Liquid-like protein interactions catalyze assembly of endocytic vesicles

During clathrin-mediated endocytosis, dozens of proteins assemble into an interconnected network at the plasma membrane. As early initiators of endocytosis, Eps15 and Fcho1 are responsible for locally concentrating downstream components on the membrane surface. However, they must also permit dynamic rearrangement of proteins within the budding vesicle. How do initiator proteins meet these competing demands? Here we show that Eps15 and Fcho1 rely on weak, liquid-like interactions to efficiently catalyze endocytosis. In reconstitution experiments, these weak interactions promote the assembly of protein droplets with liquid-like properties, including rapid coalescence and dynamic exchange of protein components. To probe the physiological role of liquid-like interactions among initiator proteins, we tuned the strength of initiator protein assembly in real time using light-inducible oligomerization of Eps15. Low light levels drove initiator proteins into liquid-like assemblies, restoring normal rates of endocytosis in mammalian Eps15 knockout cells. In contrast, initiator proteins formed solid-like assemblies upon exposure to higher light levels. Assembly of these structures stalled vesicle budding, likely owing to insufficient molecular rearrangement. These findings suggest that liquid-like assembly of early initiator proteins provides an optimal catalytic platform for endocytosis.