An important set of actin regulators initiate formation of new actin filaments by a process that is called nucleation. Spontaneous nucleation is a kinetic hurdle in the process of actin polymerization, and, therefore, factors that can accelerate or bypass this step are important for efficient actin assembly in the cell. So far, three classes of protein have been identified that initiate new filament polymerization: the actin-related protein-2/3 (ARP2/3) complex, the formins and spire. The intact ARP2/3 complex was shown to consist of a stable assembly of seven polypeptides. Two of the subunits were actin-related proteins of the ARP2 and ARP3 subfamilies, giving the complex its name. The ARP2/3 complex possesses little biochemical activity on its own.The capability of nucleation-promoting factors (NPFs) to activate the actin-related protein-2/3 (ARP2/3) complex is regulated by signal-transduction pathways that involve Rho-family GTPases and lipid second messengers.  
The mechanism of regulation has been well characterized for two class I NPFs, Wiskott–Aldrich syndrome protein (WASP and neural (N)-WASP) and SCAR (suppressor of cyclic AMP repressor; also known as WAVE (WASP-family verprolin-homologous protein)), and differs between the two protein families. Coordinated nucleation and branching by the ARP2/3 complex has an important role in several cellular processes including cell migration and adhesion, moving membranes and their cargoes.The ARP2/3 complex and its activators are therefore likely to be crucial participants in the process of tumour-cell invasion and metastasis, and might represent targets for therapeutic intervention.

References

1.Goley ED,et al. Nat Rev Mol Cell Biol. 2006 Oct;7(10):713-26.