Clathrin is well known for forming `baskets' that coat certain vesicles. However, `flat' clathrin coats also exist. These are present on endosomes and contain Hrs, a ubiquitin-binding adaptor protein that sorts ubiquitylated membrane proteins for degradation. Harald Stenmark's team now report that these coats promote sorting of proteins for degradation by concentrating Hrs in dynamic microdomains (see ). The authors show that both clathrin and the clathrin-binding domain of Hrs are required to cluster Hrs into endosomal microdomains. These microdomains rapidly exchange both proteins with the cytoplasm and acquire components of the degradative protein sorting pathway. Finally, the recruitment of clathrin to endosomes by Hrs is required for degradation of epidermal growth factor receptor. The authors propose that, by concentrating Hrs in restricted microdomains, clathrin helps retain ubiquitylated cargoes; the dynamic nature of the coat then provides transient openings into which other proteins needed for degradative protein sorting can insert. These results provide the first evidence that clathrin can function in a trafficking pathway that does not involve coated vesicles.
