Functions as an intracellular arginine sensor within the amino acid-sensing branch of the TORC1 signaling pathway (PubMed:26972053, PubMed:27487210, PubMed:33594058). As a homodimer or a heterodimer with CASTOR2, binds and inhibits the GATOR subcomplex GATOR2 and thereby mTORC1 (PubMed:26972053, PubMed:27487210, PubMed:33594058). Binding of arginine to CASTOR1 allosterically disrupts the interaction of CASTOR1-containing dimers with GATOR2 which can in turn activate mTORC1 and the TORC1 signaling pathway (PubMed:26972053, PubMed:27487210, PubMed:33594058). Forms homodimers and heterodimers with CASTOR2 (PubMed:26972053, PubMed:27487210). Interacts with the GATOR2 complex which is composed of MIOS, SEC13, SEH1L, WDR24 and WDR59; the interaction is negatively regulated by arginine (PubMed:26972053, PubMed:27487210, PubMed:33594058). Interacts with TM4SF5; the interaction is positively regulated by leucine and is negatively regulated by arginine (PubMed:30956113). Widely expressed. Based on x-ray crystallography data, the protein would be constituted of 4 tandem ACT domains instead of the 2 predicted from the sequence. Phosphorylation at Ser-14 by AKT1, promoting the interaction between CASTOR1 and RNF167. Ubiquitinated by RNF167 via 'Lys-29'-polyubiquitination, leading to its degradation, releasing the GATOR2 complex (PubMed:33594058). Ubiquitination by RNF167 is promoted by phosphorylation at Ser-14 by AKT1 (PubMed:33594058). Belongs to the GATS family. Truncated C-terminus.