Electron microscopic evidence that Aip1 disintegrates cofilin-saturated F-actin domains in the presence of coronin

Cofilin is an essential actin filament severing protein necessary for fast actin turnover dynamics. Segments of actin bound to cofilin adapt an alternative twist. This configuration is stable, but boundaries between cofilin occupied and unoccupied polymer are weak and fragment. Coronin and Aip1 are two factors that promote cofilin mediated actin filament disassembly, but whether they simply accelerate the basic cofilin severing mechanism or alter the mode of filament disassembly is still being investigated. Using electron microscopy and spectroscopy, we show that coronin accelerates phosphate release from F-actin to stimulate highly cooperative cofilin binding on to the polymer creating long stretches with a hypertwisted morphology. We find that Aip1 attacks these hypertwisted regions along their length, not just the boundaries, causing sections to disintegrate into monomers. Therefore, coronin promotes cofilin binding to F-actin to generate longer segments of polymer that are themselves the substrates for Aip1 mediated disintegration, as opposed to simply creating more heterotypic junctions that would sever. The morphological characteristics of the disassembling filaments along with spectroscopic data showing the rapid liberation of actin monomers suggest that the combination of cofilin, coronin, and Aip1 might be triggering a more catastrophic mode of filament disassembly than severing.