1
|
Hoffman AM, Chen Q, Zheng T, Nicchitta CV. Heterogeneous translational landscape of the endoplasmic reticulum revealed by ribosome proximity labeling and transcriptome analysis. J Biol Chem 2019; 294:8942-8958. [PMID: 31004035 DOI: 10.1074/jbc.ra119.007996] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2019] [Revised: 03/27/2019] [Indexed: 12/21/2022] Open
Abstract
The endoplasmic reticulum (ER) is a nexus for mRNA localization and translation, and recent studies have demonstrated that ER-bound ribosomes also play a transcriptome-wide role in regulating proteome composition. The Sec61 translocon (SEC61) serves as the receptor for ribosomes that translate secretory/integral membrane protein-encoding mRNAs, but whether SEC61 also serves as a translation site for cytosolic protein-encoding mRNAs remains unknown. Here, using a BioID proximity-labeling approach in HEK293T Flp-In cell lines, we examined interactions between ER-resident proteins and ribosomes in vivo Using in vitro analyses, we further focused on bona fide ribosome interactors (i.e. SEC61) and ER proteins (ribophorin I, leucine-rich repeat-containing 59 (LRRC59), and SEC62) previously implicated in associating with ribosomes. We observed labeling of ER-bound ribosomes with the SEC61β and LRRC59 BioID reporters, comparatively modest labeling with the ribophorin I reporter, and no labeling with the SEC62 reporter. A biotin pulse-chase/subcellular fractionation approach to examine ribosome exchange at the SEC61β and LRRC59 sites revealed that, at steady state, ribosomes at these sites comprise both rapid- and slow-exchanging pools. Global translational initiation arrest elicited by the inhibitor harringtonine accelerated SEC61β reporter-labeled ribosome exchange. RNA-Seq analyses of the mRNAs associated with SEC61β- and LRRC59-labeled ribosomes revealed both site-enriched and shared mRNAs and further established that the ER has a transcriptome-wide role in regulating proteome composition. These results provide evidence that ribosomes interact with the ER membrane via multiple modes and suggest regulatory mechanisms that control global proteome composition via ER membrane-bound ribosomes.
Collapse
Affiliation(s)
| | - Qiang Chen
- Cell Biology, Duke University School of Medicine, Durham, North Carolina 27710
| | - Tianli Zheng
- Cell Biology, Duke University School of Medicine, Durham, North Carolina 27710
| | - Christopher V Nicchitta
- From the Departments of Biochemistry and .,Cell Biology, Duke University School of Medicine, Durham, North Carolina 27710
| |
Collapse
|
2
|
Sec61β facilitates the maintenance of endoplasmic reticulum homeostasis by associating microtubules. Protein Cell 2017; 9:616-628. [PMID: 29168059 PMCID: PMC6019657 DOI: 10.1007/s13238-017-0492-5] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2017] [Accepted: 11/13/2017] [Indexed: 10/25/2022] Open
Abstract
Sec61β, a subunit of the Sec61 translocon complex, is not essential in yeast and commonly used as a marker of endoplasmic reticulum (ER). In higher eukaryotes, such as Drosophila, deletion of Sec61β causes lethality, but its physiological role is unclear. Here, we show that Sec61β interacts directly with microtubules. Overexpression of Sec61β containing small epitope tags, but not a RFP tag, induces dramatic bundling of the ER and microtubule. A basic region in the cytosolic domain of Sec61β is critical for microtubule association. Depletion of Sec61β induces ER stress in both mammalian cells and Caenorhabditis elegans, and subsequent restoration of ER homeostasis correlates with the microtubule binding ability of Sec61β. Loss of Sec61β causes increased mobility of translocon complexes and reduced level of membrane-bound ribosomes. These results suggest that Sec61β may stabilize protein translocation by linking translocon complex to microtubule and provide insight into the physiological function of ER-microtubule interaction.
Collapse
|
3
|
Snapp EL, McCaul N, Quandte M, Cabartova Z, Bontjer I, Källgren C, Nilsson I, Land A, von Heijne G, Sanders RW, Braakman I. Structure and topology around the cleavage site regulate post-translational cleavage of the HIV-1 gp160 signal peptide. eLife 2017; 6:26067. [PMID: 28753126 PMCID: PMC5577925 DOI: 10.7554/elife.26067] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2017] [Accepted: 07/26/2017] [Indexed: 12/29/2022] Open
Abstract
Like all other secretory proteins, the HIV-1 envelope glycoprotein gp160 is targeted to the endoplasmic reticulum (ER) by its signal peptide during synthesis. Proper gp160 folding in the ER requires core glycosylation, disulfide-bond formation and proline isomerization. Signal-peptide cleavage occurs only late after gp160 chain termination and is dependent on folding of the soluble subunit gp120 to a near-native conformation. We here detail the mechanism by which co-translational signal-peptide cleavage is prevented. Conserved residues from the signal peptide and residues downstream of the canonical cleavage site form an extended alpha-helix in the ER membrane, which covers the cleavage site, thus preventing cleavage. A point mutation in the signal peptide breaks the alpha helix allowing co-translational cleavage. We demonstrate that postponed cleavage of gp160 enhances functional folding of the molecule. The change to early cleavage results in decreased viral fitness compared to wild-type HIV.
Collapse
Affiliation(s)
- Erik Lee Snapp
- Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States
| | - Nicholas McCaul
- Cellular Protein Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Utrecht, Netherlands
| | - Matthias Quandte
- Cellular Protein Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Utrecht, Netherlands
| | - Zuzana Cabartova
- National Institute of Public Health, National Reference Laboratory for Viral Hepatitis, Prague, Czech Republic
| | - Ilja Bontjer
- Department of Medical Microbiology, Laboratory of Experimental Virology, Center for Infection and Immunity Amsterdam, Academic Medical Center, Amsterdam, Netherlands
| | - Carolina Källgren
- Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden.,Science for Life Laboratory, Stockholm University, Solna, Sweden
| | - IngMarie Nilsson
- Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden.,Science for Life Laboratory, Stockholm University, Solna, Sweden
| | - Aafke Land
- Cellular Protein Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Utrecht, Netherlands
| | - Gunnar von Heijne
- Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden.,Science for Life Laboratory, Stockholm University, Solna, Sweden
| | - Rogier W Sanders
- Department of Medical Microbiology, Laboratory of Experimental Virology, Center for Infection and Immunity Amsterdam, Academic Medical Center, Amsterdam, Netherlands
| | - Ineke Braakman
- Cellular Protein Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Utrecht, Netherlands
| |
Collapse
|
4
|
Lumley EC, Osborn AR, Scott JE, Scholl AG, Mercado V, McMahan YT, Coffman ZG, Brewster JL. Moderate endoplasmic reticulum stress activates a PERK and p38-dependent apoptosis. Cell Stress Chaperones 2017; 22:43-54. [PMID: 27761878 PMCID: PMC5225058 DOI: 10.1007/s12192-016-0740-2] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2016] [Revised: 10/05/2016] [Accepted: 10/07/2016] [Indexed: 02/07/2023] Open
Abstract
The endoplasmic reticulum (ER) has the ability to signal organelle dysfunction via a complex signaling network known as the unfolded protein response (UPR). In this work, hamster fibroblast cells exhibiting moderate levels of ER stress were compared to those exhibiting severe ER stress. Inhibition of N-linked glycosylation was accomplished via a temperature-sensitive mutation in the Dad1 subunit of the oligosaccharyltransferase (OST) complex or by direct inhibition with tunicamycin (Tm). Temperature shift (TS) treatment generated weak activation of ER stress signaling when compared to doses of Tm that are typically used in ER stress studies (500-1000 nM). A dose-response analysis of key ER stress signaling mediators, inositol-requiring enzyme 1 (IRE1) and protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), revealed 20-40 nM of Tm to generate activation intensity similar to TS treatment. In parental BHK21 cells, moderate (20-40 nM) and high doses (200-1000 nM) of Tm were compared to identify physiological and signaling-based differences in stress response. Inhibition of ER Ca2+ release via ITPR activity with 2-aminoethoxydiphenyl borate (2-APB) or Xestospongin C (XeC) was sufficient to protect against apoptosis induced by moderate but not higher doses of Tm. Analysis of kinase activation over a range of Tm exposures revealed the p38 stress-activated protein kinase (SAPK) to display increasing activation with Tm dosage. Interestingly, Tm induced the extracellular regulated kinases (Erk1/2) only at moderate doses of Tm. Inhibition of ER transmembrane stress sensors (IRE1, PERK) or cytosolic signaling mediators (p38, Jnk1, Erk1/2) was used to evaluate pathways involved in apoptosis activation during ER stress. Inhibition of either PERK or p38 was sufficient to reduce cell death and apoptosis induced by moderate, but not high, doses of Tm. During ER stress, cells exhibited a rapid decline in anti-apoptotic Mcl-1 and survivin proteins. Inhibition of PERK was sufficient to block this affect. This work reveals moderate doses of ER stress to generate patterns of stress signaling that are distinct from higher doses and that apoptosis activation at moderate levels of stress are dependent upon PERK and p38 signaling. Studies exploring ER stress signaling should recognize that this signaling acts as a rheostat rather than a simple switch, behaving distinctively in a dose-dependent manner.
Collapse
Affiliation(s)
- Emily C Lumley
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Acadia R Osborn
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Jessica E Scott
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Amanda G Scholl
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Vicki Mercado
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Young T McMahan
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Zachary G Coffman
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA
| | - Jay L Brewster
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu,, CA, 90263, USA.
| |
Collapse
|
5
|
Abstract
Transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi complex is highly selective. As a general rule, such transport is limited to soluble and membrane-associated secretory proteins that have reached properly folded and assembled conformations. To secure the efficiency, fidelity, and control of this crucial transport step, cells use a combination of mechanisms. The mechanisms are based on selective retention of proteins in the ER to prevent uptake into transport vesicles, on selective capture of proteins in COPII carrier vesicles, on inclusion of proteins in these vesicles by default as part of fluid and membrane bulk flow, and on selective retrieval of proteins from post-ER compartments by retrograde vesicle transport.
