Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 285, Issue 1, Pages 80-94Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M109.029637
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Funding
- National Institutes of Health [R01 DK078135]
- NIDDK [P20 RR16462]
- National Center for Research Resources
- American Diabetes Association
- American Heart Association
- Cole Foundation
- Terry Fox Foundation
- Canadian Cancer Society Research Institute [018311]
- Human Frontier Science Program
- NATIONAL CENTER FOR RESEARCH RESOURCES [P20RR016462] Funding Source: NIH RePORTER
- NATIONAL INSTITUTE OF DIABETES AND DIGESTIVE AND KIDNEY DISEASES [R01DK078135, P60DK020572] Funding Source: NIH RePORTER
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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.
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