Tredmilling and Exchange diffusion

In the process of treadmilling, the ADP-carrying subunits dissociate from the pointed end, exchange bound ADP for ATP, and then bind to the opposite, barbed end, move along the filament, and undergo hydrolysis on their way.

Exchange diffusion is a similar process, but dissociation and binding occur at the same end (barbed or pointed) rather than opposite ends, such that the hydrolyzed actin subunits (ADP- or ADP-Pi-bound) dissociate from the end, exchange ADP (or ADP-Pi) for ATP, and then bind to the same end.

Actin filaments are represented by an average filament nucleotide profile where ATP-bound actin is shown in red, ADP and inorganic phosphate (Pi) bound is shown in blue, ADP-bound actin is shown in green, and nucleotide-free actin is not shown. i is the distance from the filament end in number of subunits. The relative height of a color at any i reflects the percentage of that subunit at that i. The monomeric subunits are shown as pentagons of the corresponding color. Short arrows show association and dissociation of the actin subunits from the filament ends, exchange of the bound nucleotide by G-actin subunits, and hydrolysis of bound ATP or inorganic phosphate release by F-actin subunits. Long arrows are related to the process of treadmilling.