FIG 4.

Treadmilling as a means of motion. The illustration shows the net movement of filament to the right. To keep track, single monomeric units are represented as either lilac or tan. Treadmilling is active motion and requires an energy source such as GTP hydrolysis. In treadmilling, individual cytoskeletal monomers within the filament are stationary. However, net filament movement occurs due to reoccurring polymerization and depolymerization events at the respective positive (+) and negative (−) ends of the filament. Filament growth requires unequal rates of polymerization and depolymerization and is dependent on the cytoplasmic concentrations of cytoskeletal monomer and substrate, such as GTP. It is unknown what triggers the initial polymerization event of monomer to filament within the cell.