Collapse
Affiliation(s)
- Charles Barlowe
- Biochemistry Department, Geisel School of Medicine at Dartmouth, Hanover, New Hampshire 03755;
| | - Ari Helenius
- Institute of Biochemistry, ETH Zurich, Zurich CH-8093, Switzerland
| |
Collapse
|
6
|
Endoplasmic Reticulum: The Favorite Intracellular Niche for Viral Replication and Assembly. Viruses 2016; 8:v8060160. [PMID: 27338443 PMCID: PMC4926180 DOI: 10.3390/v8060160] [Citation(s) in RCA: 122] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2016] [Revised: 05/23/2016] [Accepted: 05/26/2016] [Indexed: 02/07/2023] Open
Abstract
The endoplasmic reticulum (ER) is the largest intracellular organelle. It forms a complex network of continuous sheets and tubules, extending from the nuclear envelope (NE) to the plasma membrane. This network is frequently perturbed by positive-strand RNA viruses utilizing the ER to create membranous replication factories (RFs), where amplification of their genomes occurs. In addition, many enveloped viruses assemble progeny virions in association with ER membranes, and viruses replicating in the nucleus need to overcome the NE barrier, requiring transient changes of the NE morphology. This review first summarizes some key aspects of ER morphology and then focuses on the exploitation of the ER by viruses for the sake of promoting the different steps of their replication cycles.
Collapse
|
7
|
Cui XA, Zhang H, Ilan L, Liu AX, Kharchuk I, Palazzo AF. mRNA encoding Sec61β, a tail-anchored protein, is localized on the endoplasmic reticulum. J Cell Sci 2015; 128:3398-410. [PMID: 26272916 PMCID: PMC4582399 DOI: 10.1242/jcs.168583] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2015] [Accepted: 08/04/2015] [Indexed: 12/30/2022] Open
Abstract
Although one pathway for the post-translational targeting of tail-anchored proteins to the endoplasmic reticulum (ER) has been well defined, it is unclear whether additional pathways exist. Here, we provide evidence that a subset of mRNAs encoding tail-anchored proteins, including Sec61β and nesprin-2, is partially localized to the surface of the ER in mammalian cells. In particular, Sec61b mRNA can be targeted to, and later maintained on, the ER using both translation-dependent and -independent mechanisms. Our data suggests that this process is independent of p180 (also known as RRBP1), a known mRNA receptor on the ER, and the transmembrane domain recognition complex (TRC) pathway components, TRC40 (also known as ASNA1) and BAT3 (also known as BAG6). In addition, our data indicates that Sec61b mRNA might access translocon-bound ribosomes. Our results show that certain tail-anchored proteins are likely to be synthesized directly on the ER, and this facilitates their membrane insertion. Thus, it is clear that mammalian cells utilize multiple mechanisms to ensure efficient targeting of tail-anchored proteins to the surface of the ER. Highlighted Article: The mRNA encoding certain tail-anchored proteins is directly localized to the surface of the endoplasmic reticulum, facilitating the insertion of newly synthesized proteins into the membrane.
Collapse
Affiliation(s)
- Xianying A Cui
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| | - Hui Zhang
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| | - Lena Ilan
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| | - Ai Xin Liu
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| | - Iryna Kharchuk
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| | - Alexander F Palazzo
- University of Toronto, Department of Biochemistry, 1 King's College Circle, MSB Room 5336, Toronto, ON, M5S 1A8, Canada
| |
Collapse
|
8
|
Stout RF, Snapp EL, Spray DC. Connexin Type and Fluorescent Protein Fusion Tag Determine Structural Stability of Gap Junction Plaques. J Biol Chem 2015; 290:23497-514. [PMID: 26265468 DOI: 10.1074/jbc.m115.659979] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2015] [Indexed: 12/22/2022] Open
Abstract
Gap junctions (GJs) are made up of plaques of laterally clustered intercellular channels and the membranes in which the channels are embedded. Arrangement of channels within a plaque determines subcellular distribution of connexin binding partners and sites of intercellular signaling. Here, we report the discovery that some connexin types form plaque structures with strikingly different degrees of fluidity in the arrangement of the GJ channel subcomponents of the GJ plaque. We uncovered this property of GJs by applying fluorescence recovery after photobleaching to GJs formed from connexins fused with fluorescent protein tags. We found that connexin 26 (Cx26) and Cx30 GJs readily diffuse within the plaque structures, whereas Cx43 GJs remain persistently immobile for more than 2 min after bleaching. The cytoplasmic C terminus of Cx43 was required for stability of Cx43 plaque arrangement. We provide evidence that these qualitative differences in GJ arrangement stability reflect endogenous characteristics, with the caveat that the sizes of the GJs examined were necessarily large for these measurements. We also uncovered an unrecognized effect of non-monomerized fluorescent protein on the dynamically arranged GJs and the organization of plaques composed of multiple connexin types. Together, these findings redefine our understanding of the GJ plaque structure and should be considered in future studies using fluorescent protein tags to probe dynamics of highly ordered protein complexes.
Collapse
Affiliation(s)
- Randy F Stout
- From the Dominick P. Purpura Department of Neuroscience and
| | - Erik Lee Snapp
- the Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461
| | - David C Spray
- From the Dominick P. Purpura Department of Neuroscience and
| |
Collapse
|
9
|
Nevalainen M, Metsikkö K. Fluvastatin delays propagation of viral infection in isolated rat FDB myofibers but does not affect exocytic membrane trafficking. Cell Biol Int 2015; 39:1307-16. [PMID: 26123964 DOI: 10.1002/cbin.10509] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2015] [Accepted: 06/25/2015] [Indexed: 11/06/2022]
Abstract
We have utilized the enveloped viral model to study the effect of fluvastatin on membrane trafficking in isolated rat myofibers. Our immunofluorescence studies constantly showed that infections in myofibers, which were treated with fluvastatin prior and during the infection with either vesicular stomatitis virus (VSV) or influenza A virus, propagated more slowly than in control myofibers without drug treatment. Experiments with a virus expressing Dad1 tagged with green fluorescent protein (GFP-Dad1) showed that fluvastatin did not affect its distribution within the ER/SR network and immunofluorescence staining for GM130 did not show any marked effect on the structure of the Golgi components. Furthermore, fluvastatin did not inhibit trafficking of the chimeric transport marker VSV temperature sensitive G protein (tsG-GFP) from the ER to the Golgi. We next subjected VSV infected myofibers for pulse-chase labeling experiments and found that fluvastatin did not slow down the ER-to-Golgi trafficking or Golgi to plasma membrane trafficking of the viral glycoprotein. These studies show that fluvastatin inhibited the propagation of viral infection in skeletal myofibers but no adverse effect on the exocytic trafficking could be demonstrated. These results suggest that other effects of statins rather than inhibition of ER-to-Golgi trafficking might be behind the myotoxic effects of the statins.
Collapse
Affiliation(s)
- Mika Nevalainen
- Division of Cancer Research and Translational Medicine, Department of Anatomy and Cell Biology, Faculty of Medicine, University of Oulu, Oulu, Finland.,Division of Musculoskeletal Imaging and Intervention, Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania, USA
| | - Kalervo Metsikkö
- Division of Cancer Research and Translational Medicine, Department of Anatomy and Cell Biology, Faculty of Medicine, University of Oulu, Oulu, Finland
| |
Collapse
|
10
|
Terasaki M, Shemesh T, Kasthuri N, Klemm RW, Schalek R, Hayworth KJ, Hand AR, Yankova M, Huber G, Lichtman JW, Rapoport TA, Kozlov MM. Stacked endoplasmic reticulum sheets are connected by helicoidal membrane motifs. Cell 2013; 154:285-96. [PMID: 23870120 PMCID: PMC3767119 DOI: 10.1016/j.cell.2013.06.031] [Citation(s) in RCA: 152] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2013] [Revised: 04/25/2013] [Accepted: 06/20/2013] [Indexed: 12/28/2022]
Abstract
The endoplasmic reticulum (ER) often forms stacked membrane sheets, an arrangement that is likely required to accommodate a maximum of membrane-bound polysomes for secretory protein synthesis. How sheets are stacked is unknown. Here, we used improved staining and automated ultrathin sectioning electron microscopy methods to analyze stacked ER sheets in neuronal cells and secretory salivary gland cells of mice. Our results show that stacked ER sheets form a continuous membrane system in which the sheets are connected by twisted membrane surfaces with helical edges of left- or right-handedness. The three-dimensional structure of tightly stacked ER sheets resembles a parking garage, in which the different levels are connected by helicoidal ramps. A theoretical model explains the experimental observations and indicates that the structure corresponds to a minimum of elastic energy of sheet edges and surfaces. The structure allows the dense packing of ER sheets in the restricted space of a cell.
