CASTOR1 (Cytosolic arginine sensor for mTORC1 subunit 1) has been identified as the cytosolic arginine sensor for the mTORC1 pathway. In the absence of arginine, CASTOR1 binds to GATOR2 and inhibits mTORC1 signaling; whereas in the presence of arginine, CASTOR1 interacts with arginine and no longer associates with GATOR2. The arginine sits in a pocket between the N-terminal domain (NTD) and the C-terminal domain (CTD) of CASTOR1. The CASTOR1-NTD on the opposite side of the arginine-binding site was identified to mediate direct physical interaction with its downstream effector GATOR2, via GATOR2 subunit Mios [1].

1: Structural mechanism for the arginine sensing and regulation of CASTOR1 in the mTORC1 signaling pathway. Gai Z, Wang Q, Yang C, Wang L, Deng W, Wu G; Cell Discov. 2016;2:16051. PMID:28066558