Studies of the Di/tripeptide Transporter in Saccharomyces cerevisiae: The N-terminal Cytoplasmic Domain of Ptr2p is Involved in Post-Translational Regulation

Throughout nature cells use peptides as a source of nutrition. For microbes, an ability to utilize peptides is especially important in nitrogen-poor environments, as peptides can be catabolized for their use as a nitrogen source. The yeast Saccharomyces cerevisiae imports di/tripeptides from the environment using the peptide transporter Ptr2p. Cellular levels of Ptr2p are highest under poor-nitrogen conditions. Here we report that the addition of a rich nitrogen source to the growth medium results in a down-regulation of Ptr2p, wherein plasma membrane Ptr2p is ubiquitinated, endocytosed, and delivered to the vacuole for destruction. We report evidence that the N-terminal portion of Ptr2p that is cytoplasmic is involved in mediating this effect. Mutations to known phosphorylation sites (Y37, S39, and S45) and suspected ubiquitination sites (K27 and K34) on Ptr2p’s Nterminal region greatly impair both the normal turnover of Ptr2p and its down-regulation in response to a rich nitrogen source. The data presented support the notion that Ptr2p turnover from the cytoplasmic membrane requires phosphorylation and ubiquitination of a discrete N-terminal cytoplasmic domain.