The TOR and EGO protein complexes control microautophagy in yeast.
Frédérique Dubouloz, Valeria Wanke, Elisabetta Cameroni, Olivier Deloche, Claudio De Virgilio
Dep. Microbiol. and Mol. Med., University of Geneva, CMU, 1 rue Michel-Servet, Geneva 4, CH-1211, Switzerland
The pathways that regulate entry of cells into quiescence as a result of downregulation of TOR and/or nutrient starvation are understood in considerable detail, yet the mechanisms that control the transition from quiescence back to proliferation have received surprisingly little attention thus far. In a first attempt to identify the corresponding control mechanisms, we provided a functional profile of the yeast genome with respect to the exit from rapamycin-induced growth arrest. Here, we focus on the characterization of three mutants, namely ego1, gtr2, and ego3, which are likely defective in the same (EGO) protein complex that is required to ensure proper e xit from rapamycin-induced g r o wth arrest. We show that Gtr2, a member of the RagA subfamily of Ras-related GTPases, acts in a vacuolar membrane-associated protein complex together with Ego1 and Ego3. We further demonstrate that this EGO complex, in conjunction with TOR, positively regulates microautophagy. This process, which counterbalances the massive rapamycin-induced, macroautophagy-mediated membrane influx towards the vacuolar membrane, appears to be essential for resumption of growth following rapamycin treatment, and therefore represents a novel growth control mechanism that originates at the vacuolar membrane. Finally, large-scale genetic analyses of the EGO complex allowed us to pinpoint glutamine as a key molecule that may act in the TOR pathway upstream of the rapamycin-sensitive TOR complex.