Collapse
Affiliation(s)
- Mark Terasaki
- Department of Cell Biology, University of Connecticut Health Center, Farmington, CT 06030, USA
- Correspondence: (M.T.), (T.A.R.)
| | - Tom Shemesh
- Department of Chemical Engineering, Columbia University, New York, NY 10027, USA
| | - Narayanan Kasthuri
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Robin W. Klemm
- Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
| | - Richard Schalek
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Kenneth J. Hayworth
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Arthur R. Hand
- Departments of Craniofacial Sciences and Cell Biology, University of Connecticut School of Dental Medicine, Farmington, CT 06032, USA
| | - Maya Yankova
- Central Electron Microscopy Facility, University of Connecticut Health Center, Farmington, CT 06030, USA
| | - Greg Huber
- Department of Cell Biology, University of Connecticut Health Center, Farmington, CT 06030, USA
- Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA 93106-4030
| | - Jeff W. Lichtman
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA
| | - Tom A. Rapoport
- Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
- Correspondence: (M.T.), (T.A.R.)
| | - Michael M. Kozlov
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel
| |
Collapse
|
11
|
Nevalainen M, Kaakinen M, Metsikkö K. Distribution of mRNA transcripts and translation activity in skeletal myofibers. Cell Tissue Res 2013; 353:539-48. [PMID: 23736382 DOI: 10.1007/s00441-013-1659-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2013] [Accepted: 05/06/2013] [Indexed: 01/26/2023]
Abstract
We examine the distribution of gene products in skeletal myofibers, which are highly differentiated multinucleated cells exhibiting a specific cellular architecture. In situ hybridization studies of adult rat myofibers with a single nucleus infected with influenza virus suggested that the viral mRNA products were distributed beneath the sarcolemma around the nucleus of origin. In situ hybridization studies with a poly-T oligonucleotide probe to detect endogenous mRNAs indicated their concentration around the nuclei and distribution beneath the sarcolemma in a cross-striated fashion at the A-I junctions (costamers). Labeling with bromouridine resulted in a similar distribution pattern. The ribosomal distribution pattern indicated concentration around the myonuclei but an intracellular component was also seen. Localization of the translating ribosomes by puromycylation revealed prominent spots perinuclearly and in the core regions of the myofibers. These spots flanked Golgi elements. Our results thus suggest that the total mRNA pool is heavily concentrated within the perinuclear and subsarcolemmal regions. However, the ribosomes and the translational activity did not follow this distribution pattern, so the mRNA transcripts were not restricted to a region beneath the sarcolemma. Furthermore, experiments utilizing green fluorescent protein showed the rapid movement of proteins within the endomembrane system, which thus facilitated proteins to reach their site of function irrespective of the site of synthesis.
Collapse
Affiliation(s)
- Mika Nevalainen
- Department of Anatomy and Cell Biology, Institute of Biomedicine, University of Oulu, P.O. Box 5000, Aapistie 7, FI-90014, Oulu, Finland.
| | | | | |
Collapse
|
12
|
Abstract
Co-chaperones regulate chaperone activities and are likely to impact a protein-folding environment as much as the chaperone itself. As co-chaperones are expressed substoichiometrically, the ability of co-chaperones to encounter a chaperone is crucial for chaperone activity. ERdj3, an abundant soluble endoplasmic reticulum (ER) co-chaperone of the Hsp70 BiP, stimulates the ATPase activity of BiP to increase BiP's affinity for client (or substrate) proteins. We investigated ERdj3 availability, how ERdj3 levels impact BiP availability, and the significance of J proteins for regulating BiP binding of clients in living cells. FRAP analysis revealed that overexpressed ERdj3-sfGFP dramatically decreases BiP-GFP mobility in a client-dependent manner. By contrast, ERdj3-GFP mobility remains low regardless of client protein levels. Native gels and co-immunoprecipitations established that ERdj3 associates with a large complex including Sec61α. Translocon binding probably ensures rapid encounters between emerging nascent peptides and stimulates BiP activity in the crucial early stages of secretory protein folding. Importantly, mutant BiP exhibited significantly increased mobility when it could not interact with any ERdjs. Thus, ERdjs appear to play the dual roles of increasing BiP affinity for clients and regulating delivery of clients to BiP. Our data suggest that BiP engagement of clients is enhanced in ER subdomains enriched in ERdj proteins.
Collapse
Affiliation(s)
- Feng Guo
- Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
| | | |
Collapse
|
13
|
Puhka M, Joensuu M, Vihinen H, Belevich I, Jokitalo E. Progressive sheet-to-tubule transformation is a general mechanism for endoplasmic reticulum partitioning in dividing mammalian cells. Mol Biol Cell 2012; 23:2424-32. [PMID: 22573885 PMCID: PMC3386207 DOI: 10.1091/mbc.e10-12-0950] [Citation(s) in RCA: 115] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
During mitosis, ER network reorganization can lead to packing of the ER into tight concentric layers at the cell cortex and occurs in tandem with rounding of the cell. Morphometric and 3D EM analysis shows that in addition to reorganization, ER sheets undergo transformation toward more fenestrated and tubular forms before anaphase in mammalian cells. The endoplasmic reticulum (ER) is both structurally and functionally complex, consisting of a dynamic network of interconnected sheets and tubules. To achieve a more comprehensive view of ER organization in interphase and mitotic cells and to address a discrepancy in the field (i.e., whether ER sheets persist, or are transformed to tubules, during mitosis), we analyzed the ER in four different mammalian cell lines using live-cell imaging, high-resolution electron microscopy, and three dimensional electron microscopy. In interphase cells, we found great variation in network organization and sheet structures among different cell lines. In mitotic cells, we show that the ER undergoes both spatial reorganization and structural transformation of sheets toward more fenestrated and tubular forms. However, the extent of spatial reorganization and sheet-to-tubule transformation varies among cell lines. Fenestration and tubulation of the ER correlates with a reduced number of membrane-bound ribosomes.
Collapse
Affiliation(s)
- Maija Puhka
- Electron Microscopy Unit, Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland
| | | | | | | | | |
Collapse
|
14
|
Lai CW, Otero JH, Hendershot LM, Snapp E. ERdj4 protein is a soluble endoplasmic reticulum (ER) DnaJ family protein that interacts with ER-associated degradation machinery. J Biol Chem 2012; 287:7969-78. [PMID: 22267725 DOI: 10.1074/jbc.m111.311290] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Protein localization within cells regulates accessibility for interactions with co-factors and substrates. The endoplasmic reticulum (ER) BiP co-factor ERdj4 is up-regulated by ER stress and has been implicated in ER-associated degradation (ERAD) of multiple unfolded secretory proteins. Several other ERdj family members tend to interact selectively with nascent proteins, presumably because those ERdj proteins associate with the Sec61 translocon that facilitates entry of nascent proteins into the ER. How ERdj4 selects and targets terminally misfolded proteins for destruction remains poorly understood. In this study, we determined properties of ERdj4 that might aid in this function. ERdj4 was reported to retain its signal sequence and to be resistant to mild detergent extraction, suggesting that it was an integral membrane protein. However, live cell photobleaching analyses of GFP-tagged ERdj4 revealed that the protein exhibits diffusion coefficients uncommonly high for an ER integral membrane protein and more similar to the mobility of a soluble luminal protein. Biochemical characterization established that the ERdj4 signal sequence is cleaved to yield a soluble protein. Importantly, we found that both endogenous and overexpressed ERdj4 associate with the integral membrane protein, Derlin-1. Our findings now directly link ERdj4 to the ERAD machinery and suggest a model in which ERjd4 could help recruit clients from throughout the ER to ERAD sites.
Collapse
Affiliation(s)
- Chunwei Walter Lai
- Department of Anatomy & Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461, USA
| | | | | | | |
Collapse
|
15
|
Roboti P, High S. Keratinocyte-associated protein 2 is a bona fide subunit of the mammalian oligosaccharyltransferase. J Cell Sci 2012; 125:220-32. [PMID: 22266900 DOI: 10.1242/jcs.094599] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
The oligosaccharyltransferase (OST) complex catalyses the N-glycosylation of polypeptides entering the endoplasmic reticulum, a process essential for the productive folding and trafficking of many secretory and membrane proteins. In eukaryotes, the OST typically comprises a homologous catalytic STT3 subunit complexed with several additional components that are usually conserved, and that often function to modulate N-glycosylation efficiency. By these criteria, the status of keratinocyte-associated protein 2 (KCP2) was unclear: it was found to co-purify with the canine OST suggesting it is part of the complex but, unlike most other subunits, no potential homologues are apparent in Saccharomyces cerevisiae. In this study we have characterised human KCP2 and show that the predominant species results from an alternative initiation of translation to form an integral membrane protein with three transmembrane spans. KCP2 localises to the endoplasmic reticulum, consistent with a role in protein biosynthesis, and has a functional KKxx retrieval signal at its cytosolic C-terminus. Native gel analysis suggests that the majority of KCP2 assembles into a distinct ~500 kDa complex that also contains several bona fide OST subunits, most notably the catalytic STT3A isoform. Co-immunoprecipitation studies confirmed a robust and specific physical interaction between KCP2 and STT3A, and revealed weaker associations with both STT3B and OST48. Taken together, these data strongly support the proposal that KCP2 is a newly identified subunit of the N-glycosylation machinery present in a subset of eukaryotes.
Collapse
Affiliation(s)
- Peristera Roboti
- Faculty of Life Sciences, The University of Manchester, Manchester, UK
| | | |
Collapse
|
16
|
Jacquier N, Choudhary V, Mari M, Toulmay A, Reggiori F, Schneiter R. Lipid droplets are functionally connected to the endoplasmic reticulum in Saccharomyces cerevisiae. J Cell Sci 2011; 124:2424-37. [PMID: 21693588 DOI: 10.1242/jcs.076836] [Citation(s) in RCA: 297] [Impact Index Per Article: 22.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Cells store metabolic energy in the form of neutral lipids that are deposited within lipid droplets (LDs). In this study, we examine the biogenesis of LDs and the transport of integral membrane proteins from the endoplasmic reticulum (ER) to newly formed LDs. In cells that lack LDs, otherwise LD-localized membrane proteins are homogenously distributed in the ER membrane. Under these conditions, transcriptional induction of a diacylglycerol acyltransferase that catalyzes the formation of the storage lipid triacylglycerol (TAG), Lro1, is sufficient to drive LD formation. Newly formed LDs originate from the ER membrane where they become decorated by marker proteins. Induction of LDs by expression of the second TAG-synthesizing integral membrane protein, Dga1, reveals that Dga1 itself moves from the ER membrane to concentrate on LDs. Photobleaching experiments (FRAP) indicate that relocation of membrane proteins from the ER to LDs is independent of temperature and energy, and thus not mediated by classical vesicular transport routes. LD-localized membrane proteins are homogenously distributed at the perimeter of LDs, they are free to move over the LD surface and can even relocate back into the ER, indicating that they are not restricted to specialized sites on LDs. These observations indicate that LDs are functionally connected to the ER membrane and that this connection allows the efficient partitioning of membrane proteins between the two compartments.
