Event Dates: August 6, 2014 - 4:00pm - 5:00pm
Gap1, the yeast general amino acid permease, is a convenient model for studying how the intracellular traffic of membrane transporters is regulated. Gap1 undergoes ubiquitylation, endocytosis, and degradation upon activation of the TORC1 kinase complex in response to an increase in internal amino acids. This downregulation is stimulated by TORC1-dependent control of the arrestin-like Bul adaptors recruiting the HECT-type Rsp5 ubiquitin ligase to Gap1. The Gap1 permease is also ubiquitylated and downregulated when cells are subjected to various stresses. This stress-induced ubiquitylation of Gap1 is also mediated by the Rsp5 ubiquitin ligase, but the mechanisms recruiting Rsp5 to Gap1 are different as additional arrestin-like adaptors acting through other cytosolic regions of Gap1 are involved. We recently uncovered a third pathway promoting Gap1 ubiquitylation, stimulated by the permease’s own substrates. This ubiquitylation is not due to intracellular accumulation of the transported amino acids but to transport catalysis itself. Following an approach based on Gap1 structural modeling, mutagenesis, and kinetic parameter analysis, we obtained evidence that this substrate-induced endocytosis requires transition of the permease to a conformational state preceding substrate release into the cell and recognized by arrestin-like adaptors of the Rsp5 ubiquitin ligase. The mechanisms controlling Gap1 ubiquitylation and endocytosis seem conserved in more complex organisms and likely play an important role in control of cell permeability in many other species.