DOCSLIB.ORG

Apoprotein B100 Has a Prolonged Interaction with the Translocon

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Apoprotein B100 Has a Prolonged Interaction with the Translocon Proc. Natl. Acad. Sci. USA Vol. 95, pp. 14733–14738, December 1998 Cell Biology Apoprotein B100 has a prolonged interaction with the translocon during which its lipidation and translocation change from dependence on the microsomal triglyceride transfer protein to independence DEBORAH M. MITCHELL*†,MINGYUE ZHOU†‡,RAJALAKSHMI PARIYARATH*, HONGXING WANG*, JOHN D. AITCHISON§,HENRY N. GINSBERG‡, AND EDWARD A. FISHER*¶ *Laboratory of Lipoprotein Research, Cardiovascular Institute, Department of Medicine, and Department of Cell Biology and Anatomy, Mount Sinai School of Medicine, 1 Gustave Levy Place, Box 1030, New York, NY 10029; ‡Department of Medicine, Columbia University College of Physicians and Surgeons, 630 West 168 Street, New York, NY 10032; and §Department of Cell Biology and Anatomy, University of Alberta, Edmonton, AB, Canada T6G 2H7 Communicated by Jan L. Breslow, The Rockefeller University, New York, NY, October 8, 1998 (received for review August 5, 1998) ABSTRACT When lipid synthesis is limited in HepG2 cells, tion. This presecretory degradation is increased when the avail- apoprotein B100 (apoB100) is not secreted but rapidly degraded ability of lipid ligands of apoB100 is limited by inadequate levels by the ubiquitin-proteasome pathway. To investigate apoB100 of either lipid synthesis or MTP-mediated lipid-transfer activity biosynthesis and secretion further, the physical and functional (7, 8). In recent studies with the human hepatocarcinoma cell line states of apoB100 destined for either degradation or lipoprotein HepG2, a standard model of lipoprotein metabolism, we and assembly were studied under conditions in which lipid synthesis, others have shown that most, if not all, of the degradation of proteasomal activity, and microsomal triglyceride transfer pro- apoB100 that occurs when lipid availability is limited is mediated tein (MTP) lipid-transfer activity were varied. Cells were pre- by the ubiquitin-proteasome pathway (9–11). treated with a proteasomal inhibitor (which remained with the Although the components of this pathway are cytosolic, a cells throughout the experiment) and radiolabeled for 15 min. number of recent reports have shown that a variety of membrane- During the chase period, labeled apoB100 remained associated associated and secretory proteins can be targeted to the protea- with the microsomes. Furthermore, by crosslinking sec61b to some for degradation in eukaryotic cells (reviewed in refs. 12 and apoB100, we showed that apoB100 remained close to the trans- 13). The precise mechanism by which these proteins become locon at the same time apoB100–ubiquitin conjugates could be substrates for a cytosolic protease has remained elusive. Based on detected. When lipid synthesis and lipoprotein assemblyy results for major histocompatibility complex class I molecules, it secretion were stimulated by adding oleic acid (OA) to the chase has been hypothesized that a protein translocated into the ER can medium, apoB100 was deubiquitinated, and its interaction with be fully ‘‘dislocated’’ back into the cytosol and subsequently sec61b was disrupted, signifying completion of translocation attacked by the proteasome (14). The need for complete dislo- concomitant with the formation of lipoprotein particles. MTP cation, which would require some form of reverse translocation, participates in apoB100 translocation and lipoprotein assembly. may not be general, given the finding that proteasomes can be In the presence of OA, when MTP lipid-transfer activity was found in association with the cytosolic face of the ER (15, 16). inhibited at the end of pulse labeling, apoB100 secretion was Thus, the initial attack of the proteasome could be directed abolished. In contrast, when the labeled apoB100 was allowed to against a protein domain that is or becomes exposed to the cytosol accumulate in the cell for 60 min before adding OA and the while other domains remain segregated by the ER membrane. inhibitor, apoB100 lipidation and secretion were no longer Consistent with this possibility is the finding that two yeast impaired. Overall, the data imply that during most of its asso- ubiquitin-conjugating enzymes essential for ER-associated pro- ciation with the endoplasmic reticulum, apoB100 is close to or teasomal degradation, Ubc6p and Ubc7p, can