101
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Zarivach R, Bashan A, Berisio R, Harms J, Auerbach T, Schluenzen F, Bartels H, Baram D, Pyetan E, Sittner A, Amit M, Hansen HAS, Kessler M, Liebe C, Wolff A, Agmon I, Yonath A. Functional aspects of ribosomal architecture: symmetry, chirality and regulation. J PHYS ORG CHEM 2004. [DOI: 10.1002/poc.831] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Raz Zarivach
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Anat Bashan
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Rita Berisio
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Joerg Harms
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Tamar Auerbach
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Frank Schluenzen
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Heike Bartels
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - David Baram
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Erez Pyetan
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Assa Sittner
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Maya Amit
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Harly A. S. Hansen
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Maggie Kessler
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Christa Liebe
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Anja Wolff
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
| | - Ilana Agmon
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
| | - Ada Yonath
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
- Max‐Planck‐Research Unit for Ribosomal Structure, 22603 Hamburg, Germany
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102
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Braunagel SC, Williamson ST, Saksena S, Zhong Z, Russell WK, Russell DH, Summers MD. Trafficking of ODV-E66 is mediated via a sorting motif and other viral proteins: facilitated trafficking to the inner nuclear membrane. Proc Natl Acad Sci U S A 2004; 101:8372-7. [PMID: 15150405 PMCID: PMC420401 DOI: 10.1073/pnas.0402727101] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The N-terminal 33 aa of the envelope protein ODV-E66 are sufficient to traffic fusion proteins to intranuclear membranes and the ODV envelope during infection with Autographa californica nucleopolyhedrovirus. This sequence has two distinct features: (i) an extremely hydrophobic sequence of 18 aa and (ii) positively charged amino acids close to the C-terminal end of the hydrophobic sequence. In the absence of infection, this sequence is sufficient to promote protein accumulation at the inner nuclear membrane. Covalent cross-linking results show that the lysines of the motif are proximal to FP25K and/or BV/ODV-E26 during transit from the endoplasmic reticulum to the nuclear envelope. We propose that the 33 aa comprise a signature for sorting proteins to the inner nuclear membrane (sorting motif) and that, unlike other resident proteins of the inner nuclear membrane, ODV-E66 and sortingmotif fusions do not randomly diffuse from their site of insertion at the endoplasmic reticulum to the nuclear envelope and viral-induced intranuclear membranes. Rather, during infection, trafficking is mediated by protein-protein interactions.
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Affiliation(s)
- Sharon C Braunagel
- Department of Biology,Texas Agricultural Experiment Station, Texas A and M University, College Station, TX 77843-2475, USA
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103
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Buskiewicz I, Deuerling E, Gu SQ, Jöckel J, Rodnina MV, Bukau B, Wintermeyer W. Trigger factor binds to ribosome-signal-recognition particle (SRP) complexes and is excluded by binding of the SRP receptor. Proc Natl Acad Sci U S A 2004; 101:7902-6. [PMID: 15148364 PMCID: PMC419529 DOI: 10.1073/pnas.0402231101] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Trigger factor (TF) and signal recognition particle (SRP) bind to the bacterial ribosome and are both crosslinked to protein L23 at the peptide exit, where they interact with emerging nascent peptide chains. It is unclear whether TF and SRP exclude one another from their ribosomal binding site(s). Here we show that SRP and TF can bind simultaneously to ribosomes or ribosome nascent-chain complexes exposing a SRP-specific signal sequence. Based on changes of the crosslinking pattern and on results obtained by fluorescence measurements using fluorescence-labeled SRP, TF binding induces structural changes in the ribosome-SRP complex. Furthermore, we show that binding of the SRP receptor, FtsY, to ribosome-bound SRP excludes TF from the ribosome. These results suggest that TF and SRP sample nascent chains on the ribosome in a nonexclusive fashion. The decision for ribosome nascent-chain complexes exposing a signal sequence to enter SRP-dependent membrane targeting seems to be determined by the binding of SRP, which is stabilized by signal sequence recognition, and promoted by the exclusion of TF due to the binding of the SRP receptor to ribosome-bound SRP.
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Affiliation(s)
- Iwona Buskiewicz
- Institute für Molekularbiologie und Physikalische Biochemie, Universität Witten/Herdecke, 58448 Witten, Germany
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104
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Abstract
Recent data highlight how eukaryotic ribosomes connect polypeptide synthesis to translational regulation and targeting. Information contained in nascent polypeptides can be transmitted by surprisingly diverse routes.
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Affiliation(s)
- Sabine Rospert
- Institut fur Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
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105
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Alder NN, Johnson AE. Cotranslational Membrane Protein Biogenesis at the Endoplasmic Reticulum. J Biol Chem 2004; 279:22787-90. [PMID: 15028726 DOI: 10.1074/jbc.r400002200] [Citation(s) in RCA: 107] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Affiliation(s)
- Nathan N Alder
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, Texas 77843-1114, USA
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106
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Woolhead CA, McCormick PJ, Johnson AE. Nascent Membrane and Secretory Proteins Differ in FRET-Detected Folding Far inside the Ribosome and in Their Exposure to Ribosomal Proteins. Cell 2004; 116:725-36. [PMID: 15006354 DOI: 10.1016/s0092-8674(04)00169-2] [Citation(s) in RCA: 293] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2003] [Revised: 01/13/2004] [Accepted: 01/20/2004] [Indexed: 11/30/2022]
Abstract
Fluorescence resonance energy transfer measurements reveal that a transmembrane sequence within a nascent membrane protein folds into a compact conformation near the peptidyltransferase center and remains folded as the sequence moves through a membrane bound ribosome into the translocon. This compact conformation is compatible with an alpha helix because nearly the same energy transfer efficiency was observed when the transmembrane sequence was integrated into the lipid bilayer. Since the transmembrane sequence unfolds upon emerging from a free ribosome, this nascent chain folding is ribosome induced and stabilized. In contrast, a nascent secretory protein is in an extended conformation in the exit tunnel. Furthermore, two ribosomal proteins photo-crosslink to nascent membrane but not secretory proteins. These interactions coincide with the previously described sequential closing and opening of the two ends of the aqueous translocon pore, thereby suggesting that ribosomal recognition of nascent chain folding controls the operational mode of the translocon at the ER membrane.
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Affiliation(s)
- Cheryl A Woolhead
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, TX 77843, USA
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107
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Raghuraman H, Pradhan SK, Chattopadhyay A. Effect of Urea on the Organization and Dynamics of Triton X-100 Micelles: A Fluorescence Approach. J Phys Chem B 2004. [DOI: 10.1021/jp0365007] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- H. Raghuraman
- Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India
| | - Suman K. Pradhan
- Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad 500 007, India
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108
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Abstract
Our studies of SecM (secretion monitor) in E. coli have revealed that some amino acid sequences can interact with ribosomal interior components, particularly with gate components of the exit tunnel, thereby interfering with their own translation elongation. Such translation arrest can be regulated by interaction of the N-terminal portion of the nascent polypeptide with other cellular components outside the ribosome. These properties of nascent proteins can in turn provide regulatory mechanisms by which the expression of genetic information at different levels is regulated.
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109
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Abstract
An ion channel protein begins life as a nascent peptide inside a ribosome, moves to the endoplasmic reticulum where it becomes integrated into the lipid bilayer, and ultimately forms a functional unit that conducts ions in a well-regulated fashion. Here, I discuss the nascent peptide and its tasks as it wends its way through ribosomal tunnels and exit ports, through translocons, and into the bilayer. We are just beginning to explore the sequence of these events, mechanisms of ion channel structure formation, when biogenic decisions are made, and by which participants. These decisions include when to exit the endoplasmic reticulum and with whom to associate. Such issues govern the expression of ion channels at the cell surface and thus the electrical activity of a cell.
