Novel role of the small GTPase Rheb: Its implication in endocytic pathway independent of the activation of mammalian target of rapamycin

The Ras-homologous GTPase Rheb that is conserved from yeast to human appears to be involved not only in cell growth but also in nutrient uptake. Recent biochemical analysis revealed that tuberous sclerosis complex (TSC), a GTPase-activating protein (GAP), deactivates Rheb and that phosphatidylinositol 3'-kinase (PI3k)-Akt/PKB kinase pathway activates Rheb through inhibition of the GAP-mediated deactivation. Although mammalian target of rapamycin (mTOR) kinase is implicated in the downstream target of Rheb, the direct effector(s) and exact functions of Rheb have not been fully elucidated. Here we identified that Rheb expression in cultured cells induces the formation of large cytoplasmic vacuoles, which are characterized as late endocytic (late endosome- and lysosome-like) components. The vacuole formation required the GTP form of Rheb, but not the activation of the downstream mTOR kinase. These results suggest that Rheb regulates endocytic trafficking pathway independent of the previously identified mTOR pathway. The physiological roles of the two Rheb-dependent signaling pathways are discussed in terms of nutrient uptake and cell growth or cell cycle progression.