treadmilling

Treadmilling is a dynamic behavior observed in some cytoskeletal polymers, in which subunits are added at one end of a filament and removed from the opposite end at roughly the same rate. This creates a steady-state situation where the filament length remains essentially constant, but a continuous flux of subunits moves through the filament, giving the appearance that the filament is “moving” through the cytoplasm.

In actin filaments, treadmilling occurs when the concentration of available actin monomers lies between the two critical concentrations for polymerization at the ends: the barbed (plus) end grows while the pointed (minus) end shrinks. ATP-actin adds predominantly at the barbed end and, after incorporation, ATP is hydrolyzed and the filament shortens at the opposite end. The result is a constant length filament with a net flux of actin subunits from the plus end toward the minus end. The phenomenon depends on cellular regulation of monomer pools and is influenced by actin-binding proteins that cap, sever, or stabilize filament ends.

Microtubules can also exhibit treadmilling under certain conditions, though they more commonly display dynamic instability. When subunit turnover is balanced such that growth at the plus end and shrinkage at the minus end proceed with a net flux of tubulin subunits, a treadmilling-like state can arise, producing movement of subunits through the filament without a persistent change in length.

Treadmilling is implicated in various cellular processes, including cell migration, organelle positioning, and mitosis. It is studied in vitro and in vivo using fluorescence imaging and reconstituted systems to understand how polymer dynamics coordinate cellular architecture.