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Affiliation(s)
- Carol Deutsch
- Department of Physiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
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110
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Heske J, Heller U, Winklhofer KF, Tatzelt J. The C-terminal globular domain of the prion protein is necessary and sufficient for import into the endoplasmic reticulum. J Biol Chem 2003; 279:5435-43. [PMID: 14645231 DOI: 10.1074/jbc.m309570200] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The mammalian prion protein (PrP) is composed of an unstructured flexible N-terminal region and a C-terminal globular domain. We examined the import of PrP into the endoplasmic reticulum (ER) of neuronal cells and show that information present in the C-terminal globular domain is required for ER import of the N terminus. N-terminal fragments of PrP, devoid of structural domains located in the C terminus, remained in the cytosol with an uncleaved signal peptide and were rapidly degraded by the proteasome. Conversely, the separate C-terminal domain of PrP, comprising the highly ordered helix 2-loop-helix 3 motif, was entirely imported into the ER. As a consequence, two PrP mutants linked to inherited prion disease in humans, PrP-W145Stop and PrP-Q160Stop, were partially retained in the cytosol. The cytosolic fraction was characterized by an uncleaved N-terminal signal peptide and was degraded by the proteasome. Our study identified a new regulatory element in the C-terminal globular domain of PrP necessary and sufficient to promote import of PrP into the ER.
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Affiliation(s)
- Johanna Heske
- Department of Cellular Biochemistry, Max-Planck-Institut für Biochemie, Am Klopferspitz 18, D-82152 Martinsried, Germany
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111
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Stockton JD, Merkert MC, Kellaris KV. A Complex of Chaperones and Disulfide Isomerases Occludes the Cytosolic Face of the Translocation Protein Sec61p and Affects Translocation of the Prion Protein†. Biochemistry 2003; 42:12821-34. [PMID: 14596596 DOI: 10.1021/bi035087q] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Secretion of newly synthesized proteins across the mammalian rough endoplasmic reticulum (translocation) is supported by the membrane proteins Sec61p and TRAM, but may also include accessory factors, depending on the particular translocation substrate. Studies designed to investigate the binding of anti-peptide antibodies to the carboxyl terminus of the alpha-subunit of Sec61 (Sec61palpha) lead us to the isolation of a complex of proteins that occlude the cytosolic face of Sec61palpha in microsomes that have been prepared by standard protocols used to study translocation in vitro [Walter, P., and Blobel, G. (1983) Methods Enzymol. 96, 84-93]. This complex was shown by nanospray tandem mass spectrometry to be composed of protein disulfide isomerase (PDI), calcium binding protein 1 (CABP1/P5), 72 kDa endoplasmic reticulum protein (ERp72), and BiP (heat shock protein A5/HSPA5), and has been named TR-PDI for "translocon-resident protein disulfide isomerase complex". This constitutes a novel location for these proteins, which are known to be major constituents of the lumen of the rough endoplasmic reticulum. We have not established the function of TR-PDI at this location, but did observe that the absence of this complex results in a relative loss of correct topology of prion protein insertion across RER membranes, indicating the possibility of a functional role in vivo.
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Affiliation(s)
- Jordan D Stockton
- Department of Chemistry, Georgetown University, Washington, DC 20007, USA
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112
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Zito CR, Oliver D. Two-stage binding of SecA to the bacterial translocon regulates ribosome-translocon interaction. J Biol Chem 2003; 278:40640-6. [PMID: 12907673 DOI: 10.1074/jbc.m308025200] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The bacterial translocon interacts with both SecA-bound preproteins and nascent chain-ribosome complexes during Sec and signal recognition particle-dependent protein translocation, respectively. In their inactive state, translocons are saturated with ribosomes and SecA protein, reflecting the inherent affinity of these components for one another. We found that SecA and ribosomes are bound simultaneously and noncompetitively to a common set of inactive translocons. Furthermore, we demonstrate that at a later stage in binding, SecA possesses a ribosome-translocon dissociation activity that is coupled to its ATP-dependent membrane insertion and retraction cycle that drives protein translocation. This novel activity is presumably important in the commitment of the translocon to the Sec-dependent pathway. These results also provide a rationale for the compatibility and regulation of multiple protein translocation pathways that each makes distinct demands on a common translocon core.
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Affiliation(s)
- Christopher R Zito
- Department of Molecular Biology and Biochemistry, Wesleyan University, Middletown, Connecticut 06459, USA
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113
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Dalley JA, Bulleid NJ. The endoplasmic reticulum (ER) translocon can differentiate between hydrophobic sequences allowing signals for glycosylphosphatidylinositol anchor addition to be fully translocated into the ER lumen. J Biol Chem 2003; 278:51749-57. [PMID: 14530277 DOI: 10.1074/jbc.m303978200] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The signal sequence within polypeptide chains that designates whether a protein is to be anchored to the membrane by a glycosylphosphatidylinositol (GPI) anchor is characterized by a carboxyl-terminal hydrophobic domain preceded by a short hydrophilic spacer linked to the GPI anchor attachment (omega) site. The hydrophobic domain within the GPI anchor signal sequence is very similar to a transmembrane domain within a stop transfer sequence. To investigate whether the GPI anchor signal sequence is translocated across or integrated into the endoplasmic reticulum membrane we studied the translocation, GPI anchor addition, and glycosylation of different variants of a model GPI-anchored protein. Our results unequivocally demonstrated that the hydrophobic domain within a GPI signal cannot act as a transmembrane domain and is fully translocated even when followed by an authentic charged cytosolic tail sequence. However, a single amino acid change within the hydrophobic domain of the GPI-signal converts it into a transmembrane domain that is fully integrated into the endoplasmic reticulum membrane. These results demonstrated that the translocation machinery can recognize and differentiate subtle changes in hydrophobic sequence allowing either full translocation or membrane integration.
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Affiliation(s)
- Jane A Dalley
- School of Biological Sciences, 2.205 Stopford Building, University of Manchester, Manchester M13 9PT, United Kingdom
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114
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Abstract
We have systematically analyzed the molecular environment of the signal sequence of a growing secretory protein from Escherichia coli using a stage- and site-specific cross-linking approach. Immediately after emerging from the ribosome, the signal sequence of pOmpA is accessible to Ffh, the protein component of the bacterial signal recognition particle, and to SecA, but it remains attached to the surface of the ribosome via protein L23. These contacts are lost upon further growth of the nascent chain, which brings the signal sequence into sole proximity to the chaperone Trigger factor (TF). In its absence, nascent pOmpA shows extended contacts with L23, and even long chains interact in these conditions proficiently with Ffh. Our results suggest that upon emergence from the ribosome, the signal sequence of an E. coli secretory protein gradually becomes sequestered by TF. Although TF thereby might control the accessibility of pOmpA's signal sequence to Ffh and SecA, it does not influence interaction of pOmpA with SecB.
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Affiliation(s)
- Gottfried Eisner
- Institut für Biochemie und Molekularbiologie, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany
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115
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Affiliation(s)
- Gunnar Von Heijne
- Department of Biochemistry and Biophysics, Stockholm University, SE-10691 Stockholm, Sweden
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116
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Affiliation(s)
- Ada Yonath
- Dept. of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel.
