Oligomeric Rings of the Sec61p Complex Induced by Ligands Required for Protein Translocation

Dorit, Hanein; Matlack, Kent E. S.; Jungnickel, Berit; Plath, Kathrin; Kalies, Kai‐Uwe; Miller, Kenneth R.; Rapoport, Tom A.; Akey, Christopher W.

doi:10.1016/s0092-8674(00)81391-4

Cited by 323 publications

(271 citation statements)

References 36 publications

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“…A possible interpretation is that this Sec61 population is in a lower oligomeric state, likely corresponding to monomers or dimers. Freeze-fracture experiments with proteoliposomes support the idea that the formation of Sec61-oligomers may be induced by ribosome binding (Hanein et al, 1996). Different quaternary states of the bacterial homologue SecYEG in the membrane have also been described, and their relative abundance may change during translocation (Bessonneau et al, 2002;Scheuring et al, 2005).…”

Section: Discussionmentioning

confidence: 78%

“…Although these connections can each break and reform, as suggested by the ATA experiments, collectively they keep the ribosome firmly bound to the membrane, explaining the extremely slow dissociation rate in the absence of ATA. Interestingly, the Sec61-oligomers in RMs are not disassembled upon removal of the ribosomes by treatment with puromycin/high salt (Hanein et al, 1996), which may explain why PK-RMs retain highaffinity sites for ribosomes. The ribosome-bound, tetrameric Sec61 population seems to be interconvertible with a free Sec61 population that binds ribosomes and RNCs lacking SRP only poorly.…”

Section: Discussionmentioning

confidence: 99%

“…© 2006 by The American Society for Cell Biology addition of ribosomes to proteoliposomes containing purified Sec61 complex (Hanein et al, 1996). Whether oligomers are required for translocation is unclear, because the x-ray structure of the archaebacterial Sec61 homologue SecYE␤ and cross-linking experiments suggest that the polypeptide chain moves through the center of a Sec61-SecY molecule (Van den Berg et al, 2004;Cannon et al, 2005).…”

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confidence: 99%

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Ribosome Binding to and Dissociation from Translocation Sites of the Endoplasmic Reticulum Membrane

Schaletzky

Rapoport

2006

MBoC

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We have addressed how ribosome-nascent chain complexes (RNCs), associated with the signal recognition particle (SRP), can be targeted to Sec61 translocation channels of the endoplasmic reticulum (ER) membrane when all binding sites are occupied by nontranslating ribosomes. These competing ribosomes are known to be bound with high affinity to tetramers of the Sec61 complex. We found that the membrane binding of RNC-SRP complexes does not require or cause the dissociation of prebound nontranslating ribosomes, a process that is extremely slow. SRP and its receptor target RNCs to a free population of Sec61 complex, which associates with nontranslating ribosomes only weakly and is conformationally different from the population of ribosome-bound Sec61 complex. Taking into account recent structural data, we propose a model in which SRP and its receptor target RNCs to a Sec61 subpopulation of monomeric or dimeric state. This could explain how RNC-SRP complexes can overcome the competition by nontranslating ribosomes. INTRODUCTIONCotranslational translocation is the major pathway in mammals by which proteins are transported across or integrated into the endoplasmic reticulum (ER) membrane. The synthesis of these proteins starts with a free ribosome in the cytosol. Once a hydrophobic signal sequence or transmembrane (TM) segment has emerged from the ribosome, the signal recognition particle (SRP) binds, forming a ribosomenascent chain (RNC)-SRP complex (for reviews, see Walter and Johnson, 1994;Egea et al., 2005). This complex binds to the ER membrane by interactions between SRP and the SRP receptor (SR) and between the ribosome and the Sec61 complex, a heterotrimeric membrane protein that forms a proteinconducting channel (Van den Berg et al., 2004). The nascent polypeptide chain forms a loop when inserting into the channel, with the signal or TM sequence intercalated into the walls of the channel and the following mature region in the pore proper (for reviews, see Rapoport et al., 2004;Osborne et al., 2005). For secretory proteins, this part of the elongating chain is moved through the channel into the ER lumen, and the signal sequence is eventually cleaved off. In membrane proteins, TM sequences are partitioned sideways through the walls of the channel into the lipid phase, leaving some parts of the polypeptide chain in the cytosol and others in the ER lumen. For both classes of proteins, the translating ribosome stays bound to the channel during the entire translocation process (Mothes et al., 1997).Previous work has shown that the Sec61 complex can bind not only RNCs but also nontranslating ribosomes with nanomolar affinity (Borgese et al., 1974;Kalies et al., 1994;Prinz et al., 2000a). Ribosomes remain bound to ER membranes after nascent chain release (Adelman et al., 1973) and do not easily dissociate from the membrane (Borgese et al., 1973;Potter and Nicchitta, 2002). Given that the concentration of free ribosomes in a secretory cell is ϳ0.3 M, 95% of the Sec61 binding sites should be occupied (Blobel and Potter, ...

