Reconstitution of active actin cortex

The actin cortex, a thin layer of an actomyosin network beneath the plasma membrane, regulates various cellular functions by generating active forces and inducing membrane deformations, including spherical membrane protrusions called blebs. Importantly, the size, number, and position of the blebs are tightly regulated within the cell. Although upstream biochemical signaling is involved in these regulations, it remains unclear to what extent downstream actomyosin molecules can control cell shape. To dissect the direct roles of the actin cortex, we reconstituted an active actin cortex inside a cell-sized liposome using purified proteins. Time-lapse observations clarified that bleb formation is induced either by membrane detachment from the actin cortex or by the rupture of the actin cortex, with the actin-membrane interaction determining which case is dominant. An agent-based computational model revealed that a sequential avalanche-like severing event of actin filaments is required to initiate bleb formation through the rupture mechanism. Furthermore, comparisons between experiments and simulations suggest that not only the dense cortical network but also the thin volume-spanning network may play active roles in regulating the number of blebs. These results provide insights into how the cell tunes the physical parameters of actomyosin networks and actin-membrane interactions to control cell shape.