Helical twist controls the thickness of F-actin bundles

Abstract

In the presence of condensing agents such as nonadsorbing polymer, multivalent counter ions, and specific bundling proteins, chiral biopolymers typically form bundles with a finite thickness, rather than phase-separating into a polymer-rich phase. Although shortrange repulsive interactions or geometrical frustrations are thought to force the equilibrium bundle size to be limited, the precise mechanism is yet to be resolved. The importance of the tight control of biopolymer bundle size is illustrated by the ubiquitous cytoskeletal actin filament bundles that are crucial for the proper functioning of cells. Using an in vitro model system, we show that size control relies on a mismatch between the helical structure of individual actin filaments and the geometric packing constraints within bundles. Small rigid actin-binding proteins change the twist of filamentous actin (F-actin) in a concentrationdependent manner, resulting in small, well defined bundle thickness up to Ϸ20 filaments, comparable to those found in filopodia. Other F-actin cross-linking proteins can subsequently link these small, well organized bundles into larger structures of several hundred filaments, comparable to those found in, for example, Drosophila bristles. The energetic tradeoff between filament twisting and cross-linker binding within a bundle is suggested as a fundamental mechanism by which cells can precisely adjust bundle size and strength. chiral aggregates ͉ finite size ͉ thickness control ͉ cytoskeleton organization

Cofilin cooperates with fascin to disassemble filopodial actin filaments

“…S1). Although fascin is known to restrict bundle thickness to ,20 filaments (Claessens et al, 2008), the small effect of fascin concentration on bundle thickness observed here is probably caused by the low total amount of actin filaments in these assays, limiting the number of actin filaments available for bundle formation. Nevertheless, the total amount of bundled actin increased with fascin concentration, but reached a maximum at 50-100 nM fascin in the reaction mixture.…”

“…The latter scenario is in line with our observation that cofilin binds to fascin-crosslinked bundles with slower kinetics than to isolated filaments. Moreover, fascin was recently shown to induce overtwisting of actin filaments upon bundling (Claessens et al, 2008). This effect might additionally account for an enhanced severing activity of cofilin due to larger differences in the twists of cofilin-bound compared with fascin-bound actin filament segments.…”

“…Notably, fascin appears to undergo frequent cycles of association and dissociation with actin bundles both in vivo and in vitro instead of stably binding to filaments, which might be an important feature in regulating the dynamic growth and shrinkage of filopodia (Vignjevic et al, 2006;Aratyn et al, 2007). Moreover, fascin was shown to induce overtwisting of actin filaments within bundles, resulting in bundles with a hexagonal geometry consisting of ,20 actin filaments at maximum (Claessens et al, 2008).…”

Power transduction of actin filaments ratcheting in vitro against a load

“…It is thus reasonable to assume that no individual filament coexists with the bundles. Authors have observed that for a molar ratio [fascin]/[actin] > 0.25, which is the case here, the number of filaments in a bundle saturates to 20 (22,23). Because about 45 bundles are visible in the image of Fig.…”

Molecular architecture of the Spire–actin nucleus and its implication for actin filament assembly

“…Most intriguingly, SpireDDD can even disrupt actin/fascin bundles very quickly (Movie S2). These bundles usually consist of about 20 actin filaments (34). Recent data showed that actin/fascin bundles are extremely stable and resistant against disruption by cofilin over days (35).…”