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Regulation of mTOR complex 1 (mTORC1) by raptor Ser863 and multisite phosphorylation

Kathryn G Foster1, Hugo A Acosta-Jaquez, Yves Romeo

  • 1Department of Cell and Developmental Biology, Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48109-2200, USA.

The Journal of Biological Chemistry
|October 30, 2009
PubMed

Insights

The mechanistic target of rapamycin complex 1 (mTORC1) pathway regulates cell growth. This study identifies key raptor phosphorylation sites, revealing that Serine 863 phosphorylation acts as a master switch controlling mTORC1 signaling. This finding offers new insights into cellular growth regulation.

Area of Science:

  • Cellular Biology
  • Molecular Signaling
  • Biochemistry

Background:

  • The mechanistic target of rapamycin complex 1 (mTORC1) is a central regulator of protein synthesis, cell growth, and proliferation.
  • mTORC1 integrates signals from growth factors and nutrients to control cellular metabolism.
  • The precise mechanisms governing mTORC1 regulation, particularly raptor phosphorylation, remain incompletely understood.

Purpose of the Study:

  • To identify and characterize phosphorylation sites on raptor, a key component of mTORC1.
  • To investigate the role of raptor phosphorylation, specifically at Serine 863, in mTORC1 signaling.
  • To elucidate how raptor phosphorylation modulates mTORC1 kinase activity.

Main Methods:

  • Tandem mass spectrometry was employed to identify raptor phosphorylation sites.
  • Phosphospecific antibodies were generated for the identified phosphorylation sites.
  • Investigated the impact of insulin, amino acids, EGF, and Rheb overexpression on raptor phosphorylation.
  • Assessed the in vitro kinase activity of mTORC1 with phosphorylation-defective raptor mutants.

Main Results:

  • Six raptor phosphorylation sites were identified, clustered in two regions.
  • Insulin signaling via the PI3K/TSC/Rheb pathway promotes raptor Serine 863 phosphorylation in a rapamycin-sensitive manner.
  • Raptor Serine 863 phosphorylation is essential for the phosphorylation of other sites, such as Serine 859 and Serine 855.
  • mTORC1 containing phosphorylation-defective raptor mutants showed reduced kinase activity toward 4E-binding protein 1 (4EBP1).

Conclusions:

  • Raptor phosphorylation, particularly at Serine 863, acts as a critical regulatory switch for mTORC1 signaling.
  • Hierarchical phosphorylation of raptor modulates mTORC1 activity in response to various cellular cues.
  • Complex raptor phosphorylation functions as a biochemical rheostat, fine-tuning mTORC1 signaling based on environmental conditions.

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