Selective trafficking of membrane proteins to lysosomes for destruction is required for proper cell signalling and metabolism. Ubiquitylation aids this process by specifying which proteins should be transported to the lysosome lumen by the multivesicular endosome pathway. The endosomal sorting complex required for transport (ESCRT) machinery sorts cargo labelled with ubiquitin into invaginations of endosome membranes. Then, through a highly conserved mechanism also used in cytokinesis and viral budding, it mediates the breaking off of the cargo-containing intraluminal vesicles from the perimeter membrane. The involvement of the ESCRT machinery in suppressing diseases such as cancer, neurodegeneration and infections underscores its importance to the cell.

After endocytosis, some membrane proteins recycle from early endosomes to the plasma membrane whereas others are transported to late endosomes and lysosomes for degradation. Conjugation with the small polypeptide ubiquitin is a signal for lysosomal sorting. Here we show that the hepatocyte growth factor-regulated tyrosine kinase substrate, Hrs, is involved in the endosomal sorting of ubiquitinated membrane proteins. Hrs contains a clathrin-binding domain, and by electron microscopy we show that Hrs localizes to flat clathrin lattices on early endosomes. We demonstrate that Hrs binds directly to ubiquitin by way of a ubiquitin-interacting motif (UIM), and that ubiquitinated proteins localize specifically to Hrs- and clathrin-containing microdomains. Whereas endocytosed transferrin receptors fail to colocalize with Hrs and rapidly recycle to the cell surface, transferrin receptors that are fused to ubiquitin interact with Hrs, localize to Hrs- and clathrin-containing microdomains and are sorted to the degradative pathway. Overexpression of Hrs strongly and specifically inhibits recycling of ubiquitinated transferrin receptors by a mechanism that requires a functional UIM. We conclude that Hrs sorts ubiquitinated membrane proteins into clathrin-coated microdomains of early endosomes, thereby preventing their recycling to the cell surface.

The endosomal sorting complexes required for transport (ESCRTs) are required to sort integral membrane proteins into intralumenal vesicles of the multivesicular body (MVB). Mutations in the ESCRT-III subunit CHMP2B were recently associated with frontotemporal dementia and amyotrophic lateral sclerosis (ALS), neurodegenerative diseases characterized by abnormal ubiquitin-positive protein deposits in affected neurons. We show here that autophagic degradation is inhibited in cells depleted of ESCRT subunits and in cells expressing CHMP2B mutants, leading to accumulation of protein aggregates containing ubiquitinated proteins, p62 and Alfy. Moreover, we find that functional MVBs are required for clearance of TDP-43 (identified as the major ubiquitinated protein in ALS and frontotemporal lobar degeneration with ubiquitin deposits), and of expanded polyglutamine aggregates associated with Huntington's disease. Together, our data indicate that efficient autophagic degradation requires functional MVBs and provide a possible explanation to the observed neurodegenerative phenotype seen in patients with CHMP2B mutations.