Name: Formation of Higher-order Actin Filaments
Uploaded: 2023-04-30T09:31:47
Description: Learn by watching this video about Formation of Higher-order Actin Filaments at JoVE.com

Actin filaments assemble to form different higher-order structures like&#160; bundles, dendritic networks, and gel-like networks.
Actin bundles are composed of straight actin filaments generated with the help of formins. Actin-crosslinking proteins such as the dimeric alpha-actinin arrange and join the filaments into a loosely packed network of oppositely oriented actin filaments allowing the insertion of myosin.
Another monomeric actin-crosslinking protein, fimbrin, forms closely packed parallel bundles of actin filaments in the same orientation.
Actin nucleating proteins Arp2/3 guide actin filaments to attach their minus-ends directly to other actin filaments and arrange them in a dendritic network.
In gel-like networks, actin filaments are interlinked through filamin, a flexible actin cross-linking protein with two actin-binding domains.
Filamin clamps two adjacent actin filaments perpendicular to one another, forming a loose viscous gel-like network.

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	<li>Lamb, M.C. and Tootle, T.L., 2020. Fascin in cell migration: more than an actin-bundling protein. Biology, 9(11), p.403.</li>
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	<li>Cronin, N.M. and DeMali, K.A., 2021. Dynamics of the Actin Cytoskeleton at Adhesion Complexes. Biology, 11(1), p.52.</li>
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formation of higher-order actin filaments

The polymerization of G-actin monomers into filamentous F-actin is a multi-step process. Once the F-actins are formed, they can bundle together in different arrangements to form higher-order networks and regulate cellular functions. Common examples include the formation of lamellipodia and filopodia at the cell&#39;s leading edge by actin reorganization in a migrating cell. The microvilli on the brush border epithelial cells are also formed through the F-actin network.
The high-order actin networks with straight F-actins are formed either through their bundled arrangement or by cross-linking them into gel-like networks. The dendritic networks are formed with the help of the branched F-actins. Actin binding proteins like fimbrin, fascin, and alpha-actinin form different types of actin filament bundles. Contrastingly filamin protein help in cross-linking the actin filaments into a gel-like network.
Actin bundling proteins
Actin-bundling proteins can arrange F-actins in either parallel or anti-parallel linear arrays. The bundles can be loose or tight depending on cellular functional requirements and the bundle&#39;s accessory protein. Monomeric proteins like fimbrin have two actin-binding domains and tightly bind to parallelly arranged adjacent actin filaments. These bundles can be found in microvilli in the small intestine. Contrastingly &#945;-actinin is a dimeric protein having one actin-binding domain on each monomer. A helical spacer separates these actin-binding domains to form loose bundles with anti-parallelly arranged F-actins.
Actin cross-linking protein
Filamin is an actin cross-linking protein having two long flexible forms with one actin-binding domain on each arm. This allows flexible movement of filamin to form perpendicularly arranged actin filaments into a gel-like network. Some small proteins like transgelin have been reported to form dense meshworks.

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