Mechanical regulation of endocytosis by protein droplet capillary forces

Abstract Clathrin-mediated endocytosis (CME) is the primary pathway for internalization of extracellular and membrane cargo in eukaryotic cells. It is characterized by a patch of the plasma membrane invaginating and pinching off to generate a cytoplasmic vesicle. Recently, experimental evidence has begun to accumulate in support of proteins assembling into liquid-like condensates at nascent CME sites, but potential functional consequences are not understood. Given that condensate capillary forces can influence membrane bending, we constructed complementary analytical and numerical model systems to probe the mechanical influence of a protein droplet on endocytosis. Model evaluations and analysis reveal that a droplet can regulate the progression of endocytosis by stalling invagination until the droplet grows past a threshold size. After the droplet-mediated brake is released, the droplet provides a force that can drive the membrane bud toward scission. Thus, we propose a “mechanical timer” mechanism as a function for protein phase separation in cellular membrane trafficking. Statement of Significance Clathrin-mediated endocytosis is the essential pathway through which a patch of plasma membrane deforms into a vesicle in all eukaryotic cells. Recent experimental studies suggest that cytoplasmic components of the endocytic machinery condense into a liquid-like droplet on the membrane prior to internalization, raising questions about the droplet’s biological function. Using complementary numerical and analytical modeling approaches, we show that an endocytic droplet can regulate the pathway via capillary force. The droplet volume sets the strength of this force, providing the cell with a handle to switch from stalling to driving membrane invagination. While the biological role of liquid condensates is typically attributed to organizing biochemistry, this work demonstrates a novel purely mechanical function.