The function of yeast epsin and Ede1 ubiquitin-binding domains during receptor internalization

The formation of a primary endocytic vesicle is a dynamic process involving the transient organization of adaptor and scaffold proteins at the plasma membrane. Epsins and Eps15-like proteins are ubiquitin-binding proteins that act early in this process. The yeast epsins, Ent1 and Ent2, carry functional ubiquitin-interacting motifs (UIMs), whereas the yeast Eps15-like protein, Ede1, has a C-terminal ubiquitin-associated (UBA) domain. Analysis of mutants lacking early endocytic adaptors reveals that the ubiquitin-binding domains (UBDs) of Ent2 and Ede1 are likely to function primarily to mediate protein-protein interactions between components of the early endocytic machinery. Cells that lack epsin and Ede1 UBDs are able to internalize activated, ubiquitinated receptors. Furthermore, under conditions in which epsin UIMs are important for receptor internalization, receptors internalized via both ubiquitin-dependent and ubiquitin-independent signals require the UIMs, indicating that UIM function is not restricted to ubiquitinated receptors. Epsin UIMs share function with non-UBD protein-protein interaction motifs in Ent2 and Ede1, and the Ede1 UBA domain appears to negatively regulate interactions between endocytic proteins. Together, our results suggest that the ubiquitin-binding domains within the yeast epsin Ent2 and Ede1 are involved in the formation and regulation of the endocytic network.