Guanine nucleotide-binding protein that plays a crucial role in the cellular response to amino acid availability through regulation of the mTORC1 signaling cascade (PubMed:18497260, PubMed:20381137, PubMed:24095279, PubMed:23723238). Forms heterodimeric Rag complexes with RRAGC or RRAGD and cycles between an inactive GDP-bound and an active GTP-bound form (PubMed:18497260, PubMed:20381137, PubMed:24095279, PubMed:23723238). In its active form participates in the relocalization of mTORC1 to the lysosomes and its subsequent activation by the GTPase RHEB (PubMed:18497260, PubMed:20381137, PubMed:23723238). Involved in the RCC1/Ran-GTPase pathway (PubMed:9394008). 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 (PubMed:11073942). In complex with RRAGC, but not with RRAGA, interacts with RPTOR; this interaction is particularly efficient with GTP-loaded RRAGB and GDP-loaded RRAGC (PubMed:18497260). 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 (PubMed:22575674). Interacts with the GATOR1 complex; inactivates RRAGB (PubMed:23723238). The Rag heterodimer interacts with SLC38A9; the probable amino acid sensor (PubMed:25561175, PubMed:25567906). Interacts with SESN1, SESN2 and SESN3 (PubMed:25259925). Belongs to the GTR/RAG GTP-binding protein family.