Ubiquitin sorts proteins into the intralumenal degradative compartment of the late-endosome/vacuole

Many studies have demonstrated a role for ubiquitin (Ub) in the down-regula tion of cell surface proteins. In yeast, down-regulation is marked by the i nternalization of proteins, followed by their delivery to the lumen of the vacuole where both the cytosolic and lumenal domains are degraded. It is ge nerally believed that the regulatory step of this process is internalizatio n from the plasma membrane and that protein delivery to the lysosome or vac uole is by default. By separating the process of internalization from degra dation, we demonstrate that incorporation of proteins into intralumenal ves icles represents a distinct sorting step along the endocytic pathway that i s controlled by recognition of ubiquitin. We show that attachment of a sing le ubiquitin can serve as a specific sorting signal for the degradative pat hway by redirecting recycling Golgi proteins and resident vacuolar proteins into intralumenal vesicles of the yeast vacuole. This pathway is independe nt of PtdIns(3,5) P2 and does not rely on the specific composition of trans membrane domain segments. These data provide a physiological basis for how ubiquitination of cell surface proteins guides their degradation and remova l from the recycling pathway.