Guanine nucleotide-binding protein that plays a crucial role in the cellular response to amino acid availability through regulation of the mTORC1 signaling cascade. Forms heterodimeric Rag complexes with RRAGC or RRAGD and cycles between an inactive GDP-bound and an active GTP-bound form. In its active form participates in the relocalization of mTORC1 to the lysosomes and its subsequent activation by the GTPase RHEB. Involved in the RCC1/Ran-GTPase pathway. GTP + H2O = GDP + H(+) + phosphate The activation of GTP-binding proteins is generally mediated by a guanine exchange factor (GEF), while inactivation through hydrolysis of bound GTP is catalyzed by a GTPase activating protein (GAP). The GATOR1 complex functions as a GAP and stimulates RRAGB GTPase activity to turn it into its inactive GDP-bound form. Interacts with RRAGC and RRAGD; heterodimerization stabilizes RRAG proteins (By similarity). In complex with RRAGC, but not with RRAGA, interacts with RPTOR; this interaction is particularly efficient with GTP-loaded RRAGB and GDP-loaded RRAGC (By similarity). Component of the lysosomal folliculin complex (LFC), composed of FLCN, FNIP1 (or FNIP2), RRAGA/RagA or RRAGB/RagB GDP-bound, RRAGC/RagC or RRAGD/RagD GTP-bound, and Ragulator (By similarity). Interacts with SH3BP4; the interaction with this negative regulator is most probably direct, preferentially occurs with the inactive GDP-bound form of RRAGB, is negatively regulated by amino acids and prevents interaction with RPTOR (By similarity). Interacts with the GATOR1 complex; inactivates RRAGB (By similarity). The Rag heterodimer interacts with SLC38A9; the probable amino acid sensor. Interacts with SESN1, SESN2 and SESN3 (PubMed:25259925). Belongs to the GTR/RAG GTP-binding protein family.