Capping Protein Dissociation Half-Life
P0 data. Capping protein (CP, the α/β heterodimer aka CapZ in muscle) binds the actin barbed end with a dissociation half-time of 30 minutes [Pollard 2016, p.4 §10].
Verbatim values
| Quantity | Value | Citation |
|---|---|---|
| Architecture | α/β heterodimer | [Pollard 2016, p.4] |
| Cellular concentration | low micromolar | [Pollard 2016, p.4] |
| Time to cap a free barbed end | seconds (at cellular [CP]) | [Pollard 2016, p.4] |
| t½ for dissociation from barbed end | 30 min | [Pollard 2016, p.4] |
Implied k_off ≈ ln(2) / (30 × 60 s) = 3.85 × 10⁻⁴ s⁻¹.
What the 30-min off-rate implies
- A capped barbed end is, on the timescale of most cellular actin dynamics (seconds to minutes), effectively permanent.
- For barbed ends to be re-opened on a faster timescale, cells use regulated uncapping:
- Polyphosphoinositides (PIP₂) — sterically block CP rebinding but do not eject CP from a filament end [Pollard 2016, p.4].
- V-1 / myotrophin — sequester free CP in the cytoplasm.
- CARMIL — allosteric weakener that lets CP bind weakly and slow elongation rather than fully cap [Pollard 2016, p.4 (Edwards 2014)].
Relevance to STRC
- Stereocilia barbed ends (tips) are NOT capped by CP — they are kept polymerization-competent (espin/EPS8/whirlin family at the tip). The CP 30-min off-rate is therefore the right comparison number to argue why stereocilia tip elongation requires constant active uncapping or local CP exclusion.
- Stereocilia pointed ends (bases) are capped by tropomodulin, not by CP — see
[[Tropomodulin Pointed End Cap Mechanism]]. The tropomodulin off-rate is slower still and stabilized by tropomyosin. - h09 hydrogel models that introduce synthetic actin-binding sequences should treat any region that mimics CP’s barbed-end binding as effectively irreversible on cellular timescales — relevant for thinking about therapeutic ototopical residence times (hours to days vs cellular seconds).
Anti-fabrication notes
- Pollard 2016 gives the 30-min half-time and a verbal “low micromolar” concentration; the exact cellular [CP] is not stated in this review. For a specific concentration, retrieve Schafer et al. 1996 J Cell Biol or Cooper & Sept 2008 Int Rev Cell Mol Biol.
- The k_off conversion above (3.85 × 10⁻⁴ s⁻¹) is a derived number, not stated in Pollard 2016. Cite as derived if used.
Connections
[source]2026-04-23-pollard-2016-actin-review-cshpb[applies]index[see-also]Tropomodulin Pointed End Cap Mechanism[see-also]Actin Polymerization Kinetics Reference Table[see-also]Actin Treadmilling Stereocilia