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A MUB E2 structure reveals E1 selectivity between cognate ubiquitin E2s in eukaryotes.

Xiaolong Lu1, Konstantin R Malley2, Caitlin C Brenner1

  • 1Department of Biology, Saint Louis University, 3507 Laclede Avenue, St Louis, MO 63103, USA.

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Summary
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Prenylated membrane-anchored ubiquitin-fold proteins (MUBs) regulate ubiquitylation by inhibiting E2 enzyme activation. This discovery reveals MUBs as key regulators of ubiquitin chain synthesis at the plasma membrane.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Cell Biology

Background:

  • Ubiquitin (Ub) modification regulates crucial cellular processes, including protein degradation and endocytosis.
  • The early-acting regulators of the ubiquitylation cascade remain largely unknown.
  • Ubiquitylation involves a three-enzyme cascade: E1 (activating enzyme), E2 (conjugating enzyme), and E3 (ligase).

Purpose of the Study:

  • To identify early-acting regulators of the ubiquitylation cascade.
  • To elucidate the mechanism by which MUBs regulate E2 enzyme activation.
  • To define the role of MUBs in ubiquitin chain synthesis.

Main Methods:

  • Co-crystallization of Arabidopsis MUB3 (AtMUB3) and an Arabidopsis E2 enzyme (AtUBC8).
  • Structural analysis of the AtMUB3:AtUBC8 complex.
  • Biochemical assays to assess E2 activation and ubiquitylation.
  • Cellular localization studies.

Main Results:

  • The prenylated membrane-anchored ubiquitin-fold protein (MUB) acts as an early regulator of subfamily-specific E2 activation.
  • A co-crystal structure revealed that MUBs inhibit E2∼Ub formation by binding to the E2 backside and using a unique loop to block E1 access.
  • MUBs tether Arabidopsis group VI E2 enzymes to the plasma membrane and inhibit E2 activation at physiological concentrations.

Conclusions:

  • MUBs are potent, plasma membrane-localized regulators of ubiquitin chain synthesis in eukaryotes.
  • This study defines a biochemical function for the conserved MUB protein family.
  • The findings demonstrate selective activation of Ub E2 enzymes, challenging the notion of constitutive E1-mediated activation.