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3.A.5.8.1
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:P36057
Protein Name:SRP Beta Chain aka SRP102 aka YKL154W aka YKL609
Length:244
Molecular Weight:26974.00
Species:Saccharomyces cerevisiae (Baker's yeast) [4932]
Number of TMSs:1
Location1 / Topology2 / Orientation3: Endoplasmic reticulum membrane1
Substrate protein

Cross database links:

DIP: DIP-4753N
RefSeq: NP_012768.1   
Entrez Gene ID: 853702   
Pfam: PF09439   
KEGG: sce:YKL154W   

Gene Ontology

GO:0030176 C:integral to endoplasmic reticulum membrane
GO:0030529 C:ribonucleoprotein complex
GO:0005785 C:signal recognition particle receptor complex
GO:0005525 F:GTP binding
GO:0005515 F:protein binding
GO:0004872 F:receptor activity
GO:0005047 F:signal recognition particle binding
GO:0006614 P:SRP-dependent cotranslational protein targe...

References (4)

[1] “DNA sequencing of a 36.2 kb fragment located between the FAS1 and LAP loci of chromosome XI of Saccharomyces cerevisiae.”  Vandenbol M.et.al.   8091859
[2] “Complete DNA sequence of yeast chromosome XI.”  Dujon B.et.al.   8196765
[3] “Global analysis of protein expression in yeast.”  Ghaemmaghami S.et.al.   14562106
[4] “Structural basis for the function of the beta subunit of the eukaryotic signal recognition particle receptor.”  Schwartz T.et.al.   12654246
Structure:
1NRJ   2GED     

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Predict TMSs (Predict number of transmembrane segments)
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FASTA formatted sequence
1:	MLSNTLIIAC LLVIGTTIAL IAVQKASSKT GIKQKSYQPS IIIAGPQNSG KTSLLTLLTT 
61:	DSVRPTVVSQ EPLSAADYDG SGVTLVDFPG HVKLRYKLSD YLKTRAKFVK GLIFMVDSTV 
121:	DPKKLTTTAE FLVDILSITE SSCENGIDIL IACNKSELFT ARPPSKIKDA LESEIQKVIE 
181:	RRKKSLNEVE RKINEEDYAE NTLDVLQSTD GFKFANLEAS VVAFEGSINK RKISQWREWI 
241:	DEKL