Collapse
Affiliation(s)
- Nicolas Jacquier
- Department of Biology, Division of Biochemistry, University of Fribourg, 1700 Fribourg, Switzerland
| | | | | | | | | | | |
Collapse
|
17
|
Shibata Y, Shemesh T, Prinz WA, Palazzo AF, Kozlov MM, Rapoport TA. Mechanisms determining the morphology of the peripheral ER. Cell 2010; 143:774-88. [PMID: 21111237 PMCID: PMC3008339 DOI: 10.1016/j.cell.2010.11.007] [Citation(s) in RCA: 383] [Impact Index Per Article: 27.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2010] [Revised: 09/03/2010] [Accepted: 10/26/2010] [Indexed: 01/08/2023]
Abstract
The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by the curvature-stabilizing proteins reticulons and DP1/Yop1p, but how the sheets are formed is unclear. Here, we identify several sheet-enriched membrane proteins in the mammalian ER, including proteins that translocate and modify newly synthesized polypeptides, as well as coiled-coil membrane proteins that are highly upregulated in cells with proliferated ER sheets, all of which are localized by membrane-bound polysomes. These results indicate that sheets and tubules correspond to rough and smooth ER, respectively. One of the coiled-coil proteins, Climp63, serves as a "luminal ER spacer" and forms sheets when overexpressed. More universally, however, sheet formation appears to involve the reticulons and DP1/Yop1p, which localize to sheet edges and whose abundance determines the ratio of sheets to tubules. These proteins may generate sheets by stabilizing the high curvature of edges.
Collapse
Affiliation(s)
- Yoko Shibata
- Howard Hughes Medical Institute, Harvard Medical School, Boston, MA 02115, USA
| | | | | | | | | | | |
Collapse
|
18
|
Malchus N, Weiss M. Anomalous diffusion reports on the interaction of misfolded proteins with the quality control machinery in the endoplasmic reticulum. Biophys J 2010; 99:1321-8. [PMID: 20713018 DOI: 10.1016/j.bpj.2010.06.020] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2009] [Revised: 06/10/2010] [Accepted: 01/14/2010] [Indexed: 11/28/2022] Open
Abstract
A multitude of transmembrane proteins enters the endoplasmic reticulum (ER) as unfolded polypeptide chains. During their folding process, they interact repetitively with the ER's quality control machinery. Here, we have used fluorescence correlation spectroscopy to probe these interactions for a prototypical transmembrane protein, VSVG ts045, in vivo. While both folded and unfolded VSVG ts045 showed anomalous diffusion, the unfolded protein had a significantly stronger anomaly. This difference subsided when unfolded VSVG ts045 was in a complex with its chaperone calnexin, or when a mutant form of VSVG ts045 with only one glycan was used. Our experimental data and accompanying simulations suggest that the folding sensor of the quality control (UGT1) oligomerizes unfolded VSVG ts045, leading to a more anomalous/obstructed diffusion. In contrast, calnexin dissolves the oligomers, rendering unfolded VSVG ts045 more mobile, and hence prevents poisoning of the ER.
Collapse
Affiliation(s)
- Nina Malchus
- Cellular Biophysics Group, German Cancer Research Center, c/o BIOQUANT, Heidelberg, Germany
| | | |
Collapse
|
19
|
Nevalainen M, Kaisto T, Metsikkö K. Mobile ER-to-Golgi but not post-Golgi membrane transport carriers disappear during the terminal myogenic differentiation. Cell Tissue Res 2010; 342:107-16. [DOI: 10.1007/s00441-010-1041-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2010] [Accepted: 08/16/2010] [Indexed: 11/29/2022]
|
20
|
Abstract
In eukaryotic cells, N-glycosylation is typically the most common protein modification that occurs in the endoplasmic reticulum (ER) lumen. N-glycosylation is facilitated by a large heterologous protein complex called the oligosaccharyltransferase (OST) that allows the attachment of a high mannose oligosaccharide from a dolichol pyrophosphate donor en bloc onto suitable asparagine residues of newly synthesized nascent chains during translocation into the ER lumen (1). While the complexity of the OST is highly conserved in eukaryotes, the role of its different subunits is poorly defined. We have investigated the function of three OST subunits, the ER translocon-associated component ribophorin I, and two isoforms of the presumptive catalytic subunit, STT3. We use a combination of siRNA-mediated knockdown of individual proteins combined with a semi-permeabilized mammalian cell system to provide a robust read out for OST subunit function during N-glycosylation of model substrates in vitro. This approach is equally applicable to the study of other cellular components.
Collapse
|
21
|
Lai CW, Aronson DE, Snapp EL. BiP availability distinguishes states of homeostasis and stress in the endoplasmic reticulum of living cells. Mol Biol Cell 2010; 21:1909-21. [PMID: 20410136 PMCID: PMC2883936 DOI: 10.1091/mbc.e09-12-1066] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
BiP availability represents a powerful tool for reporting global secretory protein misfolding levels and investigating the molecular events of ER stress in single cells, independent of traditional UPR markers. Accumulation of misfolded secretory proteins causes cellular stress and induces the endoplasmic reticulum (ER) stress pathway, the unfolded protein response (UPR). Although the UPR has been extensively studied, little is known about the molecular changes that distinguish the homeostatic and stressed ER. The increase in levels of misfolded proteins and formation of complexes with chaperones during ER stress are predicted to further crowd the already crowded ER lumen. Surprisingly, using live cell fluorescence microscopy and an inert ER reporter, we find the crowdedness of stressed ER, treated acutely with tunicamycin or DTT, either is comparable to homeostasis or significantly decreases in multiple cell types. In contrast, photobleaching experiments revealed a GFP-tagged variant of the ER chaperone BiP rapidly undergoes a reversible quantitative decrease in diffusion as misfolded proteins accumulate. BiP mobility is sensitive to exceptionally low levels of misfolded protein stressors and can detect intermediate states of BiP availability. Decreased BiP availability temporally correlates with UPR markers, but restoration of BiP availability correlates less well. Thus, BiP availability represents a novel and powerful tool for reporting global secretory protein misfolding levels and investigating the molecular events of ER stress in single cells, independent of traditional UPR markers.
Collapse
Affiliation(s)
- Chun Wei Lai
- Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
| | | | | |
Collapse
|
22
|
Vander Heyden AB, Naismith TV, Snapp EL, Hodzic D, Hanson PI. LULL1 retargets TorsinA to the nuclear envelope revealing an activity that is impaired by the DYT1 dystonia mutation. Mol Biol Cell 2009; 20:2661-72. [PMID: 19339278 DOI: 10.1091/mbc.e09-01-0094] [Citation(s) in RCA: 102] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
TorsinA (TorA) is an AAA+ ATPase in the endoplasmic reticulum (ER) lumen that is mutated in early onset DYT1 dystonia. TorA is an essential protein in mice and is thought to function in the nuclear envelope (NE) despite localizing throughout the ER. Here, we report that transient interaction of TorA with the ER membrane protein LULL1 targets TorA to the NE. FRAP and Blue Native PAGE indicate that TorA is a stable, slowly diffusing oligomer in either the absence or presence of LULL1. Increasing LULL1 expression redistributes both wild-type and disease-mutant TorA to the NE, while decreasing LULL1 with shRNAs eliminates intrinsic enrichment of disease-mutant TorA in the NE. When concentrated in the NE, TorA displaces the nuclear membrane proteins Sun2, nesprin-2G, and nesprin-3 while leaving nuclear pores and Sun1 unchanged. Wild-type TorA also induces changes in NE membrane structure. Because SUN proteins interact with nesprins to connect nucleus and cytoskeleton, these effects suggest a new role for TorA in modulating complexes that traverse the NE. Importantly, once concentrated in the NE, disease-mutant TorA displaces Sun2 with reduced efficiency and does not change NE membrane structure. Together, our data suggest that LULL1 regulates the distribution and activity of TorA within the ER and NE lumen and reveal functional defects in the mutant protein responsible for DYT1 dystonia.
Collapse
Affiliation(s)
- Abigail B Vander Heyden
- Department of Cell Biology and Physiology, Washington University School of Medicine, St Louis, MO 63110, USA
| | | | | | | | | |
Collapse
|
23
|
Ronchi P, Colombo S, Francolini M, Borgese N. Transmembrane domain-dependent partitioning of membrane proteins within the endoplasmic reticulum. ACTA ACUST UNITED AC 2008; 181:105-18. [PMID: 18391072 PMCID: PMC2287291 DOI: 10.1083/jcb.200710093] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The length and hydrophobicity of the transmembrane domain (TMD) play an important role in the sorting of membrane proteins within the secretory pathway; however, the relative contributions of protein-protein and protein-lipid interactions to this phenomenon are currently not understood. To investigate the mechanism of TMD-dependent sorting, we used the following two C tail-anchored fluorescent proteins (FPs), which differ only in TMD length: FP-17, which is anchored to the endoplasmic reticulum (ER) membrane by 17 uncharged residues, and FP-22, which is driven to the plasma membrane by its 22-residue-long TMD. Before export of FP-22, the two constructs, although freely diffusible, were seen to distribute differently between ER tubules and sheets. Analyses in temperature-blocked cells revealed that FP-17 is excluded from ER exit sites, whereas FP-22 is recruited to them, although it remains freely exchangeable with the surrounding reticulum. Thus, physicochemical features of the TMD influence sorting of membrane proteins both within the ER and at the ER-Golgi boundary by simple receptor-independent mechanisms based on partitioning.
