FIGURE 17-13 Actin nucleation by the formin FH2 domain. (a) Formins have a domain called FH2 that can form a dimer and nucleate filament assembly. The dimer binds two actin subunits step 1 and, by rocking back and forth steps 2–4, can allow insertion of additional subunits between the FH2 domain and the (+) end of the growing filament. The FH2 domain protects the (+) end from being capped by capping proteins. (b) The FH2 domain of a formin was labeled with colloidal gold (black dot) and used to nucleate assembly of an actin filament. The resulting filament was visualized by electron microscopy after staining with uranyl acetate. Formins assemble long unbranched filaments.

[Part (b) republished with permission of AAAS, from Pruyne et al., “Role of Formins in Actin Assembly: Nucleation and Barbed-End Association,” Science 297:612, 2002; permission conveyed through Copyright Clearance Center, Inc.]