![]() After this initial engagement of the nascent chain with the ER translocon, current models suggest that the hydrophobic signal sequence exits the Sec61 complex via a lateral gate. This delivery step relies on interactions with the SRP receptor and, after the release of the signal sequence from SRP, the nascent chain is extruded into the ER lumen through the Sec61 translocon coupling translocation across the ER membrane to ongoing translation at the ribosome. This signal sequence is recognized by targeting factors in the cytosol, in many cases via the actions of the signal recognition particle (SRP), which targets the precursor to the ER membrane in the form of a ribosome/nascent chain complex. The precursors for these proteins are characterized by a sequence of hydrophobic amino acids at their N-terminus that identifies them for targeting to the ER. In eukaryotes, secretory proteins synthesized in the cytosol must first translocate across the membrane of the endoplasmic reticulum (ER) to enter the secretory pathway. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Ĭompeting interests: The authors have declared that no competing interests exist. Johnson) and a grant from the Deutsche Forschungemeinschaft (IRTG1830) (S. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.įunding: This work was supported by a PhD studentship from the Biotechnology and Biological Sciences Research Council (N. ![]() Received: JAccepted: AugPublished: October 9, 2013Ĭopyright: © 2013 Johnson et al. (2013) The Signal Sequence Influences Post-Translational ER Translocation at Distinct Stages. Taken together, our data suggest that an ER signal sequence can regulate specific aspects of Sec61 mediated membrane translocation at a stage following Sec62/Sec63 dependent ER delivery.Ĭitation: Johnson N, Haßdenteufel S, Theis M, Paton AW, Paton JC, Zimmermann R, et al. Although this requirement for Sec62 and Sec63 is unaffected by the specific signal sequence that delivers a precursor to the ER, this region can influence subsequent events, including both Sec61 mediated transport and the importance of BiP for membrane translocation. Our data support a model where secretory protein precursors that fail to engage the signal recognition particle, for example because they are short, are delivered to the ER membrane via a distinct route that is dependent upon both Sec62 and Sec63. However, the signal sequence can influence the subsequent membrane translocation process, conferring sensitivity to a small molecule inhibitor and dictating reliance on the molecular chaperone BiP. This role for Sec62 and Sec63 is independent of the signal sequence that delivers the precursor to the ER. These studies suggest precursor chain length is a key factor in the post-translational translocation at the mammalian ER, and identify Sec62 and Sec63 as important components acting on this route. Having first determined the capacity of precursors to enter ER derived microsomes post-translationally, we then exploited semi-permeabilized mammalian cells specifically depleted of key membrane components using siRNA to address their contribution to the membrane translocation process. We have used two model substrates to explore the elements of a secretory protein precursor that preferentially direct it towards a co- or post-translational pathway for ER translocation. The metazoan Sec61 translocon transports polypeptides into and across the membrane of the endoplasmic reticulum via two major routes, a well-established co-translational pathway and a post-translational alternative.
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