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Background
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The mammalian Target of Rapamycin (TOR, also known as mTOR) is an evolutionarily conserved serine/threonine kinase that regulates cell growth and cell cycle progression through its ability to integrate signals from nutrient levels and growth factors. TOR regulation is accomplished through a network of various activators and repressors. It is phosphorylated by Akt, whose activity is indirectly inhibited by the lipid phosphatase PTEN. TOR is normally associated with the regulatory proteins RAPTOR, a scaffold protein whose binding by TOR substrates is necessary for effective TOR-catalyzed phosphorylation, and GΒ,L, which stimulates TORs kinase activity towards downstream proteins. It is further regulated by the proteins Rheb, TSC1 and TSC2, which act to modulate TOR activity. The downstream targets of TOR are thought to be the ribosomal protein S6 kinases and the eukaryotic initiation factor 4E binding proteins (4EBPs) whose activation leads to increased protein translation and cell growth.
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