Harada Y, Li H, Wall JS, Li H, Lennarz WJ

In Saccharomyces cerevisiae, some of the nascent chains can be post-translationally translocated into the endoplasmic reticulum through the heptameric post-translational translocon complex (post-translocon). This membrane-protein complex is composed of the protein-conducting channel and the tetrameric Sec62/63 complex. The Sec62/63 complex plays crucial roles in targeting of the signal recognition particle-independent ...

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protein substrate to the protein-conducting channel, and in assembly of the post-translocon. While the molecular mechanism of the post-translational translocation process has been well established, the structure of the post-translocon and how the channel and the Sec62/63 complex form the heptameric complex is largely uncharacterized. Here, we report a 20-A resolution cryo-electron microscopy structure of the post-translocon. The purified post-translocon was found to have a mass of 287 kDa, which is consistent with the unit stoichiometry of the seven subunits as determined by a cysteine-labeling experiment. We demonstrated that Triton X-100 dissociated the heptameric complex into three subcomplexes identified as the trimeric translocon Sec61-Sbh1-Sss1, the Sec63-Sec71-Sec72 trimer, and the heterotetramer Sec62-Sec63-Sec71-Sec72, respectively. Additionally, a role of the sixth cytosolic loop of Sec61 in assembly of the post-translocon was demonstrated. Mutations of conserved, positively charged amino acid residues in the loop caused decreased formation of the post-translocon. These studies provide the first architectural description of the yeast post-translocon.

Date: Sep. 08, 2010

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