Architecture of the protein-conducting channel associated with the translating 80S ribosome

In vitro assembled yeast ribosome-nascent chain complexes (RNCs) containing a signal sequence in the nascent chain were immunopurified and reconstitut ed with the purified protein-conducting channel (PCC) of yeast endoplasmic reticulum, the Sec61 complex. A cryo-EM reconstruction of the RNC-Sec61 com plex at 15.4 Angstrom resolution shows a tRNA in the P site. Distinct rRNA elements and proteins of the large ribosomal subunit form four connections with the PCC across a gap of about 10-20 Angstrom. Binding of the PCC influ ences the position of the highly dynamic rRNA expansion segment 27. The RNC -bound Sec61 complex has a compact appearance and was estimated to be a tri mer. We propose a binary model of cotranslational translocation entailing o nly two basic functional states of the translating ribosome-channel complex .