be localized to the within the translocon and is accessible to both the ubiquitin- ER. Ubc6p does so as an integral membrane protein (17), and proteasome and lipoprotein-assembly pathways. Furthermore, Ubc7p does so by docking onto the membrane protein Cue1p MTP lipid-transfer activity seems to be necessary only for early (18). translocation and lipidation events. The extremely large size of apoB100, its potential to pause or arrest during translocation (19, 20), as well as its multiple hydro- b Apolipoprotein B100 (apoB100) is a 4,536-aa polypeptide and is phobic -sheet domains (21), which would favor membrane the major structural protein of the liver-derived very low density interactions, led us to consider whether full translocation fol- and low density lipoproteins. Hepatic lipoprotein assembly begins lowed by complete dislocation was a likely path from the ER to when apoB100 is cotranslationally translocated across the endo- the proteasome. Key elements of the degradation pathway can be plasmic reticulum (ER) membrane (1) and interacts with the on the cytosolic side of the ER membrane; this finding suggested luminal microsomal triglyceride transfer protein (MTP; refs. that apoB100 that was incompletely translocated or partially 2–4). MTP catalyzes the initial transfer of lipid to nascent dislocated could be the proximal substrate of the ubiquitin- apoB100, with subsequent lipoprotein maturation steps occurring proteasome machinery. in the ER and possibly the Golgi apparatus (5, 6). Studies of We hypothesized, therefore, that even after translation, the cultured primary hepatocytes and transformed liver cells of apoB100 polypeptide could remain in close proximity to the human and nonhuman origin have established that significant translocon. While in close proximity, it could be either targeted control over apoB100 secretion can be exerted at the posttrans- for degradation or assembled as a lipoprotein, depending on the lational level by the targeting of the nascent protein to degrada- availability of lipid ligands. This hypothesis was tested primarily The publication costs of this article were defrayed in part by page charge Abbreviations: apoAI, apoprotein AI; apoB100, apoprotein B100; ER, endoplasmic reticulum; MTP, microsomal triglyceride transfer pro- payment. This article must therefore be hereby marked ‘‘advertisement’’ in tein; OA, oleic acid. accordance with 18 U.S.C. §1734 solely to indicate this fact. †D.M.M. and M.Z. contributed equally to this work. © 1998 by The National Academy of Sciences 0027-8424y98y9514733-6$2.00y0 ¶To whom reprint requests should be addressed. e-mail: edmd-phdo PNAS is available online at www.pnas.org. [email protected]. 14733 Downloaded by guest on September 26, 2021 14734 Cell Biology: Mitchell et al. Proc. Natl. Acad. Sci. USA 95 (1998) by examining the cellular localization and functional state of medium for 0, 20, 40, or 60 min. Cells were then disrupted by apoB100 protected from degradation by inhibition of the pro- sonication. After low-speed centrifugation (10,000 3 g, 10 min, teasome. As described below, the results not only support the 4°C) to remove nuclei and unbroken cells, the supernatant was hypothesis but further suggest that apoB100 destined to become fractionated by centrifugation at 100,000 3 g for1hat4°C. part of a lipoprotein has a prolonged interaction with the ER ApoB100 was then immunoprecipitated from the microsomal membrane; during this time, apoB100 becomes associated with its (pellet) and cytosolic (supernatant) fractions. lipid ligands in a process that depends on MTP only at an early Crosslinking of ApoB100 and sec61b. HepG2 cells were stage of the assembly process. pretreated with a proteasome inhibitor (either MG132 or lacta- cystin at a final concentration of 10 mM) for 1 h and then labeled EXPERIMENTAL PROCEDURES for 15 min by adding 100 mCiyml of [35S]methionineycysteine. Materials. Rabbit anti-human apoB100 and anti-human apo- The medium was removed, and the cells were washed twice with protein AI (apoAI) antisera were from Calbiochem; mouse chase medium and then incubated in chase medium containing anti-human apoB100 antiserum was from Caltag (South San the proteasome inhibitor for 0, 20, 40, or 60 min in the presence Francisco, CA). Rabbit anti-sec61b and anti-sec61a antisera were or absence of OA (0.8 mM). At the end of each chase period, cells gifts from Tom A. Rapoport (Harvard University, Cambridge, were washed with ice-cold PBS and harvested in crosslinking