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117
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Bashan A, Zarivach R, Schluenzen F, Agmon I, Harms J, Auerbach T, Baram D, Berisio R, Bartels H, Hansen HAS, Fucini P, Wilson D, Peretz M, Kessler M, Yonath A. Ribosomal crystallography: peptide bond formation and its inhibition. Biopolymers 2003; 70:19-41. [PMID: 12925991 DOI: 10.1002/bip.10412] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Ribosomes, the universal cellular organelles catalyzing the translation of genetic code into proteins, are protein/RNA assemblies, of a molecular weight 2.5 mega Daltons or higher. They are built of two subunits that associate for performing protein biosynthesis. The large subunit creates the peptide bond and provides the path for emerging proteins. The small has key roles in initiating the process and controlling its fidelity. Crystallographic studies on complexes of the small and the large eubacterial ribosomal subunits with substrate analogs, antibiotics, and inhibitors confirmed that the ribosomal RNA governs most of its activities, and indicated that the main catalytic contribution of the ribosome is the precise positioning and alignment of its substrates, the tRNA molecules. A symmetry-related region of a significant size, containing about two hundred nucleotides, was revealed in all known structures of the large ribosomal subunit, despite the asymmetric nature of the ribosome. The symmetry rotation axis, identified in the middle of the peptide-bond formation site, coincides with the bond connecting the tRNA double-helical features with its single-stranded 3' end, which is the moiety carrying the amino acids. This thus implies sovereign movements of tRNA features and suggests that tRNA translocation involves a rotatory motion within the ribosomal active site. This motion is guided and anchored by ribosomal nucleotides belonging to the active site walls, and results in geometry suitable for peptide-bond formation with no significant rearrangements. The sole geometrical requirement for this proposed mechanism is that the initial P-site tRNA adopts the flipped orientation. The rotatory motion is the major component of unified machinery for peptide-bond formation, translocation, and nascent protein progression, since its spiral nature ensures the entrance of the nascent peptide into the ribosomal exit tunnel. This tunnel, assumed to be a passive path for the growing chains, was found to be involved dynamically in gating and discrimination.
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Affiliation(s)
- Anat Bashan
- Department of Structural Biology, The Weizmann Institute, 76100 Rehovot, Israel
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118
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McCormick PJ, Miao Y, Shao Y, Lin J, Johnson AE. Cotranslational Protein Integration into the ER Membrane Is Mediated by the Binding of Nascent Chains to Translocon Proteins. Mol Cell 2003; 12:329-41. [PMID: 14536073 DOI: 10.1016/s1097-2765(03)00304-6] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
During cotranslational protein integration into the ER membrane, each transmembrane (TM) segment moves laterally through the translocon to reach the lipid bilayer. Photocrosslinking studies reveal that a particular surface of each nascent chain TM alpha helix and signal-anchor sequence always faces Sec61alpha in the translocon. This nonrandom and TM sequence-dependent positioning reveals that each TM segment makes specific contacts with Sec61alpha and is retained at a fixed location within the translocon, observations that are best explained by the binding of each TM sequence to a translocon protein(s). Since TM sequence hydrophobicity does not correlate with its rate of release from the translocon, nascent chain movement through the translocon appears to be mediated primarily by protein-protein interactions rather than hydrophobic nascent chain-phospholipid interactions.
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Affiliation(s)
- Peter J McCormick
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843, USA
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119
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Wirth A, Jung M, Bies C, Frien M, Tyedmers J, Zimmermann R, Wagner R. The Sec61p complex is a dynamic precursor activated channel. Mol Cell 2003; 12:261-8. [PMID: 12887911 DOI: 10.1016/s1097-2765(03)00283-1] [Citation(s) in RCA: 89] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Previous studies have shown that the rough endoplasmic reticulum (ER) contains nascent precursor polypeptide gated channels. Circumstantial evidence suggests that these channels are formed by the Sec61p complex. We reconstituted the purified Sec61p complex in a lipid bilayer and characterized its dynamics and regulation. The Sec61p complex is sufficient to form the precursor polypeptide activated channel under co- and posttranslational transport conditions. Activity of the Sec61p channel in both transport modes is induced by direct interaction with precursor protein. The Sec61p complex comprises a highly dynamic pore covering conductances corresponding to channel openings from approximately 6 to 60 A. Its properties are indistinguishable from those we observed with native ER channels, directly demonstrating that these channels are formed by the Sec61p complex.
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Affiliation(s)
- Andreas Wirth
- Biophysik, Universität Osnabrück, FB Biologie/Chemie, D-49034 Osnabrück, Germany
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120
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Beckmann R, Spahn CM, Frank J, Blobel G. The active 80S ribosome-Sec61 complex. COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY 2003; 66:543-54. [PMID: 12762056 DOI: 10.1101/sqb.2001.66.543] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- R Beckmann
- Laboratory of Cell Biology, Howard Hughes Medical Institute, Rockefeller University, New York, New York 10021, USA
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121
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Johnson AE, Chen JC, Flanagan JJ, Miao Y, Shao Y, Lin J, Bock PE. Structure, function, and regulation of free and membrane-bound ribosomes: the view from their substrates and products. COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY 2003; 66:531-41. [PMID: 12762055 DOI: 10.1101/sqb.2001.66.531] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Affiliation(s)
- A E Johnson
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, Departments of Chemistry and of Biochemistry and Biophysics, Texas A&M University, College Station, Texas, USA
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122
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Agmon I, Auerbach T, Baram D, Bartels H, Bashan A, Berisio R, Fucini P, Hansen HAS, Harms J, Kessler M, Peretz M, Schluenzen F, Yonath A, Zarivach R. On peptide bond formation, translocation, nascent protein progression and the regulatory properties of ribosomes. Derived on 20 October 2002 at the 28th FEBS Meeting in Istanbul. EUROPEAN JOURNAL OF BIOCHEMISTRY 2003; 270:2543-56. [PMID: 12787020 DOI: 10.1046/j.1432-1033.2003.03634.x] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
High-resolution crystal structures of large ribosomal subunits from Deinococcus radiodurans complexed with tRNA-mimics indicate that precise substrate positioning, mandatory for efficient protein biosynthesis with no further conformational rearrangements, is governed by remote interactions of the tRNA helical features. Based on the peptidyl transferase center (PTC) architecture, on the placement of tRNA mimics, and on the existence of a two-fold related region consisting of about 180 nucleotides of the 23S RNA, we proposed a unified mechanism integrating peptide bond formation, A-to-P site translocation, and the entrance of the nascent protein into its exit tunnel. This mechanism implies sovereign, albeit correlated, motions of the tRNA termini and includes a spiral rotation of the A-site tRNA-3' end around a local two-fold rotation axis, identified within the PTC. PTC features, ensuring the precise orientation required for the A-site nucleophilic attack on the P-site carbonyl-carbon, guide these motions. Solvent mediated hydrogen transfer appears to facilitate peptide bond formation in conjunction with the spiral rotation. The detection of similar two-fold symmetry-related regions in all known structures of the large ribosomal subunit, indicate the universality of this mechanism, and emphasizes the significance of the ribosomal template for the precise alignment of the substrates as well as for accurate and efficient translocation. The symmetry-related region may also be involved in regulatory tasks, such as signal transmission between the ribosomal features facilitating the entrance and the release of the tRNA molecules. The protein exit tunnel is an additional feature that has a role in cellular regulation. We showed by crystallographic methods that this tunnel is capable of undergoing conformational oscillations and correlated the tunnel mobility with sequence discrimination, gating and intracellular regulation.
