Rag GTPases - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 275

bound) physically interacts with and contributes to TORC1 activation. This inter-
action is dependent on amino acid availability and guanine nucleotide loading
(Binda et al. 2009 ).
12.2.2 Ragulator Complex
Rag GTPases reside on the lysosomal surface but lack a lipid-anchoring motif,
implying that Rag binding proteins must anchor them there. The Ragulator complex
was characterized shortly after identification of the Rags as a scaffold for the
heterodimeric Rag complex at the lysosome. The pentameric Ragulator complex
is composed of p18, p14, MP1, c7orf59, and HBXIP (encoded by LAMTOR1-5,
respectively) (Bar-Peled et al. 2012 ; Sancak et al. 2010 ). The heterodimeric Rag
GTPases and four Ragulator components are tethered to the lysosomal surface by
p18, which associates with the membrane via its myristoylated and palmitoylated
residues (Fig. 12.1 ). Ablation of specific Ragulator components abolishes Rag and
mTORC1 lysosomal localization, thus preventing activation of mTORC1
(Bar-Peled et al. 2012 ; Sancak et al. 2010 ). Moreover, the Rag GTPase-Ragulator
interaction is sensitive to amino acid availability. Rag GTPase-Ragulator binding
strengthens under amino acid deprivation and weakens with amino acid stimulation
(Bar-Peled et al. 2012 ). Thus, the Ragulator is essential in maintaining the
heterodimeric Rag complex along with mTORC1 at the lysosome.
Ragulator orthologues in yeast have not yet been identified. However, in yeast
TORC1 has been identified as a component of the vacuolar localized Ego complex
(EGOC). EGOC contains Ego1, Ego3, Gtr1, and Gtr2. The Ragulator and the
EGOC lack sequence similarity, but the structural conservation is noticeable.
Studies have demonstrated that, like p18, Ego1 is palmitoylated and myristoylated
and maintains the Gtr1-Gtr2 complex and TORC1 on the vacuolar membrane
(Ashrafi et al. 1998 ; Kogan et al. 2010 ). Ego3 is structurally similar to the Ragulator
components p14 and MP1 (Kogan et al. 2010 ; Zhang et al. 2012 ). Furthermore,
roadblock domains have been identified in the high-resolution crystal structures of
p14, MP1, HBXIP, and Gtr1-Gtr2 (Garcia-Saez et al. 2011 ; Kurzbauer et al. 2004 ;
Lunin et al. 2004 ). Roadblock domains also appear in the C-terminal region of the
Rag GTPases and C7orf59 based on structural predictions (Bar-Peled et al. 2012 ;
Gong et al. 2011 ). Thus, the Rag GTPases and each component of the Ragulator
complex, aside from the anchoring protein p18, appear to contain a roadblock
domain. The functional significance of this domain is unknown, but it may be
important for mediating protein-protein interactions among these proteins in the
amino acid signaling cascade to mTORC1.