunit that works in the hair-bundle membrane. The
      zebrafish imaging in this paper used an N-terminal Tmc2a fragment fused to GFP-CAAX, which is targeted to
      the membrane by the CAAX motif, so the direct evidence for full-length Tmc2a is weak; a full-length Tmc2a-mEGFP
      transgene later failed to express. Accepted because the location is supported by family-wide evidence and
      by the requirement for Tmc2a at the site of mechanotransduction.
    supported_by:
    - reference_id: PMID:25114259
      supporting_text: Next, we determined whether endogenous Pcdh15a was correctly localized in hair cells expressing
        Tmc2a 1–117 -GFP-CAAX.
    - reference_id: PMID:32371604
      supporting_text: The Tg(myo6b:tmc2a-mEGFP-pA) construct failed to express Tmc2a-mEGFP.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  supporting_entities:
  - InterPro:IPR038900
  - UniProtKB-SubCell:SL-0039
  review:
    summary: Plasma membrane from InterPro TMC family and UniProt subcellular location.
    action: ACCEPT
    reason: Consistent with the IBA and with the role of TMC subunits in the stereocilia membrane.
- term:
    id: GO:0008381
    label: mechanosensitive monoatomic ion channel activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  supporting_entities:
  - MGI:MGI:2151017
  - PANTHER:PTN001151822
  review:
    summary: Mechanosensitive ion channel activity asserted at the PANTHER TMC node.
    action: ACCEPT
    reason: 'This is the core molecular function of the family, and zebrafish genetics fit it: loss of Tmc2a reduces
      saccular microphonic potentials and abolishes transduction in the upper layer of the cristae, and loss of
      tmc1, tmc2a and tmc2b together abolishes all hair-cell transduction. The more precise child GO:0140135 (cation
      channel) is recorded in core_functions as a contribution to the MET channel complex.'
    supported_by:
    - &id003
      reference_id: PMID:32371604
      supporting_text: Interestingly, the tmc2a single mutants ( n = 8) had significantly reduced microphonic signals
        compared with WT siblings ( n = 7) and were the only single-gene mutants with such a deficit
    - &id007
      reference_id: PMID:32371604
      supporting_text: Together, our experiments reveal that, in cristae, hair cells stratify into an upper, Tmc2a-dependent
        layer of teardrop-shaped cells, and a lower, Tmc1/2b-dependent tier of gourd-shaped cells.
    - *id002
- term:
    id: GO:0008381
    label: mechanosensitive monoatomic ion channel activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  supporting_entities:
  - UniProtKB:Q8R4P4
  review:
    summary: Mechanosensitive ion channel activity transferred from mouse Tmc2.
    action: ACCEPT
    reason: Same function as the IBA row; mouse TMC2 is a pore-forming MET channel subunit and zebrafish Tmc2a is
      required for hair-cell mechanotransduction.
    supported_by:
    - *id003
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: located_in
  supporting_entities:
  - InterPro:IPR012496
  review:
    summary: Membrane, from the InterPro TMC domain.
    action: ACCEPT
    reason: Correct; less specific than plasma membrane.
- term:
    id: GO:0034220
    label: monoatomic ion transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  supporting_entities:
  - GO:0008381
  review:
    summary: Ion transmembrane transport inferred from the mechanosensitive ion channel activity.
    action: MODIFY
    reason: 'Correct in kind but under-specific: the MET channel conducts cations. The same modification was made
      for this row in the human TMC1 review.'
    proposed_replacement_terms:
    - id: GO:0098655
      label: monoatomic cation transmembrane transport
- term:
    id: GO:0048882
    label: lateral line development
  evidence_type: IEP
  original_reference_id: PMID:25114259
  qualifier: involved_in
  review:
    summary: Lateral line development, inferred from tmc2a expression in lateral-line hair cells.
    action: REMOVE
    reason: An expression pattern does not show that the gene acts in development of the organ. Tmc2a is a channel
      subunit of mature hair cells, it is barely detectable in the lateral line by in situ hybridization, and hair
      bundles form normally in tmc triple mutants. Its lateral-line role is transduction, which is covered by the
      mechanosensory detection terms.
    supported_by:
    - reference_id: PMID:29269857
      supporting_text: According to RNA in situ hybridization studies of zebrafish larva, tmc2b is robustly expressed
        in hair cells of the lateral line, but the mRNAs of the paralogs tmc2a and tmc1 are present at much lower
        levels and cannot be detected by this method.
