Structural basis for FLCN RagC GAP activation in MiT-TFE substrate-selective mTORC1 regulation

Area of Science:

• Cellular biology
• Molecular mechanisms
• Biochemistry

Background:

• The mechanistic target of rapamycin complex 1 (mTORC1) is a key regulator of cell growth and metabolism.
• The tumor suppressor folliculin (FLCN) interacts with mTORC1, influencing lysosome biogenesis and autophagy.
• FLCN's role as a GTPase-activating protein (GAP) for RagC/D GTPases is crucial for mTORC1 regulation.

Purpose of the Study:

• To determine the cryo-electron microscopy structure of the active FLCN complex (AFC).
• To elucidate the structural basis for FLCN's GAP activity towards Rag GTPases.
• To understand how FLCN regulates mTORC1 signaling specifically for MiT-TFE transcription factors.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to resolve the AFC structure.
• Biochemical assays to assess GAP activity.
• Mutagenesis studies to investigate protein-protein interactions.

Main Results:

• The AFC structure reveals a 90° reorientation of FLCN, with new contacts with RagC essential for catalysis.
• Disruption of FLCN and FNIP2 interfaces with RagC abolished GAP activity.
• Inhibition of FLCN's GAP activity led to TFE3 nuclear retention, without affecting other mTORC1 substrates like S6K or 4E-BP1.

Conclusions:

• The determined AFC structure provides critical insights into the regulation of mTORC1 substrate-specific pathways.
• This structural understanding paves the way for developing selective mTORC1 antagonists targeting the MiT-TFE pathway.
• FLCN's unique mechanism of action offers a potential therapeutic target for diseases involving dysregulated cell growth and autophagy.