- Treadmilling
Treadmilling is a phenomenon observed in many cellular cytoskeletal filaments, especially in
actin filament s andmicrotubules . 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 thecytosol . This is due to the constant removal of theprotein subunits from these filaments at one end of the filament while protein subunits are constantly added at the other end [Bruce Alberts, Dennis Bray, Julian Lewis: "Molecular Biology of the Cell", 4th Edition, Taylor & Francis, 2002, PP. 909-920, ISBN 0815340729] .Detailed Process
Dynamics of the filament
The
cytoskeleton is a highly dynamic part of a cell. Cell movement (i.e. amacrophage moving to engulf a foreign object) is often mediated by the synthesis of filaments in one direction of the cell with the front denoted as the "leading edge". Filaments are able to move through the synthesis of subunits. Although both sides of a filament are able to grow in length through this subunit addition, one end of the filament (denoted as the "positive end") will always be more dynamic and be able to grow faster than the other end (a.k.a. the "negative end") [Alberts et al. "Mole Bio of the Cell, 5th Edition." Taylor & Francis, 2008.] This difference is resultant from the fact that the negative end requires that the subunit undergo a conformational change to attach onto the end and consequently, filamental subunits are directionalized (a.k.a. polarized [Gardet A, Breton M, Trugnan G, Chwetzoff S. "Role for actin in the polarized release of rotavirus." J Virol. 2007 May;81(9):4892-4. Epub 2007 Feb 14.] ) and must be added onto the filament in the correct orientation for synthesis to occur.In the very same way, subunit removal (shrinkage) is mediated by the same process, with the positive end being able shrink faster than the negative end.
Critical Concentration
What determines whether the end grows or shrinks is entirely dependent on the concentration of available subunit monomers in the surrounding area [Schaus TE, Taylor EW, Borisy GG. "Self-organization of actin filament orientation in the dendritic-nucleation/array-treadmilling model." Proc Natl Acad Sci U S A. 2007 Apr 24;104(17):7086-91. Epub 2007 Apr 17.] . Hence there is concentration of the subunit called the "critical concentration" (CC) at which the growth rate is balanced by the shrinkage rate, and where:
* A concentration of subunit above the CC results in subunit addition
* A concentration of subunit below the CC results in subunit removalBoth the positive end and negative end have different CC values and generally, the positive end will always have a lower CC value than the negative end. This is due to the increased ease of subunit addition to the positive end, leading to faster growth [Ehrhardt DW, Shaw SL. "Microtubule dynamics and organization in the plant cortical array." Annu Rev Plant Biol. 2006;57:859-75. Review.] .
Hence, if the concentration of the filamental subunit is between the CC values of the positive end and negative end, one end will grow (usually the positive end) while the other end shrinks. This leads to the phenomenon known as treadmilling.
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 growth at the (-) end. This is deemed steady-state treadmilling in which the net length of the treadmilling filament remains unchanged.
Literature
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