25.15: 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'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'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 α-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.
