Rheb is a direct target of the tuberous sclerosis tumor suppressor proteins

Mutations in the TSC1 or TSC2 genes cause tuberous sclerosis, a benign tumour syndrome in humans1, 2. Tsc2 possesses a domain that shares homology with the GTPase-activating protein (GAP) domain of Rap1-GAP2, suggesting that a GTPase might be the physiological target of Tsc2. Here we show that the small GTPase Rheb (Ras homologue enriched in brain) is a direct target of Tsc2 GAP activity both in vivo and in vitro. Point mutations in the GAP domain of Tsc2 disrupted its ability to regulate Rheb without affecting the ability of Tsc2 to form a complex with Tsc1. Our studies identify Rheb as a molecular target of the TSC tumour suppressors

Rheb promotes cell growth as a component of the insulin/TOR signalling network

Insulin signalling is a potent inhibitor of cell growth and has been proposed to function, at least in part, through the conserved protein kinase TOR (target of rapamycin). Recent studies suggest a that the tuberous sclerosis complex Tsc1–Tsc2 may couple insulin signalling to Tor activity. However, the regulatory mechanism involved remains unclear, and additional components are most probably involved. In a screen for novel regulators of growth, we identified Rheb (Ras homologue enriched in brain), a member of the Ras superfamily of GTP-binding proteins. Increased levels of Rheb in Drosophila melanogaster promote cell growth and alter cell cycle kinetics in multiple tissues. In mitotic tissues, overexpression of Rheb accelerates passage through G1–S phase without affecting rates of cell division, whereas in endoreplicating tissues, Rheb increases DNA ploidy. Mutation of Rheb suspends larval growth and prevents progression from first to second instar. Genetic and biochemical tests indicate that Rheb functions in the insulin signalling pathway downstream of Tsc1–Tsc2 and upstream of TOR. Levels of rheb mRNA are rapidly induced in response to protein starvation, and overexpressed Rheb can drive cell growth in starved animals, suggesting a role for Rheb in the nutritional control of cell growth