MA; ref. 22) and Arthur E. Johnson (TexasA&M,College buffer (PBSy0.4% digitoniny0.5 mM EDTA). The permeable cell Station, TX). Rabbit anti-ubiquitin antiserum was from Stress- suspension was incubated on ice for1hwiththehomobifunc- Gen Biotechnologies (Victoria, Canada). Antisera and associated tional chemical crosslinking reagent dithiobis(succinimidyl pro- reagents for immunofluorescence studies are described below. pionate) (final concentration of 0.25 mM added as a concentrated [35S]methionineycysteine was from DuPontyNEN. Lactacystin solution in dimethyl sulfoxide). The reaction was quenched by was from the laboratory of E. J. Corey (Harvard University; ref. incubation in 20 mM Tris (pH 7.5) for 15 min. ApoB100–sec61b 23) and MG132
Load more

Recommended publications
  • The Signal Recognition Particle
    P1: GDL May 22, 2001 22:53 Annual Reviews AR131-22 Annu. Rev. Biochem. 2001. 70:755–75 Copyright c 2001 by Annual Reviews. All rights reserved THE SIGNAL RECOGNITION PARTICLE Robert J. Keenan1, Douglas M. Freymann2, Robert M. Stroud3, and Peter Walter3,4 1Maxygen, 515 Galveston Drive, Redwood City, California 94063; e-mail: [email protected] 2Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611; e-mail: [email protected] 3Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143; e-mail: [email protected] 4The Howard Hughes Medical Institute, University of California, San Francisco, California 94143; e-mail: [email protected] Key Words Alu domain, SRP, SRP54, Ffh, SRP receptor, FtsY, signal sequence, GTPase, SRP9/14, SRP RNA ■ Abstract The signal recognition particle (SRP) and its membrane-associated re- ceptor (SR) catalyze targeting of nascent secretory and membrane proteins to the protein translocation apparatus of the cell. Components of the SRP pathway and salient fea- tures of the molecular mechanism of SRP-dependent protein targeting are conserved in all three kingdoms of life. Recent advances in the structure determination of a number of key components in the eukaryotic and prokaryotic SRP pathway provide new insight into the molecular basis of SRP function, and they set the stage for future work toward an integrated picture that takes into account the dynamic and contextual properties of this remarkable cellular machine. CONTENTS INTRODUCTION ................................................ 756 by UNIVERSITY OF CHICAGO LIBRARIES on 11/05/07. For personal use only. COTRANSLATIONAL PROTEIN TARGETING ..........................756 Annu.
    [Show full text]
  • A Clearer Picture of the ER Translocon Complex Max Gemmer and Friedrich Förster*
    © 2020. Published by The Company of Biologists Ltd | Journal of Cell Science (2020) 133, jcs231340. doi:10.1242/jcs.231340 REVIEW A clearer picture of the ER translocon complex Max Gemmer and Friedrich Förster* ABSTRACT et al., 1986). SP-equivalent N-terminal transmembrane helices that The endoplasmic reticulum (ER) translocon complex is the main gate are not cleaved off can also target proteins to the ER through the into the secretory pathway, facilitating the translocation of nascent same mechanism. In this SRP-dependent co-translational ER- peptides into the ER lumen or their integration into the lipid membrane. targeting mode, ribosomes associate with the ER membrane via ER Protein biogenesis in the ER involves additional processes, many of translocon complexes. These membrane protein complexes them occurring co-translationally while the nascent protein resides at translocate nascent soluble proteins into the ER, integrate nascent the translocon complex, including recruitment of ER-targeted membrane proteins into the ER membrane, mediate protein folding ribosome–nascent-chain complexes, glycosylation, signal peptide and membrane protein topogenesis, and modify them chemically. In cleavage, membrane protein topogenesis and folding. To perform addition to co-translational protein import and translocation, distinct such varied functions on a broad range of substrates, the ER ER translocon complexes enable post-translational translocation and translocon complex has different accessory components that membrane integration. This post-translational pathway is widespread associate with it either stably or transiently. Here, we review recent in yeast (Panzner et al., 1995), whereas higher eukaryotes primarily structural and functional insights into this dynamically constituted use it for relatively short peptides (Schlenstedt and Zimmermann, central hub in the ER and its components.
