Tropomodulin Pointed End Cap Mechanism
P2 concept. Tropomodulin (Tmod) is the only dedicated pointed-end capping protein in eukaryotes [Pollard 2016, p.4 §10]. Important for STRC because pointed ends define the base of stereocilia actin filaments — where rootlet anchorage and mechanical stability originate.
Mechanism
| Element | Role |
|---|---|
| Tmod itself | wraps the three terminal subunits at the pointed end |
| Two tropomyosin N-termini | bind to Tmod and strengthen capping |
| Functional outcome | blocks both subunit addition and loss at the pointed end |
| In vivo behavior | still allows slow exchange of actin subunits at the pointed end |
Source: [Pollard 2016, p.4 §10] (Rao et al. 2014 Science).
Why “wraps three terminal subunits” matters
- Single-subunit-binding caps (e.g., Arp2/3 caps the pointed end of a daughter branch via the Arp2/Arp3 dimer) protect only the terminal subunit.
- Tropomodulin’s three-subunit wrap means subunit dissociation requires displacing the cap and its two-tropomyosin stabilizer simultaneously — kinetically very slow.
Isoforms
- Full-length tropomodulin caps the pointed end of muscle thin filaments (sarcomeric Z-disk math).
- Shorter Tmod isoform caps the tiny actin filaments in the spectrin–actin network of red blood cells [Pollard 2016, p.4].
Where this maps onto stereocilia
- Stereocilia actin core is a paracrystalline bundle of unidirectional actin filaments with barbed ends at the tip and pointed ends rooted in the cuticular plate.
- The pointed-end (basal) cap is occupied by Tmod1, Tmod3, or Tmod4 depending on isoform expression in hair cells (Drewes/Fowler labs; not in Pollard 2016 verbatim — retrieve before using as a load-bearing claim).
- Tropomyosin (specifically Tpm3 isoforms in hair cells) decorates stereocilia actin and reinforces the Tmod cap per the same mechanism Pollard 2016 describes for muscle.
Comparison with capping protein
| Property | Capping protein (CP) | Tropomodulin (Tmod) |
|---|---|---|
| Filament end | barbed | pointed |
| Architecture | α/β heterodimer | single chain (ELM domain + LRR) |
| Subunits contacted | 1–2 terminal | 3 terminal + 2 tropomyosin N-termini |
| Off-rate | t½ ≈ 30 min | slower (no exact number in Pollard 2016) |
| Regulation | PIP₂, V-1, CARMIL | tropomyosin association strengthens; otherwise constitutive |
For CP details see [[Capping Protein Dissociation Half-Life]].
Relevance to STRC hypotheses
- h09 hydrogel — synthetic peptides that mimic Tmod’s pointed-end-wrap geometry could provide an alternative anchoring strategy at the rootlet base. Currently not in any h09 sub-hypothesis but worth flagging.
[[STRC Stereocilia Bundle Mechanics Model]]— the boundary condition at the pointed (basal) end is “fixed” only because Tmod + tropomyosin + cuticular-plate anchorage hold it. Models that assume a free pointed end will be wrong.
Anti-fabrication notes
- Pollard 2016 does not give the Tmod off-rate numerically. “Strengthens” and “slow exchange” are verbal.
- Hair-cell-specific Tmod isoform expression is not in Pollard 2016. Retrieve from Furness & Hackney or Drewes/Fowler literature before claiming Tmod1/3/4.
Connections
[source]2026-04-23-pollard-2016-actin-review-cshpb[applies]index[see-also]Capping Protein Dissociation Half-Life[see-also]Actin Treadmilling Stereocilia[see-also]STRC Stereocilia Bundle Mechanics Model[see-also]Hair Bundle Stiffness Decomposition