Functions as an intracellular arginine sensor within the amino acid-sensing branch of the TORC1 signaling pathway. As a homodimer or a heterodimer with CASTOR2, binds and inhibits the GATOR subcomplex GATOR2 and thereby mTORC1. 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. Forms homodimers and heterodimers with CASTOR2 (By similarity). Interacts with the GATOR2 complex which is composed of MIOS, SEC13, SEH1L, WDR24 and WDR59; the interaction is negatively regulated by arginine (By similarity). Interacts with TM4SF5; the interaction is positively regulated by leucine and is negatively regulated by arginine (By similarity). 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. Ubiquitination by RNF167 is promoted by phosphorylation at Ser-14 by AKT1. Belongs to the GATS family.