Surfing the Sec61 channel: bidirectional protein translocation across the ER membrane

1999 ◽  
Vol 112 (23) ◽  
pp. 4185-4191 ◽  
Author(s):  
K. Romisch
Keyword(s):  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Signal Peptide ◽  
Protein Translocation ◽  
Transmembrane Proteins ◽  
Retrograde Transport ◽  
Conducting Channel ◽  
Peptide Cleavage ◽  
Sec61 Channel ◽  
Er Membrane

Misfolded secretory and transmembrane proteins are retained in the endoplasmic reticulum (ER) and subsequently degraded. Degradation is primarily mediated by cytosolic proteasomes and thus requires retrograde transport out of the ER back to the cytosol. The available evidence suggests that the protein-conducting channel formed by the Sec61 complex is responsible for both forward and retrograde transport of proteins across the ER membrane. For transmembrane proteins, retrograde transport can be viewed as a reversal of integration of membrane proteins into the ER membrane. Retrograde transport of soluble proteins through the Sec61 channel after signal-peptide cleavage, however, must be mechanistically distinct from signal-peptide-mediated import into the ER through the same channel.

scholarly journals Stability and flexibility of marginally hydrophobic–segment stalling at the endoplasmic reticulum translocon

2016 ◽  
Vol 27 (6) ◽  
pp. 930-940 ◽  
Author(s):  
Yuichiro Kida ◽  
Yudai Ishihara ◽  
Hidenobu Fujita ◽  
Yukiko Onishi ◽  
Masao Sakaguchi
Keyword(s):  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Lipid Phase ◽  
Endoplasmic Reticulum Membrane ◽  
Dependent Manner ◽  
Transmembrane Segment ◽  
Conducting Channel ◽  
Transmembrane Segments ◽  
Lipid Environment ◽  
Sec61 Channel

Many membrane proteins are integrated into the endoplasmic reticulum membrane through the protein-conducting channel, the translocon. Transmembrane segments with insufficient hydrophobicity for membrane integration are frequently found in multispanning membrane proteins, and such marginally hydrophobic (mH) segments should be accommodated, at least transiently, at the membrane. Here we investigated how mH-segments stall at the membrane and their stability. Our findings show that mH-segments can be retained at the membrane without moving into the lipid phase and that such segments flank Sec61α, the core channel of the translocon, in the translational intermediate state. The mH-segments are gradually transferred from the Sec61 channel to the lipid environment in a hydrophobicity-dependent manner, and this lateral movement may be affected by the ribosome. In addition, stalling mH-segments allow for insertion of the following transmembrane segment, forming an Ncytosol/Clumen orientation, suggesting that mH-segments can move laterally to accommodate the next transmembrane segment. These findings suggest that mH-segments may be accommodated at the ER membrane with lateral fluctuation between the Sec61 channel and the lipid phase.

scholarly journals Structure of the posttranslational Sec protein-translocation channel complex from yeast

Science ◽  
2018 ◽  
Vol 363 (6422) ◽  
pp. 84-87 ◽  
Author(s):  
Samuel Itskanov ◽  
Eunyong Park
Keyword(s):  
Electron Microscopy ◽  
Saccharomyces Cerevisiae ◽  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Binding Sites ◽  
Protein Translocation ◽  
Secretory Proteins ◽  
Conducting Channel ◽  
Cryo Electron Microscopy ◽  
Lateral Gate

The Sec61 protein-conducting channel mediates transport of many proteins, such as secretory proteins, across the endoplasmic reticulum (ER) membrane during or after translation. Posttranslational transport is enabled by two additional membrane proteins associated with the channel, Sec63 and Sec62, but its mechanism is poorly understood. We determined a structure of the Sec complex (Sec61-Sec63-Sec71-Sec72) from Saccharomyces cerevisiae by cryo–electron microscopy (cryo-EM). The structure 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. Furthermore, Sec63 optimally positions binding sites for cytosolic and luminal chaperones in the complex to enable efficient polypeptide translocation. Our study provides mechanistic insights into eukaryotic posttranslational protein translocation.

scholarly journals Emerging View on the Molecular Functions of Sec62 and Sec63 in Protein Translocation

2021 ◽  
Vol 22 (23) ◽  
pp. 12757
Author(s):  
Sung-jun Jung ◽  
Hyun Kim
Keyword(s):  
Endoplasmic Reticulum ◽  
Protein Targeting ◽  
Protein Translocation ◽  
Signal Recognition Particle ◽  
Signal Recognition ◽  
Conducting Channel ◽  
Evolutionarily Conserved ◽  
Translocation Pathway ◽  
Molecular Functions ◽  
Sec61 Channel

