From sensing leucine to metabolic control: The mTORC1 protein kinase complex plays central roles in regulating cell growth and metabolism and is implicated in common human diseases such as diabetes and cancer. The level of the amino acid leucine tells an organism a lot about its physiological state, including how much food is available, how much insulin is going to be needed, and whether new muscle mass can be made (see the Perspective by Buel and Blenis). Wolfson et al. identified a biochemical sensor of leucine, Sestrin2, which connects the concentration of leucine to the control of organismal metabolism and growth. When leucine bound to Sestrin2, it was released from a complex with the mTORC1 regulatory factor GATOR2, activating the mTORC1 complex. Saxton et al. describe the crystal structure of Sestrin2 and show how it specifically detects leucine. Aylett et al. determined the structure of human mTORC1 by cryoelectron microscopy and the crystal structure of a regulatory subunit, Raptor. The results reveal the structural basis for the function and intricate regulation of this important enzyme, which is also a strategic drug target. Science , this issue p. 43 , p. 48 , p. 53 ; see also p. 25

The editing enzyme double-stranded RNA adenosine deaminase includes a DNA binding domain, Zα, which is specific for left-handed Z-DNA. The 2.1 angstrom crystal structure of Zα complexed to DNA reveals that the substrate is in the left-handed Z conformation. The contacts between Zα and Z-DNA are made primarily with the “zigzag” sugar-phosphate backbone, which provides a basis for the specificity for the Z conformation. A single base contact is observed to guanine in the syn conformation, characteristic of Z-DNA. Intriguingly, the helix-turn-helix motif, frequently used to recognize B-DNA, is used by Zα to contact Z-DNA.

Structural data on the protein CASTOR1 reveal how the mTORC1 pathway senses intracellular arginine, suggesting a repurposing of an evolutionarily pre-metazoan mechanism. The mTOR pathway is a major regulator of cell growth and is deregulated in numerous diseases. It is influenced by various environmental inputs such as amino acids and growth factors. Availability of arginine can be relayed to mTORC1 through the protein CASTOR1 that interacts with the regulator GATOR2. Here, David Sabatini and colleagues present a crystal structure of arginine bound to CASTOR1. The structure and accompanying biochemistry reveal how arginine is specifically sensed by CASTOR1 and disrupts the interaction of CASTOR1 with GATOR2, triggering activation of mTORC1. CASTOR1 is structurally homologous to the lysine-binding domain of prokaryotic aspartate kinases. These results therefore establish a structural basis for arginine sensing by the mTORC1 pathway and provide insights into the evolution of a mammalian nutrient sensor. The mechanistic Target of Rapamycin Complex 1 (mTORC1) is a major regulator of eukaryotic growth that coordinates anabolic and catabolic cellular processes with inputs such as growth factors and nutrients, including amino acids1,2,3. In mammals arginine is particularly important, promoting diverse physiological effects such as immune cell activation, insulin secretion, and muscle growth, largely mediated through activation of mTORC1 (refs 4, 5, 6, 7).Arginine activates mTORC1 upstream of the Rag family of GTPases8, through either the lysosomal amino acid transporter SLC38A9 or the GATOR2-interacting Cellular Arginine Sensor for mTORC1 (CASTOR1)9,10,11,12. However, the mechanism by which the mTORC1 pathway detects and transmits this arginine signal has been elusive. Here, we present the 1.8 Å crystal structure of arginine-bound CASTOR1. Homodimeric CASTOR1 binds arginine at the interface of two Aspartate kinase, Chorismate mutase, TyrA (ACT) domains, enabling allosteric control of the adjacent GATOR2-binding site to trigger dissociation from GATOR2 and downstream activation of mTORC1. Our data reveal that CASTOR1 shares substantial structural homology with the lysine-binding regulatory domain of prokaryotic aspartate kinases, suggesting that the mTORC1 pathway exploited an ancient, amino-acid-dependent allosteric mechanism to acquire arginine sensitivity. Together, these results establish a structural basis for arginine sensing by the mTORC1 pathway and provide insights into the evolution of a mammalian nutrient sensor.