OLIGOMERIC RINGS OF THE SEC61P COMPLEX-INDUCED BY LIGANDS REQUIRED FOR PROTEIN TRANSLOCATION

The heterotrimeric Sec61p complex is a major component of the protein- conducting channel of the endoplasmic reticulum (ER) membrane, associa ting with either ribosomes or the Sec62/63 complex to perform co- and posttranslational transport, respectively. We show by electron microsc opy that purified mammalian and yeast Sec61p complexes in detergent fo rm cylindrical oligomers with a diameter of similar to 85 Angstrom and a central pore of similar to 20 Angstrom. Each oligomer contains 3-4 heterotrimers. Similar ring structures are seen in reconstituted prote oliposomes and native membranes. Oligomer formation by the reconstitut ed Sec61p complex is stimulated by its association with ribosomes or t he Sec62/63p complex. We propose that these cylindrical oligomers repr esent protein-conducting channels of the ER, formed by ligands specifi c for co- and posttranslational transport.