Short vs Long WH2 Domain Classification
Chereau et al. 2005 PNAS established that WH2 domains divide into two structural sub-families based on whether the chain extends past the conserved LKKT motif [Chereau 2005, p.16645]. The distinction matters because only short WH2 is compatible with intersubunit contacts in F-actin — the structural prerequisite for tandem-WH2 filament nucleation (Spire, WASP-CA region).
The two types
| Type | Length | Examples | Structure past LKKT | F-actin compatibility |
|---|---|---|---|---|
| Short WH2 | ~17 aa | WASP, WAVE1/2/3, Spire | Disordered after LKKT | Yes — does not clash with longitudinal actin–actin contacts in the long-pitch helix |
| Long WH2 | ~27 aa | WIP, MIM, actobindin (and Tβ-family by analogy) | Five additional residues form a β-strand parallel to a β-sheet in actin subdomain 1; an extra Ser penetrates the nucleotide cleft deeper than Tβ4 | No — extra residues track along the nucleotide cleft and would clash with neighbouring subunits in F-actin |
[Chereau 2005, p.16645, Fig. 2]
Sequence diagnostics
- Five residues C-terminal to LKKT determine type. In WIP and MIM these residues resemble Tβ4 and follow a Tβ-like path; in WASP and WAVE the chain becomes disordered immediately after LKKT.
- Long WH2 carries a Ser in the post-LKKT segment that is absent in Tβ — drives deeper penetration into the actin nucleotide cleft.
- Long WH2 ends at a position equivalent to Ser-755 of MIM, which corresponds to Ser-31 of Tβ4. Past this point, Tβ4 has a C-terminal α-helix (monomer-sequestration cap); long WH2 does not.
- Actobindin has all the features of a long WH2 (no C-terminal α-helix, short linker, extra Ser) — likely the only known protein composed purely of tandem WH2 domains [p.16645].
Why the classification matters functionally
| Function | Short WH2 (WASP/WAVE/Spire) | Long WH2 (WIP/MIM) |
|---|---|---|
| G-actin binding affinity | 52–250 nM | 160–230 nM (WIP/MIM measured) |
| Filament nucleation by tandem repeats | ✅ — short linker can space monomers along one filament strand | ✗ — extra residues collide with neighbouring subunits |
| Monomer sequestration | Weak | Stronger (more contact area) |
| Arp2/3 activation (CA region) | The C region of WASP/WAVE is itself a specialised short WH2-like tandem | Not the activation module |
Evolutionary implication
Long WH2 may be an evolutionary intermediate between short WH2 and Tβ. The five extra residues that distinguish long from short WH2 are the same residues that are conserved in Tβ and contact actin in the same way [p.16646]. This lends support to a divergent (rather than convergent) origin for the WH2/Tβ super-family.
Implications for STRC h09
Synthetic WH2-RADA16 scaffolds for the hydrogel HTC must use the short form if the intent is to bundle/nucleate F-actin. A long-WH2 design would sequester monomers more efficiently but cannot template along a filament strand — wrong mode for stereocilia-bundle exoprosthesis. Confirm sequence type before committing AF3 jobs (Phase 4b) or wet-lab synthesis (Phase 2c).
Connections
[source]2026-04-23-chereau-wh2-actin-pnas[see-also]Tandem WH2 Filament Nucleation Mechanism[see-also]Recipe — Short WH2 Actin Binder Design[see-also]2026-04-23-dominguez-2016-wh2-nucleation-review[applies]h09 hub[part-of]actin-kinetics