Glucose-induced monoubiquitination of the Saccharomyces cerevisiae galactose transporter is sufficient to signal its internalization

In Saccharomyces cerevisiae, the addition of glucose to cells growing on ga lactose induces internalization of the galactose transporter Gal2p and its subsequent proteolysis in the vacuole. Here we report that the essential st ep in Gal2p down-regulation is its ubiquitination through the Ubc1p-Ubc4p-U bc5p triad of ubiquitin-conjugating enzymes and Npi1/Rsp5p ubiquitin-protei n ligase. Moreover, Gal2p appears to be stabilized in mutant cells defectiv e in the ubiquitin-hydrolase Npi2p/Doa4p, and the mutant phenotype can be r eversed by overexpression of ubiquitin. An analysis of the fate of Gal2p in cells overexpressing wild-type ubiquitin as well as its variants incompete nt to form polyubiquitin chains showed that monoubiquitination of Gal2p is sufficient to signal internalization of the protein into the endocytic path way.