The role of phosphorylation in the regulation of the mammalian target of rapamycin
A key regulator of translation is the mammalian target of rapamycin (mTOR), a protein kinase member of the family of phosphatidylinositol kinase (PIK)-related kinases. mTOR is dually regulated by growth factors and nutrient availability, though the precise mechanisms by which mTOR is regulated are not well understood. The C-terminal of the mTOR catalytic domain has been of regulatory interest by the identification of the insulin stimulated and nutrient sensitive S2448 phosphorylation site. The functional significance of S2448 phosphorylation on the mTOR downstream targets p70 S6 kinase (S6K1) and eIF4E-binding protein 1 (4E-BP1) are unclear. A novel nutrient responsive mTOR phosphorylation site has been identified at T2446. In contrast to S2448 phosphorylation, T2446 is dephosphorylated when CHO-IR cells are insulin stimulated and phosphorylated when cells are nutrient deprived. Studies show that activation of AMP-activated kinase (AMPK) is concomitant with an increase in mTOR T2446 phosphorylation, paralleled by a decrease in S6K1 phosphorylation. Regulation of T2446 phosphorylation may involve AMPK. Phosphorylation at T2446 and S2448 is mutually exclusive. The functional significance of phosphorylation at T2446 and S2448 on the downstream target S6K1 was investigated by a mutational strategy where each site was substituted with non-phosphorylatable alanine or phospho-mimic glutamic acid. Evidence indicates that although phosphorylation of T2446 and S2448 is mutually exclusive in response to growth factors and nutrients, their individual phosphorylation may not be enough to have a direct effect on downstream S6K1 activity. Additionally, the tuberous sclerosis complex (TSC) may have positive regulatory effects on insulin signalling. Loss of TSC2 impairs insulin signalling by down- regulating the turnover of insulin receptor substrate-1 (IRS-1) protein, affecting associated class 1a phosphoinositide 3-kinase (PI3K) activity and downstream signalling; including suppression of PKB activation and mTOR S2448 phosphorylation.