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promoting the exchange of guanosine nucleotides, GDP to GTP. In contrast, GAPs
act antagonistically to deactivate GTPases by increasing their intrinsic rate of GTP
hydrolysis (Bos et al.
2007
). As previously noted, the active Rag heterodimeric
complex consists of RagA/B bound to GTP and RagC/D bound to GDP. Thus, the
orchestration of the GEFs and GAPs for Rags GTPases will supply a better
understanding of mTORC1 regulation by amino acids and cell growth.
12.3.1 Ragulator
Shortly after the Ragulator complex was identified as an anchor for the Rag
GTPases at the lysosome, it was discovered to also be a GEF for RagA/B, promot-
ing the exchange of GDP for GTP (Bar-Peled et al.
2012
). RagA/B GTP bound is
essential for the activation of the Rag complex and for mTORC1 lysosomal
localization and activation (Kim et al.
2008
; Sancak et al.
2008
). The identification
of the Ragulator as a GEF for RagA/B provided more mechanistic detail for the
amino acid signaling cascade to mTORC1 and Rag guanine nucleotide loading. All
five subunits of the Ragulator complex are required for RagA/B GEF activity
(Bar-Peled et al.
2012
). However, it appears that none of the Ragulator components
contain any GEF-like catalytic domains. Guanine nucleotide-free RagA/B favors
association with the Ragulator over guanine nucleotide-bound RagA/B, which is a
common characteristic found in other GEF-GTPase interactions (Bos et al.
2007
;
Feig
1999
). The Ragulator does not appear to have any GEF activity towards RagC/
D (Bar-Peled et al.
2012
). This is perhaps due to differences in the switch I and
switch II regions between RagA/B and RagC/D. These switch regions are known to
act as recognition motifs for GEF and GTPase interactions (Goldberg
1998
).
12.3.2 Vam6
There are no identified orthologues of the Ragulator components in yeast. However,
Vam6 has been reported to function as a Gtr1 (RagA/B orthologue) GEF in
Saccharomyces cerevisiae
(Binda et al.
2009
; Valbuena et al.
2012
). In addition
to Vam6 promoting nucleotide exchange of Gtr1, Gtr1 interacts with Ego1 in a
manner dependent on the GTP loading of Gtr1. Moreover, the association between
Gtr1 and Ego1 was dramatically decreased in a Vam6-deleted strain (Binda
et al.
2009
). The mammalian orthologue of Vam6, VPS39, is involved in promoting
late endosome to lysosome fusion. VPS39 does not appear to bind to or function as
a GEF for RagA/B in mammals, possibly suggesting that the GEF for Gtr1 in yeast
and RagA/B in mammals has diverged (Bar-Peled et al.
2012
). There are other
homologues of VPS39 in mammalian cells that have not been assessed as potential
GEFs for RagA/B, such as TGF
RAP1 (Messler et al.
2011
). Given the high
sequence similarity between Gtr1 and RagA/B, and the importance of guanine
β
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