Intracellular trafficking and ubiquitination of the Schizosaccharomyces pombe amino acid permease Aat1p

In Schizosaccharomyces pombe, neither intracellular sorting nor ubiquitination of amino acid permeases is well understood. In the present study, we show that intracellular sorting of the amino acid permease Aat1p in S. pombe depends on the presence of a nitrogen source in the growth medium. Under nitrogen-sufficient conditions, Aat1p appeared to be stably localized at the Golgi apparatus. In contrast, under nitrogen-insufficient conditions, Aat1p was sorted to the plasma membrane. Over time, plasma membrane-localized Aat1p was internalized and sorted to the lumen of the vacuole, where it was degraded. Sorting of Aat1p to the vacuolar lumen was dependent on the ESCRT (endosomal sorting complex required for transport) complex, which is required for formation of the multivesicular body. S. pombe has three genes (pub1 ⁺, pub2 ⁺ and pub3 ⁺) that are homologous to the ubiquitin ligase RSP5. Under nitrogen-sufficient conditions, Aat1-GFP was missorted to the plasma membrane in pub1D cells and ubiquitinated Aat1p was not detected. These results suggest that Pub1p-mediated ubiquitination is required for retention of Aat1 at the Golgi under nitrogen-sufficient conditions. The Aat1p lysine mutant Aat1 ^{K18, 26, 27} was completely missorted to the plasma membrane under nitrogen-rich conditions. Furthermore, Aat1 ^{K4, 18R}, Aat1 ^{K4, 26, 27R} and Aat1 ^{K18, 26, 27K} mutants were severely blocked in endocytosis. These results indicate that ubiquitination is an important determinant for localization and regulation of the Aat1p permease in S. pombe.