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.