Role of formins in actin assembly: Nucleation and barbed-end association

Nucleation of branched actin laments by the Arp2/3 complex is a conserved process in eukaryotic cells, yet the source of unbranched actin laments has remained obscure. In yeast, formins stimulate assembly of actin cables independently of Arp2/3. Here, the conserved core of formin homology domains 1 and 2 of Bni1p (Bni1pFH1FH2) was found to nucleate unbranched actin laments in vitro. Bni1pFH2 provided the minimal region sufficient for nucleation. Unique among actin nucleators, Bni1pFH1FH2 remained associated with the growing barbed ends of laments. This combination of properties suggests a direct role for formins in regulating nucleation and polarization of unbranched filamentous actin structures.