Spontaneous membrane protrusion and cell morphogenesis via self-propelled actin filaments.

Cells frequently undergo spontaneous morphogenesis, yet the underlying mechanisms remain incompletely understood. While actin filaments are central to cell morphogenesis and are typically regulated by biochemical signaling, cells can form protrusions even without clear external cues, suggesting the existence of intrinsic mechanisms. Here, we report that actin filament assemblies undergo directional movement driven by their directional polymerization and disassembly. These filament assemblies move as discrete "particles" and exhibit random yet directional motion. Since this motion resembles that of self-propelled "particles" rather than the previously reported reaction-diffusion "waves", we have termed them Self-propelled Treadmilling Actin filaments (SpTAs). SpTA arrival at the cell periphery drives membrane protrusion by orienting their polymerizing ends outwards. Furthermore, SpTAs spontaneously accumulate at cell protrusions, guided by membrane curvature. This SpTA accumulation, further boosts the growth and expansion of protrusions, driving cellular polarization for migration. Our findings establish that the assembly of actin filaments as a novel class of biological active particle, and provides new insight into how molecular-scale motion orchestrates complex higher-order organization.