We have recently reported that the yeast plasma membrane uracil permease undergoes cell-surface ubiquitination, which is dependent on the Npi1/Rsp5 ubiquitin-protein ligase. Ubiquitination of this permease, like that of some other transporters and receptors, signals endocytosis of the protein, leading to its subsequent vacuolar degradation, This process does not involve the proteasome, which binds and degrades ubiquitin-protein conjugates carrying Lys48-linked ubiquitin chains, The data presented here show that ubiquitination and endocytosis of uracil permease are impaired in yeast cells lacking the Doa4p ubiquitin-isopeptidase. Both processes were rescued by overexpression of wild-type ubiquitin, Mutant ubiquitins carrying Lys-->Arg mutations at Lys29 and Lys48 restored normal permease ubiquitination, In contrast, a ubiquitin mutated at Lys63 did not restore permease polyubiquitination. Ubiquitin-permease conjugates are therefore extended through the Lys63 of ubiquitin, When polyubiquitination through Lys63 is blocked, the permease still undergoes endocytosis, but at a reduced rate, We have thus identified a natural target of Lys63-linked ubiquitin chains, We have also shown that monoubiquitination is sufficient to induce permease endocytosis, but that Lys63-linked ubiquitin chains appear to stimulate this process.