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Section: Discussionmentioning

confidence: 78%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Ribosome Binding to and Dissociation from Translocation Sites of the Endoplasmic Reticulum Membrane

Schaletzky

Rapoport

2006

MBoC

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“…The Sec61 channel is formed by four or five Sec61 complexes that assemble in the ER membrane for protein import into the ER in response to the presence of a functional signal peptide or in the presence of the Sec63 complex, which is required for posttranslational protein import into the yeast ER (Hanein et al, 1996). The Sec61 channel is sealed at both ends to maintain the permeability barrier across the ER membrane (Hamman et al, 1998).…”

Section: Introductionmentioning

confidence: 99%

O-Mannosylation Protects Mutant Alpha-Factor Precursor from Endoplasmic Reticulum-associated Degradation

Harty

Strahl

Römisch

2001

MBoC

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Secretory proteins that fail to fold in the endoplasmic reticulum (ER) are transported back to the cytosol and degraded by proteasomes. It remains unclear how the cell distinguishes between folding intermediates and misfolded proteins. We asked whether misfolded secretory proteins are covalently modified in the ER before export. We found that a fraction of mutant alpha-factor precursor, but not the wild type, was progressively O-mannosylated in microsomes and in intact yeast cells by protein O-mannosyl transferase 2 (Pmt2p). O-Mannosylation increased significantly in vitro under ER export conditions, i.e., in the presence of ATP and cytosol, and this required export-proficient Sec61p in the ER membrane. Deletion of PMT2, however, did not abrogate mutant alpha-factor precursor degradation but, rather, enhanced its turnover in intact yeast cells. In vitro, O-mannosylated mutant alpha-factor precursor was stable and protease protected, and a fraction was associated with Sec61p in the ER lumen. Thus, prolonged ER residence allows modification of exposed O-mannosyl acceptor sites in misfolded proteins, which abrogates misfolded protein export from the ER at a posttargeting stage. We conclude that there is a limited window of time during which misfolded proteins can be removed from the ER before they acquire inappropriate modifications that can interfere with disposal through the Sec61 channel.

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“…In both of these protein export machines, microscopic, biochemical and biophysical studies have shown the translocon complexes to be arranged around a central cavity estimated to range between 9 and 60 Å , depending on the translocation status of the channel. (8)(9)(10)(11) To maintain the permeability barrier provided by the membrane, passage through pores of such large diameter must be controlled. The mode of regulation is dependent on whether translocation is coupled to protein translation or rather occurs following the translation event.…”

Section: Introductionmentioning

confidence: 99%

Move it on over: getting proteins across biological membranes

Eichler¹,

Irihimovitch²

2003

BioEssays

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SummaryThe translocation of proteins across membranes is a central problem in biology. Regardless of the system in question, delivering proteins across a given membrane relies on many of the same basic themes. At the same time, however, each membrane translocation system, be it signal-gated or signal-assembled, makes use of components unique to that system. The latest findings on protein translocation across a variety of biological membranes have been presented in a recent review article. (1)

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Oligomeric Rings of the Sec61p Complex Induced by Ligands Required for Protein Translocation

Cited by 323 publications

References 36 publications

Ribosome Binding to and Dissociation from Translocation Sites of the Endoplasmic Reticulum Membrane

Ribosome Binding to and Dissociation from Translocation Sites of the Endoplasmic Reticulum Membrane

O-Mannosylation Protects Mutant Alpha-Factor Precursor from Endoplasmic Reticulum-associated Degradation

Move it on over: getting proteins across biological membranes

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