Cofilin Filament Twist Geometry Change
P0 data. When cofilin binds the side of an actin filament, both the helical twist and the long-pitch repeat change. This is a mechanical-state change that any model of cofilin-regulated actin (including stereocilia rootlet remodeling) must carry.
Verbatim values
| Parameter | Free filament | Cofilin-decorated filament | Δ |
|---|---|---|---|
| Short-pitch twist per subunit | 167° | 162° | −5° |
| Long-pitch helix repeat | 36 nm | 27 nm | −25 % |
| Outer-domain rotation (subdomains 1+2) | 0° (reference) | ~30° from flattened conformation | +30° |
| Result | ”even more skewed than monomers” | — | — |
Source: [Pollard 2016, p.3 §7], summarizing Galkin 2011 PNAS and McCullough 2011 Biophys J.
Mechanism (per Galkin 2011, McCullough 2011)
- The cofilin domain anchors in the barbed-end groove of one actin subunit (same site as twinfilin’s cofilin domain — see Paavilainen 2008).
- A second cofilin contact reaches subdomain 2 of the next actin subunit along the long-pitch helix (toward the pointed end).
- To accommodate this contact without steric clash between successive subunits along the short-pitch helix, the inter-subunit twist relaxes from 167° to 162°.
- The compounding effect is a 25 % shortening of the long-pitch repeat (36 → 27 nm) and a more flexible filament.
Mechanical / functional consequence
- Filaments saturated with cofilin are stable (uniform elastic modulus).
- Filaments with partial cofilin decoration sever preferentially at the boundary between bare and decorated segments — because of the abrupt mechanical mismatch (Elam 2013; Ngo 2015).
- This is why severing is substoichiometric-optimal — see
[[Cofilin Severing Substoichiometric Optimum]].
Relevance to STRC
- Stereocilia actin rootlets and shaft are continuously pruned at the base, where cofilin/ADF concentrations are higher [Drummond 2015, broader literature]. Any model that simulates rootlet thinning under ADF/cofilin must use the cofilin-decorated geometry, not the free-filament geometry.
- For the bundle-stiffness decomposition
[[Hair Bundle Stiffness Decomposition]], K_SP includes the bending modulus of the actin rootlet — which scales (roughly) with the persistence length of the filament. A 27-nm-repeat cofilin-decorated filament has lower persistence length than the 36-nm free filament; this is a specific quantitative input that should not be ignored.
Anti-fabrication notes
- Pollard 2016 reports the geometry change but does not give a numerical persistence-length ratio. McCullough 2011 reports filaments are “more flexible”; for a specific persistence-length ratio, retrieve McCullough 2011 directly.
- The “30° outer-domain rotation” is approximate (“~30°” verbatim).
Connections
[source]2026-04-23-pollard-2016-actin-review-cshpb[applies]index[see-also]Cofilin Severing Substoichiometric Optimum[see-also]Hair Bundle Stiffness Decomposition[see-also]Actin Treadmilling Stereocilia[see-also]STRC Stereocilia Bundle Mechanics Model