Adaptor interactions trigger Pan1 self-assembly during clathrin-mediated endocytosis in budding yeast
Clathrin-mediated endocytosis requires the coordinated assembly of a highly dynamic protein network that couples membrane remodeling to actin-driven force generation. The essential budding yeast protein, Pan1, scaffolds the endocytic protein network by linking adaptor proteins, and other coat components, and actin assembly regulators. How Pan1 coordinates all these different proteins in space and time is not well understood. We show that Pan1 undergoes biomolecular condensation. Elevated Pan1 expression induced the formation of condensates that exhibited partial rapid molecular exchange, temperature-dependent reversibility, and sensitivity to disruption of weak hydrophobic interactions. The Pan1 assemblies are compositionally selective, preferentially enriching late-stage endocytic factors while excluding early adaptor proteins. Truncation analysis demonstrated that condensation depends on cooperative contributions from intrinsically disordered regions, the EH2 domain, and the oligomerization module, whereas a C-terminal region negatively regulates condensation. Deletion of Pan1 regions promoting its self-assembly perturbed Pan1 assembly and function at endocytic sites. Disruption of interactions between Pan1 and endocytic adaptors did not alter the dynamics of endocytic events but instead reduced the number of endocytic events marked by Pan1 and caused the formation of ectopic Pan1 condensates. This indicates that adaptor-mediated interactions spatially constrain Pan1 localization by seeding Pan1 assembly at endocytic sites. Our findings support a model in which adaptor proteins seed Pan1 assembly at the endocytic sites while multivalent interactions drive Pan1 self-assembly to build the higher-order molecular network of the late endocytic coat.