TPRN

TPRN (taperin) is a vertebrate F-actin bundling protein that maintains the structural integrity of inner ear hair cell stereocilia, where it localizes specifically to the taper/pivot region at the base of the stereocilium (PMID:20170899, PMID:27693694). Purified TPRN cross-links F-actin into bendable bundles through its N-terminal region and connects the actin core to the plasma membrane via the membrane receptor PTPRQ, anchoring the stereocilium (PMID:40471101). Within the taper region, TPRN assembles with CLIC5 and PTPRQ into concentric rings; this architecture also depends on a complex with CLIC5, radixin, and myosin VI, and loss of CLIC5 mislocalizes TPRN from the stereocilia base (PMID:24285636, PMID:37952086). Functionally, TPRN is required to preserve stereociliary rootlet integrity and to retain the rootlet-associated proteins TRIOBP-5 and ANKRD24 at the pivot point (PMID:30159668, PMID:40471101). Loss of TPRN causes progressive degeneration of hair cell stereocilia and progressive sensorineural hearing loss, and AAV-mediated TPRN gene replacement restores auditory function in Tprn-mutant (DFNB79) mice (PMID:27693694, PMID:37952086, PMID:41923624). Beyond its stereociliary role, taperin docks the PP1α phosphatase isoform through a canonical RVxF motif and suppresses its activity, shuttles between nucleus and cytoplasm, and is recruited to sites of DNA damage in association with Ku70, Ku80, PARP, and topoisomerases I and IIα (PMID:23213405).

1. 2010 Medium

   Establishing where TPRN acts began with showing the protein concentrates at the taper region of hair cell stereocilia, pointing to a structural role at the stereocilium base rather than diffuse function.

   Evidence Immunolocalization in mouse cochlea

   PMID:20170899

   Open questions at the time

   • Localization alone did not define molecular activity
   • No binding partners identified at this stage
2. 2011 Medium

   A parallel line of work asked whether taperin has biochemical activity beyond the ear, showing it docks PP1α via an RVxF motif and suppresses its phosphatase activity, that it shuttles between nucleus and cytoplasm complexed with PP1, and that it is recruited to DNA damage sites with Ku70/Ku80/PARP and topoisomerases.

   Evidence In vitro phosphatase assay, RVxF mutagenesis, subcellular fractionation, SILAC mass spectrometry, live-cell DNA damage imaging

   PMID:23213405

   Open questions at the time

   • Functional consequence of PP1 suppression in hair cells not established
   • Role of DNA damage recruitment not linked to a phenotype
   • Single-lab interactome without reciprocal validation in vivo
3. 2013 Medium

   To place TPRN in a molecular assembly, it was shown to form a complex with CLIC5, radixin, and myosin VI at the stereocilia base, with CLIC5 required to position TPRN there, defining an interdependent base-of-stereocilium machine.

   Evidence Co-IP/pulldown, confocal microscopy, CLIC5-deficient jitterbug mice

   PMID:24285636

   Open questions at the time

   • Direct versus indirect nature of each interaction not resolved
   • Stoichiometry and order of assembly unknown
4. 2016 Medium

   Whether TPRN is actually required for hearing was tested by knockout, demonstrating progressive stereocilia degeneration and sensorineural hearing loss and establishing taperin as a maintenance factor for stereocilia.

   Evidence TALEN knockout mice, ABR, scanning electron microscopy, phalloidin labeling

   PMID:27693694

   Open questions at the time

   • Molecular mechanism of degeneration not defined
   • Did not distinguish development from maintenance defect
5. 2018 Medium

   The structural target of TPRN was localized to the rootlet, with knockout causing curved rootlet shafts, hollow cores, and dispersed radixin plus reduced β-actin, implicating taperin in rootlet integrity.

   Evidence CRISPR/Cas9 knockout, transmission electron microscopy, immunofluorescence, Western blot

   PMID:30159668

   Open questions at the time

   • Whether rootlet defect is direct or secondary not resolved
   • Mechanism linking TPRN loss to reduced actin levels unknown
6. 2023 High

   Super-resolution imaging resolved the supramolecular architecture, showing TPRN, CLIC5, and PTPRQ form concentric rings whose disruption causes degeneration and hearing loss, while AAV restoration of TPRN rescued auditory function.

   Evidence Co-IP, dual-STED microscopy, competitive inhibition, AAV delivery in Tprn KO mice, ABR

   PMID:37952086

   Open questions at the time

   • Molecular basis of ring self-organization not defined
   • Why overexpression is also disruptive not mechanistically explained
7. 2025 High

   The biochemical activity underlying the structural role was defined by reconstitution: purified TPRN cross-links F-actin into bendable bundles via its N-terminal region and binds PTPRQ to link the actin core to the membrane, and TPRN loss depletes TRIOBP-5 and ANKRD24 from the pivot point.

   Evidence In vitro F-actin bundling with recombinant protein, domain mapping, PTPRQ co-IP, TPRN-deficient and overexpression mice, immunofluorescence

   PMID:40471101

   Open questions at the time

   • Structural basis of actin cross-linking not determined
   • Whether TRIOBP-5/ANKRD24 retention is via direct binding unknown
8. 2026 Medium

   Translational potential was demonstrated by hair cell-specific AAV delivery of TPRN restoring near wild-type hearing in DFNB79 mice, confirming gene replacement reverses the loss-of-function phenotype.

   Evidence AAV gene therapy with coProB2 promoter in DFNB79 mouse model, ABR

   PMID:41923624

   Open questions at the time

   • Durability and developmental-window dependence not addressed
   • Single-lab rescue not yet in additional models

• How taperin's nuclear PP1/DNA-damage functions relate to its stereociliary actin-bundling role, and whether they operate in the same or distinct cell contexts, remains unresolved.
• No integration of nuclear and stereociliary functions
• No structural model of the actin-bundling N-terminal domain
• Mechanism of recruitment to DNA damage sites not defined