Receptors and other plasma membrane proteins bound for destruction are tagged with one or more ubiquitin (Ub) molecules before they are internalized in vesicles that fuse with early endosomes. To be fully degraded when the endosome fuses with the vacuole, the tagged proteins must bud off a second time to form a multivesiculated body (MVB)—a structure that has vesicles inside the lumen of the endosome. Proteins that escape this step never reach the inside of the vacuole and may still be recycled back to the cell surface. It turns out that Ub is a potent sorting signal for the incorporation of proteins into the lumenal vesicles of the MVB.
With the new results, the authors have identified the protein Hse1p, which, coupled with Vps27p, mediates the lumenal internalization decision in yeast. A complex of Hse1p and Vps27p localized to endosomal compartments and bound Ub through several Ub interaction motifs (UIMs). Mutation of HSE1 resulted in a class E phenotype, which is shared by a set of vacuolar protein sorting mutants that are defective in recycling Golgi proteins, forming MVBs, and sorting ubiquitinated proteins into the vacuole. Mutation of the Hse1p and Vps27p UIMs alone abolished sorting of Ub-tagged proteins into the vacuole, but did not cause the other class E phenotypes.
Separation of these phenotypes indicates that the Vps27p–Hse1p complex is involved in several aspects of endosomal function, one of which is to sort tagged proteins into the lumen of the endosome by binding Ub and dragging the cargo inside for destruction. Though it is not yet known how the complex mediates internalization, regulation of its activity could provide another means for controlling receptor levels. “For the most part, attention has been focused on how internalization of receptors is controlled at the plasma membrane,” says Piper. “But this is not enough. Internalization happens all the time, yet there must also be a mechanism to decide whether the receptors are recycled or degraded.” ▪