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Affiliation(s)
- Ilana Agmon
- Department of Structural Biology, The Weizmann Institute, Rehovot, Israel
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123
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Nilsson I, Kelleher DJ, Miao Y, Shao Y, Kreibich G, Gilmore R, von Heijne G, Johnson AE. Photocross-linking of nascent chains to the STT3 subunit of the oligosaccharyltransferase complex. J Cell Biol 2003; 161:715-25. [PMID: 12756234 PMCID: PMC2199356 DOI: 10.1083/jcb.200301043] [Citation(s) in RCA: 116] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
In eukaryotic cells, polypeptides are N glycosylated after passing through the membrane of the ER into the ER lumen. This modification is effected cotranslationally by the multimeric oligosaccharyltransferase (OST) enzyme. Here, we report the first cross-linking of an OST subunit to a nascent chain that is undergoing translocation through, or integration into, the ER membrane. A photoreactive probe was incorporated into a nascent chain using a modified Lys-tRNA and was positioned in a cryptic glycosylation site (-Q-K-T- instead of -N-K-T-) in the nascent chain. When translocation intermediates with nascent chains of increasing length were irradiated, nascent chain photocross-linking to translocon components, Sec61alpha and TRAM, was replaced by efficient photocross-linking solely to a protein identified by immunoprecipitation as the STT3 subunit of the OST. No cross-linking was observed in the absence of a cryptic sequence or in the presence of a competitive peptide substrate of the OST. As no significant nascent chain photocross-linking to other OST subunits was detected in these fully assembled translocation and integration intermediates, our results strongly indicate that the nascent chain portion of the OST active site is located in STT3.
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Affiliation(s)
- IngMarie Nilsson
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, TX 77843, USA
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124
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Flanagan JJ, Chen JC, Miao Y, Shao Y, Lin J, Bock PE, Johnson AE. Signal recognition particle binds to ribosome-bound signal sequences with fluorescence-detected subnanomolar affinity that does not diminish as the nascent chain lengthens. J Biol Chem 2003; 278:18628-37. [PMID: 12621052 DOI: 10.1074/jbc.m300173200] [Citation(s) in RCA: 116] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The binding of signal recognition particle (SRP) to ribosome-bound signal sequences has been characterized directly and quantitatively using fluorescence spectroscopy. A fluorescent probe was incorporated cotranslationally into the signal sequence of a ribosome.nascent chain complex (RNC), and upon titration with SRP, a large and saturable increase in fluorescence intensity was observed. Spectral analyses of SRP and RNC association as a function of concentration allowed us to measure, at equilibrium, K(d) values of 0.05-0.38 nm for SRP.RNC complexes with different signal sequences. Competitive binding experiments with nonfluorescent RNC species revealed that the nascent chain probe did not alter SRP affinity and that SRP has significant affinity for both nontranslating ribosomes (K(d) = 71 nm) and RNCs that lack an exposed signal sequence (K(d) = 8 nm). SRP can therefore distinguish between translating and nontranslating ribosomes. The very high signal sequence-dependent SRP.RNC affinity did not decrease as the nascent chain lengthened. Thus, the inhibition of SRP-dependent targeting of RNCs to the endoplasmic reticulum membrane observed with long nascent chains does not result from reduced SRP binding to the signal sequence, as widely thought, but rather from a subsequent step, presumably nascent chain interference of SRP.RNC association with the SRP receptor and/or translocon.
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Affiliation(s)
- John J Flanagan
- Department of Medical Biochemistry and Genetics, Health Science Center, Texas A&M University, College Station, Texas 77843, USA
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125
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Tyedmers J, Lerner M, Wiedmann M, Volkmer J, Zimmermann R. Polypeptide-binding proteins mediate completion of co-translational protein translocation into the mammalian endoplasmic reticulum. EMBO Rep 2003; 4:505-10. [PMID: 12704426 PMCID: PMC1319181 DOI: 10.1038/sj.embor.embor826] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2003] [Revised: 03/10/2003] [Accepted: 03/13/2003] [Indexed: 11/08/2022] Open
Abstract
The first step in the secretion of most mammalian proteins is their transport into the lumen of the endoplasmic reticulum (ER). Transport of pre-secretory proteins into the mammalian ER requires signal peptides in the precursor proteins and a protein translocase in the ER membrane. In addition, hitherto unidentified lumenal ER proteins have been shown to be required for vectorial protein translocation. This requirement was confirmed in this study by using proteoliposomes that were made from microsomal detergent extracts and contained either low or high concentrations of lumenal ER proteins. Furthermore, immunoglobulin-heavy-chain-binding protein (BiP) was shown to be able to substitute for the full set of lumenal proteins and, in the case of biotinylated precursor proteins, avidin was found to be able to substitute for lumenal proteins. Thus, the polypeptide-chain-binding protein BiP was identified as one lumenal protein that is involved in efficient vectorial protein translocation into the mammalian ER.
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Affiliation(s)
- Jens Tyedmers
- Medizinische Biochemie, Universität des Saarlandes, D-66421 Homburg, Germany
| | - Monika Lerner
- Medizinische Biochemie, Universität des Saarlandes, D-66421 Homburg, Germany
| | - Martin Wiedmann
- Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA
| | - Jörg Volkmer
- Medizinische Biochemie, Universität des Saarlandes, D-66421 Homburg, Germany
| | - Richard Zimmermann
- Medizinische Biochemie, Universität des Saarlandes, D-66421 Homburg, Germany
- Tel: +49 6841 1626510; fax: +49 6841 1626288;
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126
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Berisio R, Schluenzen F, Harms J, Bashan A, Auerbach T, Baram D, Yonath A. Structural insight into the role of the ribosomal tunnel in cellular regulation. Nat Struct Mol Biol 2003; 10:366-70. [PMID: 12665853 DOI: 10.1038/nsb915] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2002] [Accepted: 03/06/2003] [Indexed: 11/09/2022]
Abstract
Nascent proteins emerge out of ribosomes through an exit tunnel, which was assumed to be a firmly built passive path. Recent biochemical results, however, indicate that the tunnel plays an active role in sequence-specific gating of nascent chains and in responding to cellular signals. Consistently, modulation of the tunnel shape, caused by the binding of the semi-synthetic macrolide troleandomycin to the large ribosomal subunit from Deinococcus radiodurans, was revealed crystallographically. The results provide insights into the tunnel dynamics at high resolution. Here we show that, in addition to the typical steric blockage of the ribosomal tunnel by macrolides, troleandomycin induces a conformational rearrangement in a wall constituent, protein L22, flipping the tip of its highly conserved beta-hairpin across the tunnel. On the basis of mutations that alleviate elongation arrest, the tunnel motion could be correlated with sequence discrimination and gating, suggesting that specific arrest motifs within nascent chain sequences may induce a similar gating mechanism.
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Affiliation(s)
- Rita Berisio
- Max-Planck-Research Unit for Ribosomal Structure, Hamburg, Germany
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127
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Gu SQ, Peske F, Wieden HJ, Rodnina MV, Wintermeyer W. The signal recognition particle binds to protein L23 at the peptide exit of the Escherichia coli ribosome. RNA (NEW YORK, N.Y.) 2003; 9:566-73. [PMID: 12702815 PMCID: PMC1370422 DOI: 10.1261/rna.2196403] [Citation(s) in RCA: 115] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/16/2002] [Accepted: 02/10/2003] [Indexed: 05/19/2023]
Abstract
The signal recognition particle (SRP) from Escherichia coli, composed of Ffh protein and 4.5S RNA, mediates membrane targeting of translating ribosomes displaying a signal or signal-anchor sequence. SRP binds at the peptide exit of the large ribosomal subunit. Structural details of the interaction are not known. Here, the position of Ffh or SRP on the ribosome was probed by using site-specific UV-induced crosslinking by p-azidophenacyl bromide (AzP) attached to a number of cysteine residues engineered into surface positions of Ffh. Efficient crosslinking to vacant ribosomes took place from two positions (AzP17 and AzP25) in the N domain of Ffh, both with Ffh and SRP. Both AzP17 and AzP25 were predominantly crosslinked to ribosomal protein L23 that is located at the peptide exit of the 50S subunit. The SRP receptor, FtsY, did not change the crosslink pattern, whereas the presence of a nascent signal peptide on the ribosome resulted in a second crosslink between Ffh(AzP17) and protein L23, indicating that binding to the nascent signal peptide induced a slightly different arrangement of SRP on the ribosome. These results indicate a model of the topographical arrangement of SRP at the peptide exit of the 50S ribosomal subunit.
