Epsin1 enforces a condensation-dependent checkpoint for ubiquitylated cargo during clathrin-mediated endocytosis.

Clathrin-mediated endocytosis internalizes proteins and lipids from the cell surface. A flexible condensate of initiator proteins catalyzes assembly of clathrin-coated vesicles in diverse organisms. Here we reveal that an endocytic adaptor protein, Epsin1, conditionally stabilizes this network, creating a cargo-dependent endocytic checkpoint. Epsin1 recruits ubiquitylated cargo to endocytic sites. Using in vitro assays, we demonstrate that Epsin1 destabilizes condensation of initiator proteins in the absence of ubiquitin. However, when polyubiquitin is present, Epsin1 binds to both ubiquitin and initiator proteins, stabilizing condensation. Similarly, in mammalian cells, endocytosis is disrupted by removal of either ubiquitin or Epsin1. When both components are removed simultaneously, endocytic defects are largely rescued, although the ability to preferentially internalize ubiquitylated cargo is lost. These results suggest that Epsin1 tunes protein condensation to internalize ubiquitylated cargo. More broadly, these findings illustrate how a balance of attractive and repulsive molecular interactions can exert dynamic control over cellular events.