Tuberous sclerosis (TSC) is an autosomal dominant disorder caused by a mutation in either the TSC1 or TSC2 tumour suppressor genes. The disease ischaracterised by hamartomas in one or more organs (including brain, skin,heart and kidney) giving rise to a broad phenotypic spectrum (including seizures, mental retardation, renal dysfunction and dermatological abnormalities. TSC2 encodes tuberin, a putative GTPase activatingprotein for rap1 and rab5. The TSC1 gene was recently identified and codesfor hamartin, a novel protein with no significant similarity to tuberin orany other known vertebrate protein []. Hamartin and tuberin have been shown to associate physically in vivo, their interaction being mediated by predicted coiled-coil domains. It is thought that hamartin and tuberin function in the same complex, rather than in separate pathways.Moreover, because oligomerisation of the hamartin C-terminal coiled coildomain is inhibited by the presence of tuberin, it is possible that tuberinacts as a chaperone, preventing hamartin self-aggregation [].Tuberin is a widely expressed 1784-amino-acid protein. Expression of the wild-type gene in TSC2 mutant tumour cells inhibits proliferation andtumorigenicity. This "suppressor"activity is encoded by a functionaldomain in the C terminus that shares similarity with the GTPase activatingprotein Rap1GAP []. It is thought that tuberin functions as a Rab5GAP in vivo to negatively regulate Rab5-GTP activity in endocytosis []. It also acts as a GTPase-activating protein (GAP) for the small GTPase RheB, a direct activator of the protein kinase activity of mTORC1 [, ].
