title: mTORC1 signaling requires proteasomal function and the involvement of CUL4-DDB1 ubiquitin E3 ligase.
abstract: The mammalian target-of-rapamycin (mTOR) signaling pathway serves as a major regulator of cell growth, cell size and metabolism. In vivo, mTOR exists in two complexes, both of which contain the catalytic subunit mTOR, the invariable subunit mLST8, and a complex specific subunit Raptor or Rictor, forming either the rapamycin-sensitive mTORC1 or rapamycin-insensitive mTORC2, respectively. The exact functions of Raptor or Rictor in these complexes are still unclear. Here we demonstrate that mTORC1-mediated signaling events require the function of the 26S proteasome. Inhibition of the 26S proteasome by MG132 leads to the rapid inhibition of phosphorylation of the mTORC1 substrates S6 kinase and 4E-BP1. We have further discovered that the WD40 repeat proteins Raptor and mLST8 bind the CUL4-DDB1 ubiquitin E3 ligase. Loss of CUL4B or DDB1 specifically blocks the phosphorylation of S6 kinase at threonine 389 and 4E-BP1 at serine 65 and threonines 37 and 46, while loss of CUL4B enhances the phosphorylation of AKT at serine 473. These phosphorylation effects are identical to those resulting from the inactivation of Raptor. Our data suggest that the CUL4-DDB1 ubiquitin ligase interacts with Raptor and regulates the mTORC1- mediated signaling pathway through ubiquitin-dependent proteolysis.
authors.affiliation: Laboratory of Cancer Genomics, Division of Basic Science, Nevada Cancer Institute, Las Vegas, Nevada 89135, USA.
authors.forename: Papia
authors.initials: P
authors.lastname: Ghosh
authors.forename: Min
authors.initials: M
authors.lastname: Wu
authors.forename: Hui
authors.initials: H
authors.lastname: Zhang
authors.forename: Hong
authors.initials: H
authors.lastname: Sun
mesh_terms: D000595:Amino Acid Sequence; D002460:Cell Line; D002478:Cells, Cultured; D044842:Cullin Proteins / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D006801:Humans; D006868:Hydrolysis; D000076222:Mechanistic Target of Rapamycin Complex 1; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes; D010447:Peptide Hydrolases / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000502:physiology*; D011506:Proteins; D015398:Signal Transduction / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000502:physiology*
chemical_list: C114234:CUL4A protein, human; C517832:CUL4B protein, human; D044842:Cullin Proteins; C091435:DDB1 protein, human; D004268:DNA-Binding Proteins; D046912:Multiprotein Complexes; D011506:Proteins; D014157:Transcription Factors; D044767:Ubiquitin-Protein Ligases; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases; D010447:Peptide Hydrolases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease