Mechanism of arginine sensing by CASTOR1 upstream of mTORC1

Abstract

SummaryThe 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 acids1–3. In mammals, arginine is particularly important and promotes diverse physiological effects including immune cell activation, insulin secretion, and muscle growth, largely through activation of mTORC14–7. Arginine activates mTORC1 upstream of the Rag GTPases8, through either the lysosomal amino acid transporter SLC38A9 or the GATOR2-interacting CASTOR1 (Cellular Arginine Sensor for mTORC1)9–12. However, the mechanism by which the mTORC1 pathway detects and transmits the 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 ACT domains, enabling allosteric control of the adjacent GATOR2-binding site to trigger dissociation from GATOR2 and the 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.

mTOR Signaling in Growth, Metabolism, and Disease

“…Much like Sestrin2, CASTOR1 binds and inhibits GATOR2 in the absence of arginine, and dissociates upon arginine binding to enable the activation of mTORC1 Saxton et al, 2016b). Thus, both leucine and arginine stimulate mTORC1 activity at least in part by releasing inhibitors from GATOR2, establishing GATOR2 as a central node in the signaling of amino acids to mTORC1.…”

mTORC1 Activator SLC38A9 Is Required to Efflux Essential Amino Acids from Lysosomes and Use Protein as a Nutrient

“…Two such complexes are GATOR1, which is a GTPase activating protein (GAP) for Rag A/B and inhibits the mTORC1 pathway, and GATOR2, a positive component of the pathway of unknown molecular function that acts upstream of or in parallel to GATOR1 (Bar-Peled et al, 2013). The best-characterized amino acid sensors, the Sestrin and CASTOR families of proteins bind cytosolic leucine and arginine, respectively, and interact with and suppress GATOR2 in the absence of their cognate amino acids (Chantranupong et al, 2016; Chantranupong et al, 2014; Kim et al, 2015; Parmigiani et al, 2014; Saxton et al, 2016a; Saxton et al, 2016b; Wolfson et al, 2016). A third is the FLCN-FNIP complex, a GAP for Rag C/D, which translocates to the lysosomal surface in the absence of amino acids, but for which the amino acid sensing mechanism is unknown (Petit et al, 2013; Tsun et al, 2013).…”

The BIF Domain in Plant bHLH Proteins Is an ACT-Like Domain

“…The ACT-like domains in plant bHLH proteins, including those in R and DYT1, show a bbabba topology (Feller et al, 2006;Cui et al, 2016) (Figure 1), compared with the more characteristic babbab topology of ACT domains (Curien et al, 2008). However, three independent studies conducted on the crystal structure of arginine-bound CASTOR1 proposed somewhat different ACT domain topologies (babbab, bbabba, and other variants) (Gai et al, 2016;Saxton et al, 2016;Xia et al, 2016), suggesting some flexibility in the structure of the domain.…”

“…Besides their presence in bHLH factors, ACT-like domains have been found in additional proteins in Arabidopsis and other plants, for example, as part of the ACR (ACT domain repeats) protein family (Hsieh and Goodman, 2002;Liu, 2006;Sung et al, 2011). More recently, four tandem ACT domains were identified in the human Cellular Arginine Sensor for mTORC1 (CASTOR) proteins and shown to participate in arginine sensing Saxton et al, 2016). To date, the protein family database (Pfam PF01842, http:// pfam.xfam.org/family/ACT) shows 15,117 sequences containing ACT domains from 4123 species with 133 different domain architectures and 159 diverse structures.…”