Figure 4.

Reaction schemes including force-dependent processes. (A) Given here are simplified schemes for stair-stepping and stepping second models. The polymerization or depolymerization processes of two actin subunits (shown as n − 1 to n + 1) are shown. The processes in rapid equilibrium are shown by broken arrows. The parts corresponding to FH2 dimer-half translocation are surrounded by a rectangle. C, closed state; O, open state. In the stepping second model, the two types of open state, polymerizable and depolymerizable, are shown as O and O^∗, respectively. GA, G-actin. (B) Shown here is a schematic illustration of the reaction scheme parts corresponding to FH2 dimer-half translocation. White spheres are actin molecules; magenta/gray rectangles are FH2 dimer halves. Front (magenta) rectangle translocation corresponds to FH2 dimer transition from open to closed states (stair-stepping model) or depolymerizable to polymerizable open states (stepping second model). We assumed that the FH2 dimer half moves over the distance (5.5 nm) during the closed-to-open state transition. Although the FH2 dimer often dissociates from the barbed end during the FH2-half translocation in the stepping second model (19), the dissociation process is not included for simplification. To see this figure in color, go online.