Collapse
Affiliation(s)
- Paolo Ronchi
- Consiglio Nazionale delle Ricerche, Institute of Neuroscience, Cellular and Molecular Pharmacology, University of Milan, 20129 Milan, Italy
| | | | | | | |
Collapse
|
24
|
Lavoie C, Paiement J. Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data. Histochem Cell Biol 2008; 129:117-28. [PMID: 18172663 PMCID: PMC2228376 DOI: 10.1007/s00418-007-0370-y] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/06/2007] [Indexed: 11/20/2022]
Abstract
The endoplasmic reticulum (ER) is a key organelle of the secretion pathway involved in the synthesis of both proteins and lipids destined for multiple sites within and without the cell. The ER functions to both co- and post-translationally modify newly synthesized proteins and lipids and sort them for housekeeping within the ER and for transport to their sites of function away from the ER. In addition, the ER is involved in the metabolism and degradation of specific xenobiotics and endogenous biosynthetic products. A variety of proteomics studies have been reported on different subcompartments of the ER providing an ER protein dictionary with new data being made available on many protein complexes of relevance to the biology of the ER including the ribosome, the translocon, coatomer proteins, cytoskeletal proteins, folding proteins, the antigen-processing machinery, signaling proteins and proteins involved in membrane traffic. This review examines proteomics and cytological data in support of the presence of specific molecular machines at specific sites or subcompartments of the ER.
Collapse
Affiliation(s)
- Christine Lavoie
- Département de pharmacologie, Faculté de Médecine, Université de Sherbrooke, Sherbrooke, QC, Canada, J1H 5N4
| | | |
Collapse
|
25
|
Kaakinen M, Papponen H, Metsikkö K. Microdomains of endoplasmic reticulum within the sarcoplasmic reticulum of skeletal myofibers. Exp Cell Res 2007; 314:237-45. [PMID: 17999928 DOI: 10.1016/j.yexcr.2007.10.009] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2007] [Revised: 09/14/2007] [Accepted: 10/06/2007] [Indexed: 10/22/2022]
Abstract
The relationship between the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of skeletal muscle cells has remained obscure. In this study, we found that ER- and SR-specific membrane proteins exhibited diverse solubility properties when extracted with mild detergents. Accordingly, the major SR-specific protein Ca(2+)-ATPase (SERCA) remained insoluble in Brij 58 and floated in sucrose gradients while typical ER proteins were partially or fully soluble. Sphingomyelinase treatment rendered SERCA soluble in Brij 58. Immunofluorescence staining for resident ER proteins revealed dispersed dots over I bands contrasting the continuous staining pattern of SERCA. Infection of isolated myofibers with enveloped viruses indicated that interfibrillar protein synthesis occurred. Furthermore, we found that GFP-tagged Dad1, able to incorporate into the oligosaccharyltransferase complex, showed the dot-like structures but the fusion protein was also present in membranes over the Z lines. This behaviour mimics that of cargo proteins that accumulated over the Z lines when blocked in the ER. Taken together, the results suggest that resident ER proteins comprised Brij 58-soluble microdomains within the insoluble SR membrane. After synthesis and folding in the ER-microdomains, cargo proteins and non-incorporated GFP-Dad1 diffused into the Z line-flanking compartment which likely represents the ER exit sites.
Collapse
Affiliation(s)
- Mika Kaakinen
- Department of Anatomy and Cell Biology, P.O. Box 5000 (Aapistie 7), FIN-90014 University of Oulu, Finland
| | | | | |
Collapse
|
26
|
Daigle N, Ellenberg J. LambdaN-GFP: an RNA reporter system for live-cell imaging. Nat Methods 2007; 4:633-6. [PMID: 17603490 DOI: 10.1038/nmeth1065] [Citation(s) in RCA: 174] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2007] [Accepted: 05/31/2007] [Indexed: 11/08/2022]
Abstract
We describe a GFP-based RNA reporter system (lambdaN-GFP) to visualize RNA molecules in live mammalian cells. It consists of GFP fused to an arginine-rich peptide derived from the phage lambda N protein, lambdaN22, which binds a unique minimal RNA motif and can be used to tag any RNA molecule. LambdaN-GFP uses a small and easy to engineer RNA tag, reducing the likelihood of perturbing the function of the tagged RNA molecule.
Collapse
Affiliation(s)
- Nathalie Daigle
- Gene Expression Unit, EMBL, Meyerhofstr. 1, D-69117 Heidelberg, Germany
| | | |
Collapse
|
27
|
Nikonov AV, Hauri HP, Lauring B, Kreibich G. Climp-63-mediated binding of microtubules to the ER affects the lateral mobility of translocon complexes. J Cell Sci 2007; 120:2248-58. [PMID: 17567679 DOI: 10.1242/jcs.008979] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Microtubules are frequently seen in close proximity to membranes of the endoplasmic reticulum (ER), and the membrane protein CLIMP-63 is thought to mediate specific interaction between these two structures. It was, therefore, of interest to investigate whether these microtubules are in fact responsible for the highly restricted lateral mobility of the translocon complexes in M3/18 cells as described before. As determined by fluorescence recovery after photobleaching, the breakdown of microtubules caused by drug treatment or by overexpression of the microtubule-severing protein spastin, resulted in an increased lateral mobility of the translocons that are assembled into polysomes. Also, the expression of a CLIMP-63 mutant lacking the microtubule-binding domain resulted in a significant increase of the lateral mobility of the translocon complexes. The most striking increase in the diffusion rate of the translocon complexes was observed in M3/18 cells transfected with a siRNA that effectively knocked down the expression of the endogenous CLIMP-63. It appears, therefore, that interaction of microtubules with the ER results in the immobilization of translocon complexes that are part of membrane-bound polysomes, and may play a role in the mechanism that segregates the rough and smooth domains of the ER.
Collapse
Affiliation(s)
- Andrei V Nikonov
- Department of Cell Biology, New York University School of Medicine, New York, NY 10016, USA
| | | | | | | |
Collapse
|
28
|
Heine C, Quitsch A, Storch S, Martin Y, Lonka L, Lehesjoki AE, Mole SE, Braulke T. Topology and endoplasmic reticulum retention signals of the lysosomal storage disease-related membrane protein CLN6. Mol Membr Biol 2007; 24:74-87. [PMID: 17453415 DOI: 10.1080/09687860600967317] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
CLN6 is a polytopic membrane protein of unknown function resident in the endoplasmic reticulum (ER). Mutant CLN6 causes the lysosomal storage disorder neuronal ceroid lipofuscinosis. Defining the topology of CLN6, and the structural domains and motifs required for interaction with cytosolic and luminal proteins may allow insights into its function. In this study we analysed the topology, ER retention and oligomerization of CLN6. We demonstrated, by differential membrane permeabilization of transfected BHK cells using specific detergents and two distinct antibodies, that CLN6 contains an N-terminal cytoplasmic domain, seven transmembrane domains, and a luminal C terminus. Mutational analyses and confocal immunofluorescence microscopy showed that changes of potential ER localization signals in the N- or C-terminal domain (a triple arginine cluster, and a dileucine motif) did not alter the subcellular localization of CLN6. The deletion of a dilysine motif impaired partially the ER localization of CLN6. Furthermore, expression analyses of fusion and deletion constructs in non-neuronal and neuronal cells suggested that two portions of CLN6 contributed to its retention within the ER. We showed that the N-terminal domain was necessary but not sufficient for ER retention of CLN6 and that deletion of transmembrane domains 6 and 7 was accompanied with the loss of ER localization and, in some instances, trafficking to the cisGolgi. From these data we concluded that CLN6 maintains its ER localization by expressing retention signals present in both the N-terminal cytosolic domain and in the carboxy-proximal transmembrane domains 6 and 7. Additionally, the ability of CLN6 to homodimerize may also prevent exit from the ER via an interaction with membrane-associated factors.
Collapse
Affiliation(s)
- Claudia Heine
- Department of Biochemistry, University Hospital Hamburg Eppendorf, Children's Hospital, Hamburg, Germany
| | | | | | | | | | | | | | | |
Collapse
|
29
|
Braun DP, Ding J, Shaheen F, Willey JC, Rana N, Dmowski WP. Quantitative expression of apoptosis-regulating genes in endometrium from women with and without endometriosis. Fertil Steril 2007; 87:263-8. [PMID: 17094974 DOI: 10.1016/j.fertnstert.2006.06.026] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2005] [Revised: 06/13/2006] [Accepted: 06/13/2006] [Indexed: 11/19/2022]
Abstract
OBJECTIVE To quantitate antiapoptotic and proapoptotic gene expression in endometrial cells (ECs) of women with and without endometriosis. DESIGN Determination of transcript abundance (TA) of apoptosis-regulating genes in eutopic and ectopic endometrial cells. SETTING Institute for the Study and Treatment of Endometriosis, Chicago, Illinois, and university-based research laboratories. PATIENT(S) Women with (n = 10) and without (n = 6) endometriosis. INTERVENTION(S) None. MAIN OUTCOME MEASURE(S) Quantitative virtually multiplexed transcript abundance measurement (VMTA) of the BCL2, BCLxL, defender against cell death-1 (DAD-1), BCLxS, P53, Caspase-1, and proliferating cell nuclear antigen (PCNA) genes. RESULT(S) The TA ratio of antiapoptotic to proapoptotic isoforms of the BCL-X gene favors survival in eutopic and ectopic ECs from women with endometriosis, but not control ECs. This was found throughout the menstrual cycle for ectopic ECs. Eutopic but not ectopic ECs also expressed increased TA of the antiapoptotic DAD-1 gene in endometriosis. Eutopic and ectopic ECs from women with endometriosis expressed decreased TA of p53 and Caspase-1 compared to ECs from women without endometriosis. Expression of these genes was not correlated with the proliferative state of ECs based on TA of the PCNA gene. CONCLUSION(S) Dysregulation in expression of pro- and antiapoptotic regulatory genes characterizes eutopic and ectopic ECs from women with endometriosis. These results are consistent with apoptotic resistance and enhanced survival of ECs in endometriosis.
