What they found
The benchmark actin polymerization kinetics paper. Measured rate constants for ATP-actin and ADP-actin association/dissociation at both barbed and pointed ends using Limulus sperm acrosomal processes as nuclei and electron microscopy. Defines critical concentrations for both ends and both nucleotide states.
Numbers that matter
ATP-actin:
- Barbed end k_on = 11.6 μM⁻¹s⁻¹, k_off = 1.4 s⁻¹, Kd (critical conc.) = 0.12 μM
- Pointed end k_on = 1.3 μM⁻¹s⁻¹, k_off = 0.16 s⁻¹, Kd = 0.12 μM
ADP-actin:
- Barbed end k_on = 3.8 μM⁻¹s⁻¹, k_off = 7.2 s⁻¹, Kd = 1.9 μM
- Pointed end k_on = 0.16 μM⁻¹s⁻¹, k_off = 0.27 s⁻¹, Kd = 1.7 μM
- Overall critical concentration for ADP-actin polymerization = 1.8 μM
Physiological context:
- Cells maintain ~50–200 μM total actin; ~25–100 μM unpolymerized (monomer pool)
- Critical concentration at barbed end (ATP-actin) = ~0.1 μM
- Treadmilling occurs because pointed-end Kd (1.7 μM, ADP) >> barbed-end Kd (0.12 μM, ATP)
Relevance to stereocilia model:
- Stereocilia actin is largely stable (slow turnover along shaft, active only at tips)
- The actin treadmilling model applies at stereocilia tip region (~0.5 μm zone)
- Local [F-actin] in OHC stereocilium: ~3,500 μM (200 nm dia, 3 μm long, 200,000 molecules) — far above critical concentration; essentially all actin in shaft is filamentous
Connections
- Hydrogel Phase 4d F-actin Bundling Model — barbed/pointed end rate constants for actin assembly in stereocilia context
- Actin Treadmilling Stereocilia — sets rate constants used in tip-turnover estimates
- STRC H09 WH2 F-actin Bundling Hypothesis — benchmark for comparison with WH2 on/off rates