    - &id004
      reference_id: PMID:32371604
      supporting_text: Together, these results suggest that hair bundles are intact and central components of the
        MET complex are properly localized in tmc triple mutants.
- term:
    id: GO:0050910
    label: detection of mechanical stimulus involved in sensory perception of sound
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  supporting_entities:
  - MGI:MGI:2151017
  - PANTHER:PTN002761430
  - UniProtKB:Q8TDI7
  - ZFIN:ZDB-GENE-060526-262
  - ZFIN:ZDB-GENE-060526-280
  review:
    summary: Detection of mechanical stimulus involved in sensory perception of sound, from the PANTHER TMC2 node.
    action: ACCEPT
    reason: 'Core process, and zebrafish data support it directly for this copy: tmc2a single mutants are the only
      single tmc mutants with reduced saccular microphonics, the authors rank Tmc2a first in a Tmc2a > Tmc2b > Tmc1
      hierarchy for sound detection, and tmc2a/tmc2b double mutants hear only intermittently. The WITH list includes
      the zebrafish genes themselves, which is expected when their own experimental rows seeded the node.'
    supported_by:
    - *id003
    - &id006
      reference_id: PMID:32371604
      supporting_text: Together, these results demonstrate a Tmc2a > Tmc2b > Tmc1 hierarchy in the detection of
        sound.
    - reference_id: PMID:32167554
      supporting_text: By examining tmc2b cwr2 tmc2a cwr3 double and tmc2b cwr8 tmc1 cwr4 tmc2a cwr6 triple mutants,
        we demonstrate that the double mutant has attenuated hearing attributable to diminished mechanotransduction,
        but the triple mutant is deaf owing to extinguished mechanotransduction
- term:
    id: GO:0050910
    label: detection of mechanical stimulus involved in sensory perception of sound
  evidence_type: IGI
  original_reference_id: PMID:25114259
  qualifier: acts_upstream_of_or_within
  supporting_entities:
  - ZFIN:ZDB-GENO-150116-4
  review:
    summary: Hearing-related mechanosensory detection from the Maeda 2014 genotype (dominant-negative Tmc2a N-terminal
      fragment line).
    action: ACCEPT
    reason: The paper shows that expressing the Tmc2a N-terminus in hair cells mislocalizes Pcdh15a and reduces
      mechanically evoked calcium responses (tested in lateral-line hair cells). The assay is a dominant-negative
      interference with the Tmc-Pcdh15a complex rather than loss of tmc2a alone. The process itself is independently
      confirmed for tmc2a by later loss-of-function alleles that reduce saccular microphonics, so the annotation
      stands.
    supported_by:
    - *id001
    - *id003
- term:
    id: GO:0050910
    label: detection of mechanical stimulus involved in sensory perception of sound
  evidence_type: IGI
  original_reference_id: PMID:29269857
  qualifier: acts_upstream_of_or_within
  supporting_entities:
  - ZFIN:ZDB-GENO-190604-15
  review:
    summary: Sound-detection term from the tmc2a/tmc2b double mutant of Chou et al. 2017.
    action: MODIFY
    reason: Chou et al. assayed the double mutant only in the lateral line, where microphonic potentials and dye
      uptake were abolished; the lateral line detects water motion, not sound. The finding is correct but belongs
      under the general mechanosensory detection term. Hearing in the double mutant was measured later (Chen et
      al. 2020, attenuated hearing) and is covered by the IBA row.
    proposed_replacement_terms:
    - id: GO:0050974
      label: detection of mechanical stimulus involved in sensory perception
    supported_by:
    - reference_id: PMID:29269857
      supporting_text: Stimulus-evoked microphonic potentials were not detectable in neuromasts with A–P orientations
        in the PLL of tmc2a −/− /tmc2b −/− larvae (Fig. 8a, b) at 6 dpf, yet robust signatures were apparent in
        controls.
    - &id008
      reference_id: PMID:29269857
      supporting_text: Overall, these findings indicate that the residual activity in the tmc2b −/− mutant is dependent
        on Tmc2a.
