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The general secretory pathway (Sec-SRP) complex. The Yet1 and Yet3 proteins interact directly with the Sec translocon (Wilson & Barlowe et al., 2010). The Sss1/Sec61γ protein (80aas) has two domains. The cytosolic domain is required for Sec61p interaction while the transmembrane clamp domain is required to complete activation of the translocon after precursor targeting to Sec61p (Wilkinson et al., 2010). However, the apolar surfrace area determines the efficiency of translocon-mediated membrane-protein integration into the endoplasmic reticulum (Öjemalm et al., 2011). The essential Sec62, Sec63 and non-essential Sec66 and Sec72 proteins may comprise an SRP-independent tetrameric translocon enlisting the lumenal chaperone, BiP/Kar2 to ""ratchet"" its substrates into the ER (Feldheim and Schekman 1994; Ast et al. 2013). Cytosolic segments of the Sec61 complex important for promoting the structural transition between the closed and open conformations of the complex have been identified (Mandon et al. 2018). Positively charged residues in multiple cytosolic segments, as well as bulky hydrophobic residues in the L6/7-TMS7 junction may be required for cotranslational translocation or integration of membrane proteins by the Sec61 complex (Mandon et al. 2018). The structure of the yeast post-translational Sec complex (Sec61-Sec63-Sec71-Sec72) by cryo-EM shows that Sec63 tightly associates with Sec61 through interactions in cytosolic, transmembrane, and ER-luminal domains, prying open Sec61's lateral gate and translocation pore, and thus activating the channel for substrate engagement.  Sec63 optimally positions binding sites for cytosolic and luminal chaperones in the complex to enable efficient polypeptide translocation (Itskanov and Park 2019). Further, post-translational translocation is mediated by the association of the Sec61 channel with the membrane protein complex, the Sec62-Sec63 complex, and substrates move through the channel by the luminal BiP ATPase. Wu et al. 2019 determined the cryoEM structure of the S. cerevisiae Sec complex, 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 both with cytosolic loops in the C-terminal half of Sec61 and transmembrane 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. The structure thus shows how the Sec61 channel is activated for post-translational protein translocation (Wu et al. 2019).

Accession Number:P35179
Protein Name:Sec61g aka SSS1 aka YDR086C aka D4475
Molecular Weight:8944.00
Species:Saccharomyces cerevisiae (Baker's yeast) [4932]
Number of TMSs:1
Location1 / Topology2 / Orientation3: Endoplasmic reticulum membrane1 / Single-pass membrane protein2
Substrate protein

Cross database links:

DIP: DIP-2432N
RefSeq: NP_010371.1   
Entrez Gene ID: 851659   
Pfam: PF00584   
KEGG: sce:YDR086C   

Gene Ontology

GO:0016021 C:integral to membrane
GO:0005784 C:Sec61 translocon complex
GO:0071261 C:Ssh1 translocon complex
GO:0015450 F:P-P-bond-hydrolysis-driven protein transmem...
GO:0005198 F:structural molecule activity
GO:0031204 P:posttranslational protein targeting to memb...
GO:0006616 P:SRP-dependent cotranslational protein targe...

References (9)

[1] “The yeast SSS1 gene is essential for secretory protein translocation and encodes a conserved protein of the endoplasmic reticulum.”  Esnault   8223425
[2] “The nucleotide sequence of Saccharomyces cerevisiae chromosome IV.”  Jacq   9169867
[3] “Approaching a complete repository of sequence-verified protein-encoding clones for Saccharomyces cerevisiae.”  Hu   17322287
[4] “Oligomeric rings of the Sec61p complex induced by ligands required for protein translocation.”  Hanein   8929540
[5] “The aqueous pore through the translocon has a diameter of 40-60 A during cotranslational protein translocation at the ER membrane.”  Hamman   9160745
[6] “The translocon: a dynamic gateway at the ER membrane.”  Johnson   10611978
[7] “Evolutionarily conserved binding of ribosomes to the translocation channel via the large ribosomal RNA.”  Prinz   10775273
[8] “Global analysis of protein localization in budding yeast.”  Huh   14562095
[9] “Subunits of the translocon interact with components of the oligosaccharyl transferase complex.”  Chavan   15831493
2WW9   2WWA   6N3Q   6ND1     

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