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Science  09 Nov 2001:
Vol. 294, Issue 5545, pp. 1245a
DOI: 10.1126/science.294.5545.1245a

Atomic-resolution structures of prokaryotic ribosomes have added to our understanding of protein biosynthesis, but less structural information is available for eukaryotic ribosomes. Some eukaryotic proteins are inserted into or translocated across the endoplasmic reticulum (ER) membrane during translation. To accomplish this, the ribosome binds to a protein-conducting channel (PCC), which contains the heterotrimeric protein Sec61.

Spahn et al. present a cryoelectron microscopy reconstruction of the yeast 80S ribosome, which in comparison to the bacterial structure is homologous in core regions with expansion segments at surface sites. Beckmann et al. describe a translating yeast ribosome with the PCC bound at the outlet of a tunnel that conducts the nascent polypeptide. The diameter of the PCC is consistent with a complex of three Sec61 trimers, and Sec61 is connected to the ribosome by four contacts with ribosomal RNA and proteins. Surprisingly, the PCC shows an indentation instead of a central pore. Interaction with the signal sequence may not be sufficient to induce an open channel conformation, and the channel itself may act as a seal to prevent ion leakage across the ER membrane.—VV

Cell 107, 361; 373 (2001).

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