Plasma membrane localization of paralogous leucine permeases Bap2 and Bap3 is regulated by Bul1

Timeliness in expression and degradation of the nutrient permeases is crucial for any organism. In Saccharomyces cerevisiae, post translational regulation of nutrient permeases such as trafficking and turnover are poorly understood. We found that loss of a leucine permease BAP2, but not other permeases lead to severe growth retardation when the carbon source is glucose or galactose but not glycerol and lactate. Leucine prototrophy suppressed the retardation, showing BAP2 and LEU2 are synthetically lethal. We discovered that loss of BUL1, an arrestin involved in trafficking of diverse permeases suppressed this lethality. The suppression required another leucine permease, BAP3. Our results suggest that BUL1 downregulate permeases BAP2 and BAP3 present in plasma membrane through Rsp5 dependent endocytosis. We speculate that by regulating leucine import BUL1 regulates the activity of TORC1.