The Vam6 GEF Controls TORC1 by Activating the EGO Complex

Abstract

The target of rapamycin complex 1 (TORC1) is a central regulator of eukaryotic cell growth that is activated by a variety of hormones (e.g., insulin) and nutrients (e.g., amino acids) and is deregulated in various cancers. Here, we report that the yeast Rag GTPase homolog Gtr1, a component of the vacuolar-membrane-associated EGO complex (EGOC), interacts with and activates TORC1 in an amino-acid-sensitive manner. Expression of a constitutively active (GTP-bound) Gtr1(GTP), which interacted strongly with TORC1, rendered TORC1 partially resistant to leucine deprivation, whereas expression of a growth inhibitory, GDP-bound Gtr1(GDP), caused constitutively low TORC1 activity. We also show that the nucleotide-binding status of Gtr1 is regulated by the conserved guanine nucleotide exchange factor (GEF) Vam6. Thus, in addition to its regulatory role in homotypic vacuolar fusion and vacuole protein sorting within the HOPS complex, Vam6 also controls TORC1 function by activating the Gtr1 subunit of the EGO complex.

dRAGging Amino Acid-mTORC1 Signaling by SH3BP4

“…Vam6p/Vps39p (vacuolar protein sorting 39) is a component of homotypic fusion and vacuole protein sorting (HOPS) complex and plays a role as a guanine nucleotide exchange factor (GEF) for Ypt7p, an orthologue of Rab7, in yeast (Binda et al, 2009;Wurmser et al, 2000). In yeast, deletion of Vam6 suppresses the cellular capacity to recover from rapamycin-induced growth arrest.…”

“…In yeast, deletion of Vam6 suppresses the cellular capacity to recover from rapamycin-induced growth arrest. Vam6p is important for the binding of Ego1p to Gtr1p and for the activation of TORC1 (Binda et al, 2009;Dubouloz et al, 2005). Vam6p binds to Gtr1p and acts as a guanine nucleotide exchange factor (GEF) for Gtr1p (Binda et al, 2009), thus positively regulating amino acid-TORC1 signaling.…”

“…In yeast, Gtr1p and Gtr2p, which are orthologues of RagA and RagC, form a heterodimeric complex playing a similar role as that played by mammalian Rag GTPase complexes (Binda et al, 2009;Dubouloz et al, 2005;Valbuena et al, 2012). Gtr1p and Gtr2p exist in a large multiprotein complex termed the EGO complex together with Ego1p and Ego3p.…”

“…Gtr1p and Gtr2p exist in a large multiprotein complex termed the EGO complex together with Ego1p and Ego3p. The EGO complex binds and activates TORC1 in response to amino acid availability (Binda et al, 2009;Dubouloz et al, 2005). Ego1p and TORC1 preferentially interact with GTP-bound forms of Gtr1p, and this interaction is important for TORC1 activation.…”

“…RagA and RagB recruit mTORC1 via binding to raptor under amino acid-enriched conditions (Binda et al, 2009;Sancak et al, 2010). Once mTORC1 is recruited to the lysosome, it is activated by Rheb that resides on the lysosomal membrane (Sancak et al, 2010).…”