Fig. 6

Model for cooperative binding of cofilin to the actin filament. The P-end is at the top. a One strand of F-actin. The ID–ID and OD–ID intrastrand interactions are presented by blue and red arrows, respectively. b Model of one cofilin (Cof1) binding to Ac1 and Ac2. Ac1 and Ac2 are replaced by the cofilactin structure by aligning the inner domains of the actin subunits. Because of the structural transition of Ac2, the OD–ID interaction between Ac2 and Ac3 is eliminated, creating a space (orange ellipse) for Gi-site binding of another cofilin (Cof2). The form of each actin subunit, C-form or F-form, is indicated in camel and cyan, respectively. At the boundary of Ac1/Ac0, the ID–ID and OD–ID intrastrand interactions between the actin subunits (Fig. 5a, blue and red arrows, respectively) remain because there was no significant structural change inside the ID of Ac1 (C-form) from the F-form and because Ac0 is in the F-form. c Model of Cof2 binding to the Gi-site of Ac3 in b