Editing Microfilament (section)

==''In vitro'' self-assembly==
Measuring approximately 6 [[nanometer|nm]] in [[diameter]],<ref>{{Cite journal |vauthors=Fuchs E, Cleveland DW |date=January 1998 |title=A structural scaffolding of intermediate filaments in health and disease |journal=Science |volume=279 |issue=5350 |pages=514–9 |bibcode=1998Sci...279..514F |doi=10.1126/science.279.5350.514 |pmid=9438837}}</ref> microfilaments are the thinnest fibers of the cytoskeleton. They are [[polymers]] of [[actin]] subunits (globular actin, or G-actin), which as part of the fiber are referred to as filamentous actin, or F-actin. Each microfilament is made up of two [[Helix|helical]], interlaced strands of subunits. Much like [[microtubule]]s, actin filaments are polarized. [[Electron micrograph]]s have provided evidence of their fast-growing barbed-ends and their slow-growing pointed-end. This polarity has been determined by the pattern created by the [[binding (molecular)|binding]] of myosin S1 fragments: they themselves are subunits of the larger myosin II [[Protein quaternary structure|protein complex]]. The pointed end is commonly referred to as the minus (−) end and the barbed end is referred to as the plus (+) end.{{cn|date=June 2024}}

''In vitro'' actin polymerization, or [[nucleation]], starts with the self-association of three G-actin monomers to form a [[trimer (biochemistry)|trimer]]. [[Adenosine triphosphate|ATP]]-bound actin then itself binds the barbed end, and the ATP is subsequently [[Hydrolysis|hydrolyzed]]. ATP hydrolysis occurs with a [[Half time (physics)|half time]] of about 2 seconds,<ref name="cell biology">{{Cite book |title=Cell Biology |vauthors=Pollard TD, Earnshaw WD |publisher=SAUNDERS |year=2004 |isbn=978-1-4160-2388-3 |edition=First}}</ref> while the half time for the dissociation of the [[inorganic phosphate]] is about 6 minutes.<ref name="cell biology" /> This [[Autocatalysis|autocatalyzed]] event reduces the binding strength between neighboring subunits, and thus generally destabilizes the filament. ''[[In vivo]]'' actin polymerization is catalyzed by a class of filament end-tracking molecular motors known as [[actoclampins]]. Recent evidence suggests that the rate of ATP hydrolysis and the rate of monomer incorporation are strongly coupled.{{cn|date=December 2024}}

Subsequently, [[Adenosine diphosphate|ADP]]-actin dissociates slowly from the pointed end, a process significantly accelerated by the actin-binding protein, [[cofilin]]. ADP bound cofilin severs ADP-rich regions nearest the (−)-ends. Upon release, the free actin monomer slowly dissociates from ADP, which in turn rapidly binds to the free ATP [[diffusion|diffusing]] in the [[cytosol]], thereby forming the ATP-actin monomeric units needed for further barbed-end filament elongation. This rapid turnover is important for the cell's movement. End-capping proteins such as [[CapZ]] prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus.{{cn|date=December 2024}}

Actin polymerization together with capping proteins were recently used to control the 3-dimensional growth of protein filament so as to perform 3D topologies useful in technology and the making of electrical interconnect. Electrical conductivity is obtained by metallisation of the protein 3D structure.<ref name="Thery2013">{{Cite journal |vauthors=Galland R, Leduc P, Guérin C, Peyrade D, Blanchoin L, Théry M |date=May 2013 |title=Fabrication of three-dimensional electrical connections by means of directed actin self-organization |journal=Nature Materials |volume=12 |issue=5 |pages=416–21 |bibcode=2013NatMa..12..416G |doi=10.1038/nmat3569 |pmid=23396247}}</ref><ref>US Patent US 9070702, Method for obtaining three-dimensional actin structures and uses thereof, Jean-Christophe Gabriel, Laurent Blanchoin, Manuel Thery, Remi Galland</ref>