Abstract
The organization of filamentous actin (F-actin) in resilient networks is coordinated by various F-actin cross-linking proteins. The relative tolerance of cells to null mutations of genes that code for a single actin cross-linking protein suggests that the functions of those proteins are highly redundant. This apparent functional redundancy may, however, reflect the limited resolution of available assays in assessing the mechanical role of F-actin cross-linking/bundling proteins. Using reconstituted F-actin networks and rheological methods, we demonstrate how alpha-actinin and fascin, two F-actin cross-linking/bundling proteins that co-localize along stress fibers and in lamellipodia, could synergistically enhance the resilience of F-actin networks in vitro. These two proteins can generate microfilament arrays that "yield" at a strain amplitude that is much larger than each one of the proteins separately. F-actin/alpha-actinin/fascin networks display strain-induced hardening, whereby the network "stiffens" under shear deformations, a phenomenon that is non-existent in F-actin/fascin networks and much weaker in F-actin/alpha-actinin networks. Strain-hardening is further enhanced at high rates of deformation and high concentrations of actin cross-linking proteins. A simplified model suggests that the optimum results of the competition between the increased stiffness of bundles and their decreased density of cross-links. Our studies support a re-evaluation of the notion of functional redundancy among cytoskeletal regulatory proteins.
Highlights
The dynamic assembly and disassembly of filamentous actin (F-actin)1 and its organization in ordered arrays are coordinated by various accessory proteins [1]
Using reconstituted F-actin networks and rheological methods, we demonstrate how ␣-actinin and fascin, two F-actin cross-linking/bundling proteins that co-localize along stress fibers and in lamellipodia, could synergistically enhance the resilience of F-actin networks in vitro
Strain Hardening of F-actin/␣-Actinin/Fascin Networks— Using rheological methods, we analyzed the mechanical response of reconstituted F-actin networks in the presence of either ␣-actinin, fascin, or both
Summary
The dynamic assembly and disassembly of filamentous actin (F-actin) and its organization in ordered arrays are coordinated by various accessory proteins [1]. These organelles, which coordinate cell migration and cell spreading [3], often contain more than one actin cross-linking protein. The relative tolerance of cells to null mutation of genes that code for a single cross-linking protein suggests that the functions of actin cross-linking proteins are highly redundant [7,8,9]. This apparent functional redundancy may, reflect the limited resolution of available assays in assessing the mechanical role of F-actin cross-linking/bundling proteins in vitro and in vivo. How singed and forked proteins can complement one another to produce the propulsive forces required for the development of sensory bristles is unknown
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