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Affiliation(s)
- Shan-Qing Gu
- Institute of Molecular Biology, University of Witten/Herdecke, 58448 Witten, Germany
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128
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Affiliation(s)
- R J Turner
- Membrane Biology Section, Gene Therapy and Therapeutics Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, DHHS, 10 Center Drive MSC 1190, Bethesda, MD 20892, USA.
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129
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Schnell DJ, Hebert DN. Protein translocons: multifunctional mediators of protein translocation across membranes. Cell 2003; 112:491-505. [PMID: 12600313 DOI: 10.1016/s0092-8674(03)00110-7] [Citation(s) in RCA: 195] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Protein translocation systems consist of complex molecular machines whose activities are not limited to unidirectional protein targeting. Protein translocons and their associated receptor systems can be viewed as dynamic modular units whose interactions, and therefore functions, are regulated in response to specific signals. This flexibility allows translocons to interact with multiple signal receptor systems to manage the targeting of topologically distinct classes of proteins, to mediate targeting to different suborganellar compartments, and to respond to stress and developmental cues. Furthermore, the activities of translocons are tightly coordinated with downstream events, thereby providing a direct link between targeting and protein maturation.
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Affiliation(s)
- Danny J Schnell
- Program in Plant Biology, University of Massachusetts, Amherst, MA 01003, USA.
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130
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Morgan DG, Ménétret JF, Neuhof A, Rapoport TA, Akey CW. Structure of the mammalian ribosome-channel complex at 17A resolution. J Mol Biol 2002; 324:871-86. [PMID: 12460584 DOI: 10.1016/s0022-2836(02)01111-7] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The co-translational translocation of proteins into the endoplasmic reticulum (ER) lumen and the biogenesis of membrane proteins require ribosome binding to a membrane channel formed by the Sec61p complex. We now report the 17A structure of a mammalian ribosome-channel complex derived from ER membranes. Atomic models of the ribosomal subunits were aligned to the programmed ribosome from Thermus thermophilus, to provide a common reference frame. The T.thermophilus ribosome, and by extension all known high resolution subunit models, were then docked within our map of the ribosome-channel complex. The structure shows that the ribosome contains a putative tRNA in the exit site, and a comparison with a non-programmed, yeast ribosome suggests that the L1 stalk may function as a gate in the tRNA exit path. We have localized six major expansion segments in the large subunit of the vertebrate ribosome including ES27, and suggest a function for ES30. The large ribosomal subunit is linked to the channel by four connections. We identified regions in the large subunit rRNA and four proteins that may help form the connections. These regions of the ribosome probably serve as a template to guide the assembly of the asymmetric translocation channel. Three of the connections form a picket fence that separates the putative translocation pore from the attachment site of an additional membrane component. The ribosome-channel connections also create an open junction that would allow egress of a nascent chain into the cytosol. At a threshold that is appropriate for the entire complex, the channel is rather solid and the lumenal half of the putative translocation pore is closed. These data suggest that the flow of small molecules across the membrane may be impeded by the channel itself, rather than the ribosome-channel junction.
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Affiliation(s)
- David Gene Morgan
- Department of Physiology and Biophysics, Boston University School of Medicine, 700 Albany St., Boston, MA 02118-2526, USA
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131
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Meacock SL, Lecomte FJL, Crawshaw SG, High S. Different transmembrane domains associate with distinct endoplasmic reticulum components during membrane integration of a polytopic protein. Mol Biol Cell 2002; 13:4114-29. [PMID: 12475939 PMCID: PMC138620 DOI: 10.1091/mbc.e02-04-0198] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
We have been studying the insertion of the seven transmembrane domain (TM) protein opsin to gain insights into how the multiple TMs of polytopic proteins are integrated at the endoplasmic reticulum (ER). We find that the ER components associated with the first and second TMs of the nascent opsin polypeptide chain are clearly distinct. The first TM (TM1) is adjacent to the alpha and beta subunits of the Sec61 complex, and a novel component, a protein associated with the ER translocon of 10 kDa (PAT-10). The most striking characteristic of PAT-10 is that it remains adjacent to TM1 throughout the biogenesis and membrane integration of the full-length opsin polypeptide. TM2 is also found to be adjacent to Sec61alpha and Sec61beta during its membrane integration. However, TM2 does not form any adducts with PAT-10; rather, a transient association with the TRAM protein is observed. We show that the association of PAT-10 with opsin TM1 does not require the N-glycosylation of the nascent chain and occurs irrespective of the amino acid sequence and transmembrane topology of TM1. We conclude that the precise makeup of the ER membrane insertion site can be distinct for the different transmembrane domains of a polytopic protein. We find that the environment of a particular TM can be influenced by both the "stage" of nascent chain biosynthesis reached, and the TM's relative location within the polypeptide.
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Affiliation(s)
- Suzanna L Meacock
- School of Biological Sciences, University of Manchester, Manchester, M13 9PT United Kingdom
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132
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Abstract
In the October issue of Molecular Cell, Kowarik et al. examine cotranslational translocation and folding during the synthesis of secretory and integral membrane proteins. Their results suggest that these potentially competing processes are regulated in surprising ways and reveal novel insights into the mechanisms by which proteins are assembled into membranes.
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Affiliation(s)
- David W Andrews
- Department of Biochemistry, McMaster University, 1200 Main Street West, Hamilton, Ontario L8N 3Z5, Canada
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133
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Carveth K, Buck T, Anthony V, Skach WR. Cooperativity and flexibility of cystic fibrosis transmembrane conductance regulator transmembrane segments participate in membrane localization of a charged residue. J Biol Chem 2002; 277:39507-14. [PMID: 12186867 DOI: 10.1074/jbc.m205759200] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Polytopic protein topology is established in the endoplasmic reticulum (ER) by sequence determinants encoded throughout the nascent polypeptide. Here we characterize 12 topogenic determinants in the cystic fibrosis transmembrane conductance regulator, and identify a novel mechanism by which a charged residue is positioned within the plane of the lipid bilayer. During cystic fibrosis transmembrane conductance regulator biogenesis, topology of the C-terminal transmembrane domain (TMs 7-12) is directed by alternating signal (TMs 7, 9, and 11) and stop transfer (TMs 8, 10, and 12) sequences. Unlike conventional stop transfer sequences, however, TM8 is unable to independently terminate translocation due to the presence of a single charged residue, Asp(924), within the TM segment. Instead, TM8 stop transfer activity is specifically dependent on TM7, which functions both to initiate translocation and to compensate for the charged residue within TM8. Moreover, even in the presence of TM7, the N terminus of TM8 extends significantly into the ER lumen, suggesting a high degree of flexibility in establishing TM8 transmembrane boundaries. These studies demonstrate that signal sequences can markedly influence stop transfer behavior and indicate that ER translocation machinery simultaneously integrates information from multiple topogenic determinants as they are presented in rapid succession during polytopic protein biogenesis.
