include cortactin (Weaver et al., 2001), fungal myosin-I (Evangelista et al.,
2000; Lechler et al., 2000; Lee et al., 2000), fungal Abp1p (Goode et al., 2001)
and fungal Pan1p (Duncan et al., 2001).
Nucleation-promoting factors activate the Arp2/3 complex, generally
employing a sequence of acidic residues with a key tryptophan to bind the
Arp2/3 complex. Most also bind one or two actin monomers which
presumably become the first subunit(s) in the new filament. The activating
part of WASp/Scar proteins is located near their C-terminus, consisting of a
'V' motif (for verprolin homology), a 'C' motif (for central or connecting) and
an 'A' motif (for acidic). The VC region binds the actin monomer and the CA
region binds Arp2/3 complex. In isolation, the VCA domain constitutively
activates the Arp2/3 complex. The corresponding functional regions are
located in the middle of the sequence of ActA.
Some nucleation-promoting factors are constitutively active such as
ActA, but the WASp/Scar family are tightly regulated and responsive to
activation by signalling pathways (Figure 1.3). WASp and N-WASp are
auto-inhibited by virtue of an intramolecular interaction of the C motif
Figure 1.3 WASp domains and activation. Intramolecular binding of the C region to the
GTPase binding domain (GBD) strongly auto-inhibits the nucleation promoting activity of
the VCA domains at the C-terminus. Binding of the GBD to Cdc42 and the basic region to
PIP 2 release the VCA domains, so that the VC region can bind an actin monomer and the
CA region can bind Arp2/3 complex. Binding of this ternary complex to the side of an actin
filament complete the activation process and initiates the formation of a new filament.
(Redrawn from Higgs and Pollard, 2001)