Collapse
Affiliation(s)
- Donald P Braun
- Cancer Institute, Departments of Surgery and Medicine, Medical University of Toledo, Toledo, Ohio 43614-5809, USA.
| | | | | | | | | | | |
Collapse
|
30
|
Wilson CM, High S. Ribophorin I acts as a substrate-specific facilitator of N-glycosylation. J Cell Sci 2007; 120:648-57. [PMID: 17264154 DOI: 10.1242/jcs.000729] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The mammalian oligosaccharyltransferase (OST) complex is composed of about eight subunits and mediates the N-glycosylation of nascent polypeptide chains entering the endoplasmic reticulum (ER). The conserved STT3 subunit of eukaryotic OST complexes has been identified as its catalytic centre, yet although many other subunits are equally well conserved their functions are unknown. We used RNA interference to investigate the function of ribophorin I, an ER-translocon-associated subunit of the OST complex previously shown to associate with newly synthesised membrane proteins. We show that ribophorin I dramatically enhances the N-glycosylation of selected membrane proteins and provide evidence that it is not essential for N-glycosylation per se. Parallel studies confirm that STT3 is essential for transferase activity of the complex, but reveal that the two mammalian isoforms are not functionally equivalent when modifying bona fide polypeptide substrates. We propose a new model for OST function where ribophorin I acts as a chaperone or escort to promote the N-glycosylation of selected substrates by the catalytic STT3 subunits.
Collapse
Affiliation(s)
- Cornelia M Wilson
- Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester, M13 9PT, UK
| | | |
Collapse
|
31
|
Tu L, Kong XP, Sun TT, Kreibich G. Integrity of all four transmembrane domains of the tetraspanin uroplakin Ib is required for its exit from the ER. J Cell Sci 2006; 119:5077-86. [PMID: 17158912 DOI: 10.1242/jcs.03285] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The surface of the mammalian urinary bladder is covered by a crystalline, asymmetric unit membrane (AUM) structure that contains the four major uroplakins (UPs): Ia, Ib, II and IIIa. UPIa and UPIb belong to the family of tetraspanins. Although UPIa and UPIb are structurally conserved, only UPIb could exit from the endoplasmic reticulum (ER) and reach the cell surface when expressed alone in 293T cells. Modifications of the large extracellular loop of UPIb, such as mutation of the N-glycosylation site or the cysteines involved in the formation of three disulfide bridges, or exchanging the large luminal loop of UPIb with that of UPIa did not affect the ability of UPIb to reach the cell surface. However, modifications of any of the four transmembrane domains of UPIb led to ER retention, suggesting that the proper formation of helical bundles consisting of the tetraspanin transmembrane domains is a prerequisite for UPIb to exit from the ER. Results of sedimentation analysis suggested that aggregate formation is a mechanism for ER retention.
Collapse
Affiliation(s)
- Liyu Tu
- Department of Cell Biology, The Ronald O Perelman, NYU Cancer Institute, New York University School of Medicine, New York, NY 10016, USA
| | | | | | | |
Collapse
|
32
|
Abstract
The endoplasmic reticulum (ER) has distinct morphological domains composed of sheets and tubules, which differ in their characteristic membrane curvature. Key proteins may drive the formation of these structural morphologies, which in turn could generate the rough and smooth functional domains of the ER.
Collapse
Affiliation(s)
- Yoko Shibata
- Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
| | | | | |
Collapse
|
33
|
Sprocati T, Ronchi P, Raimondi A, Francolini M, Borgese N. Dynamic and reversible restructuring of the ER induced by PDMP in cultured cells. J Cell Sci 2006; 119:3249-60. [PMID: 16847050 DOI: 10.1242/jcs.03058] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
In many cells, the endoplasmic reticulum (ER) contains segregated smooth and rough domains, but the mechanism of this segregation is unclear. Here, we used a HeLa cell line, inducibly expressing a GFP fusion protein [GFP-b(5)tail] anchored to the ER membrane, as a tool to investigate factors influencing ER organisation. Induction of GFP-b(5)tail expression caused proliferation of the ER, but its normal branching polygonal meshwork architecture was maintained. Experiments designed to test the effects of drugs that alter ceramide levels revealed that treatment of these cells with Phenyl-2-decanoyl-amino-3-morpholino-1-propanol-hydrocholride (PDMP) generated patches of segregated smooth ER, organised as a random tubular network, which rapidly dispersed after removal of the drug. The effect of PDMP was independent of its activity as sphingolipid synthesis inhibitor, but could be partially reversed by a membrane-permeant Ca(2+) chelator. Although the smooth ER patches maintained connectivity with the remaining ER, they appeared to represent distinct domains differing in protein and lipid composition from the remaining ER. PDMP did not cause detachment of membrane-bound ribosomes, indicating that smooth ER patch generation was due to a reorganisation of pre-existing ribosome-free areas. Our results demonstrate a dynamic relationship between smooth and rough ER and have implications for the mechanisms regulating ER architecture.
Collapse
Affiliation(s)
- Teresa Sprocati
- Consiglio Nazionale delle Ricerche Institute of Neuroscience and Department of Medical Pharmacology, via Vanvitelli 32, University of Milano, 20129 Milano, Italy
| | | | | | | | | |
Collapse
|
34
|
Snapp EL, Sharma A, Lippincott-Schwartz J, Hegde RS. Monitoring chaperone engagement of substrates in the endoplasmic reticulum of live cells. Proc Natl Acad Sci U S A 2006; 103:6536-41. [PMID: 16617114 PMCID: PMC1458919 DOI: 10.1073/pnas.0510657103] [Citation(s) in RCA: 94] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The folding environment in the endoplasmic reticulum (ER) depends on multiple abundant chaperones that function together to accommodate a range of substrates. The ways in which substrate engagement shapes either specific chaperone dynamics or general ER attributes in vivo remain unknown. In this study, we have evaluated how changes in substrate flux through the ER influence the diffusion of both the lectin chaperone calreticulin and an inert reporter of ER crowdedness. During acute changes in substrate load, the inert probe revealed no changes in ER organization, despite significant changes in calreticulin dynamics. By contrast, inhibition of the lectin chaperone system caused rapid changes in the ER environment that could be reversed over time by easing new substrate burden. Our findings provide insight into the normal organization and dynamics of an ER chaperone and characterize the capacity of the ER to maintain homeostasis during acute changes in chaperone activity and availability.
Collapse
Affiliation(s)
- Erik L Snapp
- Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, 18 Library Drive, Building 18, Room 101, Bethesda, MD 20892, USA.
| | | | | | | |
Collapse
|
35
|
Chavan M, Lennarz W. The molecular basis of coupling of translocation and N-glycosylation. Trends Biochem Sci 2006; 31:17-20. [PMID: 16356726 DOI: 10.1016/j.tibs.2005.11.010] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2005] [Revised: 10/31/2005] [Accepted: 11/24/2005] [Indexed: 11/20/2022]
Abstract
Protein translocation and N-glycosylation are essential coordinated cellular processes that are mediated by the translocon and the oligosaccharyl transferase (OT), respectively. The recent identification of several specific interactions between the OT subunits and the translocon provides a molecular basis for the coupling of these two processes. Data suggest that multiple OT isoforms with different affinities for the translocon and ribosome and with heterogeneous subunit composition might exist in the endoplasmic reticulum (ER) membrane, thereby providing a means of regulating protein N-glycosylation.
Collapse
Affiliation(s)
- Manasi Chavan
- Department of Biochemistry and Cell Biology and Institute for Cell and Developmental Biology, Stony Brook University, Stony Brook, NY 11794, USA
| | | |
Collapse
|
36
|
Cruttwell C, Bernard J, Hilly M, Nicolas V, Tunwell REA, Mauger JP. Dynamics of the Ins(1,4,5)P3 receptor during polarization of MDCK cells. Biol Cell 2005; 97:699-707. [PMID: 15730344 DOI: 10.1042/bc20040503] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
BACKGROUND INFORMATION The uneven distribution of the Ins(1,4,5)P3R [Ins(1,4,5)P3 receptor] within the ER (endoplasmic reticulum) membrane generates spatially complex Ca2+ signals. The ER is a dynamic network, which allows the rapid diffusion of membrane proteins from one part of the cell to another. However, little is known about the localization and the dynamics of the Ins(1,4,5)P3R in the ER of living cells. We have used a MDCK (Madin-Darby canine kidney) clone stably expressing the Ins(1,4,5)P3R1-GFP (where GFP stands for green fluorescent protein) to investigate the effect of cell polarity on the lateral mobility of the Ins(1,4,5)P3R. RESULTS In non-confluent MDCK cells, the chimaera is homogeneously distributed throughout the ER and the nuclear envelope. FRAP (fluorescence recovery after photobleaching) experiments showed that the receptor can move freely in the ER with a diffusion constant (D=0.01 microm2/s) approx. ten times lower than other ER membrane proteins. In confluent polarized cells, two populations of receptor can be defined: one population is distributed in the cytoplasm and is mobile but with a slower diffusion constant (D=0.004 microm2/s) compared with non-confluent cells, whereas the other population is concentrated at the periphery of the cells and is apparently immobile. CONCLUSIONS The observed differences in the mobility of the Ins(1,4,5)P3R are most probably due to its interactions with stable protein complexes that form at the periphery of the polarized cells.
Collapse
Affiliation(s)
- Caroline Cruttwell
- Inserm U-442, Université Paris Sud, Bâtiment 443, F-91405 Orsay Cedex, France
| | | | | | | | | | | |
Collapse
|
37
|
Luedeke C, Frei SB, Sbalzarini I, Schwarz H, Spang A, Barral Y. Septin-dependent compartmentalization of the endoplasmic reticulum during yeast polarized growth. ACTA ACUST UNITED AC 2005; 169:897-908. [PMID: 15967812 PMCID: PMC2171641 DOI: 10.1083/jcb.200412143] [Citation(s) in RCA: 123] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Polarized cells frequently use diffusion barriers to separate plasma membrane domains. It is unknown whether diffusion barriers also compartmentalize intracellular organelles. We used photobleaching techniques to characterize protein diffusion in the yeast endoplasmic reticulum (ER). Although a soluble protein diffused rapidly throughout the ER lumen, diffusion of ER membrane proteins was restricted at the bud neck. Ultrastructural studies and fluorescence microscopy revealed the presence of a ring of smooth ER at the bud neck. This ER domain and the restriction of diffusion for ER membrane proteins through the bud neck depended on septin function. The membrane-associated protein Bud6 localized to the bud neck in a septin-dependent manner and was required to restrict the diffusion of ER membrane proteins. Our results indicate that Bud6 acts downstream of septins to assemble a fence in the ER membrane at the bud neck. Thus, in polarized yeast cells, diffusion barriers compartmentalize the ER and the plasma membrane along parallel lines.
