Identification of a conserved motif required for mTOR signaling.
BACKGROUND
The mammalian target of rapamycin (mTOR) controls the translation machinery via activation of S6 kinases 1 and 2 (S6K1/2) and inhibition of the eukaryotic initiation factor 4E (eIF4E) binding proteins 1, 2, and 3 (4E-BP1/2/3). S6K1 and 4E-BP1 are regulated by nutrient-sensing and mitogen-activated pathways. The molecular basis of mTOR regulation of S6K1 and 4E-BP1 remains controversial.


RESULTS
We have identified a conserved TOR signaling (TOS) motif in the N terminus of all known S6 kinases and in the C terminus of the 4E-BPs that is crucial for phosphorylation and regulation S6K1 and 4E-BP1 activities. Deletion or mutations within the TOS motif significantly inhibit S6K1 activation and the phosphorylation of its hydrophobic motif, Thr389. In addition, this sequence is required to suppress an inhibitory activity mediated by the S6K1 C terminus. The TOS motif is essential for S6K1 activation by mTOR, as mutations in this motif mimic the effect of rapamycin on S6K1 phosphorylation, and render S6K1 insensitive to changes in amino acids. Furthermore, only overexpression of S6K1 with an intact TOS motif prevents 4E-BP1 phosphorylation by a common mTOR-regulated modulator of S6K1 and 4E-BP1.


CONCLUSIONS
S6K1 and 4E-BP1 contain a conserved five amino acid sequence (TOS motif) that is crucial for their regulation by the mTOR pathway. mTOR seems to regulate S6K1 by two distinct mechanisms. The TOS motif appears to function as a docking site for either mTOR itself or a common upstream activator of S6K1 and 4E-BP1.
Schalm
Stefanie S
SS
Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
Blenis
John
J
D048868:Adaptor Proteins, Signal Transducing; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002460:Cell Line; D017124:Conserved Sequence; D004789:Enzyme Activation; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D010750:Phosphoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D010769:Phosphothreonine / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D010750:Phosphoproteins; D010769:Phosphothreonine; D011494:Protein Kinases; C546842:MTOR protein, human; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases