Actin Treadmilling

Actin treadmilling is the continuous addition of G-actin monomers at the plus-end of F-actin filaments while simultaneously removing them from the minus-end. At steady-state, this process maintains constant filament length while allowing newly added actin monomers to move forward through the filament structure. Treadmilling enables rapid cellular processes like cell migration and endocytosis while consuming up to 50% of cellular energy.

Critical concentration (Cc) is the free G-actin monomer concentration at which polymerization and depolymerization rates are equal. During treadmilling, the cytosolic G-actin concentration remains between the Cc values of the plus and minus-ends, allowing faster addition at the plus-end and continuous removal at the minus-end. This concentration gradient is essential for maintaining directional monomer flow through the filament.

ATP-G-actin monomers bind rapidly at the plus-end and undergo gradual ATP hydrolysis, creating three distinct actin forms within F-actin: ATP-actin, ADP-phosphate-actin, and ADP-actin. ADP-actins are weakly bound and readily dissociate from the minus-end, driving continuous depolymerization. This ATP hydrolysis cycle is fundamental to the directional movement of monomers through the filament.

ATP-G-actins bind faster at the plus or barbed end due to its structural polarity, while the minus or pointed end has slower polymerization kinetics. At the plus-end, polymerization rate exceeds depolymerization, whereas at the minus-end, depolymerization dominates. This polarity difference enables the directional treadmilling mechanism that drives the role of actin and myosin in non-muscle cells.

Actin treadmilling is crucial for cell migration, endocytosis, and exocytosis in eukaryotic cells. Treadmilling occurs continuously even in resting cells, enabling dynamic cytoskeletal remodeling necessary for these processes. The constant monomer flux through filaments allows cells to rapidly reorganize their cytoskeleton in response to environmental signals and cellular demands.

ATP-actin monomers are tightly bound within F-actin filaments, while ADP-actin monomers are weakly associated and readily dissociate. During treadmilling, ATP-actins gradually convert to ADP-phosphate-actins and then ADP-actins through hydrolysis. This progressive weakening of binding affinity concentrates depolymerization at the minus-end, where ADP-actins predominate.

Actin treadmilling depends on the ATP hydrolysis rate within F-actin, the polarity difference in polymerization and depolymerization rates at the plus and minus-ends, and the cytosolic concentration of free G-actins. These three factors work together to establish the concentration gradient and directional monomer flow that sustains treadmilling. Without any one factor, the continuous cycling of monomers cannot be maintained.