The structure of ribosome-channel complexes engaged in protein translocation.
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.
Menetret
J F
JF
Department of Physiology and Biophysics Boston University School of Medicine, Boston, MA 02118, USA.
Neuhof
A
A
Morgan
D G
DG
Plath
K
K
Radermacher
M
M
Rapoport
T A
TA
Akey
C W
CW
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
D005656:Fungal Proteins; D046911:Macromolecular Substances; D008565:Membrane Proteins; D012333:RNA, Messenger; D000069816:SEC Translocation Channels