RESUMO
The mammalian target of rapamycin (mTOR) integrates signals from nutrients and insulin via two distinct complexes, mTORC1 and mTORC2. Disruption of mTORC2 impairs the insulin-induced activation of Akt, an mTORC2 substrate. Here, we found that mTORC2 can also regulate insulin signaling at the level of insulin receptor substrate-1 (IRS-1). Despite phosphorylation at the mTORC1-mediated serine sites, which supposedly triggers IRS-1 downregulation, inactive IRS-1 accumulated in mTORC2-disrupted cells. Defective IRS-1 degradation was due to attenuated expression and phosphorylation of the ubiquitin ligase substrate-targeting subunit, Fbw8. mTORC2 stabilizes Fbw8 by phosphorylation at Ser86, allowing the insulin-induced translocation of Fbw8 to the cytosol where it mediates IRS-1 degradation. Thus, mTORC2 negatively feeds back to IRS-1 via control of Fbw8 stability and localization. Our findings reveal that in addition to persistent mTORC1 signaling, heightened mTORC2 signals can promote insulin resistance due to mTORC2-mediated degradation of IRS-1.
Assuntos
Proteínas F-Box/metabolismo , Proteínas Substratos do Receptor de Insulina/metabolismo , Complexos Multiproteicos/metabolismo , Processamento de Proteína Pós-Traducional , Serina-Treonina Quinases TOR/metabolismo , Animais , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Células Cultivadas , Ativação Enzimática , Proteínas F-Box/genética , Expressão Gênica , Regulação da Expressão Gênica , Técnicas de Inativação de Genes , Meia-Vida , Insulina/fisiologia , Proteínas Substratos do Receptor de Insulina/genética , Alvo Mecanístico do Complexo 1 de Rapamicina , Alvo Mecanístico do Complexo 2 de Rapamicina , Camundongos , Complexos Multiproteicos/antagonistas & inibidores , Fosforilação , Proteína Quinase C/metabolismo , Estabilidade Proteica , Proteínas/metabolismo , Proteólise , Proteínas Proto-Oncogênicas c-akt/metabolismo , Proteínas Quinases S6 Ribossômicas 90-kDa/metabolismo , Transdução de Sinais , Sirolimo/farmacologia , Serina-Treonina Quinases TOR/antagonistas & inibidoresRESUMO
The mechanisms that couple translation and protein processing are poorly understood in higher eukaryotes. Although mammalian target of rapamycin (mTOR) complex 1 (mTORC1) controls translation initiation, the function of mTORC2 in protein synthesis remains to be defined. In this study, we find that mTORC2 can colocalize with actively translating ribosomes and can stably interact with rpL23a, a large ribosomal subunit protein present at the tunnel exit. Exclusively during translation of Akt, mTORC2 mediates phosphorylation of the nascent polypeptide at the turn motif (TM) site, Thr450, to avoid cotranslational Akt ubiquitination. Constitutive TM phosphorylation occurs because the TM site is accessible, whereas the hydrophobic motif (Ser473) site is concealed in the ribosomal tunnel. Thus, mTORC2 can function cotranslationally by phosphorylating residues in nascent chains that are critical to attain proper conformation. Our findings reveal that mTOR links protein production with quality control.