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Structure of the ER membrane complex, a transmembrane-domain insertase.

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The endoplasmic reticulum membrane complex (EMC) structure reveals how it inserts proteins into the ER membrane. This finding offers insights into integral membrane protein biogenesis and evolutionary conservation.

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Area of Science:

  • Cell Biology
  • Structural Biology
  • Protein Biochemistry

Background:

  • The Endoplasmic Reticulum Membrane Complex (EMC) is crucial for inserting transmembrane helices (TMHs) of integral membrane proteins into the ER membrane.
  • The precise mechanism by which EMC facilitates this insertion, particularly in cooperation with the Sec61 translocon, has remained largely undefined.

Purpose of the Study:

  • To determine the high-resolution structure of the eukaryotic EMC.
  • To elucidate the molecular mechanisms underlying EMC-mediated TMH insertion into the ER membrane.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) was used to determine the structure of the Saccharomyces cerevisiae EMC.
  • Mutational analyses were performed to assess the functional importance of specific EMC features.

Main Results:

  • The cryo-EM structure revealed the eight-subunit composition of the yeast EMC, detailing its lumenal, cytosolic, and transmembrane regions.
  • A five-transmembrane helix (TMH) fold, conserved with prokaryotic YidC insertases, was identified, enclosing a hydrophilic client protein pocket.
  • The mobility of the Emc4 subunit and the hydrophilicity of the client pocket were found to be essential for EMC function.

Conclusions:

  • The determined EMC structure provides a molecular framework for understanding eukaryotic TMH insertion.
  • The findings suggest a conserved mechanism for TMH insertion between prokaryotes and eukaryotes.
  • This work advances our comprehension of the biogenesis of integral membrane and tail-anchored proteins.