Editors' ChoiceCell Biology

A lysosomal amino acid sensor for mTORC1

Sci. Signal.  31 Mar 2015:
Vol. 8, Issue 370, pp. ec78
DOI: 10.1126/scisignal.aab2107

The mechanistic target of rapamcyin complex 1 (mTORC1) regulates cell metabolism and growth in response to nutrients, growth factors, and cellular energy status. Amino acids inside the lysosome promote activation of mTORC1 at the lysosomal surface. Rebsamen et al. found that a human amino acid transporter, member 9 of the solute carrier family 38 (SLC38A9), cooperated with RAG GTPases and the Ragulator complex to activate mTORC1. In human cell lines, tagged SLC38A9 colocalized with late endosomal and lysosomal markers, and knocking down SLC38A9 reduced cell size, cell proliferation, and amino acid–induced mTORC1 activation. Endogenous SLC38A9 associated with all four RAG GTPases and all members of the Ragulator complex in several different human and mouse cell lines, whereas other SLC superfamily members did not. Mutations in three short motifs in the N-terminal tail of SLC38A9 abolished the interaction with RAG GTPases and Ragulator components. These motifs are conserved in SLC38A9 homologs from other vertebrates, but not in other members of the SLC38 family. SLC38A9 transported radiolabeled glutamine, arginine, and asparagine out of liposomes, with reduced activity compared to other amino acid transporters, suggesting that SLC38A9 may act primarily as an amino acid sensor. Binding of SLC38A9 to RAG GTPases depended on the nucleotide-binding ability of the RAG GTPases and was influenced by the presence of the nucleotide-sensing transmembrane region of SLC38A9, suggesting that binding of amino acids by SLC38A9 may constitute an allosteric switch that alters the nucleotide-binding properties, and therefore the activity, of RAG GTPases. Amino acid stimulation promotes mTORC1 signaling, and amino acid starvation inactivates mTORC1. In HEK293T cells overexpressing SLC38A9, mTORC1 activity persisted when the cells were starved for amino acids, resulting in reduced starvation-induced autophagy. The mTOR inhibitor Torin1 inhibited mTORC1 signaling in amino acid-starved SLC38A9-overexpressing cells, but the vacuolar H+–adenosine triphosphatase ATPase (v-ATPase) inhibitor concanamycin A did not. In contrast, concanamycin A inhibited mTORC1 activation in response to amino acid stimulation in cells not overexpressing SLC38A9. These findings suggest that v-ATPase and SLC38A9 cooperate to control mTORC1 activation at the lysosomal surface in response to the presence of amino acids.

M. Rebsamen, L. Pochini, T. Stasyk, M. E. G. de Araújo, M. Galluccio, R. K. Kandasamy, B. Snijder, A. Fauster, E. L. Rudashevskaya, M. Bruckner, S. Scorzoni, P. A. Filipek, K. V. M. Huber, J. W. Bigenzahn, L. X. Heinz, C. Kraft, K. L. Bennett, C. Indiveri, L. A. Huber, G. Superti-Furga, SLC38A9 is a component of the lysosomal amino acid sensing machinery that controls mTORC1. Nature 519, 477–481 (2015). [PubMed]

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