Cell growth is a fundamental biological process whereby cells accumulate mass and increase in size. The mammalian TOR pathway regulates growth by coordinating energy and nutrient signals with growth factor-derived signals. mTOR is a large protein kinase with two different complexes. One complex contains mTOR, GβL and raptor, which is a target of rapamycin. The other complex, insensitive to rapamycin, includes mTOR, GβL, Sin1, and rictor. The mTOR-rictor complex phosphorylates Ser473 of Akt/PKB in vitro. This phosphorylation is essential for full Akt/PKB activation. Furthermore, an siRNA knockdown of rictor inhibits Ser473 phosphorylation in 3T3-L1 adipocytes. This complex has also been shown to phosphorylate the rapamycin-resistant mutants of S6K1, another effector of mTOR.
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