Oncogenic MAPK signaling stimulates mTORC1 activity by promoting RSK-mediated raptor phosphorylation.
BACKGROUND
The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase that controls cell growth in response to mitogens, as well as amino acid and energy sufficiency. The scaffolding protein Raptor binds to mTOR and recruits substrates to the rapamycin-sensitive mTOR complex 1 (mTORC1). Although Raptor has been shown to be essential for mTORC1 activity, the mechanisms regulating Raptor function remain unknown.


RESULTS
Here, we demonstrate that Raptor becomes highly phosphorylated on RXRXXpS/T consensus motifs after activation of the Ras/mitogen-activated protein kinase (MAPK) pathway. Using pharmacological inhibitors and RNA interference, we show that the p90 ribosomal S6 kinases (RSKs) 1 and 2 are required for Raptor phosphorylation in vivo and directly phosphorylate Raptor in vitro. Quantitative mass spectrometry and site-directed mutagenesis revealed that RSK specifically phosphorylates Raptor within an evolutionarily conserved region with no previously known function. Interestingly, expression of oncogenic forms of Ras and MEK that elevate mTORC1 activity induced strong and constitutive phosphorylation of Raptor on these residues. Importantly, we demonstrate that expression of Raptor mutants lacking RSK-dependent phosphorylation sites markedly reduced mTOR phosphotransferase activity, indicating that RSK-mediated phosphorylation of Raptor is important for mTORC1 activation by the Ras/MAPK pathway.


CONCLUSIONS
We propose a unique mode of mTOR regulation in which RSK-mediated phosphorylation of Raptor regulates mTORC1 activity and thus suggest a means by which the Ras/MAPK pathway might promote rapamycin-sensitive signaling independently of the PI3K/Akt pathway.
Carrière
Audrey
A
Department of Pathology and Cell Biology, Faculty of Medicine, Université de Montréal, P.O. Box 6128, Station Centre-ville, Montréal Québec, H3C 3J7 , Canada.
Cargnello
Marie
M
Julien
Louis-André
LA
Gao
Huanhuan
H
Bonneil
Eric
E
Thibault
Pierre
P
Roux
Philippe P
PP
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D002460:Cell Line; D006367:HeLa Cells; D006801:Humans; D020935:MAP Kinase Signaling System / Q000502:physiology*; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008954:Models, Biological; D046912:Multiprotein Complexes; D041681:NIH 3T3 Cells; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D000076223:Regulatory-Associated Protein of mTOR; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000502:physiology*; D012694:Serine / Q000378:metabolism; D013379:Substrate Specificity; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D046912:Multiprotein Complexes; D011506:Proteins; C463659:RPTOR protein, human; D000076223:Regulatory-Associated Protein of mTOR; D014157:Transcription Factors; D012694:Serine; D011494:Protein Kinases; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D000076222:Mechanistic Target of Rapamycin Complex 1; C069329:RPS6KA1 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D058570:TOR Serine-Threonine Kinases; C069330:ribosomal protein S6 kinase, 90kDa, polypeptide 3