The Vps27p–Hse1p complex binds ubiquitin and mediates endosomal protein sorting

Membrane proteins that are degraded in the vacuole of Saccharomyces cerevisiae are sorted into discrete intralumenal vesicles, analogous to the internal membranes of multi-vesiculated bodies (MVBs). Recently, it has shownthat the attachment of ubiquitin (Ub) mediates sorting into lumenal membranes 1 . We describe a complex of Vps27p and Hselp that localizes to endosomal compartments and is required for the recycling of Golgi proteins, formation of lumenal membranes and sorting of ubiquitinated proteins into those membranes. The Vps27p-Hselp complex binds to Ub and requires multiple Ub Interaction Motifs (UIMs). Mutation of these motifs results in specific defects in the sorting of ubiquitinated proteins into the vacuolar lumen. However, the recycling of Golgi proteins and the generation of lumenal membranes proceeds normally in AUIM mutants. These data support a model in which the Vps27p-Hselp complex has multiple functions at the endosome, one of which is as a sorting receptor for ubiquitinated membrane proteins destined for degradation.