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:23723238, PubMed:24095279). Forms heterodimeric Rag complexes with RagC/RRAGC or RagD/RRAGD and cycles between an inactive GDP-bound and an active GTP-bound form: RagB/RRAGB is in its active form when GTP-bound RagB/RRAGB forms a complex with GDP-bound RagC/RRAGC (or RagD/RRAGD) and in an inactive form when GDP-bound RagB/RRAGB heterodimerizes with GTP-bound RagC/RRAGC (or RagD/RRAGD) (PubMed:18497260, PubMed:20381137, PubMed:23723238, PubMed:24095279). In its GTP-bound active form, promotes the recruitment 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). {ECO:0000269|PubMed:18497260, ECO:0000269|PubMed:20381137, ECO:0000269|PubMed:23723238, ECO:0000269|PubMed:24095279, ECO:0000269|PubMed:9394008}. This is the function of ENSG00000083750 (RRAGB, Ras related GTP binding B).