Liquid-like protein interactions catalyse assembly of endocytic vesicles

Abstract

During clathrin-mediated endocytosis, dozens of proteins assemble into an interconnected network at the plasma membrane. As initiators of endocytosis, Eps15 and Fcho1/2 concentrate downstream components, while permitting dynamic rearrangement within the budding vesicle. How do initiator proteins meet these competing demands? Here we show that Eps15 and Fcho1/2 rely on weak, liquid-like interactions to catalyze endocytosis. In vitro, these weak interactions promote the assembly of protein droplets with liquid-like properties. To probe the physiological role of these liquid-like networks, we tuned the strength of initiator protein assembly in real time using light-inducible oligomerization of Eps15. Low light levels drove 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, which stalled vesicle budding, likely owing to insufficient molecular rearrangement. These findings suggest that liquid-like assembly of initiator proteins provides an optimal catalytic platform for endocytosis.

Unconventional COPII-mediated secretory protein trafficking continues in the absence of the Sar1 GTPase

“…This idea is consistent with in vitro studies showing that the large hydrodynamic radii of disordered domains from epsin1 or AP180 are adequate to drive membrane tubulation when either is tethered to a bilayer (52,53). In addition, the liquid-like state of COPII condensates may also facilitate further membrane remodeling events that enable the release of coated transport carriers from the ER, in a manner similar to that shown recently for Eps15-Fcho liquid droplets during endocytosis (54). Considering our finding that the mobility of COPII subunits within condensates formed following Sar1 depletion is highly similar to that observed in control cells, our data raise the intriguing possibility that COPII phase separation normally contributes to transport carrier biogenesis and secretory efflux from the ER (ie., when Sar1 is present).…”

Physics of compartmentalization: How phase separation and signaling shape membrane and organelle identity

“…The main parameter controlling the efficiency of the sorting process is the strength of the effective intermolecular interaction driving phase separation, and optimal sorting is realized for intermediate values of the interaction strength (neither too low nor too high) [83] . A similar picture emerges from recent work, where optimal endocytic efficiency was obtained by tuning the strength of protein assembly at intermediate values using light-inducible oligomerization [107] .…”

Clathrin packets move in slow axonal transport and deliver functional payloads to synapses

“…Finally, the behavior of synaptic clathrin resembles the movement of soluble molecules within liquid condensates and may be conceptually related to the contemporary idea that SV clusters have fluid-like properties (Milovanovic et al, 2018). Interestingly, recent studies in non-neuronal cells indicate that CME also relies on the formation of liquid condensates (Day et al, 2021;Witkowska and Haucke, 2021), and future experiments focused on these issues might bring more clarity.…”