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Identification of a conserved motif required for mTOR signaling
Stefanie S Schalm1, John Blenis
1Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
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.
Insights
Researchers discovered a conserved TOR signaling (TOS) motif in S6 kinases and 4E-binding proteins. This motif is essential for regulating protein activity by the mTOR pathway, acting as a docking site for mTOR signaling.
Area of Science:
- Cellular Biology
- Molecular Biology
- Signal Transduction
Background:
- The mammalian target of rapamycin (mTOR) pathway regulates protein synthesis.
- mTOR controls S6 kinases (S6K1/2) activation and eukaryotic initiation factor 4E (eIF4E) binding proteins (4E-BP1/2/3) inhibition.
- The precise molecular mechanisms of mTOR regulation on S6K1 and 4E-BP1 are debated.
Purpose of the Study:
- To identify the molecular basis of mTOR pathway regulation on S6K1 and 4E-BP1.
- To investigate the role of conserved sequence motifs in mTOR signaling.
- To elucidate the mechanism by which mTOR controls translation machinery.
Main Methods:
- Sequence analysis to identify conserved motifs.
- Site-directed mutagenesis of identified motifs in S6K1 and 4E-BP1.
- Phosphorylation assays to assess protein activity.
- Overexpression studies to evaluate pathway regulation.
Main Results:
- A conserved TOR signaling (TOS) motif was identified in S6 kinases and 4E-binding proteins.
- Mutations or deletions in the TOS motif impaired S6K1 activation and Thr389 phosphorylation.
- The TOS motif is critical for S6K1 activation by mTOR and is required for suppressing C-terminal inhibitory activity.
- Overexpression of S6K1 with an intact TOS motif prevented 4E-BP1 phosphorylation.
Conclusions:
- A conserved five-amino acid TOS motif is crucial for mTOR pathway regulation of S6K1 and 4E-BP1.
- The TOS motif likely functions as a docking site for mTOR or a common upstream activator.
- mTOR employs distinct mechanisms to regulate S6K1, involving the TOS motif.