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Affiliation(s)
- Kristin Carveth
- Division of Molecular Medicine, Oregon Health Sciences University, Portland, Oregon 97201, USA
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134
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Lingappa VR, Rutkowski DT, Hegde RS, Andersen OS. Conformational control through translocational regulation: a new view of secretory and membrane protein folding. Bioessays 2002; 24:741-8. [PMID: 12210535 DOI: 10.1002/bies.10130] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
We suggest a new view of secretory and membrane protein folding that emphasizes the role of pathways of biogenesis in generating functional and conformational heterogeneity. In this view, heterogeneity results from action of accessory factors either directly binding specific sequences of the nascent chain, or indirectly, changing the environment in which a particular domain is synthesized. Entrained by signaling pathways, these variables create a combinatorial set of necessary-but-not-sufficient conditions that enhance synthesis and folding of particular alternate, functional, conformational forms. We therefore propose that protein conformation is productively regulated by the cell during translocation across the endoplasmic reticulum (ER), a concept that may account for currently poorly understood aspects of physiological function, natural selection, and disease pathogenesis.
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Affiliation(s)
- Vishwanath R Lingappa
- Department of Physiology, University of California, San Francisco, CA 94143-0444, USA.
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135
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Kanner EM, Klein IK, Friedlander M, Simon SM. The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally. Biochemistry 2002; 41:7707-15. [PMID: 12056902 DOI: 10.1021/bi0256882] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Opsin, a member of the G-protein-coupled receptor family, is a polytopic membrane protein that does not encode a cleaved amino-terminal signal sequence. The amino terminus of opsin precedes the first known targeting information, suggesting that it translocates across the endoplasmic reticulum (ER) membrane after synthesis, uncoupled from translation. However, translocation across the mammalian ER is believed to be coupled to protein synthesis. In this study we show that opsin, within a range of nascent peptide lengths, targets and translocates equally efficiently co- and posttranslationally. Longer nascent opsin peptides have a lower efficiency of cotranslational translocation but an even lower efficiency of posttranslational translocation. We also show that SRP is required for both co- and posttranslational targeting.
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Affiliation(s)
- Elliott M Kanner
- Laboratory of Cellular Biophysics, The Rockefeller University, New York, New York 10021, USA
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136
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Dudek J, Volkmer J, Bies C, Guth S, Müller A, Lerner M, Feick P, Schäfer KH, Morgenstern E, Hennessy F, Blatch GL, Janoscheck K, Heim N, Scholtes P, Frien M, Nastainczyk W, Zimmermann R. A novel type of co-chaperone mediates transmembrane recruitment of DnaK-like chaperones to ribosomes. EMBO J 2002; 21:2958-67. [PMID: 12065409 PMCID: PMC126068 DOI: 10.1093/emboj/cdf315] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2001] [Revised: 04/02/2002] [Accepted: 04/29/2002] [Indexed: 11/12/2022] Open
Abstract
Recently, the homolog of yeast protein Sec63p was identified in dog pancreas microsomes. This pancreatic DnaJ-like protein was shown to be an abundant protein, interacting with both the Sec61p complex and lumenal DnaK-like proteins, such as BiP. The pancreatic endoplasmic reticulum contains a second DnaJ-like membrane protein, which had been termed Mtj1p in mouse. Mtj1p is present in pancreatic microsomes at a lower concentration than Sec63p but has a higher affinity for BiP. In addition to a lumenal J-domain, Mtj1p contains a single transmembrane domain and a cytosolic domain which is in close contact with translating ribosomes and appears to have the ability to modulate translation. The interaction with ribosomes involves a highly charged region within the cytosolic domain of Mtj1p. We propose that Mtj1p represents a novel type of co-chaperone, mediating transmembrane recruitment of DnaK-like chaperones to ribosomes and, possibly, transmembrane signaling between ribosomes and DnaK-like chaperones of the endoplasmic reticulum.
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Affiliation(s)
| | | | | | | | | | | | - Peter Feick
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
| | - Karl-Herbert Schäfer
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
| | - Eberhard Morgenstern
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
| | - Fritha Hennessy
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
| | - Gregory L. Blatch
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
| | | | | | | | | | | | - Richard Zimmermann
- Medizinische Biochemie und Molekularbiologie,
Physiologie, Anatomie and Zellbiologie, Universität des Saarlandes, D-66421 Homburg, Germany and Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa Corresponding author e-mail:
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137
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Abstract
Over recent years, much progress has been made in the identification and characterization of factors involved in the biosynthesis of integral membrane proteins of the helix-bundle type. In addition, our knowledge of membrane protein structure and the forces stabilizing helix-helix interactions in a lipid environment is expanding rapidly. However, it is still not clear how a membrane protein folds into its final form in vivo, nor what constraints there are on the folded structure that results from the mechanistic details of translocon-mediated assembly rather than simply from the thermodynamics of protein-lipid interactions.
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Affiliation(s)
- Chen-Ni Chin
- Department Molecular Biophysics and Biochemistry, Yale University, Bass 429, 266 Whitney Avenue, New Haven, CT 06520, USA
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138
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Abstract
Accumulating evidence for nascent-peptide-mediated regulation of translation suggests that all nascent peptides do not necessarily interact with the ribosome in a similar manner. Recent studies have helped to elucidate the exit route of the nascent chain and its interactions with the ribosome.
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Affiliation(s)
- Tanel Tenson
- Institute of Molecular and Cell Biology, Tartu University, Riia 23, Tartu, Estonia.
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139
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Haigh NG, Johnson AE. A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane. J Cell Biol 2002; 156:261-70. [PMID: 11807091 PMCID: PMC2199230 DOI: 10.1083/jcb.200110074] [Citation(s) in RCA: 99] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
In mammalian cells, most membrane proteins are inserted cotranslationally into the ER membrane at sites termed translocons. Although each translocon forms an aqueous pore, the permeability barrier of the membrane is maintained during integration, even when the otherwise tight ribosome-translocon seal is opened to allow the cytoplasmic domain of a nascent protein to enter the cytosol. To identify the mechanism by which membrane integrity is preserved, nascent chain exposure to each side of the membrane was determined at different stages of integration by collisional quenching of a fluorescent probe in the nascent chain. Comparing integration intermediates prepared with intact, empty, or BiP-loaded microsomes revealed that the lumenal end of the translocon pore is closed by BiP in an ATP-dependent process before the opening of the cytoplasmic ribosome-translocon seal during integration. This BiP function is distinct from its previously identified role in closing ribosome-free, empty translocons because of the presence of the ribosome at the translocon and the nascent membrane protein that extends through the translocon pore and into the lumen during integration. Therefore, BiP is a key component in a sophisticated mechanism that selectively closes the lumenal end of some, but not all, translocons occupied by a nascent chain. By using collisional quenchers of different sizes, the large internal diameter of the ribosome-bound aqueous translocon pore was found to contract when BiP was required to seal the pore during integration. Therefore, closure of the pore involves substantial conformational changes in the translocon that are coupled to a complex sequence of structural rearrangements on both sides of the ER membrane involving the ribosome and BiP.
