MTOR (mammalian target of rapamycin) is a 289 kD member of the PI3 kinase family, containing HEAT, FRB, and FATC domains. mTOR is expressed in the nucleus and cytoplasm, and is a target for cell-cycle arrest and the immunosuppressive effects of FKBP12-rapamycin complex. mTOR controls ribosome biogenesis and cell growth, and acts as an ATP sensor. mTOR is repressed by nutrient deprivation and mitochondrial uncoupling. mTOR is the target of PI3K-regulated protein kinase, AKT/PKB (phosphorylated at Thr 2446 and Ser2448). mTOR has also been shown to bind RAPTOR, which negatively regulates mTOR kinase activity, and is also inhibited by tuberin-hamartin tumor suppressor complex. The binding of RAPTOR to mTOR is necessary for the mTOR-catalyzed phosphorylation of 4EBP1 in vitro, and it strongly enhances the mTOR kinase activity toward p70-alpha.