    [Show full text]
  • Structure of the Human Signal Peptidase Complex Reveals the Determinants for Signal Peptide Cleavage
    bioRxiv preprint doi: https://doi.org/10.1101/2020.11.11.378711; this version posted November 11, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Structure of the Human Signal Peptidase Complex Reveals the Determinants for Signal Peptide Cleavage A. Manuel Liaci1, Barbara Steigenberger2,3, Sem Tamara2,3, Paulo Cesar Telles de Souza4, Mariska Gröllers-Mulderij1, Patrick Ogrissek1,5, Siewert J. Marrink4, Richard A. Scheltema2,3, Friedrich Förster1* Abstract The signal peptidase complex (SPC) is an essential membrane complex in the endoplasmic reticulum (ER), where it removes signal peptides (SPs) from a large variety of secretory pre-proteins with exquisite specificity. Although the determinants of this process have been established empirically, the molecular details of SP recognition and removal remain elusive. Here, we show that the human SPC exists in two functional paralogs with distinct proteolytic subunits. We determined the atomic structures of both paralogs using electron cryo- microscopy and structural proteomics. The active site is formed by a catalytic triad and abuts the ER membrane, where a transmembrane window collectively formed by all subunits locally thins the bilayer. This unique architecture generates specificity for thousands of SPs based on the length of their hydrophobic segments. Keywords Signal Peptidase Complex, Signal Peptide, Protein Maturation, Membrane Thinning, cryo-EM, Crosslinking Mass Spectrometry, Molecular Dynamics Simulations, Protein Secretion, ER Translocon 1Cryo-Electron Microscopy, Bijvoet Centre for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
    [Show full text]
  • The Chloroplast Protein Import System: from Algae to Trees☆
    CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Biochimica et Biophysica Acta 1833 (2013) 314–331 Contents lists available at SciVerse ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbamcr Review The chloroplast protein import system: From algae to trees☆ Lan-Xin Shi, Steven M. Theg ⁎ Department of Plant Biology, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA article info abstract Article history: Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plas- Received 2 July 2012 tids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable Received in revised form 7 September 2012 of a limited amount of semiautonomous protein synthesis – their genomes encode ~100 proteins – they must Accepted 1 October 2012 import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an Available online 9 October 2012 N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc com- Keywords: Toc/Tic complex plex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral compo- Chloroplast nent, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Protein import Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, Protein conducting channel appears to operate in chloroplasts in all green plants, albeit with significant modifications.
    [Show full text]
  • The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein- Folding Capacity of the Endoplasmic Reticulum
    UCSF UC San Francisco Previously Published Works Title The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein- Folding Capacity of the Endoplasmic Reticulum. Permalink https://escholarship.org/uc/item/80r6147q Journal Cold Spring Harbor perspectives in biology, 11(9) ISSN 1943-0264 Authors Karagöz, G Elif Acosta-Alvear, Diego Walter, Peter Publication Date 2019-09-03 DOI 10.1101/cshperspect.a033886 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Downloaded from http://cshperspectives.cshlp.org/ at UNIV OF CALIF-SF on January 30, 2019 - Published by Cold Spring Harbor Laboratory Press The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein- Folding Capacity of the Endoplasmic Reticulum G. Elif Karagöz,1 Diego Acosta-Alvear,2 and Peter Walter1 1Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94143 2Department of Molecular, Cellular, and Developmental Biology, University of California at Santa Barbara, Santa Barbara, California 93106 Correspondence: [email protected] Most of the secreted and plasma membrane proteins are synthesized on membrane-bound ribosomes on the endoplasmic reticulum (ER). They require engagement of ER-resident chap- erones and foldases that assist in their folding and maturation. Since protein homeostasis in the ER is crucial for cellular function, the protein-folding status in the organelle’s lumen is continually surveyed by a network of signaling pathways, collectively called the unfolded protein response (UPR). Protein-folding imbalances, or “ER stress,” are detected by highly conserved sensors that adjust the ER’s protein-folding capacity according to the physiological needs of the cell.
