Fig. 7.

Model for severing. The P-end is at the top. a F-actin. The F-form actin subunits are represented in blue. The intrastrand actin-actin contact is represented by a pair of thick blue lines, one for the ID–ID interactions and the other for the OD–ID interactions. The interstrand actin-actin contacts are also represented by thick blue lines. b Upon binding of the first cofilin molecule (orange oval) to one strand of the F-actin, two actin subunits undergo conformational transition into the C-form (camel), which is associated with a twist change in the actin strand, and therefore changes in relative positions of subunits. We assume that these positional changes weaken the interstrand contacts, as indicated by the thin gray lines. c The second cofilin binds next to the first cofilin on the same actin strand. Collision occurs between the strands, as indicated by an “X”. Accumulated position mismatches between actin subunits with and without bound cofilin may require adapter subunits at either boundary (gray). d As the size of the cofilin-bound section in the cofilin-bound strand increases, the helical twist transition propagates to the opposite strand. At this stage, the strain accumulated in the cofilin-bound strand and between the strands would be relieved. The cofilin-free actin subunits with the cofilactin helical twist are represented in magenta. The P-end boundary of the section is a candidate for severing (red arrows). e The helical twist change in the cofilin-free strand facilitates the binding of cofilin molecules to the strand, forming a double-stranded cofilin-bound cluster. The potential severing site remains (red arrows). Orange lines represent the interstrand interactions in cofilactin (Fig. 3e). f When another cofilin joins at the boundary of the cofilin cluster before severing, the cofilin cluster grows. The candidate severing site remains at the P-end boundary of the cluster (red arrowheads). g, h Severed filaments