- term:
    id: GO:0060005
    label: vestibular reflex
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  supporting_entities:
  - MGI:MGI:2151017
  - PANTHER:PTN002761430
  review:
    summary: Vestibular reflex, from the PANTHER TMC2 node.
    action: KEEP_AS_NON_CORE
    reason: Organism-level reflex downstream of vestibular hair-cell transduction. In zebrafish larvae the vestibular-induced
      eye movement is reduced in tmc2a/tmc2b and tmc1/tmc2b double mutants but not in tmc2a single mutants, so tmc2a
      contributes redundantly. Kept as a correct but non-core consequence of the channel function.
    supported_by:
    - reference_id: PMID:32371604
      supporting_text: With regard to vestibular function of the utricle/anterior macula, we observed a reduction
        in VIEM in both tmc1/2b and tmc2a/2b double mutants, whereas all other genotypes yielded responses that
        were not statistically different from their siblings
- term:
    id: GO:0060119
    label: inner ear receptor cell development
  evidence_type: IEP
  original_reference_id: PMID:25114259
  qualifier: involved_in
  review:
    summary: Inner ear receptor cell development, inferred from tmc2a expression in nascent otic hair cells.
    action: REMOVE
    reason: Expression in developing hair cells is not evidence of a developmental role. Tmc2a is a transduction
      channel subunit; hair bundles and the other MET components are in place in tmc triple mutants.
    supported_by:
    - reference_id: PMID:25114259
      supporting_text: We observed that tmc2a is expressed in the nascent hair cells of the developing ear as early
        as 1 dpf, and expression in the inner ear continues throughout development
    - *id004
- term:
    id: GO:0070588
    label: calcium ion transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: involved_in
  supporting_entities:
  - GO:0005245
  - GO:0005262
  review:
    summary: Calcium ion transmembrane transport inferred from the channel activity terms.
    action: ACCEPT
    reason: Calcium entry through the MET channel is a direct consequence of the channel activity; Tmc2a-expressing
      neuromast bundles show larger calcium signals.
    supported_by:
    - *id005
- term:
    id: GO:0071260
    label: cellular response to mechanical stimulus
  evidence_type: IMP
  original_reference_id: PMID:33490059
  qualifier: acts_upstream_of_or_within
  supporting_entities:
  - ZFIN:ZDB-GENO-230726-20
  review:
    summary: 'Cellular response to mechanical stimulus from tmc2a mutants (Zhu et al. 2020): short hair cells of
      the cristae lose mechanotransduction-dependent dye uptake.'
    action: ACCEPT
    reason: The paper measures cell-level transduction (styryl dye uptake) and shows that short hair cells need
      Tmc2a. The term is general but accurate for this assay; the more specific process, detection of mechanical
      stimulus in sensory perception, is recorded in core_functions.
    supported_by:
    - reference_id: PMID:33490059
      supporting_text: These findings confirm that short hair cells mainly depend on Tmc2 isoforms, and Tmc2a plays
        a significant role in these cells.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of
    sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on on inter-ontology links
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:25114259
  title: Tip-link protein protocadherin 15 interacts with transmembrane channel-like proteins TMC1 and TMC2.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. Names the pair; Tmc2a N-terminus binds Pcdh15a CD1/CD3; dominant-negative fragment;
      in situ and RT-PCR expression of tmc1/2a/2b.
- id: PMID:29269857
  title: A molecular basis for water motion detection by the mechanosensory lateral line of zebrafish.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. tmc2b single (TALEN) and tmc2a/tmc2b double (CRISPR) mutants; lateral-line data only
      for the double mutant.
- id: PMID:33490059
  title: Tmc Reliance Is Biased by the Hair Cell Subtype and Position Within the Ear.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. Short crista hair cells depend on Tmc2 isoforms, with a significant Tmc2a role; tall
      cells on Tmc1.
- id: PMID:32167554
  title: Tmc proteins are essential for zebrafish hearing where Tmc1 is not obligatory.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. tmc2a single reduces saccular microphonics; double mutant attenuated hearing; triple
      deaf; no paralog upregulation in tmc1 mutants.
- id: PMID:32371604
  title: Disruption of tmc1/2a/2b Genes in Zebrafish Reveals Subunit Requirements in Subtypes of Inner Ear Hair
    Cells.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. Tmc2a > Tmc2b > Tmc1 hierarchy for hearing; crista teardrop cells Tmc2a-dependent;
      Tmc2b-mEGFP rescues triple mutants; Tmc2a transgene failed to express.
