STRC Research

Tandem WH2 Filament Nucleation Mechanism

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Tandem WH2 Filament Nucleation Mechanism

Chereau et al. 2005 PNAS proposed and structurally justified how tandem short WH2 domains seed new actin filaments without requiring Arp2/3 [Chereau 2005, p.16646–16648]. The mechanism rests on two structural facts established by the WH2–actin crystal structures and is the basis on which STRC h09 hydrogel bundling modeling stands.

Two enabling structural facts

  1. Short WH2 (~17 aa) is filament-compatible. Superimposing a WH2–actin structure onto two consecutive subunits of Holmes’ actin filament model shows that the WH2 binding interface on the actin monomer (subdomain 1/3 cleft) does not clash with longitudinal actin–actin contacts along the long-pitch helix. The same is not true of long WH2 (WIP/MIM) — see Short vs Long WH2 Domain Classification.
  2. Short linkers force one-strand spacing. The linkers between consecutive short WH2 domains in tandem-repeat proteins (Spire, WASP-CA) are too short to bridge subunits on opposite strands of the actin double helix. So consecutive WH2-bound monomers must occupy the same filament strand.

[Chereau 2005, p.16647, Fig. 4B]

Nucleation logic

       monomer 1        monomer 2         monomer 3
       (strand A)       (strand A)        (strand A)
          │                │                  │
        [WH2-1] ─────── [WH2-2] ─────── [WH2-3]
          ↑                ↑                  ↑
                short linkers (≤ ~5 aa)

Tandem WH2 domains act as a “ruler” — the number of repeats sets the size of the nucleus. Spire has four tandem WH2 domains, sufficient to template a four-monomer single-strand seed that then condenses into a filament. This explains why Spire’s nucleation activity tracks WH2 copy-number [p.16647, citing Quinlan et al. 2005].

WASP-CA region as a specialised tandem

The C region of WASP/WAVE proteins shares sequence and structural features with short WH2 — N-terminal amphipathic α-helix, similar binding mode in the actin cleft. Chereau proposed the WH2–C arrangement is itself a specialised tandem repeat whose role is to line up one actin subunit alongside Arp2 during Arp2/3 nucleation, just like Spire lines up two adjacent actin monomers [p.16647]. This unifies “Arp2/3-dependent” and “Arp2/3-independent” nucleation under one geometric principle.

Conditions on the geometry

RequirementWhyConsequence if violated
Short WH2 (≤ ~17 aa, ends at LKKT)Long-form residues track the nucleotide cleft and clash with adjacent subunitsLong WH2 tandems cannot template filament strand — work as monomer-sequestration units instead
Short linkers (≤ ~5 aa typical)Spacing between consecutive G-actin sites along one strand is ~5.5 nmLong linkers (e.g. RADA16 spacers) span opposite strands or skip subunits — break the single-strand template
ATP-actin preferred substrateWH2 has higher affinity for ATP-actin than ADP-actin (Table 3 in Chereau 2005)ADP-actin pool nucleates more slowly via WH2

A Pro-rich sequence immediately N-terminal to WH2 (common in WASP, WAVE, VASP) binds profilin–actin. The structures of profilin–actin and WH2–actin overlap partially at the actin barbed face — profilin can hand its actin to WH2 with a short release step. This is the proposed structural basis for the actoclampin model of processive elongation: WH2 acts as a “clamp” stepping along the barbed end while profilin loads new monomers via the upstream Pro-rich site [Chereau 2005, p.16647–16648, Fig. 4C]. ATP-hydrolysis on actin (≈ 0.1–0.3 s⁻¹) provides the energy for processive stepping because WH2 prefers ATP- over ADP-actin.

Implications for STRC h09 hydrogel

The h09 strategy depends on the same geometry working in reverse — a multi-WH2 RADA16 scaffold must space WH2 domains correctly to either (a) bundle pre-existing F-actin via avidity across a filament face or (b) template synthetic bundles. Two non-trivial constraints fall out of this note:

  1. Short WH2 only. Confirm any WH2 used in the scaffold is ≤ ~17 aa, ends at LKKT (per Recipe — Short WH2 Actin Binder Design).
  2. Linker spacing on RADA16. RADA16 inter-WH2 spacing must produce contact points on the same filament face. Mis-spaced WH2 → the avidity gain that the entire h09 model depends on collapses to single-WH2 affinity (5–10 mM equivalent for F-actin side-binding — see actin-kinetics).

Neither constraint has been wet-lab validated; both gate Phase 2c.

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