This animation describes the process of Actin Filament Treadmilling. Treadmilling is a phenomenon observed in many cellular cytoskeletal filaments, especially in actin filaments and microtubules. It occurs when one end of a filament grows in length while the other end shrinks resulting in a section of filament seemingly “moving” across a stratum or the cytosol. This is due to the constant removal of the protein subunits from these filaments at one end of the filament while protein subunits are constantly added at the other end.
Cytoskeletal protein of most cells is actin. Individual actin molecules, also called G actin,
are globular proteins of 375 amino acids. G actin is described as having a pointed end
and a barbed end. G actin monomers polymerize to form actin filaments. The assembly begins with a nucleation of three monomers to form a trimer.
Polymerization is reversible and proceeds from both ends, yielding a filament called
F actin. Polymerization is a reversible process, in which monomers both, associate with and dissociate from both ends of the filament.
At high concentrations of free subunits, the filament grows at each end, but growth is faster at the barbed end. When the concentration of free actin monomers falls below a certain level, the barbed end will continue to increase in length, but the pointed end will decrease in length, resulting in a phenomenon called treadmilling.
Treadmilling can be depicted in a cycle diagram, in which there is a net gain of subunits on
the barbed end and an equivalent net loss of subunits on the pointed end. The treadmilling
cycle depends on ATP, which is bound to monomers that add to the ends of the filament. Actin monomers at the barbed end are therefore bound to ATP, however soon after polymerization, ATP is hydrolized to ADP resulting in a zone of ADP bound monomers toward the pointed end of the filament. ADP bound actin which is primarily the pointed end of the filament, dissociates more readily from the filament than ATP bound actin. In contrast, actin bound to ATP associates more readily with the rapidly growing barbed
ends.
ATP binding and hydrolysis play a key role in the dynamic behavior of actin filaments.
In the cell, two types of proteins including one called Formin, determine where filaments
are formed. Formin binds to monomers and facilitates the nucleation process in which three monomers are joined to form a trimer. After nucleation, polymerization proceeds quickly. It is thought that Formins nucleate long unbranched actin
filaments, such as those in a muscle cell’s thin filaments. At the leading edge of moving cells, actin filaments treadmill and branch extensively.
The branches are nucleated by the Arp2/3 complex, which binds near the barbed ends of microfilaments, and forms a new branch. Arp2/3 helps promote the remodeling of the actin cytoskeleton that’s required for cell movements and changes in
cell shape.
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