25.12:
Actin Treadmilling
Actin treadmilling is the continuous addition of G-actin monomers at F-actin's plus-end and the simultaneous removal from the minus-end. At steady-state, this keeps the filament length unchanged but allows the newly added actin monomers to move forward within the filament.
Treadmilling depends on the critical concentration or Cc of the free monomer, which is the concentration at which the rates of polymerization and depolymerization are equal.
The actin filament grows if the free monomer concentration is higher than the Cc and shrinks if it is lower.
During actin treadmilling, the monomer concentration remains in between the Cc values for the plus and minus-ends, allowing faster and continuous addition of ATP-G-actins only at the plus-end.
These ATP-G-actin monomers undergo gradual ATP hydrolysis with slow phosphate release, resulting in three different forms of actin within F-actin: ATP-actin, ADP-phosphate-actin, and ADP-actin.
The ADP-actins are weakly bound and readily dissociate, resulting in continuous depolymerization at the minus-end of F-actin.
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.
Suggested Reading
- Neuhaus, J.M., Wanger, M., Keiser, T. and Wegner, A., 1983. Treadmilling of actin. Journal of Muscle Research & Cell Motility, 4(5), pp.507-527.
- Bugyi, B. and Carlier, M.F., 2010. Control of actin filament treadmilling in cell motility. Annual review of biophysics, 39(1), pp.449-470.