Formation of actin–cofilin rods by depletion forces

Abstract

Various stress conditions induce the formation of actin–cofilin rods in either the nucleus or the cytoplasm, although the mechanism of rod formation is unclear. In this study, we constituted actin–cofilin rods using purified actin, cofilin and actin interacting protein 1 (AIP1) in the presence of a physiological buffer containing a crowding agent, 0.8% methylcellulose (MC), which led to bundled actin filaments formed by depletion forces. Most of the F-actin bundles formed with methylcellulose were linear, whereas cofilin-bound F-actin bundles often had bent, looped, and often ring-like shapes. Increasing the amount of AIP1 shortened actin–cofilin bundles into rod-like bundles with tapering at both ends. As much shorter actin–cofilin filaments were formed in the presence of AIP1 before MC was added to the mixture, the rod-like bundle might be a mass of those short filaments. Furthermore, the small rods fused with each other to become larger rods, indicating that these rods were anisotropic liquid droplets. Several minutes after the addition of MC to the F-actin–cofilin–AIP1 mixture, we observed some long bundles in which the thick and thin parts appear alternately, reminiscent of a Plateau–Rayleigh instability observed in fluid columns. Simultaneously, we found images in which thin parts were interrupted, but the thick parts were arranged in a row in the longitudinal direction. These structures were also observed in cytoplasmic actin–cofilin rods in cells overexpressing cofilin–GFP, suggesting that cytoplasmic actin–cofilin rods have the same structure formation process as the rods reconstituted in vitro.