What they found
Highly sensitive single-cell proteomics (SCoPE-MS) applied to E15 chick utricle hair cells. Quantified relative molar fractions of ~1500 proteins per cell type. Provides the most precise molecular-level census of hair cell protein composition, including actin copy numbers that can be used to estimate local [F-actin] in stereocilia.
Numbers that matter
Actin copy numbers:
- E15 chick utricle hair cell: ~15,000,000 total actin molecules per cell
- Per stereocilium: ~200,000 actin molecules (72 stereocilia counted per utricle hair cell)
- E20 (later development): ~400,000 actin molecules per stereocilium (estimate)
- ACTG1 group (total actin) = 0.043 ± 0.001 molar fraction in FM1-43high cells
Derived local [F-actin] in stereocilia (calculation, not in paper):
- OHC stereocilium: ~200 nm diameter, ~3 μm long → volume ≈ 9.4 × 10⁻¹⁷ L
- 200,000 molecules / (6.022×10²³ × 9.4×10⁻¹⁷) ≈ 3,500 μM total actin
- Assuming 90% filamentous → [F-actin] ≈ 3,150 μM ≈ 3.15 mM
- Cross-check with ~100 nm diameter: [actin] ≈ 14,000 μM (extreme upper bound, likely for developing stereocilia with denser core)
- Practical working estimate: [F-actin] in mature stereocilium core = 1–5 mM
Critical implication for WH2 × F-actin bundling model:
- [F-actin] is 1–5 mM — this is far above any plausible WH2_KD_FACTIN_M
- If WH2_KD_FACTIN_M = 5 μM (as in model), fractional occupancy at 1 mM [F-actin] would approach 100% — but this is irrelevant if the binding event doesn’t exist in the first place
- The high local [F-actin] is not a bottleneck; the question is purely whether WH2 can side-bind at all
STRC/stereocilin: Not quantified in this study (chick utricle dataset; STRC is mammalian cochlear-specific in its critical role).
Connections
- Hydrogel Phase 4d F-actin Bundling Model — provides 200,000 actin/stereocilium baseline for [F-actin] calculation
- STRC normal OHC concentration (parameter, see
literature/stereocilia-mechanics.md) — STRC_NORMAL_OHC_M needs own source; this paper doesn’t cover STRC - Barr-Gillespie lab — Stereocilia Atlas (external resource) — anchor paper for quantitative hair cell proteomics