Identification of novel proteins that regulate the amino acid-sensitive mTORC1 pathway

Abstract

mTOR is a serine-threonine kinase that, as the catalytic subunit of mTORC1, promotes growth and anabolism. Due to its central role in metabolism, the local and systemic environment surrounding the cell tightly regulate mTORC1. Growth factors and nutrients are each required to activate mTORC1 and promote growth. Activation of mTORC1 by growth factors has been well-elucidated, but it is only recently becoming clear how nutrients, specifically amino acids, activate mTORC1. The presence of amino acids leads to the recruitment of mTORC1 from the cytosol to the surface of the lysosomal membrane, allowing it to be activated downstream of growth factors. This amino acid-induced translocation is mediated by the Rag GTPases and Ragulator (the scaffold for the Rag GTPases and mTORC1 on the lysosomal membrane). Here we describe the identification of two new components of Ragulator, HBXIP and c7orf59, that are required for the lysosomal localization of both the Rag GTPases and mTORC1 and that allowed us to identify new functions that Ragulator fulfills. We also characterized RagA-null mice and RagA-null mouse embryonic fibroblasts (MEFs). RagA is required for embryonic development, and, surprisingly, its deletion in adult mice leads to an expansion of monocytes. MEFs derived from RagA-null embryos display atypical, nutrient-insensitive mTORC1 activation. Finally, we identified c17orf59, a new Ragulator-interacting protein that inhibits the interaction between the Rag GTPases and Ragulator, inhibiting mTORC1 activation by amino acids. We report here our progress in characterizing the components of the amino acid-sensitive mTORC1 pathway and their physiological roles and we discuss the many open questions that remain to be studied regarding how amino acid sufficiency promotes the lysosomal localization of mTORC1.