Collapse
Affiliation(s)
- Cosima Luedeke
- Biology Department, Institute of Biochemistry, Swiss Federal Institute of Technology (ETH), ETH-Hönggerberg, 8093 Zürich, Switzerland
| | | | | | | | | | | |
Collapse
|
38
|
Vainauskas S, Menon AK. Endoplasmic Reticulum Localization of Gaa1 and PIG-T, Subunits of the Glycosylphosphatidylinositol Transamidase Complex. J Biol Chem 2005; 280:16402-9. [PMID: 15713669 DOI: 10.1074/jbc.m414253200] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
After integration into the endoplasmic reticulum (ER) membrane, ER-resident membrane proteins must be segregated from proteins that are exported to post-ER compartments. Here we analyze how human Gaa1 and PIG-T, two of the five subunits of the ER-localized glycosylphosphatidylinositol transamidase complex, are retained in the ER. Neither protein contains a known ER localization signal. Gaa1 is a polytopic membrane glycoprotein with a cytoplasmic N terminus and a large luminal loop between its first two transmembrane spans; PIG-T is a type I membrane glycoprotein. To simplify our analyses, we studied Gaa1 and PIG-T constructs that could not interact with other subunits of the transamidase. We now show that Gaa1(282), a truncated protein consisting of the first TM domain and luminal loop of Gaa1, is correctly oriented, N-glycosylated, and ER-localized. Removal of a potential ER localization signal in the form of a triple arginine cluster near the N terminus of Gaa1 or Gaa1(282) had no effect on ER localization. Fusion proteins consisting of different elements of Gaa1(282) appended to alpha2,6-sialyltransferase or transferrin receptor could exit the ER, indicating that Gaa1(282), and by implication Gaa1, does not contain any dominant ER-sorting determinants. The data suggest that Gaa1 is passively retained in the ER by a signalless mechanism. In contrast, similar analyses of PIG-T revealed that it is ER-localized because of information in its transmembrane span; fusion of the PIG-T transmembrane span to Tac antigen, a plasma membrane-localized protein, caused the fusion protein to remain in the ER. These data are discussed in the context of models that have been proposed to account for retention of ER membrane proteins.
Collapse
Affiliation(s)
- Saulius Vainauskas
- Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706-1544, USA.
| | | |
Collapse
|
39
|
Shen J, Snapp EL, Lippincott-Schwartz J, Prywes R. Stable binding of ATF6 to BiP in the endoplasmic reticulum stress response. Mol Cell Biol 2005; 25:921-32. [PMID: 15657421 PMCID: PMC543992 DOI: 10.1128/mcb.25.3.921-932.2005] [Citation(s) in RCA: 165] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Endoplasmic reticulum (ER) stress-induced activation of ATF6, an ER membrane-bound transcription factor, requires a dissociation step from its inhibitory regulator, BiP. It has been generally postulated that dissociation of the BiP-ATF6 complex is a result of the competitive binding of misfolded proteins generated during ER stress. Here we present evidence against this model and for an active regulatory mechanism for dissociation of the complex. Contradictory to the competition model that is based on dynamic binding of BiP to ATF6, our data reveal relatively stable binding. First, the complex was easily isolated, in contrast to many chaperone complexes that require chemical cross-linking. Second, ATF6 bound at similar levels to wild-type BiP and a BiP mutant form that binds substrates stably because of a defect in its ATPase activity. Third, ER stress specifically induced the dissociation of BiP from ER stress transducers while the competition model would predict dissociation from any specific substrate. Fourth, the ATF6-BiP complex was resistant to ATP-induced dissociation in vitro when isolated without detergents, suggesting that cofactors stabilize the complex. In favor of an active dissociation model, one specific region within the ATF6 lumenal domain was identified as a specific ER stress-responsive sequence required for ER stress-triggered BiP release. Together, our results do not support a model in which competitive binding of misfolded proteins causes dissociation of the BiP-ATF6 complex in stressed cells. We propose that stable BiP binding is essential for ATF6 regulation and that ER stress dissociates BiP from ATF6 by actively restarting the BiP ATPase cycle.
Collapse
Affiliation(s)
- Jingshi Shen
- Department of Biological Sciences, Columbia University, Fairchild 813B, MC 2420, 1212 Amsterdam Avenue, New York, NY 10027, USA
| | | | | | | |
Collapse
|
40
|
Affiliation(s)
- Aixin Yan
- Department of Biochemistry and Cell Biology and Institute for Cell and Developmental Biology, State University of New York, Stony Brook, New York 11794, USA
| | | |
Collapse
|
41
|
Niederer KE, Morrow DK, Gettings JL, Irick M, Krawiecki A, Brewster JL. Cypermethrin blocks a mitochondria-dependent apoptotic signal initiated by deficient N-linked glycosylation within the endoplasmic reticulum. Cell Signal 2005; 17:177-86. [PMID: 15494209 DOI: 10.1016/j.cellsig.2004.06.009] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2004] [Accepted: 06/30/2004] [Indexed: 11/28/2022]
Abstract
The endoplasmic reticulum (ER) serves as a critical site of protein synthesis and processing. The temperature-sensitive hamster fibroblast cell line (tsBN7) displays deficient N-linked glycosylation activity at the restrictive temperature and activates cellular apoptosis. Temperature-shifted tsBN7 cells display induction of Grp78 and Gadd153, genes known to be induced by ER stress, and activate apoptosis via the release of cytochrome c from the mitochondria. Cyclosporin A (CsA), a potent blocker of the mitochondrial permeability transition pore (PTP), was sufficient to block cytochrome c release and to rescue tsBN7 cells from apoptosis. CsA-treated cells displayed Grp78 induction at the restrictive temperature, consistent with an ER stress signal being carried to the nucleus, while the apoptosis-associated transcription factor, Gadd153, displayed only a mild induction. Cypermethrin, a type II pyrethroid known to perturb Ca(2+) signaling in neuronal cells, was sufficient to arrest apoptosis under these conditions. This work identifies type II pyrethroids as a valuable new tool in the characterization of cellular stress signaling pathways.
Collapse
Affiliation(s)
- Katherine E Niederer
- Natural Science Division, Pepperdine University, 24255 Pacific Coast Highway, Malibu, CA 90263, USA
| | | | | | | | | | | |
Collapse
|
42
|
Abstract
The study of intracellular membrane trafficking and organelle dynamics has been revolutionized with the advent of GFP technology, which allows individual proteins to be tagged with GFP and monitored in living cells. Using GFP-based techniques such as selective photobleaching and time-lapse imaging of different GFP color variants, it is now possible to address a host of questions related to protein retention within the organelles, the origin and fate of transport intermediates, and the extent of trafficking through different transport pathways. This has provided unexpected new insights into the organization and function of secretory and endocytic pathways within cells.
Collapse
Affiliation(s)
- Jennifer Lippincott-Schwartz
- Section on Organelle Biology, Cell Biology & Metabolism Branch, Division of Intramural Research, NICHD, NIH, Building 18T, Room 101, 18 Library Drive, Bethesda, MD 20892-5430, USA.
| |
Collapse
|
43
|
Wilson CM, Kraft C, Duggan C, Ismail N, Crawshaw SG, High S. Ribophorin I associates with a subset of membrane proteins after their integration at the sec61 translocon. J Biol Chem 2004; 280:4195-206. [PMID: 15556939 DOI: 10.1074/jbc.m410329200] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The biosynthesis of membrane proteins at the endoplasmic reticulum (ER) involves the integration of the polypeptide at the Sec61 translocon together with a number of maturation events, such as N-glycosylation and signal sequence cleavage, that can occur both during and after synthesis. To better understand the events occurring after the release of the nascent chain from the ER translocon, we investigated the ER components adjacent to the transmembrane-spanning domain of a well characterized fragment of the amyloid precursor protein. Using individual cysteine residues as site-specific cross-linking targets, we found that several ER components can be cross-linked to the fully integrated polypeptide. We identified strong adducts with both the ribophorin I subunit of the oligosaccharyltransferase complex and the 25-kDa subunit of the signal peptidase complex. Focusing on the association with ribophorin I, we found that adduct formation occurred exclusively after the exit of the nascent chain from the Sec61 translocon and was unaffected by the N-glycosylation status of the associated precursor. Only a subset of newly made membrane proteins associated with ribophorin I in vitro, and we could recapitulate a specific association between the amyloid precursor protein fragment and ribophorin I in vivo. Taken together, our data suggest a model where ribophorin I may function to retain potential substrates in close proximity to the catalytic subunit of the oligosaccharyltransferase and thereby stochastically improve the efficiency of the N-glycosylation reaction in vivo. Alternatively ribophorin I may be multifunctional and facilitate additional processes, for example, ER quality control.
