Clathrin-mediated endocytosis is initiated by a network of weakly interacting proteins through a poorly understood mechanism. Ede1, the yeast homologue of mammalian Eps15, is an early-arriving endocytic protein and a key initiation factor. In the absence of Ede1, most other early endocytic proteins lose their punctate localization and endocytic uptake is decreased. We show that in yeast cells, cytosolic concentration of Ede1 is buffered at a critical level. Excess amounts of Ede1 form large condensates which recruit other endocytic proteins and exhibit properties of phase-separated liquid droplets. We demonstrate that the central region of Ede1, containing a coiled-coil and a prion-like region, is essential for both the condensate formation and the function of Ede1 in endocytosis. The functionality of Ede1 mutants lacking the central region can be partially rescued by an insertion of heterologous prion-like domains. Conversely, fusion of a heterologous lipid-binding domain with the central region of Ede1 can promote clustering into stable plasma membrane domains. We propose that the ability of Ede1 to form condensed networks supports the clustering of early endocytic proteins and promotes the initiation of endocytosis.

Human insulin is a pivotal protein hormone controlling metabolism, growth, and aging and whose malfunctioning underlies diabetes, some cancers, and neurodegeneration. Despite its central position in human physiology, the in vivo oligomeric state and conformation of insulin in its storage granules in the pancreas are not known. In contrast, many in vitro structures of hexamers of this hormone are available and fall into three conformational states: T6, T3Rf3, and R6. As there is strong evidence for accumulation of neurotransmitters, such as serotonin and dopamine, in insulin storage granules in pancreatic β-cells, we probed by molecular dynamics (MD) and protein crystallography (PC) if these endogenous ligands affect and stabilize insulin oligomers. Parallel studies independently converged on the observation that serotonin binds well within the insulin hexamer (site I), stabilizing it in the T3R3 conformation. Both methods indicated serotonin binding on the hexamer surface (site III) as well. MD, but not PC, indicated that dopamine was also a good site III ligand. Some of the PC studies also included arginine, which may be abundant in insulin granules upon processing of pro-insulin, and stable T3R3 hexamers loaded with both serotonin and arginine were obtained. The MD and PC results were supported further by in solution spectroscopic studies with R-state-specific chromophore. Our results indicate that the T3R3 oligomer is a plausible insulin pancreatic storage form, resulting from its complex interplay with neurotransmitters, and pro-insulin processing products. These findings may have implications for clinical insulin formulations.

Clathrin-mediated endocytosis is a major pathway that eukaryotic cells use to produce transport vesicles from the plasma membrane. The assembly of the endocytic coat is initiated by a dynamic network of weakly interacting proteins, but the exact mechanism of initiation is unknown. Ede1, the yeast homologue of mammalian Eps15, is one of the early-arriving endocytic proteins and a key initiation factor. In the absence of Ede1, most other early endocytic proteins lose their punctate localization and the frequency of endocytic initiation is decreased. We show here that in mutants with increased amounts of cytoplasmic Ede1, the excess protein forms large condensates which exhibit properties of phase separated liquid protein droplets. These Ede1 condensates recruit many other early-arriving endocytic proteins. Their formation depends on the core region of Ede1 that contains a coiled coil and a low-complexity domain. We demonstrate that Ede1 core region is essential for the endocytic function of Ede1. The core region can also promote clustering of a heterologous lipid-binding domain into discrete sites on the plasma membrane that initiate endocytic events. We propose that the clustering of the early endocytic proteins and cargo depend on phase separation mediated by Ede1.

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