New structures of mTORC1: Focus on Rag GTPases

mTORC1 is a central player in cell growth, a process that is tightly regulated by the availability of nutrients and that controls various aspects of metabolism in the normal cell and in severe diseases such as cancers. mTORC1 is a large multiprotein complex, composed of the kinase subunit mTOR, of Ragulator, which attaches mTOR to the lysosome membrane, of the atypical Rag GTPases and the small GTPase RheB, whose nucleotide states directly dictate its localization to the lysosome and its kinase activity, and of RAPTOR, an adaptor that assembles the complex. The activity of the Rag GTPases is further controlled by the GATOR1 and folliculin complexes, which regulate their GTP/GDP conversion. Here, we review recent structures of important components of the mTORC1 machinery, determined by cryo-electron microscopy for the most part, which allow to reconstitute the architecture of active mTORC1 at near atomic resolution. Notably, we discuss how these structures shed new light on the roles of Rag GTPases and their regulators in mTORC1 regulation, and the perspectives that they open towards understanding the inner workings of mTORC1 on the lysosomal membrane.