title: The structure of ribosome-channel complexes engaged in protein translocation.
abstract: Cotranslational translocation of proteins requires ribosome binding to the Sec61p channel at the endoplasmic reticulum (ER) membrane. We have used electron cryomicroscopy to determine the structures of ribosome-channel complexes in the absence or presence of translocating polypeptide chains. Surprisingly, the structures are similar and contain 3-4 connections between the ribosome and channel that leave a lateral opening into the cytosol. Therefore, the ribosome-channel junction may allow the direct transfer of polypeptides into the channel and provide a path for the egress of some nascent chains into the cytosol. Moreover, complexes solubilized from mammalian ER membranes contain an additional membrane protein that has a large, lumenal protrusion and is intercalated into the wall of the Sec61p channel. Thus, the native channel contains a component that is not essential for translocation.
authors: Department of Physiology and Biophysics Boston University School of Medicine, Boston, MA 02118, USA.
authors: J F
authors: JF
authors: Menetret
authors: A
authors: A
authors: Neuhof
authors: D G
authors: DG
authors: Morgan
authors: K
authors: K
authors: Plath
authors: M
authors: M
authors: Radermacher
authors: T A
authors: TA
authors: Rapoport
authors: C W
authors: CW
authors: Akey
mesh_terms: D000818:Animals; D020285:Cryoelectron Microscopy; D003593:Cytoplasm / Q000378:metabolism; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism; D007425:Intracellular Membranes / Q000737:chemistry / Q000378:metabolism; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008954:Models, Biological; D008958:Models, Molecular; D010441:Peptide Chain Elongation, Translational; D011487:Protein Conformation; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011817:Rabbits; D012270:Ribosomes / Q000737:chemistry* / Q000378:metabolism*; D000069816:SEC Translocation Channels; D015003:Yeasts / Q000737:chemistry / Q000166:cytology / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D046911:Macromolecular Substances; D008565:Membrane Proteins; D012333:RNA, Messenger; D000069816:SEC Translocation Channels