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Heterotrimeric G proteins are guanine nucleotide-binding proteins. As the name suggests, heterotrimeric G proteins are composed of three subunits: alpha, beta, and gamma. They remain GDP-bound or GTP-bound inside the cells and switch between inactive/active states. The Gα subunit possesses the nucleotide-binding pocket that binds guanine nucleotides and switches between GDP or GTP-bound states. In contrast, the Gꞵ and Gγ subunits are always bound together with high...
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Comparing the Affinity of GTPase-binding Proteins using Competition Assays
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The small GTPase MRAS is a broken switch.

Gabriela Bernal Astrain1,2, Regina Strakhova1,2, Chang Hwa Jo1

  • 1Institute for Research in Immunology and Cancer (IRIC), Université de Montréal, Montréal, QC, H3T 1J4, Canada.

Nature Communications
|January 14, 2025
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Summary
This summary is machine-generated.

MRAS GTPase does not function as a molecular switch, remaining GDP-loaded even with activating mutations. This challenges the traditional model for RAS GTPases and suggests reinterpretation of prior MRAS research.

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Area of Science:

  • Molecular Biology
  • Cellular Signaling
  • Biochemistry

Background:

  • RAS superfamily proteins are critical signaling molecules.
  • Small GTPases are generally understood to function as molecular switches.
  • MRAS GTPase is a close homolog of NRAS and KRAS, regulating MAPK activity.

Purpose of the Study:

  • To investigate the nucleotide-binding and switching mechanism of MRAS GTPase.
  • To determine if MRAS functions as a classical GDP/GTP molecular switch.
  • To re-evaluate the function of MRAS in cellular signaling pathways.

Main Methods:

  • In vitro nucleotide exchange assays in solution and on lipid bilayers.
  • Analysis of MRAS interaction with GEF SOS1.
  • NMR spectroscopy of isotopically labeled MRAS in live cells.
  • Characterization of synthetic activating mutations.

Main Results:

  • MRAS is unable to exchange GDP for GTP in solution or on lipid bilayers.
  • The guanine nucleotide exchange factor SOS1 does not rescue this defect.
  • Synthetic mutations thought to activate MRAS do not increase nucleotide exchange but promote effector binding from a GDP-loaded state.
  • Live-cell NMR confirmed MRAS remains GDP-loaded, even in supposed activated mutants.

Conclusions:

  • MRAS does not operate as a classical GDP/GTP molecular switch.
  • The observed functions of MRAS may be independent of GTP loading.
  • Current understanding and experimental approaches to MRAS may require reinterpretation.