Regulation of mTOR complex 1 (mTORC1) by raptor Ser863 and multisite phosphorylation.
The rapamycin-sensitive mTOR complex 1 (mTORC1) promotes protein synthesis, cell growth, and cell proliferation in response to growth factors and nutritional cues. To elucidate the poorly defined mechanisms underlying mTORC1 regulation, we have studied the phosphorylation of raptor, an mTOR-interacting partner. We have identified six raptor phosphorylation sites that lie in two centrally localized clusters (cluster 1, Ser(696)/Thr(706) and cluster 2, Ser(855)/Ser(859)/Ser(863)/Ser(877)) using tandem mass spectrometry and generated phosphospecific antibodies for each of these sites. Here we focus primarily although not exclusively on raptor Ser(863) phosphorylation. We report that insulin promotes mTORC1-associated phosphorylation of raptor Ser(863) via the canonical PI3K/TSC/Rheb pathway in a rapamycin-sensitive manner. mTORC1 activation by other stimuli (e.g. amino acids, epidermal growth factor/MAPK signaling, and cellular energy) also promote raptor Ser(863) phosphorylation. Rheb overexpression increases phosphorylation on raptor Ser(863) as well as on the five other identified sites (e.g. Ser(859), Ser(855), Ser(877), Ser(696), and Thr(706)). Strikingly, raptor Ser(863) phosphorylation is absolutely required for raptor Ser(859) and Ser(855) phosphorylation. These data suggest that mTORC1 activation leads to raptor multisite phosphorylation and that raptor Ser(863) phosphorylation functions as a master biochemical switch that modulates hierarchical raptor phosphorylation (e.g. on Ser(859) and Ser(855)). Importantly, mTORC1 containing phosphorylation site-defective raptor exhibits reduced in vitro kinase activity toward the substrate 4EBP1, with a multisite raptor 6A mutant more strongly defective that single-site raptor S863A. Taken together, these data suggest that complex raptor phosphorylation functions as a biochemical rheostat that modulates mTORC1 signaling in accordance with environmental cues.
Department of Cell and Developmental Biology, Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48109-2200, USA.
Kathryn G
KG
Foster
Hugo A
HA
Acosta-Jaquez
Yves
Y
Romeo
Bilgen
B
Ekim
Ghada A
GA
Soliman
Audrey
A
Carriere
Philippe P
PP
Roux
Bryan A
BA
Ballif
Diane C
DC
Fingar
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D002460:Cell Line; D004815:Epidermal Growth Factor / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D020935:MAP Kinase Signaling System / Q000187:drug effects; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D020559:Monomeric GTP-Binding Proteins / Q000378:metabolism; D046912:Multiprotein Complexes; D009479:Neuropeptides / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011506:Proteins / Q000737:chemistry* / Q000378:metabolism*; D000076205:Ras Homolog Enriched in Brain Protein; D051381:Rats; D000076223:Regulatory-Associated Protein of mTOR; D012694:Serine / Q000378:metabolism*; D013329:Structure-Activity Relationship; D058570:TOR Serine-Threonine Kinases; D053719:Tandem Mass Spectrometry; D013816:Thermodynamics; D014157:Transcription Factors / Q000378:metabolism*; D000077004:Tuberous Sclerosis Complex 1 Protein; D025521:Tumor Suppressor Proteins / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004815:Epidermal Growth Factor; D007328:Insulin; D000076222:Mechanistic Target of Rapamycin Complex 1; D020559:Monomeric GTP-Binding Proteins; D046912:Multiprotein Complexes; D009479:Neuropeptides; D019869:Phosphatidylinositol 3-Kinases; D011506:Proteins; D000076205:Ras Homolog Enriched in Brain Protein; D000076223:Regulatory-Associated Protein of mTOR; D012694:Serine; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors; D000077004:Tuberous Sclerosis Complex 1 Protein; D025521:Tumor Suppressor Proteins; C463659:RPTOR protein, human; C490213:Rheb protein, mouse; C540162:Rptor protein, mouse