FIGURE 17-10 Actin treadmilling. ATP-actin subunits are added faster at the (+) end than at the (−) end of an actin filament, resulting in a lower critical concentration at the (+) end and treadmilling at steady state. (a) The rate of addition of ATP–G-actin is much faster at the (+) end than at the (−) end, whereas the rate of dissociation of ADP–G-actin is similar at the two ends. This difference results in a lower critical concentration at the (+) end. At steady state, ATP-actin is added preferentially at the (+) end, giving rise to a short region of the filament containing ATP-actin and regions containing ADP-P_i–actin and ADP-actin toward the (−) end. (b) At steady state, ATP–G-actin subunits add preferentially to the (+) end, while ADP–G-actin subunits disassemble from the (−) end, giving rise to treadmilling of subunits. Two actin subunits are colored blue as a reference point within the filament.