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Abstract
Many plasma membrane transporters are downregulated by ubiquitylation, endocytosis, and delivery to the lysosome in response to various stimuli. We report here that two amino acid transporters of Saccharomyces cerevisiae, the general amino acid permease (Gap1) and the arginine-specific permease (Can1), undergo ubiquitin-dependent downregulation in response to their substrates and that this downregulation is not due to intracellular accumulation of the transported amino acids but to transport catalysis itself. Following an approach based on permease structural modeling, mutagenesis, and kinetic parameter analysis, we obtained evidence that substrate-induced endocytosis requires transition of the permease to a conformational state preceding substrate release into the cell. Furthermore, this transient conformation must be stable enough, and thus sufficiently populated, for the permease to undergo efficient downregulation. Additional observations, including the constitutive downregulation of two active Gap1 mutants altered in cytosolic regions, support the model that the substrate-induced conformational transition inducing endocytosis involves remodeling of cytosolic regions of the permeases, thereby promoting their recognition by arrestin-like adaptors of the Rsp5 ubiquitin ligase. Similar mechanisms might control many other plasma membrane transporters according to the external concentrations of their substrates.
SUPPLEMENTAL MATERIAL
Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00699-14.
ACKNOWLEDGMENTS
We are grateful to Catherine Jauniaux for her contribution to isolating strains and plasmids. We also thank members of the laboratory for fruitful discussions and Lydia Spedale for her technical assistance.
M.P. is senior research associate and E.-M.K. is a postdoctoral researcher of the FNRS. This work was supported by an FRSM grant (3.4.592.08.F) and an ARC grant (AUWB 2010-15-2) from the Fédération Wallonie-Bruxelles.