What they found
Comprehensive review/research article from the Carlier lab (MF Carlier is the authoritative actin dynamics biochemist) on the β-thymosin/WH2 module. Used chimeric protein engineering, actin polymerization assays, ITC, NMR, and SAXS to dissect how a single βT/WH2 domain can function as G-actin sequesterer, filament barbed-end deliverer, nucleator, or severer depending on C-terminal sequence context.
Numbers that matter
Thymosin-β4 (Tβ4) × G-actin:
- Kd = 1 μM for ATP-actin (moderate affinity, the reference baseline)
- Kd = 80–100 μM for ADP-actin (50–100× weaker — nucleotide-state dependent)
Tβ4 × F-actin (side-binding):
- Kd = 5–10 mM (weak cooperative binding at [Tβ4] > 20 μM)
- This is 5,000–10,000× weaker than G-actin binding
- Mechanism: C-terminal α-helix of Tβ4 interferes with filament contacts
Implication for WH2 × F-actin:
- WH2 lacks the C-terminal α-helix that interferes with F-actin in Tβ4
- But WH2 canonical binding site (barbed-end groove) is also involved in longitudinal actin–actin contacts in filament — still not a clean side-binding site
- No direct measurement of WH2 × F-actin Kd reported in this paper
- The paper does not support the existence of a low-μM WH2 × F-actin interaction
Context for model parameter WH2_KD_FACTIN_M = 5 μM:
- Based on all available literature, 5 μM would be VERY optimistic for a WH2 × F-actin side-binding event. It is possible if constructs are specifically engineered, but not for a canonical isolated WH2 peptide. True value is likely ≥ 1 mM or unmeasurable.
Connections
[source]STRC H09 WH2 F-actin Bundling Hypothesis — Tβ4/WH2 Kd values and F-actin binding data; shows F-actin side-binding is mM-range or absent- Hydrogel Phase 4d F-actin Bundling Model — key constraint on WH2_KD_FACTIN_M: the 5 μM value in model is 1000× more optimistic than Tβ4 literature
- Actin Treadmilling Stereocilia — Carlier lab is primary authority on actin dynamics constants