Fascin-induced bundling protects actin filaments from disassembly by cofilin

Actin filament turnover plays a central role in shaping actin networks, yet the feedback mechanism between network architecture and filament assembly dynamics remains unclear. The activity of ADF/cofilin, the main protein family responsible for filament disassembly, has been mainly studied at the single filament level. Here, we report that fascin, by crosslinking filaments into bundles, strongly slows down filament disassembly by cofilin. We show that this is mainly due to a slower nucleation of the first cofilin clusters, which occurs up to 100-fold slower on large bundles compared to single filaments. In contrast, severing at cofilin cluster boundaries is unaffected by fascin bundling. After the nucleation of an initial cofilin cluster on a filament of a bundle, we observe the local removal of fascin. Surprisingly, the nucleation of cofilin clusters on adjacent filaments is highly enhanced, locally. We propose that this inter-filament cooperativity in cofilin binding arises from the local propagation of the cofilin-induced change in helicity from one filament to the other filaments of the bundle. Taken together, these observations reveal the molecular events explaining why, despite inter-filament cooperativity, fascin crosslinking protects actin filaments from cofilin-induced disassembly. These findings highlight the important role played by crosslinkers in organizing actin networks and modulating the activity of other regulatory proteins.