Most secreted and membrane proteins are targeted to and translocated across the endoplasmic reticulum (ER) membrane through the Sec61 protein-conducting channel. Evolutionarily conserved Sec62 and Sec63 associate with the Sec61 channel, forming the Sec complex and mediating translocation of a subset of proteins. For the last three decades, it has been thought that ER protein targeting and translocation occur via two distinct pathways: signal recognition particle (SRP)-dependent co-translational or SRP-independent, Sec62/Sec63 dependent post-translational translocation pathway. However, recent studies have suggested that ER protein targeting and translocation through the Sec translocon are more intricate than previously thought. This review summarizes the current understanding of the molecular functions of Sec62/Sec63 in ER protein translocation.

TMCC3 localizes at the three-way junctions for the proper tubular network of the endoplasmic reticulum

2019 ◽  
Vol 476 (21) ◽  
pp. 3241-3260
Author(s):  
Sindhu Wisesa ◽  
Yasunori Yamamoto ◽  
Toshiaki Sakisaka
Keyword(s):  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Membrane Protein ◽  
Mammalian Cells ◽  
Coiled Coil ◽  
Transmembrane Domains ◽  
Domain Family ◽  
Coiled Coil Domain ◽  
U2os Cells ◽  
Er Membrane

The tubular network of the endoplasmic reticulum (ER) is formed by connecting ER tubules through three-way junctions. Two classes of the conserved ER membrane proteins, atlastins and lunapark, have been shown to reside at the three-way junctions so far and be involved in the generation and stabilization of the three-way junctions. In this study, we report TMCC3 (transmembrane and coiled-coil domain family 3), a member of the TEX28 family, as another ER membrane protein that resides at the three-way junctions in mammalian cells. When the TEX28 family members were transfected into U2OS cells, TMCC3 specifically localized at the three-way junctions in the peripheral ER. TMCC3 bound to atlastins through the C-terminal transmembrane domains. A TMCC3 mutant lacking the N-terminal coiled-coil domain abolished localization to the three-way junctions, suggesting that TMCC3 localized independently of binding to atlastins. TMCC3 knockdown caused a decrease in the number of three-way junctions and expansion of ER sheets, leading to a reduction of the tubular ER network in U2OS cells. The TMCC3 knockdown phenotype was partially rescued by the overexpression of atlastin-2, suggesting that TMCC3 knockdown would decrease the activity of atlastins. These results indicate that TMCC3 localizes at the three-way junctions for the proper tubular ER network.

Spatial localization of unknown proteins in the endoplasmic reticulum predicts functions

2007 ◽  
Vol 30 (4) ◽  
pp. 84
Author(s):  
Michael D. Jain ◽  
Hisao Nagaya ◽  
Annalyn Gilchrist ◽  
Miroslaw Cygler ◽  
John J.M. Bergeron
Keyword(s):  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Protein Synthesis ◽  
Protein Function ◽  
Protein Translocation ◽  
Spatial Localization ◽  
Predict Protein Function ◽  
Insight Into ◽  
Soluble Fractions ◽  
Er Membrane

Protein synthesis, folding and degradation functions are spatially segregated in the endoplasmic reticulum (ER) with respect to the membrane and the ribosome (rough and smooth ER). Interrogation of a proteomics resource characterizing rough and smooth ER membranes subfractionated into cytosolic, membrane, and soluble fractions gives a spatial map of known proteins involved in ER function. The spatial localization of 224 identified unknown proteins in the ER is predicted to give insight into their function. Here we provide evidence that the proteomics resource accurately predicts the function of new proteins involved in protein synthesis (nudilin), protein translocation across the ER membrane (nicalin), co-translational protein folding (stexin), and distal protein folding in the lumen of the ER (erlin-1, TMX2). Proteomics provides the spatial localization of proteins and can be used to accurately predict protein function.

scholarly journals Septin-dependent compartmentalization of the endoplasmic reticulum during yeast polarized growth

2005 ◽  
Vol 169 (6) ◽  
pp. 897-908 ◽  
Author(s):  
Cosima Luedeke ◽  
Stéphanie Buvelot Frei ◽  
Ivo Sbalzarini ◽  
Heinz Schwarz ◽  
Anne Spang ◽  
...  
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Membrane Proteins ◽  
Diffusion Barriers ◽  
Yeast Cells ◽  
Dependent Manner ◽  
Protein Diffusion ◽  
Ultrastructural Studies ◽  
Bud Neck ◽  
Er Membrane