- id: PMID:37952940
  title: Transmembrane Channel-Like (Tmc) Subunits Contribute to Frequency Sensitivity in the Zebrafish Utricle.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Full text. tmc2a striolar, needed for high-frequency utricular responses.
- id: PMID:38035267
  title: Differential expression of mechanotransduction complex genes in auditory/vestibular hair cells in zebrafish.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. HCR maps of tmc1/2a/2b in inner ear zones at 1-15 dpf.
- id: PMID:36905930
  title: Asymmetric mechanotransduction by hair cells of the zebrafish lateral line.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text. Tmc2a only in P-to-A neuromast hair cells; tmc2a mutants lose response asymmetry.
- id: PMID:40825768
  title: prdm1a drives a fate switch between hair cells of different mechanosensory organs.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Full text. tmc2a is an ear hair-cell gene repressed by prdm1a in the lateral line; tmc2b unaffected.
- id: PMID:40086870
  title: Esrp1 and Esrp2 Regulate the Stability of tmc1/2a mRNAs in Zebrafish Sensory Hair Cells.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Full text. Esrp1/2 stabilize tmc1 and tmc2a but not tmc2b mRNA.
- id: PMID:39773557
  title: Complexes of vertebrate TMC1/2 and CIB2/3 proteins form hair-cell mechanotransduction cation channels.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Full text. Family-level statement that TMC1/2 are pore-forming MET subunits; CIB2/3 needed in
      zebrafish.
- id: file:DANRE/tmc2a/tmc2a-bioinformatics/RESULTS.md
  title: tmc2a / tmc2b identity, conserved synteny and expression records
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Manual audit verified the sidecar supports a TGD-consistent origin and public hair-cell expression
      records for both copies; it does not distinguish TGD from segmental duplication by synteny alone.
  findings: []
core_functions:
- description: Pore-forming subunit of the hair-cell mechanoelectrical transduction channel at the stereocilia,
    coupled to the tip-link protein Pcdh15a through its N-terminus; recorded as a contribution because the working
    channel also needs other subunits and can be formed by Tmc1 or Tmc2b.
  contributes_to_molecular_function:
    id: GO:0140135
    label: mechanosensitive monoatomic cation channel activity
  directly_involved_in:
  - id: GO:0050910
    label: detection of mechanical stimulus involved in sensory perception of sound
  - id: GO:0050974
    label: detection of mechanical stimulus involved in sensory perception
  - id: GO:0098655
    label: monoatomic cation transmembrane transport
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - *id003
  - *id006
  - *id007
  - *id008
  - *id001
suggested_questions:
- question: Does full-length Tmc2a localize to stereocilia tips and rescue tmc2a or tmc triple mutants when expressed
    from a hair-cell promoter, and can Tmc2a rescue the Tmc2b-only neuromast hair cells? The reciprocal Tmc2b transgene
    rescues all end organs, but the Tmc2a construct never expressed.
- question: Do Tmc2a and Tmc2b form heteromeric channels in the cells that express both, and does Tmc2a confer different
    conductance or calcium permeability (larger responses in P-to-A neuromast cells, broader frequency range in
    the utricle)?
- question: Is the tmc2a/tmc2b pair a product of the teleost genome duplication although both copies are on zebrafish
    chromosome 5? Are the medaka co-orthologs on different chromosomes, and does a gar-anchored double-conserved-synteny
    analysis place both in the same ancestral block?
suggested_experiments:
- hypothesis: The Tmc2a-dependent crista and saccular phenotypes reflect expression, not a Tmc2a-specific protein
    property.
  description: Express Tmc2b (and Tmc2a, if a stable construct can be made) under a tmc2a enhancer in tmc2a mutants
    and measure dye uptake in upper-layer crista hair cells and saccular microphonics.
  experiment_type: cross-rescue
- hypothesis: Tmc2a changes single-channel conductance or calcium selectivity relative to Tmc2b.
  description: Record MET currents from neuromast hair cells of tmc2b mutants (Tmc2a-only P-to-A cells) and tmc2a
    mutants (Tmc2b-only cells) and compare conductance and calcium permeability.
  experiment_type: electrophysiology