Fig. 5. (A) Two different actin nucleators. Actin nucleation by FH2 occurs by dimer stabilization. Comparison between Arp2/3-mediated nucleation and FH2-mediated nucleation indicates that the Arp2/3 complex, in its activated state, is a more efficient nucleator since it only requires the addition of one monomer to create a stable nucleus. (B) The Arp2/3 complex (left panel, green ovals) nucleates a new filament on the side of a pre-existing filament at a 70° angle. The Arp2/3 complex also caps the slowing-growing end of the new filament and allows it to grow from its barbed end, ultimately generating a branched network that is crosslinked by the Arp2/3 complex. The FH2 domain (right panel, blue rectangle) of Bni1p nucleates an actin filament but stays associated with the barbed growing end, thereby regulating filament elongation. The filaments that form are unbranched, polarized by the FH2 domain and stabilized by tropomyosin (purple). (C) Model for how the FH1 domain and profilin-actin contributes to formin-induced nucleation of actin filaments. First, the FH1 domain localizes profilin, which sequesters an actin monomer, to the nucleation vicinity. The FH1 domain may cause profilin to release its bound actin, which the FH2 domain can then utilize to nucleate an actin filament. The filament elongates at the barbed end, a process that also requires profilin-actin.

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