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Affiliation(s)
- Nora G Haigh
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, TX 77843-1114, USA
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140
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Hermansson M, Monné M, von Heijne G. Formation of helical hairpins during membrane protein integration into the endoplasmic reticulum membrane. Role of the N and C-terminal flanking regions. J Mol Biol 2001; 313:1171-9. [PMID: 11700072 DOI: 10.1006/jmbi.2001.5108] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The helical hairpin, two closely spaced transmembrane helices separated by a short turn, is a common structural element in integral membrane proteins. Previous studies on the sequence determinants of helical hairpin formation have focussed on the role of polar and charged residues placed centrally in a long stretch of hydrophobic residues, and have yielded a "propensity scale" for the relative efficiency with which different residues promote the formation of helical hairpins. In this study, we shift our attention to the role of charged residues flanking the hydrophobic stretch. Clusters of charged residues are known to hinder membrane translocation, and thus flanking charged residues may conceivably force a long hydrophobic segment to form a helical hairpin even if there are no or only weakly turn-promoting residues in the hydrophobic stretch. We indeed find that Lys and, more surprisingly, Asp residues strongly affect helical hairpin formation when placed next to a poly-Leu-based transmembrane segment. We also find that a cluster of four consecutive Lys residues can affect the efficiency of helical hairpin formation even when placed approximately 30 residues downstream of the hydrophobic stretch. These observations have interesting implications for the way we picture membrane protein topogenesis within the context of the endoplasmic reticulum (ER) translocon.
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Affiliation(s)
- M Hermansson
- Department of Biochemistry and Biophysics, Stockholm University, S-106 91 Stockholm, Sweden
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141
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Lin J, Liang Z, Zhang Z, Li G. Membrane topography and topogenesis of prenylated Rab acceptor (PRA1). J Biol Chem 2001; 276:41733-41. [PMID: 11535589 PMCID: PMC1350924 DOI: 10.1074/jbc.m103475200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The mouse prenylated Rab acceptor (mPRA1) is associated with the Golgi membrane at steady state and interacts with Rab proteins. It contains two internal hydrophobic domains (34 residues each) that have enough residues to form four transmembrane (TM) segments. In this study, we have determined the membrane topography of mPRA1 in both intact cells and isolated microsomes. The putative TM segments of mPRA1 were used to substitute for a known TM segment of a model membrane protein to determine whether the mPRA1 segments integrate into the membrane. Furthermore, N-linked glycosylation scanning methods were used to distinguish luminal domains from cytoplasmic domains of mPRA1. The data demonstrate that mPRA1 is a polytopic membrane protein containing four TM segments. These TM segments act cooperatively during the translocation and integration at the endoplasmic reticulum membrane. All hydrophilic domains are in the cytoplasm, including the N-terminal domain, the linker domain between the two hydrophobic domains, and the C-terminal domain. As a result, the bulk of mPRA1 is located in the cytoplasm, supporting its postulated role in regulating Rab membrane targeting and intracellular trafficking.
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Affiliation(s)
- Jialing Lin
- ‡ To whom correspondence may be addressed: Dept. of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, 940 S. L. Young Blvd., BMSB 853, Oklahoma City, OK 73104. G. L.: Tel.: 405-271-2227 (ext. 1232); Fax: 405-271-3139; E-mail:; J.L.: Tel.: 405-271-2227 (ext. 1216); Fax: 405-271-3139; E-mail:
| | | | | | - Guangpu Li
- ‡ To whom correspondence may be addressed: Dept. of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, 940 S. L. Young Blvd., BMSB 853, Oklahoma City, OK 73104. G. L.: Tel.: 405-271-2227 (ext. 1232); Fax: 405-271-3139; E-mail:; J.L.: Tel.: 405-271-2227 (ext. 1216); Fax: 405-271-3139; E-mail:
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142
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Nilsson I, Ohvo-Rekilä H, Slotte JP, Johnson AE, von Heijne G. Inhibition of protein translocation across the endoplasmic reticulum membrane by sterols. J Biol Chem 2001; 276:41748-54. [PMID: 11535595 DOI: 10.1074/jbc.m105823200] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cholesterol and related sterols are known to modulate the physical properties of biological membranes and can affect the activities of membrane-bound protein complexes. Here, we report that an early step in protein translocation across the endoplasmic reticulum (ER) membrane is reversibly inhibited by cholesterol levels significantly lower than those found in the plasma membrane. By UV-induced chemical cross-linking we further show that high cholesterol levels prevent cross-linking between ribosome-nascent chain complexes and components of the Sec61 translocon, but have no effect on cross-linking to the signal recognition particle. The inhibiting effect on translocation is different between different sterols. Our data suggest that the protein translocation machinery may be sensitive to changes in cholesterol levels in the ER membrane.
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Affiliation(s)
- I Nilsson
- Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden
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143
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Beckmann R, Spahn CM, Eswar N, Helmers J, Penczek PA, Sali A, Frank J, Blobel G. Architecture of the protein-conducting channel associated with the translating 80S ribosome. Cell 2001; 107:361-72. [PMID: 11701126 DOI: 10.1016/s0092-8674(01)00541-4] [Citation(s) in RCA: 311] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
In vitro assembled yeast ribosome-nascent chain complexes (RNCs) containing a signal sequence in the nascent chain were immunopurified and reconstituted with the purified protein-conducting channel (PCC) of yeast endoplasmic reticulum, the Sec61 complex. A cryo-EM reconstruction of the RNC-Sec61 complex at 15.4 A resolution shows a tRNA in the P site. Distinct rRNA elements and proteins of the large ribosomal subunit form four connections with the PCC across a gap of about 10-20 A. Binding of the PCC influences the position of the highly dynamic rRNA expansion segment 27. The RNC-bound Sec61 complex has a compact appearance and was estimated to be a trimer. We propose a binary model of cotranslational translocation entailing only two basic functional states of the translating ribosome-channel complex.
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Affiliation(s)
- R Beckmann
- Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.
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144
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Rukmini R, Rawat SS, Biswas SC, Chattopadhyay A. Cholesterol organization in membranes at low concentrations: effects of curvature stress and membrane thickness. Biophys J 2001; 81:2122-34. [PMID: 11566783 PMCID: PMC1301684 DOI: 10.1016/s0006-3495(01)75860-2] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Cholesterol is often found distributed nonrandomly in domains in biological and model membranes and has been reported to be distributed heterogeneously among various intracellular membranes. Although a large body of literature exists on the organization of cholesterol in plasma membranes or membranes with high cholesterol content, very little is known about organization of cholesterol in membranes containing low amounts of cholesterol. Using a fluorescent cholesterol analog (25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol, or NBD-cholesterol), we have previously shown that cholesterol may exhibit local organization even at very low concentrations in membranes, which could possibly be attributable to transbilayer tail-to-tail dimers. This is supported by similar observations reported by other groups using cholesterol or dehydroergosterol, a naturally occurring fluorescent cholesterol analog which closely mimics cholesterol. In this paper, we have tested the basic features of cholesterol organization in membranes at low concentrations using spectral features of dehydroergosterol. More importantly, we have investigated the role of membrane surface curvature and thickness on transbilayer dimer arrangement of cholesterol using NBD-cholesterol. We find that dimerization is not favored in membranes with high curvature. However, cholesterol dimers are observed again if the curvature stress is relieved. Further, we have monitored the effect of membrane thickness on the dimerization process. Our results show that the dimerization process is stringently controlled by a narrow window of membrane thickness. Interestingly, this type of local organization of NBD-cholesterol at low concentrations is also observed in sphingomyelin-containing membranes. These results could be significant in membranes that have very low cholesterol content, such as the endoplasmic reticulum and the inner mitochondrial membrane, and in trafficking and sorting of cellular cholesterol.
