TEKT4

TEKT4 is a tektin-family structural protein that incorporates into axonemal doublet microtubules of sperm flagella, where it stabilizes microtubule architecture and coordinates flagellar beat waveform rather than being required for axonemal assembly (PMID:17244819, PMID:24823476). Loss of TEKT4 in mice causes uncoordinated flagellar waveform propagation, drastically reduced forward progressive velocity, and ~10-fold higher intracellular ATP consumption without gross ultrastructural defects, and TEKT4 functions non-redundantly with TEKT3, as double-null mice exhibit greater subfertility than either single knockout (PMID:17244819, PMID:18951373). TEKT4 expression is transcriptionally regulated by TBX5, which directly binds the TEKT4 promoter (PMID:36936432). Germline TEKT4 variants destabilize microtubules and antagonize paclitaxel-induced stabilization in breast cancer cells, conferring drug resistance (PMID:24823476).

1. 2007 High

   The fundamental question of whether TEKT4 is needed for flagellar assembly versus flagellar function was resolved: TEKT4 regulates the mechanics and energy efficiency of the flagellar beat without being required for axonemal ultrastructure.

   Evidence Tekt4-knockout mouse with TEM, motility analysis, ATP measurement, and immunolocalization

   PMID:17244819

   Open questions at the time

   • Molecular mechanism by which TEKT4 coordinates dynein-driven sliding or waveform propagation is unknown
   • Whether TEKT4 directly modulates mechanochemical coupling or acts indirectly through other axonemal components is unresolved
2. 2009 High

   It was unknown whether TEKT4 and its paralog TEKT3 serve redundant roles; double-knockout epistasis analysis established that they have partially non-redundant functions in sperm motility.

   Evidence Tekt3/Tekt4 double-knockout mice with genetic epistasis and motility assays

   PMID:18951373

   Open questions at the time

   • Whether TEKT3 and TEKT4 occupy distinct positions within the doublet microtubule or interact directly is unknown
   • The molecular basis for their functional divergence has not been determined
3. 2012 Low

   Whether TEKT4 deficiency is relevant to human male infertility was open; reduced TEKT4 mRNA and protein levels were found in sperm from idiopathic asthenozoospermic men, supporting a correlative link.

   Evidence RT-PCR and Western blot in human ejaculated spermatozoa from asthenozoospermic versus normozoospermic men

   PMID:22774605

   Open questions at the time

   • Correlative association only; no causal or interventional evidence in human samples
   • Sample sizes and replication across cohorts not established
   • Whether TEKT4 reduction is cause or consequence of asthenozoospermia is unresolved
4. 2014 Medium

   Beyond flagella, it was unknown whether TEKT4 influences cytoplasmic microtubule stability; germline TEKT4 variants were shown to destabilize microtubules and antagonize paclitaxel-induced stabilization, establishing TEKT4 as a microtubule-stabilizing factor relevant to drug resistance.

   Evidence Ectopic expression of variant TEKT4 in breast cancer cell lines with microtubule stability assays and exome sequencing

   PMID:24823476

   Open questions at the time

   • Whether wild-type TEKT4 is endogenously expressed in breast epithelial cells at functional levels is unclear
   • The structural basis for variant-mediated destabilization has not been defined
5. 2018 Low

   Whether TEKT4 has signaling roles beyond structural microtubule functions was unexplored; knockdown in thyroid cancer cells suppressed PI3K/Akt signaling, proliferation, and invasion, suggesting a broader oncogenic role.

   Evidence siRNA knockdown in papillary thyroid cancer cell lines with proliferation, colony formation, migration/invasion, and pathway analysis

   PMID:30251060

   Open questions at the time

   • No rescue experiment or direct binding evidence linking TEKT4 to PI3K/Akt components; single knockdown approach without orthogonal validation
   • Whether PI3K/Akt effects are indirect consequences of microtubule disruption is unresolved
   • Not independently confirmed
6. 2023 Medium

   Upstream transcriptional regulation of TEKT4 was unknown; TBX5 was identified as a direct transcriptional activator that binds the TEKT4 promoter, providing the first defined regulatory input for TEKT4 expression.

   Evidence ChIP for TBX5 on TEKT4 promoter in HL-1 cardiomyocytes with TBX5 knockdown and qPCR validation

   PMID:36936432

   Open questions at the time

   • Whether TBX5-driven TEKT4 expression has functional significance in cardiomyocytes or is relevant to cilia/flagella contexts is unknown
   • Other transcription factors controlling TEKT4 in spermatogenic cells have not been identified

• The structural basis for how TEKT4 integrates into doublet microtubules and mechanistically coordinates dynein-driven flagellar waveform remains undefined, and the physiological relevance of TEKT4 in non-ciliary/flagellar tissues lacks robust evidence.
• No cryo-EM or high-resolution structural model of TEKT4 within the axonemal doublet
• Direct binding partners within the outer doublet beyond tubulin are not identified
• Whether TEKT4 functions outside flagellar/ciliary contexts under normal physiology is unestablished