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A single Sec61-complex functions as a protein-conducting channel.

Kai-Uwe Kalies1, Vivica Stokes, Enno Hartmann

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Mammalian endoplasmic reticulum (ER) protein translocation channels are formed by single Sec61-complexes, not large rings. This single complex facilitates nascent polypeptide movement during cotranslational translocation.

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Protein Trafficking

Background:

  • During cotranslational translocation, ribosomes bind to Sec61-complexes to facilitate protein entry into the endoplasmic reticulum (ER).
  • Two models exist for Sec61-complexes forming protein-conducting channels: a four-complex ring (electron microscopy) or a single complex (bacterial crystal structure).

Purpose of the Study:

  • To investigate the structural basis of mammalian protein-conducting channels in the ER.
  • To determine whether a single Sec61-complex or a larger assembly forms the active translocation channel.

Main Methods:

  • Protease protection assays were employed to examine the structure of mammalian protein-conducting channels.
  • Quantitative immunoblotting was used in conjunction with protease protection assays.

Main Results:

  • A single Sec61alpha molecule is protected by a ribosome-bound membrane in native ER membranes, irrespective of nascent polypeptides.
  • Nascent polypeptides destabilize a potential ring-like translocation apparatus of four Sec61-complexes.
  • A single Sec61-complex, even after ER membrane solubilization, protects nascent polypeptides from proteases and allows their movement upon puromycin release.

Conclusions:

  • The active protein-conducting channel in the ER is formed by a single Sec61-complex.
  • This finding resolves the discrepancy between existing models regarding the oligomeric state of the Sec61 translocation channel.