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Affiliation(s)
- R Rukmini
- Centre for Cellular & Molecular Biology, Hyderabad 500 007, India
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145
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Wilkinson BM, Tyson JR, Stirling CJ. Ssh1p determines the translocation and dislocation capacities of the yeast endoplasmic reticulum. Dev Cell 2001; 1:401-9. [PMID: 11702951 DOI: 10.1016/s1534-5807(01)00043-0] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Sec61p is required both for protein translocation and dislocation across the membrane of the endoplasmic reticulum (ER). However, the cellular role of the Sec61p homolog Ssh1p has not been clearly defined. We show that deltassh1 mutant cells have strong defects in both SRP-dependent and -independent translocation. Moreover, these cells were also found to be induced for the unfolded protein response and to be defective in dislocation of a misfolded ER protein. In addition, deltassh1 mutant cells rapidly became respiratory deficient. The other defects discussed above were suppressed in the respiratory-deficient state or under conditions where the rate of polypeptide translation was artificially reduced. These data identify Ssh1p as a component of a second, functionally distinct translocon in the yeast ER membrane.
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Affiliation(s)
- B M Wilkinson
- School of Biological Sciences, University of Manchester, United Kingdom
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146
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Rutkowski DT, Lingappa VR, Hegde RS. Substrate-specific regulation of the ribosome- translocon junction by N-terminal signal sequences. Proc Natl Acad Sci U S A 2001; 98:7823-8. [PMID: 11416167 PMCID: PMC35426 DOI: 10.1073/pnas.141125098] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Amino-terminal signal sequences target nascent secretory and membrane proteins to the endoplasmic reticulum for translocation. Subsequent interactions between the signal sequence and components of the translocation machinery at the endoplasmic reticulum are thought to be important for the productive engagement of the translocon by the ribosome-nascent chain complex. However, it is not clear whether all signal sequences carry out these posttargeting steps identically, or if there are differences in the interactions directed by one signal sequence versus another. In this study, we find substantial differences in the ability of signal sequences from different substrates to mediate closure of the ribosome--translocon junction early in translocation. We also show that these differences in some cases necessitate functional coordination between the signal sequence and mature domain for faithful translocation. Accordingly, the translocation of some proteins is sensitive to replacement of their signal sequences. In a particularly dramatic example, the topology of the prion protein was found to depend highly on the choice of signal sequence used to direct its translocation. Taken together, our results reveal an unanticipated degree of substrate-specific functionality encoded in N-terminal signal sequences.
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Affiliation(s)
- D T Rutkowski
- Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0444, USA
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147
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Gabashvili IS, Gregory ST, Valle M, Grassucci R, Worbs M, Wahl MC, Dahlberg AE, Frank J. The polypeptide tunnel system in the ribosome and its gating in erythromycin resistance mutants of L4 and L22. Mol Cell 2001; 8:181-8. [PMID: 11511371 DOI: 10.1016/s1097-2765(01)00293-3] [Citation(s) in RCA: 163] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Variations in the inner ribosomal landscape determining the topology of nascent protein transport have been studied by three-dimensional cryo-electron microscopy of erythromycin-resistant Escherichia coli 70S ribosomes. Significant differences in the mouth of the 50S subunit tunnel system visualized in the present study support a simple steric-hindrance explanation for the action of the drug. Examination of ribosomes in different functional states suggests that opening and closing of the main tunnel are dynamic features of the large subunit, possibly accompanied by changes in the L7/L12 stalk region. The existence and dynamic behavior of side tunnels suggest that ribosomal proteins L4 and L22 might be involved in the regulation of a multiple exit system facilitating cotranslational processing (or folding or directing) of nascent proteins.
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Affiliation(s)
- I S Gabashvili
- Wadsworth Center, State University of New York at Albany, P.O. Box 509, Albany, NY 12201, USA
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148
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Fulga TA, Sinning I, Dobberstein B, Pool MR. SRbeta coordinates signal sequence release from SRP with ribosome binding to the translocon. EMBO J 2001; 20:2338-47. [PMID: 11331598 PMCID: PMC125438 DOI: 10.1093/emboj/20.9.2338] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Protein targeting to the endoplasmic reticulum (ER) membrane is regulated by three GTPases, the 54 kDa subunit of the signal recognition particle (SRP) and the alpha- and beta-subunits of the SRP receptor (SR). Using a soluble form of SR and an XTP-binding mutant of SRbeta, we show that SRbeta is essential for protein translocation across the ER membrane. SRbeta can be cross-linked to a 21 kDa ribosomal protein in its empty and GDP-bound state, but not when GTP is bound. GTP binding to SRbeta is required to induce signal sequence release from SRP. This is achieved by the presence of the translocon, which changes the interaction between the 21 kDa ribosomal protein and SRbeta and thereby allows SRbeta to bind GTP. We conclude that SRbeta coordinates the release of the signal sequence from SRP with the presence of the translocon.
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Affiliation(s)
| | - Irmgard Sinning
- Structural Biology Programme, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg and
Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), D-69120 Heidelberg, Germany Present address: Biochemiezentrum der Universität Heidelberg (BZH), D-69120 Heidelberg, Germany Corresponding author e-mail:
| | - Bernhard Dobberstein
- Structural Biology Programme, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg and
Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), D-69120 Heidelberg, Germany Present address: Biochemiezentrum der Universität Heidelberg (BZH), D-69120 Heidelberg, Germany Corresponding author e-mail:
| | - Martin R. Pool
- Structural Biology Programme, European Molecular Biology Laboratory (EMBL), D-69117 Heidelberg and
Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), D-69120 Heidelberg, Germany Present address: Biochemiezentrum der Universität Heidelberg (BZH), D-69120 Heidelberg, Germany Corresponding author e-mail:
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149
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Abstract
Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.
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Affiliation(s)
- A M Benham
- Department of Bio-Organic Chemistry, Utrecht University, The Netherlands
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150
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Vidugiriene J, Vainauskas S, Johnson AE, Menon AK. Endoplasmic reticulum proteins involved in glycosylphosphatidylinositol-anchor attachment: photocrosslinking studies in a cell-free system. EUROPEAN JOURNAL OF BIOCHEMISTRY 2001; 268:2290-300. [PMID: 11298746 DOI: 10.1046/j.1432-1327.2001.02106.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Assembly of glycosylphosphatidylinositol (GPtdIns)-anchored proteins requires translocation of the nascent polypeptide chain across the endoplasmic reticulum (ER) membrane and replacement of the C-terminal signal sequence with a GPtdIns moiety. The anchoring reaction is carried out by an ER enzyme, GPtdIns transamidase. Genetic studies with yeast indicate that the transamidase consists of a dynamic complex of at least two subunits, Gaa1p and Gpi8p. To study the GPtdIns-anchoring reaction, we used a small reporter protein that becomes GPtdIns-anchored when the corresponding mRNA is translated in the presence of microsomes, in conjunction with site-specific photocrosslinking to identify ER membrane components that are proximal to the reporter during its conversion to a GPtdIns-anchored protein. We generated variants of the reporter protein such that upon in vitro translation in the presence of Nepsilon-(5-azido-2-nitrobenzoyl)-lysyl-tRNA, photoreactive lysine residues would be incorporated in the protein specifically near the GPtdIns-attachment site. We analyzed photoadducts resulting from UV irradiation of the samples. We show that proproteins can be crosslinked to the transamidase subunit Gpi8p, as well as to ER proteins of molecular mass approximately 60 kDa, approximately 70 kDa, and approximately 120 kDa. The identification of a photoadduct between a proprotein and Gpi8p provides the first direct evidence of an interaction between a proprotein substrate and one of the genetically identified transamidase subunits. The approximately 70-kDa protein that we identified may correspond to the other subunit Gaa1p, while the other proteins possibly represent additional, hitherto unidentified subunits of the mammalian GPtdIns transamidase complex.
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Affiliation(s)
- J Vidugiriene
- Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
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