Treadmilling
Encyclopedia
Treadmilling is a phenomenon observed in many cellular
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 cytoskeletal
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 filaments
Protein filament
In biology, a filament is a "long chain of proteins, such as those found in hair, muscle, or in flagella". They are often bundled together for strength and rigidity. Some cellular examples include:*Actin filaments*Microtubules*Intermediate 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
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

. This is due to the constant removal of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 subunits from these filaments at one end of the filament while protein subunits are constantly added at the other end.

Dynamics of the filament

The cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 is a highly dynamic part of a cell
and cytoskeletal filaments constantly grow and shrink through addition and removal of subunits.
Directed crawling motion of cells such as macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s relies on
directed growth of actin filaments at the cell front (leading edge).
The two ends of an actin filament differ in their dynamics of subunit addition and removal.
They are thus referred to as the plus end (with faster dynamics, also called barbed end)
and the minus end (with slower dynamics, also called pointed end)
This difference results from the fact that subunit addition at the minus end requires a conformational change
of the subunits [reference?].
Note that each subunit is structurally polar and has to attach to the filament
in a particular orientation
As a consequence, the actin filaments are also structurally polar.
Remarkably, the polymerization dynamics is not only faster at the plus end,
but, in addition, the critical concentration
is higher at the plus end than at the minus end.
The critical concentration (CC) characterizes the concentration of free monomer subunits
at which net filament growth equals net shrinkage.
This peculiar property is crucial for the phenomenon of treadmilling, see below.
According to the thermodynamic law of microscopic reversibility,
the equilibrium constants should be equal at both filament ends in an equilibrium system (such as a dead cell deprived of the chemical fuel ATP).
In a living cell, however, a non-equilibrium steady-state is maintained and ATP is constantly
consumed (hydrolyzed) to power the addition of new subunits.

The situation for microtubules is similar.

Critical Concentration

What determines whether the ends grow or shrink is entirely dependent on the cytosolic concentration of available monomer subunits in the surrounding area. Both the plus and the minus ends have a different critical concentration (CC). Examples in which the cytosolic concentration can affect the critical concentrations are as followed:
  • A cytosolic concentration of subunits above both the CC+ and CC- ends results in subunit addition at both ends
  • A cytosolic concentration of subunits below both the CC+ and CC- ends results in subunit removal at both ends


Both the plus and minus ends have different CC values and generally, the plus end will always have a lower CC value than the minus end. This is due to the increased ease of subunit addition to the plus end, leading to faster growth.

Note that the cytosolic concentration of the monomer subunit between the CC+ and CC- ends is what is defined as treadmilling in which there is growth at the plus end, and shrinking on the minus end.

Steady-state treadmilling

While treadmilling may occur at different speeds at both ends, there is a concentration at which the speed of growth at the (+) end is equal to the rate of shrinkage at the (-) end. This is deemed steady-state treadmilling in which the net length of the treadmilling filament remains unchanged.
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