How does the cellular machinery decide whether proteins should be monoubiquitylated rather than polyubiquitylated? This decision is crucial because being tagged with a single copy of ubiquitin determines whether proteins become involved in controlling endocytosis rather than being targeted for destruction by the proteasome.

Now, three reports shed light on this process by showing that ubiquitin recognition and monoubiquitylation depend on the ubiquitin-interacting motif (UIM), which is found in many groups of proteins, including endocytic proteins and ubiquitin ligases.

Polo and colleagues report in Nature that the determinants that are necessary for monoubiquitylation of the endocytosis-regulating proteins Eps15 and eps15R mapped to the carboxy-terminal regions of the proteins — a region that contains two UIM domains. Deletion analysis of this region showed that one of these domains was crucial for monoubiquitylation and binding to ubiquitin-containing proteins, whereas the other domain was crucial only for monoubiquitylation. The function of the UIM domain was further illustrated by the demonstration that the ubiquitin ligase Nedd4 — which catalyses the transfer of ubiquitin — could monoubiquitylate eps15 but not a UIM-deleted mutant.

Two reports in Nature Cell Biology showed that other proteins that are involved in endocytosis — epsins, which interact directly with clathrin and are involved in internalization, and Hrs (hepatocyte growth-factor regulated tyrosine kinase substrate), which is involved in recruiting clathrin to endosomes — also function through the UIM domain. Hicke and colleagues, and Stenmark and colleagues, found that UIM domains in epsins and Hrs, respectively, were required for ubiquitin binding. Stenmark and colleagues also found that ubiquitylated proteins localize specifically to Hrs- and clathrin-coated microdomains, which indicates that the Hrs/clathrin coat concentrates ubiquitylated proteins before the formation of vesicles.

There is obviously more to be learnt about UIM and its function, but as Riezman concludes in a News and Views article, this work “marks the beginning of an exciting quest”.