STRC Research

Actin and Actin-Binding Proteins

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What they found

Comprehensive authoritative review of actin biochemistry and all major actin-binding protein families. Covers sequestering proteins (profilin, thymosin-β4), nucleators (Arp2/3, formins, WH2-domain proteins), elongators, cappers, severers, and cross-linkers. Full-text open access on PMC.

Numbers that matter

Profilin × G-actin:

  • Kd = 0.1 μM for ATP-actin — sets benchmark for high-affinity sequestration

Thymosin-β4 × G-actin:

  • Micromolar affinity (Kd ~1 μM); cellular concentration > 100 μM → sequesters large monomer pool

Actin-binding domains (ABDs, calponin-homology type) × F-actin:

  • “ABDs typically have relatively low affinity for actin filaments (Kd ~10 μM)”
  • This is the Pollard consensus for ABD-class F-actin binders
  • WH2 is NOT an ABD (different fold, different binding site) but the comparison is instructive: even canonical F-actin cross-linkers have Kd ~10 μM

WH2 × actin (from this review):

  • WH2 binds in the barbed-end groove (actin subdomains 1/3 cleft)
  • Functions: deliver actin to barbed end, template nucleation via tandem repeats
  • No Kd measurements given for WH2 directly; references Chereau 2005

Barbed end kinetics (ATP-actin):

  • k_on ≈ 10 μM⁻¹s⁻¹, k_off ≈ 1 s⁻¹, critical concentration ≈ 0.1 μM

Total cellular actin:

  • 50–200 μM in eukaryotic cells
  • ~half unpolymerized at any moment (maintained by profilin + thymosin-β4 pool)

Critical concentration for ADP-actin = 1.8 μM (Pollard 1986; consistent)

Relevance to hydrogel model

The “Kd ~10 μM for ABD-class cross-linkers” is the best available proxy for WH2 × F-actin side-binding in the absence of direct measurement. The model’s WH2_KD_FACTIN_M = 5 μM is therefore in the right ballpark as an optimistic estimate but has no primary measurement backing. A more conservative estimate would be 10–100 μM or higher.

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