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Spatio-Temporal Manipulation of Small GTPase Activity at Subcellular Level and on Timescale of Seconds in Living Cells
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Structural mechanism of a Rag GTPase activation checkpoint by the lysosomal folliculin complex

Rosalie E Lawrence1,2, Simon A Fromm1, Yangxue Fu1

  • 1Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA 94720, USA.

Science (New York, N.Y.)
|November 2, 2019
PubMed

Insights

The tumor suppressor folliculin (FLCN) regulates mTORC1 signaling. Its lysosomal function, revealed by cryo-EM, acts as a checkpoint controlling lysosomal biogenesis.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • The tumor suppressor folliculin (FLCN) is crucial for nutrient-dependent activation of mTORC1 signaling.
  • FLCN exhibits guanosine triphosphatase (GTPase) activating protein (GAP) activity towards RagC, a key regulator of mTORC1.
  • During starvation, FLCN relocates from the cytosol to lysosomes, suggesting a role in lysosomal function.

Purpose of the Study:

  • To elucidate the lysosomal function of FLCN by determining the structure of the human lysosomal FLCN complex (LFC).
  • To understand how FLCN's GAP activity is regulated within the lysosomal environment.
  • To investigate the role of the LFC in controlling mTORC1 signaling and lysosomal biogenesis.

Main Methods:

  • Reconstitution of the human lysosomal FLCN complex (LFC) including FLCN, FNIP2, RagAGDP:RagCGTP, and the Ragulator complex.
  • Determination of the LFC's cryo-electron microscopy (cryo-EM) structure at 3.6 angstrom resolution.
  • Analysis of FLCN's RagC-GAP activity within the reconstituted LFC.

Main Results:

  • The cryo-EM structure revealed that FLCN's RagC-GAP activity is inhibited within the LFC due to steric hindrance of a catalytic arginine residue.
  • Disassembly of the LFC releases FLCN's GAP activity.
  • This regulated GAP activity influences mTORC1-dependent regulation of transcription factor E3, a master regulator of lysosomal biogenesis.

Conclusions:

  • The lysosomal FLCN complex (LFC) acts as a critical checkpoint in mTORC1 signaling.
  • FLCN's inhibitory state within the LFC prevents premature mTORC1 activation during nutrient deprivation.
  • The LFC's regulated disassembly allows for proper control of lysosomal biogenesis in response to cellular cues.

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