This domain is found in Cytosolic arginine sensor for mTORC1 subunit 1 from humans (CASTOR1) and in similar proteins predominantly from vertebrates. CASTOR1 functions as an intracellular arginine sensor within the amino acid-sensing branch of the TORC1 signaling pathway. This entry represents the third out of the four tandem ACT domains present in this protein. This domain adopts the typical beta-alpha- beta-beta-alpha-beta configuration of the ACT domain, although it lacks the last beta-strand. It folds together with the first ACT domain to form an eight-stranded beta-sheet flanked by four alpha- helices on one side, making up the dimerization domain [1,4].

1: Structural insight into the arginine-binding specificity of CASTOR1 in amino acid-dependent mTORC1 signaling. Xia J, Wang R, Zhang T, Ding J; Cell Discov. 2016;2:16035. PMID:27648300

2: Crystal structures of arginine sensor CASTOR1 in arginine-bound and ligand free states. Zhou Y, Wang C, Xiao Q, Guo L; Biochem Biophys Res Commun. 2019;508:387-391. PMID:30503338

3: 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

4: Mechanism of arginine sensing by CASTOR1 upstream of mTORC1. Saxton RA, Chantranupong L, Knockenhauer KE, Schwartz TU, Sabatini DM; Nature. 2016;536:229-233. PMID:27487210