Collapse
Affiliation(s)
- Cornelia M Wilson
- Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester M13 9PT, United Kingdom
| | | | | | | | | | | |
Collapse
|
44
|
Ferreri-Jacobia M, Mak DOD, Foskett JK. Translational mobility of the type 3 inositol 1,4,5-trisphosphate receptor Ca2+ release channel in endoplasmic reticulum membrane. J Biol Chem 2004; 280:3824-31. [PMID: 15537642 DOI: 10.1074/jbc.m409462200] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The inositol 1,4,5-trisphosphate receptor (InsP3R) is an integral membrane protein in the endoplasmic reticulum (ER) which functions as a ligand-gated Ca2+ release channel. InsP3-mediated Ca2+ release modulates the cytoplasmic free Ca2+ concentration ([Ca2+]i), providing a ubiquitous intracellular signal with high temporal and spatial specificity. Precise localization of the InsP3R is believed to be important for providing local [Ca2+] regulation and for ensuring efficient functional coupling between Ca2+ release sites by enabling graded recruitment of channels with increasing stimulus strength in the face of the intrinsically unstable regenerative process of Ca2+-induced Ca2+ release. Highly localized Ca2+ release has been attributed to the ability of the InsP3R channels to cluster and to be localized to discrete areas, suggesting that mechanisms may exist to restrict their movement. Here, we examined the lateral mobility of the type 3 isoform of the InsP3R (InsP3R3) in the ER membrane by performing confocal fluorescence recovery after photobleaching of an InsP3R3 with green fluorescent protein fused to its N terminus. In Chinese hamster ovary and COS-7 cells, the diffusion coefficient D was approximately 4 x 10(-10) cm2/s at room temperature, a value similar to that determined for other ER-localized integral membrane proteins, with a high fraction (approximately 75%) of channels mobile. D was modestly increased at 37 degrees C, and it as well as the mobile fraction were reversibly reduced by ATP depletion. Although disruption of the actin cytoskeleton (latrunculin) was without effect, disruption of microtubules (nocodazole) reduced D by half without affecting the mobile fraction. We conclude that the entire ER is continuous in these cells, with the large majority of InsP3R3 channels free to diffuse throughout it, at rates that are comparable with those measured for other polytopic ER integral membrane proteins. The observed InsP3R3 mobility may be higher than its intrinsic diffusional mobility because of additional ATP- and microtubule-facilitated motility of the channel.
Collapse
Affiliation(s)
- Michelle Ferreri-Jacobia
- Department of Physiology, University of Pennsylvania, School of Medicine, Philadelphia, Pennsylvania 19104, USA
| | | | | |
Collapse
|
45
|
Snapp EL, Reinhart GA, Bogert BA, Lippincott-Schwartz J, Hegde RS. The organization of engaged and quiescent translocons in the endoplasmic reticulum of mammalian cells. ACTA ACUST UNITED AC 2004; 164:997-1007. [PMID: 15051734 PMCID: PMC2172055 DOI: 10.1083/jcb.200312079] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Protein translocons of the mammalian endoplasmic reticulum are composed of numerous functional components whose organization during different stages of the transport cycle in vivo remains poorly understood. We have developed generally applicable methods based on fluorescence resonance energy transfer (FRET) to probe the relative proximities of endogenously expressed translocon components in cells. Examination of substrate-engaged translocons revealed oligomeric assemblies of the Sec61 complex that were associated to varying degrees with other essential components including the signal recognition particle receptor TRAM and the TRAP complex. Remarkably, these components not only remained assembled but also had a similar, yet distinguishable, organization both during and after nascent chain translocation. The persistence of preassembled and complete translocons between successive rounds of transport may facilitate highly efficient translocation in vivo despite temporal constraints imposed by ongoing translation and a crowded cellular environment.
Collapse
Affiliation(s)
- Erik L Snapp
- Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, 18 Library Dr., Bldg. 18, Rm. 101, Bethesda, MD 20892, USA
| | | | | | | | | |
Collapse
|
46
|
Biehn SE, Czymmek KJ, Leavens KF, Karin NJ. Expression of the sarco/endoplasmic Ca2+-ATPase, SERCA1a, in fibroblasts induces the formation of organelle membrane arrays. Exp Cell Res 2004; 292:78-88. [PMID: 14720508 DOI: 10.1016/j.yexcr.2003.08.016] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Members of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) family are transmembrane proteins that are essential for the function of intracellular Ca(2+) storage organelles. We found that overexpression of avian muscle SERCA1a in transfected mouse fibroblasts led to the appearance of tubular membrane bundles that we termed plaques. These structures were generated in transfected cells when SERCA1a protein expression approached the endogenous level measured in chicken skeletal muscle. Plaque membranes had associated ribosomes and contained endoplasmic reticulum (ER) proteins. Endogenous ER protein levels were not elevated in SERCA1a-expressing cells, indicating that plaques were not generalized proliferations of ER but rather a reorganization of existing organelle membrane. Plaque formation also was observed in cells expressing a green fluorescent protein-SERCA1a fusion protein (GFP-SERCA1a). GFP-SERCA1a molecules displayed extensive lateral mobility between plaques, suggesting the presence of membrane continuities between these structures. Plaques were induced in cells expressing cDNA encoding a catalytically silent SERCA1a mutant indicating that ER redistribution was driven by a structural feature of the enzyme. SERCA1a-induced plaque formation shares some characteristics of sarcoplasmic reticulum (SR) biogenesis during muscle differentiation, and high-level SERCA1a expression in vivo may contribute to the formation of SR from ER during embryonic myogenesis.
Collapse
Affiliation(s)
- Suzanne E Biehn
- Department of Biological Sciences, University of Delaware, Newark, DE 19716, USA
| | | | | | | |
Collapse
|
47
|
Scheper W, Thaminy S, Kais S, Stagljar I, Römisch K. Coordination of N-glycosylation and protein translocation across the endoplasmic reticulum membrane by Sss1 protein. J Biol Chem 2003; 278:37998-8003. [PMID: 12860997 DOI: 10.1074/jbc.m300176200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Secretory proteins are translocated across the endoplasmic reticulum (ER) membrane through a channel formed by three proteins, namely Sec61p, Sbh1p, and Sss1p (Johnson, A. E., and van Waes, M. A. (1999) Annu. Rev. Cell Dev. Biol. 15, 799-842). Sec61p and Sss1p are essential for translocation (Esnault, Y., Blondel, M. O., Deshaies, R. J., Schekman, R., and Kepes, F. (1993) EMBO J. 12, 4083-4093). Sec61p is a polytopic membrane protein that lines the protein translocation channel. The role of Sss1p is unknown. During import into the ER through the Sec61p channel, many proteins are N-glycosylated before translocation is completed. In addition, both the Sec61 channel and oligosaccharyl transferase (OST) copurify with ribosomes from rough ER, suggesting that OST is located in close proximity to the Sec61 channel (Gorlich, D., Prehn, S., Hartmann, E., Kalies, K.-U., and Rapoport, T. A. (1992) Cell 71, 489-503 and Wang, L., and Dobberstein, B. (1999) FEBS Lett. 457, 316-322). Here, we demonstrate a direct interaction between Sss1p and a subunit of OST, Wbp1p, using the split-ubiquitin system and co-immunoprecipitation. We generated mutants in the cytoplasmic domain of Sss1p that disturb the interaction with OST and are viable but display a translocation defect specific for proteins with glycosylation acceptor sites. Our data suggest that Sss1p coordinates translocation across the ER membrane and N-linked glycosylation of secretory proteins.
Collapse
Affiliation(s)
- Wiep Scheper
- University of Cambridge, Cambridge Institute for Medical Research, Hills Road, Cambridge CB2 2XY, United Kingdom
| | | | | | | | | |
Collapse
|
48
|
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.
Collapse
Affiliation(s)
- IngMarie Nilsson
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, TX 77843, USA
| | | | | | | | | | | | | | | |
Collapse
|
49
|
Lippincott-Schwartz J, Patterson GH. Development and use of fluorescent protein markers in living cells. Science 2003; 300:87-91. [PMID: 12677058 DOI: 10.1126/science.1082520] [Citation(s) in RCA: 773] [Impact Index Per Article: 36.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
The ability to visualize, track, and quantify molecules and events in living cells with high spatial and temporal resolution is essential for understanding biological systems. Only recently has it become feasible to carry out these tasks due to the advent of fluorescent protein technology. Here, we trace the development of highly visible and minimally perturbing fluorescent proteins that, together with updated fluorescent imaging techniques, are providing unprecedented insights into the movement of proteins and their interactions with cellular components in living cells.
Collapse
Affiliation(s)
- Jennifer Lippincott-Schwartz
- Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
| | | |
Collapse
|
50
|
Yabal M, Brambillasca S, Soffientini P, Pedrazzini E, Borgese N, Makarow M. Translocation of the C terminus of a tail-anchored protein across the endoplasmic reticulum membrane in yeast mutants defective in signal peptide-driven translocation. J Biol Chem 2003; 278:3489-96. [PMID: 12446686 DOI: 10.1074/jbc.m210253200] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
C-tail-anchored proteins are defined by an N-terminal cytosolic domain followed by a transmembrane anchor close to the C terminus. Their extreme C-terminal polar residues are translocated across membranes by poorly understood post-translational mechanism(s). Here we have used the yeast system to study translocation of the C terminus of a tagged form of mammalian cytochrome b(5), carrying an N-glycosylation site in its C-terminal domain (b(5)-Nglyc). Utilization of this site was adopted as a rigorous criterion for translocation across the ER membrane of yeast wild-type and mutant cells. The C terminus of b(5)-Nglyc was rapidly glycosylated in mutants where Sec61p was defective and incapable of translocating carboxypeptidase Y, a well known substrate for post-translational translocation. Likewise, inactivation of several other components of the translocon machinery had no effect on b(5)-Nglyc translocation. The kinetics of translocation were faster for b(5)-Nglyc than for a signal peptide-containing reporter. Depletion of the cellular ATP pool to a level that retarded Sec61p-dependent post-translational translocation still allowed translocation of b(5)-Nglyc. Similarly, only low ATP concentrations (below 1 microm), in addition to cytosolic protein(s), were required for in vitro translocation of b(5)-Nglyc into mammalian microsomes. Thus, translocation of tail-anchored b(5)-Nglyc proceeds by a mechanism different from that of signal peptide-driven post-translational translocation.
Collapse
Affiliation(s)
- Monica Yabal
- Program of Cellular Biotechnology, Institute of Biotechnology, University of Helsinki, Viikinkaari 9, 00710 Helsinki, Finland
| | | | | | | | | | | |
Collapse
|