Although the most commonly described nucleators are the Arp2/3 complex, and the formins, however a third group has also been identified. This third group is known as ‘tandem-monomer-binding nucleators’ as each member possesses tandem repeats of G-actin binding motifs.

Included in this group of nucleators are the Spire proteins, Cordon-bleu (Cobl), Leiomodin (Lmod-2), JMY and adenomatous polyposis coli (APC). Common to each of these proteins are repeats of the actin binding motif Wiskott-Aldrich Syndrome protein (WASp) homology 2 (WH2) domain. However, additional actin binding motifs may be present in the individual members, providing variation not their mechanisms of nucleation, but importantly, in the cellular functions they facilitate. 

Animation used by permission. Source: Janet Iwasa