    [Show full text]
  • Evidence Supporting an Antimicrobial Origin of Targeting Peptides to Endosymbiotic Organelles
    cells Article Evidence Supporting an Antimicrobial Origin of Targeting Peptides to Endosymbiotic Organelles Clotilde Garrido y, Oliver D. Caspari y , Yves Choquet , Francis-André Wollman and Ingrid Lafontaine * UMR7141, Institut de Biologie Physico-Chimique (CNRS/Sorbonne Université), 13 Rue Pierre et Marie Curie, 75005 Paris, France; [email protected] (C.G.); [email protected] (O.D.C.); [email protected] (Y.C.); [email protected] (F.-A.W.) * Correspondence: [email protected] These authors contributed equally to this work. y Received: 19 June 2020; Accepted: 24 July 2020; Published: 28 July 2020 Abstract: Mitochondria and chloroplasts emerged from primary endosymbiosis. Most proteins of the endosymbiont were subsequently expressed in the nucleo-cytosol of the host and organelle-targeted via the acquisition of N-terminal presequences, whose evolutionary origin remains enigmatic. Using a quantitative assessment of their physico-chemical properties, we show that organelle targeting peptides, which are distinct from signal peptides targeting other subcellular compartments, group with a subset of antimicrobial peptides. We demonstrate that extant antimicrobial peptides target a fluorescent reporter to either the mitochondria or the chloroplast in the green alga Chlamydomonas reinhardtii and, conversely, that extant targeting peptides still display antimicrobial activity. Thus, we provide strong computational and functional evidence for an evolutionary link between organelle-targeting and antimicrobial peptides. Our results support the view that resistance of bacterial progenitors of organelles to the attack of host antimicrobial peptides has been instrumental in eukaryogenesis and in the emergence of photosynthetic eukaryotes. Keywords: Chlamydomonas; targeting peptides; antimicrobial peptides; primary endosymbiosis; import into organelles; chloroplast; mitochondrion 1.
    [Show full text]
  • A Signal-Anchor Sequence Stimulates Signal Recognition Particle Binding to Ribosomes from Inside the Exit Tunnel
    A signal-anchor sequence stimulates signal recognition particle binding to ribosomes from inside the exit tunnel Uta Berndta,b,1, Stefan Oellerera,b,c,1, Ying Zhanga,b,c, Arthur E. Johnsond, and Sabine Rosperta,b,2 aInstitute of Biochemistry and Molecular Biology and bCenter for Biological Signalling Studies, University of Freiburg, Stefan-Meier-Strasse 17, D-79104 Freiburg, Germany; cFakulta¨t fu¨ r Biologie, University of Freiburg, Scha¨nzlestrasse 1, D-79104 Freiburg, Germany; and dDepartment of Molecular and Cellular Medicine, Texas A&M Health Science Center, 116 Reynolds Medical Building, College Station, TX 77843 Edited by Arthur Horwich, Yale University School of Medicine, New Haven, CT, and approved December 15, 2008 (received for review August 29, 2008) Sorting of eukaryotic membrane and secretory proteins depends direct interaction between SRP and the nascent chain. Previous on recognition of ribosome-bound nascent chain signal sequences studies have addressed the question of whether or not specific by the signal recognition particle (SRP). The current model suggests amino acid sequences of segments inside the tunnel can further that the SRP cycle is initiated when a signal sequence emerges from the affinity of SRP for RNCs (9, 10). Because signal sequences the ribosomal tunnel and binds to SRP. Then elongation is slowed would be prime candidates for such effects, this possibility was until the SRP-bound ribosome–nascent chain complex (RNC) is tested in the eukaryotic system by using RNCs carrying prep- targeted to the SRP receptor in the endoplasmic reticulum (ER) rolactin, a secreted protein with a cleavable signal sequence. membrane. The RNC is then transferred to the translocon, SRP is However, when nascent preprolactin was too short to exit the released, and translation resumes.
