Tuberous sclerosis complex (TSC) is caused by mutations in the TSC1 and TSC2 tumor suppressor genes. The gene products hamartin and tuberin form the TSC complex that acts as GTPase-activating protein for Rheb and negatively regulates the mammalian target of rapamycin complex 1 (mTORC1). Tuberin contains a RapGAP homology domain responsible for inactivation of Rheb, but functions of other protein domains remain elusive. Here we show that the TSC2 N terminus interacts with the TSC1 C terminus to mediate complex formation. The structure of the TSC2 N-terminal domain from Chaetomium thermophilum and a homology model of the human tuberin N terminus are presented. We characterize the molecular requirements for TSC1-TSC2 interactions and analyze pathological point mutations in tuberin. Many mutations are structural and produce improperly folded protein, explaining their effect in pathology, but we identify one point mutant that abrogates complex formation without affecting protein structure. We provide the first structural information on TSC2/tuberin with novel insight into the molecular function.In eukaryotic cells growth factor signaling is responsible to adjust proliferation and metabolism to environmental and developmental cues. Growth factor receptors regulate, among other targets, activation of the mammalian target of rapamycin complex 1 (mTORC1), 2 considered the master regulator of cellular growth (1). This pathway involves the protein kinase B/AKT, the tuberous sclerosis complex (TSC), and the small GTPase Rheb (2). Under resting conditions, Rheb is kept inactive by TSC, which represents the cognate GTPase-activating protein (GAP) (3). Rheb undergoes the classical GTPase conversion and cycles between an active GTP-bound and an inactive GDP-bound state. Active Rheb is required for full activation of mTORC1 at the lysosomal membrane (4). Upon stimulation of growth factor signaling and the downstream kinase AKT, TSC gets inactivated by phosphorylation, thus leading to increasing levels of active Rheb and mTORC1.The TSC complex consists of the 130-kDa subunit hamartin (encoded by TSC1) (5), the 200-kDa protein tuberin (TSC2 gene product) (6), and the recently identified TBC1D7 (7, 8), which form high oligomeric assemblies in cells (9) and have a similar architecture as the RalGAP complexes (10, 11). Tuberin contains a RapGAP-like domain that catalyzes the hydrolysis of Rheb⅐GTP to Rheb⅐GDP (12). In addition to the GAP domain, hamartin and tuberin show no sequence homology to other known proteins, but the N terminus of tuberin has been shown to mediate interaction with harmartin (13-16). TBC1D7 is a RabGAP domain-containing protein with specificity for Rab17 involved in trafficking from endosomes to cilia (17). In the context of the TSC complex, TBC1D7 stabilizes the hamartin dimerization and the interaction with tuberin and increases the GAP activity of the complex (7,18,19). Activity of TSC is in part regulated by its recruitment to Rheb on the lysosome, which has been reported to be promoted by Rag GTPases upon ...