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Published on: October 23, 2018
Structural basis for growth factor and nutrient signal integration on the lysosomal membrane by mTORC1
Zhicheng Cui1,2, Alessandra Esposito3,4, Gennaro Napolitano3,4,5
1Department of Molecular and Cell Biology, University of California Berkeley; Berkeley CA 94720, USA.
Abstract:
Mechanistic target of rapamycin complex 1 (mTORC1), which consists of mTOR, Raptor, and mLST8, receives signaling inputs from growth factor signals and nutrients. These signals are mediated by the Rheb and Rag small GTPases, respectively, which activate mTORC1 on the cytosolic face of the lysosome membrane. We biochemically reconstituted the activation of mTORC1 on membranes by physiological submicromolar concentrations of Rheb, Rags, and Ragulator. We determined the cryo-EM structure and found that Raptor and mTOR directly interact with the membrane at anchor points separated by up to 230 Å across the membrane surface. Full engagement of the membrane anchors is required for maximal activation, which is brought about by alignment of the catalytic residues in the mTOR kinase active site. The observations show at the molecular and atomic scale how converging signals from growth factors and nutrients drive mTORC1 recruitment to and activation on the lysosomal membrane in a three-step process, consisting of (1) Rag-Ragulator-driven recruitment to within ~100 Å of the lysosomal membrane, (2) Rheb-driven recruitment to within ~40 Å, and finally (3) direct engagement of mTOR and Raptor with the membrane. The combination of Rheb and membrane engagement leads to full catalytic activation, providing a structural explanation for growth factor and nutrient signal integration at the lysosome.
Insights
Mechanistic target of rapamycin complex 1 (mTORC1) activation on lysosomal membranes requires direct engagement of mTOR and Raptor with the membrane. This process integrates growth factor and nutrient signals for full catalytic activation.
Area of Science:
- Cellular signaling pathways
- Molecular mechanisms of cell growth
- Lysosome biology and function
Background:
- Mechanistic target of rapamycin complex 1 (mTORC1) integrates growth factor and nutrient signals.
- mTORC1 activation occurs on the lysosomal membrane, mediated by Rheb and Rag GTPases.
- Understanding the structural basis of mTORC1 activation is crucial for deciphering cellular growth regulation.
Purpose of the Study:
- To biochemically reconstitute and structurally elucidate mTORC1 activation on lysosomal membranes.
- To determine the role of membrane engagement and GTPase signaling in mTORC1 activation.
- To provide a molecular-level explanation for signal integration at the lysosome.
Main Methods:
- Biochemical reconstitution of mTORC1 activation using purified components.
- Cryo-electron microscopy (cryo-EM) for structural determination.
- Analysis of protein-membrane interactions and catalytic site alignment.
Main Results:
- mTORC1 activation involves a three-step process: Rag-Ragulator recruitment, Rheb-mediated recruitment, and direct membrane engagement by mTOR and Raptor.
- Direct membrane interaction via anchor points up to 230 Å apart is essential for maximal activation.
- Full engagement of membrane anchors aligns catalytic residues in the mTOR active site, leading to activation.
Conclusions:
- Converging growth factor and nutrient signals are integrated at the lysosome through a stepwise recruitment and membrane-engagement mechanism.
- The structure reveals how Rheb and membrane interactions synergize to fully activate mTORC1's catalytic function.
- This provides a detailed structural framework for understanding nutrient and growth factor signaling in mTORC1 regulation.
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