25.12: Actin Treadmilling
Actin filaments undergo polymerization and depolymerization from either end. The polymerization and depolymerization rates depend on the cytosolic concentration of free G-actins. The polymerization rate is generally higher at the plus or barbed end, while the depolymerization rate is higher at the minus or pointed end. At a steady state, critical concentration describes the concentration of free G-actin monomers at which the polymerization rate at the plus end is equal to that of the depolymerization rate from the minus ends. Actin treadmilling is the continuous addition at one end and dissociation from the other end of the G-actin monomers.
Actin treadmilling is crucial to several functions in eukaryotic cells, such as cell migration, endocytosis, and exocytosis. Treadmilling occurs continuously, even in resting cells, and is responsible for up to 50% of energy consumption in most cells.
Actin treadmilling depends primarily on three factors, the ATP hydrolysis rate within the F-actin, the polarity in the polymerization and depolymerization rate at both plus and minus ends, and the cytosolic concentration of free G-actins. ATP-G-Actins bind faster at the plus end and undergo gradual ATP hydrolysis, forming intermittent ADP-Pi-Actins and, eventually, ADP-Actins. ADP-G-actins are loosely associated with the monomers with the F-actins and readily dissociate. At the plus end, the polymerization rate is higher than depolymerization, while at the minus end, the depolymerization rate of G-actin is higher than their polymerization rate. The cytosolic G-actin concentration during the actin treadmilling remains intermediate between that of the critical concentrations of plus and minus ends of F-actin.
