Oligomeric Rings of the Sec61p Complex Induced by Ligands Required for Protein Translocation

TLDR

The Sec61p heterotrimeric complex is the principal ER protein‑conducting channel, associating with ribosomes or the Sec62/63 complex for co‑ and post‑translational translocation. The authors propose that ligand‑induced cylindrical oligomers of Sec61p form the ER protein‑conducting channels for co‑ and post‑translational transport. Electron microscopy shows purified Sec61p complexes assemble into 85‑Å diameter, 20‑Å pore cylindrical oligomers of 3–4 heterotrimers, with similar ring structures in proteoliposomes and native membranes, and oligomerization is stimulated by ribosome or Sec62/63 association.

Abstract

The heterotrimeric Sec61p complex is a major component of the protein-conducting channel of the endoplasmic reticulum (ER) membrane, associating with either ribosomes or the Sec62/63 complex to perform co- and posttranslational transport, respectively. We show by electron microscopy that purified mammalian and yeast Sec61p complexes in detergent form cylindrical oligomers with a diameter of ∼85 Å and a central pore of ∼20 Å. Each oligomer contains 3–4 heterotrimers. Similar ring structures are seen in reconstituted proteoliposomes and native membranes. Oligomer formation by the reconstituted Sec61p complex is stimulated by its association with ribosomes or the Sec62/63p complex. We propose that these cylindrical oligomers represent protein-conducting channels of the ER, formed by ligands specific for co- and posttranslational transport.