Functions as a negative regulator of the TORC1 signaling pathway through the GATOR complex. As part of homodimers or heterodimers with CASTOR1, directly binds and inhibits the GATOR subcomplex GATOR2 and thereby mTORC1. Does not directly bind arginine, but binding of arginine to CASTOR1 disrupts the interaction of CASTOR2- containing heterodimers with GATOR2 which can in turn activate mTORC1 and the TORC1 signaling pathway. . This is the function of CASTOR2 (cytosolic arginine sensor for mTORC1 subunit 2, ENSG00000274070).