WH2 Domain Actin Nucleation
WH2 (Wiskott-Aldrich syndrome protein homology domain 2) is a ~20 amino acid actin-binding motif found in a huge range of cytoskeletal proteins: WASP, N-WASP, WAVE1-3, WIP, spire, VASP. It’s the minimal unit that grabs actin monomers.
Chéreau et al. 2005 solved the crystal structures of WH2–actin for WASP, WAVE2, and WIP. All three bind in the same cleft between actin subdomains 1 and 3.
The key structural fact: WH2 is shorter than thymosin-β (Tβ) and lacks its C-terminal α-helix. That missing helix is what makes Tβ a monomer sequesterer: it caps both ends of actin and prevents polymerization. WH2, being shorter, does not interfere with intersubunit contacts in F-actin. This is the structural basis for why WH2 functions in nucleation while Tβ sequesters.
Tandem WH2 = nucleation scaffold: Short WH2 domains connected by short linkers can coexist with F-actin contacts. Multiple WH2 domains (4× in spire) line up actin monomers along one strand of the double helix — essentially pre-forming a filament seed. Linker length determines which strand spacing is matched.
Affinity: WH2 binds actin ~10× stronger than Tβ, despite smaller interface. The N-terminal amphiphilic α-helix accounts for most of the affinity — it embeds hydrophobic residues in the subdomain 1/3 cleft. WAVE is the strongest binder because it has two Arg in the LKKT motif instead of Lys, forming extra salt bridges.
For stereocilia: OHCs maintain their F-actin stereocilia bundles throughout life. WH2-containing proteins (WASP, WAVE, spire) are part of the maintenance machinery. Loss of STRC destabilizes the membrane attachment context — understanding the actin nucleators that work in this region tells you what compensatory machinery might be preservable.
Connections
[source]2005-chereau-wh2-actin-pnas[about]Stereocilia Actin Cytoskeleton[applies]STRC Research Portal[see-also]STRC Gene Therapy sphere