CATABOLITE INACTIVATION OF THE YEAST MALTOSE TRANSPORTER REQUIRES UBIQUITIN-LIGASE NPI1 RSP5 AND UBIQUITIN-HYDROLASE NPI2/DOA4/

The maltose transporter in Saccharomyces cerevisiae is degraded in the vacuole after internalization by endocytosis when protein synthesis i s impaired and a fermentable substrate is present. The possible implic ation of the ubiquitin pathway in this inactivation, known as cataboli te inactivation, has been investigated. Using mutants deficient in npi 1/rsp5 ubiquitin-protein ligase and npi2/doa4 ubiquitin-protein hydrol ase, we have shown that these two enzymes are required for normal endo cytosis and degradation of the transporter. These facts indicate that the ubiquitin pathway is involved in catabolite inactivation of the ma ltose transporter. The results also revealed that both enzymes act in the internalization step of endocytosis.