    [Show full text]
  • Protein Translocation: the Sec61/Secyeg Translocon
    Dispatch R317 4. Read, B.A., Kegel, J., Klute, M.J., Kuo, A., 11. Quince, C., Lanzen, A., Davenport, R.J., and 17. Mangot, J.F., Domaizon, I., Taib, N., Marouni, N., Lefebvre, S.C., Maumus, F., Mayer, C., Turnbaugh, P.J. (2011). Removing noise from Duffaud, E., Bronner, G., and Debroas, D. (2013). Miller, J., Monier, A., Salamov, A., et al. (2013). pyrosequenced amplicons. BMC Short-term dynamics of diversity patterns: Pan genome of the phytoplankton Emiliania Bioinformatics 12, 38. evidence of continual reassembly within underpins its global distribution. Nature 499, 12. Koeppel, A.F., and Wu, M. (2013). Surprisingly lacustrine small eukaryotes. Environ. Microbiol. 209–213. extensive mixed phylogenetic and ecological 15, 1745–1758. 5. Sogin, M.L., Morrison, H.G., Huber, J.A., Mark signals among bacterial Operational 18. Nolte, V., Pandey, R.V., Jost, S., Medinger, R., Welch, D., Huse, S.M., Neal, P.R., Arrieta, J.M., Taxonomic Units. Nucleic Acids Res. 41, Ottenwalder, B., Boenigk, J., and and Herndl, G.J. (2006). Microbial diversity 5175–5188. Schlotterer, C. (2010). Contrasting seasonal in the deep sea and the underexplored ‘‘rare 13. Stoeck, T., Bass, D., Nebel, M., Christen, R., niche separation between rare and abundant biosphere’’. Proc. Natl. Acad. Sci. USA 103, Jones, M.D., Breiner, H.W., and Richards, T.A. taxa conceals the extent of protist diversity. 12115–12120. (2010). Multiple marker parallel tag Mol. Ecol. 19, 2908–2915. 6. Pedros-Alio, C. (2012). The rare bacterial environmental DNA sequencing reveals a 19. Chow, C.E., Sachdeva, R., Cram, J.A., biosphere. Annu.
    [Show full text]
  • The Dynamic Secyeg Translocon
    fmolb-08-664241 April 11, 2021 Time: 10:46 # 1 REVIEW published: 15 April 2021 doi: 10.3389/fmolb.2021.664241 The Dynamic SecYEG Translocon Julia Oswald1, Robert Njenga1,2, Ana Natriashvili1,2, Pinku Sarmah1,2 and Hans-Georg Koch1* 1 Institute for Biochemistry and Molecular Biology, Zentrum für Biochemie und Molekulare Medizin (ZMBZ), Faculty of Medicine, Albert Ludwigs Universität Freiburg, Freiburg, Germany, 2 Faculty of Biology, Albert Ludwigs Universität Freiburg, Freiburg, Germany The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in Edited by: part determined by its ability to interact with multiple targeting factors, chaperones Kür ¸sadTurgay, and accessory proteins. These interactions are crucial for the assisted passage of Max Planck Unit for the Science of Pathogens, newly synthesized proteins from the cytosol into the different bacterial compartments. Max-Planck-Gesellschaft (MPG), In this review, we summarize the current knowledge about SecYEG-mediated protein Germany transport, primarily in the model organism Escherichia coli, and describe the dynamic Reviewed by: Damon Huber, interaction of the SecYEG translocon with its multiple partner proteins. We furthermore University of Birmingham, highlight how protein transport is regulated and explore recent developments in using United Kingdom the SecYEG translocon as an antimicrobial target.
