Ludwig Institute for Cancer Research, S-171 77 Stockholm, Sweden-
Abstract: Nutrient uptake in the yeast Saccharomyces cerevisiae is a highly regulated process- Cells adjust levels of nutrient transporters within the plasma membrane at multiple stages of the secretory and endosomal pathways- In the absence of the ER-membrane-localized chaperone Shr3, amino acid permeases -AAP- inefficiently fold and are largely retained in the ER- Consequently, shr3 null mutants exhibit greatly reduced rates of amino acid uptake due to lower levels of AAPs in their plasma membranes- To further our understanding of mechanisms affecting AAP localization, we identified SSH4 and RCR2 as high-copy suppressors of shr3 null mutations- The overexpression of SSH4, RCR2, or the RCR2 homolog RCR1 increases steady-state AAP levels, whereas the genetic inactivation of these genes reduces steady-state AAP levels- Additionally, the overexpression of any of these suppressor genes exerts a positive effect on phosphate and uracil uptake systems- Ssh4 and Rcr2 primarily localize to structures associated with the vacuole; however, Rcr2 also localizes to endosome-like vesicles- Our findings are consistent with a model in which Ssh4, Rcr2, and presumably Rcr1, function within the endosome-vacuole trafficking pathway, where they affect events that determine whether plasma membrane proteins are degraded or routed to the plasma membrane-