Activates the protein kinase activity of mTORC1, and thereby plays a role in the regulation of apoptosis. Stimulates the phosphorylation of S6K1 and EIF4EBP1 through activation of mTORC1 signaling. Has low intrinsic GTPase activity (By similarity). Alternates between an inactive form bound to GDP and an active form bound to GTP. Inactivated by TSC1-TSC2 via the GTPase activating protein (GAP) domain of TSC2 (By similarity). Binds to mTORC1 in a guanyl nucleotide-independent manner. Interacts directly with MTOR, MLST8 and RPTOR. Interacts with TSC2 (By similarity). Interacts (when prenylated) with PDE6D; this promotes release from membranes (By similarity). Farnesylation is important for efficiently activating mTORC1-mediated signaling. Phosphorylation by MAPKAPK5 impairs GTP-binding and inactivation. The conserved catalytic Gln-64 found in other Ras-like GTPases seems not to be involved in GTP hydrolysis in RHEB. Belongs to the small GTPase superfamily. Rheb family.