Different ubiquitin signals act at the Golgi and plasma membrane to direct GAP1 trafficking.

The high capacity general amino acid permease, Gap1p, in Saccharomyces cerevisiae is distributed between the plasma membrane and internal compartments according to availability of amino acids. When internal amino acid levels are low, Gap1p is localized to the plasma membrane where it imports available amino acids from the medium. When sufficient amino acids are imported, Gap1p at the plasma membrane is endocytosed and newly synthesized Gap1p is delivered to the vacuole; both sorting steps require Gap1p ubiquitination. Although it has been suggested that identical trans-acting factors and Gap1p ubiquitin acceptor sites are involved in both processes, we define unique requirements for each of the ubiquitin-mediated sorting steps involved in delivery of Gap1p to the vacuole upon amino acid addition. Our finding that distinct ubiquitin-mediated sorting steps employ unique trans-acting factors, ubiquitination sites on Gap1p, and types of ubiquitination demonstrates a previously unrecognized level of specificity in ubiquitin-mediated protein sorting.