Ragulator Is a GEF for the Rag GTPases that Signal Amino Acid Levels to mTORC1

Abstract

The mTOR Complex 1 (mTORC1) pathway promotes cell growth in response to a diverse set of cues, including growth factors as well as energy and amino acid levels. Amino acids signal through the Rag GTPases to promote the translocation of mTORC1 to the lysosomal surface, its site of activation. The four mammalian Rag proteins form obligate heterodimers consisting of RagA or RagB bound to RagC or RagD. A key upstream component of the Rag GTPases is Ragulator, a trimeric complex that tethers them to the lysosome and also interacts with the v-ATPase, which is necessary for amino acid sensing by mTORC1. Amino acids stimulate the binding of GTP to RagB, a critical step in the sensing mechanism, but the factors that regulate Rag nucleotide loading are unknown. Here, we identify the proteins encoded by the HBXIP and C7orf59 genes as novel Ragulator components that are required for mTORC1 activation by amino acids. The pentameric Ragulator has nucleotide exchange activity towards RagA and RagB and interacts with the Rag heterodimers in an amino acid- and v-ATPase-dependent fashion. Thus, we provide mechanistic insight into how mTORC1 senses amino acids by revealing Ragulator to be a scaffold with guanine nucleotide exchange factor (GEF) activity for the Rag GTPases.

Pharmacological inhibition of lysosomes activates the MTORC1 signaling pathway in chondrocytes in an autophagy-independent manner

“…V-ATPase acts downstream of amino acids, but upstream of the Rag GTPases and mediates amino acid sensing by interacting with the Ragulator complex. 4,9,24 In HEK 293 cells amino acid starvation strengthens, and re-stimulation weakens, the interaction between the v-ATPase and the Ragulator, whereas v-ATPase inhibition locks in the interaction in the amino acid-free state. 9,24 Since the v-ATPase is downstream of amino acids, a nutrition-independent effect of MTORC1 activation by Baf in chondrocytes is again pointing toward a v-ATPase-dependent mechanism.…”

“…This is a very surprising observation, which is opposite to the current understanding of MTORC1 regulation and we discuss it in the next paragraph below. 4,9,24 MTORC1 is a well-known regulator of cell size; 1 however, it is not clear if MTORC1 is regulating Figure 6. The stimulation of RPS6 phosphorylation by Baf is nutrition independent.…”

dRAGging Amino Acid-mTORC1 Signaling by SH3BP4

“…Rag GTPases interact with a lysosomal multiprotein complex termed Ragulator that consists of MP1, p14, p18, HBXIP and C7orf59 (Bar-Peled et al, 2012;Sancak et al, 2010). Ragulator is necessary for the localization of Rag GTPases to the lysosomal surface.…”

“…The interaction between Ragulator and Rag GTPases is important for activation of mTORC1 since Rag GTPases need to recruit mTORC1 to the lysosome for the activation of mTORC1 by Rheb. The interaction between Ragulator and Rag GTPases is enhanced under conditions when amino acid levels are high (Bar-Peled et al, 2012). Ragulator was determined to function as a guanine nucleotide exchange factor (GEF) for Rag GTPases (Bar-Peled et al, 2012).…”

“…Ragulator was determined to function as a guanine nucleotide exchange factor (GEF) for Rag GTPases (Bar-Peled et al, 2012). The GEF function facilitates the exchange of GDP for GTP of RagA or RagB resulting in the formation of the active Rag heterodimeric complex (Bar-Peled et al, 2012).…”