Structure of the post-translational protein translocation machinery of the ER membrane

Abstract

Many proteins must translocate through the protein-conducting Sec61 channel in the eukaryotic endoplasmic reticulum membrane or the SecY channel in the prokaryotic plasma membrane 1 , 2 . Proteins with hydrophobic signal sequences are first recognized by the signal recognition particle (SRP) 3 , 4 and then moved co-translationally through the Sec61/SecY channel by the associated translating ribosome. Substrates with less hydrophobic signal sequences bypass SRP and are moved through the channel post-translationally 5 , 6 . In eukaryotic cells, post-translational translocation is mediated by the association of the Sec61 channel with another membrane protein complex, the Sec62/Sec63 complex 7 – 9 , and substrates are moved through the channel by the luminal BiP ATPase 9 . How the Sec62/63 complex activates the Sec61 channel for post-translational translocation is unclear. Here, we report the electron cryo-microscopy (cryo-EM) structure of the Sec complex from S. cerevisiae , consisting of the Sec61 channel and the Sec62, Sec63, Sec71, and Sec72 proteins. Sec63 causes wide opening of the lateral gate of the Sec61 channel, priming it for the passage of low-hydrophobicity signal sequences into the lipid phase, without displacing the channel’s plug domain. Lateral channel opening is triggered by Sec63 interacting with both cytosolic loops in the C-terminal half of Sec61 and trans-membrane (TM) segments in the N-terminal half of the Sec61 channel. The cytosolic Brl domain of Sec63 blocks ribosome binding to the channel and recruits Sec71 and Sec72, positioning them for the capture of polypeptides associated with cytosolic Hsp70 (ref. 10 ). Our structure shows how the Sec61 channel is activated for post-translational protein translocation.

ER import of small human presecretory proteins: components and mechanisms

“…Channel opening occurs in two stages, a priming step, which involves the ribosome as allosteric channel‐effector in cotranslational transport , and possibly, Sec62 with or without Sec63 in post‐translational transport (Figs and ). In yeast, however, there is the so‐called SEC‐complex, a permanent assembly of a heterotrimeric Sec61 complex plus the heterotetrameric Sec62/Sec63/Sec71/Sec72‐complex that is dedicated to post‐translational protein import . In contrast, the mammalian complex of Sec61, Sec62, and Sec63 appears to be assembled on demand rather than permanently, which was observed for cotranslational ER import of the precursors of the prion protein and the ER‐lumenal protein ERj3 .…”

“…So far, there are no structural data on the mammalian Sec62/Sec63-complex. However, the recent structural analysis of the yeast heptameric SEC-complex elucidated extensive interactions between Sec63 and the Sec61 complex including contacts in their cytosolic, membrane and lumenal domains [70,71], which is perfectly in line with the above-discussed intrinsic Sec63 activity in ER import of prestatherin [64]. Notably, the yeast SEC-complex includes in addition to the heterotrimeric Sec61 complex and the heterodimeric Sec62/Sec63 complex the heterodimeric Sec71/Sec72-complex and is supposedly involved only in post-translational protein import into the ER.…”

A Molecular Mechanism for Turning off IRE1α Signaling During Endoplasmic Reticulum Stress

“…Our studies show that IRE1α tightly associates with Sec61/Sec63 through luminal its juxtamembrane and transmebrane regions. Recent structural studies suggest that Sec63 binding to the translocon sterically hinders the ribosome binding to the translocon (Wu et al, 2019; Itskanov and Park, 2019). Future studies are warranted to determine whether Sec63 is dissociated from the translocon when the ribosome-nascent chain complex is recruited to the Sec61/IRE1α complex.…”

Prion Protein Folding Mechanism Revealed by Pulling Force Studies

“…Sec62/Sec63 has been implicated in the cotranslational ER import of a subset of proteins with less and/or lacking hydrophobic domains . Sec63 has been shown to be in the vicinity of ribosome-translocon complexes, acting indirectly through Sec62 (Müller et al 2010;; Wu, Cabanos, and Rapoport 2019;; Jan, Williams, and Weissman 2014), but its role in cotranslational translocation remains to be shown.…”