    [Show full text]
  • Friday XIII Adapt to the Highly Variable H-Region2
    news and views brane lipids and its h-region has a helical proteins such as calmodulin9, or they are pre- conformation3. Enter the signal peptidase. sented to the immune system by molecules of Lys Its job: to skim the lumenal (or periplasmic) the major histocompatibility complex on the 145 COOH surface of the membrane, looking surface of the cell10. So, some signal peptides Ser 90 NH O H 2 H2N for suitably exposed signal-peptide cleavage probably have a second signalling function, Periplasm -1 COOH sites. Paetzel and colleagues’ structure of the distinct from their role in targeting. c -3 periplasmic domain of the E. coli enzyme1 We are experiencing a new wave of struc- tells us how this is accomplished. tural and biochemical work on protein tar- h First, an extended hydrophobic patch geting, which is finally showing us the int8i- n surrounds the active site, suggesting that this mate details of the life and death of signal Cytoplasm + + part of the enzyme is immersed in the outer peptides and the machineries that they put in N leaflet of the lipid bilayer. Second, the struc- motion. The key is the h-region — a simple ture of the active site readily explains the stretch of about ten hydrophobic residues Figure 1 The signal peptidase of Escherichia coli, requirement for small residues such as ala- that primes the SRP, unlocks the translocon the structure of which has been solved by Paetzel nine in positions 11 and 13, upstream of and positions the signal peptide for cleavage. et al.1. Two amino-terminal transmembrane the cleavage site, and also shows that the The messenger is the message.
    [Show full text]
  • The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein- Folding Capacity of the Endoplasmic Reticulum
    Downloaded from http://cshperspectives.cshlp.org/ at UNIV OF CALIF-SF on January 30, 2019 - Published by Cold Spring Harbor Laboratory Press The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein- Folding Capacity of the Endoplasmic Reticulum G. Elif Karagöz,1 Diego Acosta-Alvear,2 and Peter Walter1 1Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94143 2Department of Molecular, Cellular, and Developmental Biology, University of California at Santa Barbara, Santa Barbara, California 93106 Correspondence: [email protected] Most of the secreted and plasma membrane proteins are synthesized on membrane-bound ribosomes on the endoplasmic reticulum (ER). They require engagement of ER-resident chap- erones and foldases that assist in their folding and maturation. Since protein homeostasis in the ER is crucial for cellular function, the protein-folding status in the organelle’s lumen is continually surveyed by a network of signaling pathways, collectively called the unfolded protein response (UPR). Protein-folding imbalances, or “ER stress,” are detected by highly conserved sensors that adjust the ER’s protein-folding capacity according to the physiological needs of the cell. We review recent developments in the field that have provided new insights into the ER stress-sensing mechanisms used by UPR sensors and the mechanisms by which they integrate various cellular inputs to adjust the folding capacity of the organelle to accom- modate to fluctuations in ER protein-folding demands. THE ENDOPLASMIC RETICULUM AS to the outside of the cell (Devos et al. 2004; Baum A PROTEIN-FOLDING COMPARTMENT and Baum 2014).
    [Show full text]
  • Efficient Integration of Transmembrane Domains Depends on the Folding Properties of the Upstream Sequences
    Efficient integration of transmembrane domains depends on the folding properties of the upstream sequences Marco Janoschkea, Mirjam Zimmermanna,b, Anna Brunauera, Raffael Humbela, Tina Junnea, and Martin Spiessa,1 aBiozentrum, University of Basel, 4056 Basel, Switzerland; and bInstitute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria Edited by Tom A. Rapoport, Harvard University, Boston, MA, and approved June 27, 2021 (received for review February 9, 2021) The topology of most membrane proteins is defined by the successive C-terminal, half of the translocon, leaving a gap open toward the integration of α-helical transmembrane domains at the Sec61 trans- cytosol (3). As the protein is synthesized, its transmembrane locon. The translocon provides a pore for the transfer of polypeptide segments are inserted successively into the lipid bilayer. segments across the membrane while giving them lateral access to Three distinct membrane integration processes can be distin- the lipid. For each polypeptide segment of ∼20 residues, the com- guished (schematically shown in Fig. 1A). First, the signal bined hydrophobicities of its constituent amino acids were previously (anchor) activates the translocon by intercalating between the gate shown to define the extent of membrane integration. Here, we dis- helices and exiting toward the lipid phase (2). In the process, the covered that different sequences preceding a potential transmem- hydrophilic flanking sequence is inserted into the pore for transfer brane domain substantially affect its hydrophobicity requirement into the lumen, and the plug is pushed away. This has been eluci- for integration. Rapidly folding domains, sequences that are intrinsi- dated by a number of structures of translocons engaged with signal cally disordered or very short or capable of binding chaperones with sequences (3–5).
    [Show full text]