Polarized cells frequently use diffusion barriers to separate plasma membrane domains. It is unknown whether diffusion barriers also compartmentalize intracellular organelles. We used photobleaching techniques to characterize protein diffusion in the yeast endoplasmic reticulum (ER). Although a soluble protein diffused rapidly throughout the ER lumen, diffusion of ER membrane proteins was restricted at the bud neck. Ultrastructural studies and fluorescence microscopy revealed the presence of a ring of smooth ER at the bud neck. This ER domain and the restriction of diffusion for ER membrane proteins through the bud neck depended on septin function. The membrane-associated protein Bud6 localized to the bud neck in a septin-dependent manner and was required to restrict the diffusion of ER membrane proteins. Our results indicate that Bud6 acts downstream of septins to assemble a fence in the ER membrane at the bud neck. Thus, in polarized yeast cells, diffusion barriers compartmentalize the ER and the plasma membrane along parallel lines.

scholarly journals Two Endoplasmic Reticulum (ER) Membrane Proteins That Facilitate ER-to-Golgi Transport of Glycosylphosphatidylinositol-anchored Proteins

1999 ◽  
Vol 10 (4) ◽  
pp. 1043-1059 ◽  
Author(s):  
Wolfgang P. Barz ◽  
Peter Walter
Keyword(s):  
Endoplasmic Reticulum ◽  
Cell Surface ◽  
Secretory Pathway ◽  
Protein Transport ◽  
Protein Translocation ◽  
Sequence Similarity ◽  
Surface Proteins ◽  
Golgi Transport ◽  
Er Membrane

Many eukaryotic cell surface proteins are anchored in the lipid bilayer through glycosylphosphatidylinositol (GPI). GPI anchors are covalently attached in the endoplasmic reticulum (ER). The modified proteins are then transported through the secretory pathway to the cell surface. We have identified two genes inSaccharomyces cerevisiae, LAG1 and a novel gene termed DGT1 (for “delayed GPI-anchored protein transport”), encoding structurally related proteins with multiple membrane-spanning domains. Both proteins are localized to the ER, as demonstrated by immunofluorescence microscopy. Deletion of either gene caused no detectable phenotype, whereas lag1Δ dgt1Δ cells displayed growth defects and a significant delay in ER-to-Golgi transport of GPI-anchored proteins, suggesting thatLAG1 and DGT1 encode functionally redundant or overlapping proteins. The rate of GPI anchor attachment was not affected, nor was the transport rate of several non–GPI-anchored proteins. Consistent with a role of Lag1p and Dgt1p in GPI-anchored protein transport, lag1Δ dgt1Δ cells deposit abnormal, multilayered cell walls. Both proteins have significant sequence similarity to TRAM, a mammalian membrane protein thought to be involved in protein translocation across the ER membrane. In vivo translocation studies, however, did not detect any defects in protein translocation in lag1Δ dgt1Δcells, suggesting that neither yeast gene plays a role in this process. Instead, we propose that Lag1p and Dgt1p facilitate efficient ER-to-Golgi transport of GPI-anchored proteins.

scholarly journals HRDGene Dependence of Endoplasmic Reticulum-associated Degradation

2000 ◽  
Vol 11 (5) ◽  
pp. 1697-1708 ◽  
Author(s):  
Sharon Wilhovsky ◽  
Richard Gardner ◽  
Randolph Hampton
Keyword(s):  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Protein Degradation ◽  
Degradation Pathway ◽  
Encoded Proteins ◽  
Absolute Dependence ◽  
Coa Reductase ◽  
Ubiquitin Conjugating Enzymes ◽  
Er Membrane

Work from several laboratories has indicated that many different proteins are subject to endoplasmic reticulum (ER) degradation by a common ER-associated machinery. This machinery includes ER membrane proteins Hrd1p/Der3p and Hrd3p and the ER-associated ubiquitin-conjugating enzymes Ubc7p and Ubc6p. The wide variety of substrates for this degradation pathway has led to the reasonable hypothesis that the HRD (Hmg CoA reductase degradation) gene-encoded proteins are generally involved in ER protein degradation in eukaryotes. We have tested this model by directly comparing the HRD dependency of the ER-associated degradation for various ER membrane proteins. Our data indicated that the role of HRD genes in protein degradation, even in this highly defined subset of proteins, can vary from absolute dependence to complete independence. Thus, ER-associated degradation can occur by mechanisms that do not involve Hrd1p or Hrd3p, despite their apparently broad envelope of substrates. These data favor models in which the HRD gene-encoded proteins function as specificity factors, such as ubiquitin ligases, rather than as factors involved in common aspects of ER degradation.

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