Reconstitution of actomyosin networks in cell-sized liposomes dissects distinct mechanisms of membrane blebbing and symmetry breaking.

The actin cortex, a thin layer of actomyosin network beneath the plasma membrane, regulates various cell functions by generating active forces and inducing membrane deformations, including blebs. Although upstream signaling is involved in regulating cell shape, the extent to which downstream actomyosin molecules can control the shape remains elusive. Here, using a minimal reconstituted system combined with an agent-based computational model, we show that actin-membrane coupling strength determines the magnitude of membrane deformation, while its balance with actin network connectivity governs the bleb initiation mechanism, either by detachment of the cortex from the membrane or by rupture of the cortex. This balance also modulates single versus multiple bleb formation, thereby regulating symmetry breaking. Furthermore, our results suggest that not only the dense cortical network but also the sparse volume-spanning network actively contributes to the regulation of bleb number. These findings provide mechanistic insights into how cells tune actin network organization to control their shape and polarity.