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Mutations in TSC1 or TSC2 cause tuberous sclerosis complex (TSC), an autosomal dominant disorder characterized by the occurrence of benign tumors in various vital organs and tissues. TSC1 and TSC2, the TSC1 and TSC2 gene products, form the TSC protein complex that senses specific cellular growth conditions to control mTORC1 signaling. TBC1D7 is the third subunit of the TSC complex, and helps to stabilize the TSC1-TSC2 complex through its direct interaction with TSC1. Homozygous inactivation of TBC1D7 causes intellectual disability and megaencephaly. Here we report the crystal structure of a TSC1-TBC1D7 complex and biochemical characterization of the TSC1-TBC1D7 interaction. TBC1D7 interacts with the C-terminal region of the predicted coiled-coil domain of TSC1. The TSC1-TBC1D7 interface is largely hydrophobic, involving the ␣4 helix of TBC1D7. Each TBC1D7 molecule interacts simultaneously with two parallel TSC1 helices from two TSC1 molecules, suggesting that TBC1D7 may stabilize the TSC complex by tethering the C-terminal ends of two TSC1 coiled-coils.Tuberous sclerosis complex (TSC) 4 is a multisystem genetic disease that is characterized by the development of hamartomas or benign tumors in various organs, including skin, brain, and kidneys, and is frequently associated with epilepsy, intellectual disability, and autism (1, 2). TSC is caused by mutations to either of two genes, TSC1 and TSC2, which encode for proteins hamartin (TSC1) and tuberin (TSC2), respectively. TSC1 and TSC2 form a protein complex that acts as a GTPase-activating protein (GAP) for the small G-protein Rheb, which in turn serves as a crucial activator of the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1). The TSC complex is a tumor suppressor, inhibiting the Rheb-mTORC1-dependent stimulation of anabolic metabolism and cell growth (2-11). TSC2 contains a GAP domain and is the catalytic subunit of the complex. TSC1 is believed to enhance TSC2 function by activating TSC2 GAP activity, stabilizing TSC2, and/or maintaining the correct intracellular localization of the TSC complex (12-19). Recent work indicates that TSC1 might also have TSC2-independent activities in the cell (20 -22).Tre2-Bub2-Cdc16 domain family member 7 (TBC1D7) is a TSC1-binding protein (23-25), and has been recently identified as the third subunit of the TSC complex (25). TSC1, TSC2, and TBC1D7 form the core of the TSC protein complex in the cell, which migrates in size exclusion chromatography with a peak mass of ϳ2 MDa (26). Consistent with its important role in the TSC protein complex, knockdown of TBC1D7 results in decreased association of TSC1 and TSC2, leading to decreased Rheb-GAP activity, increased mTORC1 signaling, delayed induction of autophagy, and enhanced cell growth under poor growth conditions (25). Intriguingly, overexpression of TBC1D7 was also reported to increase mTORC1 activity (23). TBC1D7 mutations have not been found in TSC patients, but homozygous loss of TBC1D7 causes intellectual disability and megalencephaly, a developmental disorder a...