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SecA interacts with ribosomes in order to facilitate posttranslational translocation in bacteria. Mol Cell 2011; 41:343-53. [PMID: 21292166 DOI: 10.1016/j.molcel.2010.12.028] [Citation(s) in RCA: 78] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2010] [Revised: 08/06/2010] [Accepted: 12/10/2010] [Indexed: 11/21/2022]
Abstract
In Escherichia coli, translocation of exported proteins across the cytoplasmic membrane is dependent on the motor protein SecA and typically begins only after synthesis of the substrate has already been completed (i.e., posttranslationally). Thus, it has generally been assumed that the translocation machinery also recognizes its protein substrates posttranslationally. Here we report a specific interaction between SecA and the ribosome at a site near the polypeptide exit channel. This interaction is mediated by conserved motifs in SecA and ribosomal protein L23, and partial disruption of this interaction in vivo by introducing mutations into the genes encoding SecA or L23 affects the efficiency of translocation by the posttranslational pathway. Based on these findings, we propose that SecA could interact with its nascent substrates during translation in order to efficiently channel them into the "posttranslational" translocation pathway.
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52
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Wowor AJ, Yu D, Kendall DA, Cole JL. Energetics of SecA dimerization. J Mol Biol 2011; 408:87-98. [PMID: 21315086 DOI: 10.1016/j.jmb.2011.02.006] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2010] [Revised: 01/17/2011] [Accepted: 02/02/2011] [Indexed: 10/18/2022]
Abstract
Transport of many proteins to extracytoplasmic locations occurs via the general secretion (Sec) pathway. In Escherichia coli, this pathway is composed of the SecYEG protein-conducting channel and the SecA ATPase. SecA plays a central role in binding the signal peptide region of preproteins, directing preproteins to membrane-bound SecYEG and promoting translocation coupled with ATP hydrolysis. Although it is well established that SecA is crucial for preprotein transport and thus cell viability, its oligomeric state during different stages of transport remains ill defined. We have characterized the energetics of SecA dimerization as a function of salt concentration and temperature and defined the linkage of SecA dimerization and signal peptide binding using analytical ultracentrifugation. The use of a new fluorescence detector permitted an analysis of SecA dimerization down to concentrations as low as 50 nM. The dimer dissociation constants are strongly dependent on salt. Linkage analysis indicates that SecA dimerization is coupled to the release of about five ions, demonstrating that electrostatic interactions play an important role in stabilizing the SecA dimer interface. Binding of signal peptide reduces SecA dimerization affinity, such that K(d) increases about 9-fold from 0.28 μM in the absence of peptide to 2.68 μM in the presence of peptide. The weakening of the SecA dimer that accompanies signal peptide binding may poise the SecA dimer to dissociate upon binding to SecYEG.
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Affiliation(s)
- Andy J Wowor
- Department of Pharmaceutical Sciences, University of Connecticut, Storrs, CT 06269, USA
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53
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du Plessis DJF, Nouwen N, Driessen AJM. The Sec translocase. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1808:851-65. [PMID: 20801097 DOI: 10.1016/j.bbamem.2010.08.016] [Citation(s) in RCA: 195] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/24/2010] [Revised: 08/19/2010] [Accepted: 08/20/2010] [Indexed: 11/18/2022]
Abstract
The vast majority of proteins trafficking across or into the bacterial cytoplasmic membrane occur via the translocon. The translocon consists of the SecYEG complex that forms an evolutionarily conserved heterotrimeric protein-conducting membrane channel that functions in conjunction with a variety of ancillary proteins. For posttranslational protein translocation, the translocon interacts with the cytosolic motor protein SecA that drives the ATP-dependent stepwise translocation of unfolded polypeptides across the membrane. For the cotranslational integration of membrane proteins, the translocon interacts with ribosome-nascent chain complexes and membrane insertion is coupled to polypeptide chain elongation at the ribosome. These processes are assisted by the YidC and SecDF(yajC) complex that transiently interacts with the translocon. This review summarizes our current understanding of the structure-function relationship of the translocon and its interactions with ancillary components during protein translocation and membrane protein insertion. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.
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Affiliation(s)
- David J F du Plessis
- Department of Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute and the Zernike Institute for Advanced Materials, University of Groningen, 9751NN Haren, The Netherlands
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Auclair SM, Moses JP, Musial-Siwek M, Kendall DA, Oliver DB, Mukerji I. Mapping of the signal peptide-binding domain of Escherichia coli SecA using Förster resonance energy transfer. Biochemistry 2010; 49:782-92. [PMID: 20025247 DOI: 10.1021/bi901446r] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Identification of the signal peptide-binding domain within SecA ATPase is an important goal for understanding the molecular basis of SecA preprotein recognition as well as elucidating the chemo-mechanical cycle of this nanomotor during protein translocation. In this study, Forster resonance energy transfer methodology was employed to map the location of the SecA signal peptide-binding domain using a collection of functional monocysteine SecA mutants and alkaline phosphatase signal peptides labeled with appropriate donor-acceptor fluorophores. Fluorescence anisotropy measurements yielded an equilibrium binding constant of 1.4 or 10.7 muM for the alkaline phosphatase signal peptide labeled at residue 22 or 2, respectively, with SecA, and a binding stoichiometry of one signal peptide bound per SecA monomer. Binding affinity measurements performed with a monomer-biased mutant indicate that the signal peptide binds equally well to SecA monomer or dimer. Distance measurements determined for 13 SecA mutants show that the SecA signal peptide-binding domain encompasses a portion of the preprotein cross-linking domain but also includes regions of nucleotide-binding domain 1 and particularly the helical scaffold domain. The identified region lies at a multidomain interface within the heart of SecA, surrounded by and potentially responsive to domains important for binding nucleotide, mature portions of the preprotein, and the SecYEG channel. Our FRET-mapped binding domain, in contrast to the domain identified by NMR spectroscopy, includes the two-helix finger that has been shown to interact with the preprotein during translocation and lies at the entrance to the protein-conducting channel in the recently determined SecA-SecYEG structure.
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Affiliation(s)
- Sarah M Auclair
- Department of Molecular Biology and Biochemistry, Molecular Biophysics Program, Wesleyan University, Middletown, Connecticut 06459, USA
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56
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Yuan J, Zweers JC, van Dijl JM, Dalbey RE. Protein transport across and into cell membranes in bacteria and archaea. Cell Mol Life Sci 2010; 67:179-99. [PMID: 19823765 PMCID: PMC11115550 DOI: 10.1007/s00018-009-0160-x] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2009] [Revised: 09/13/2009] [Accepted: 09/21/2009] [Indexed: 12/21/2022]
Abstract
In the three domains of life, the Sec, YidC/Oxa1, and Tat translocases play important roles in protein translocation across membranes and membrane protein insertion. While extensive studies have been performed on the endoplasmic reticular and Escherichia coli systems, far fewer studies have been done on archaea, other Gram-negative bacteria, and Gram-positive bacteria. Interestingly, work carried out to date has shown that there are differences in the protein transport systems in terms of the number of translocase components and, in some cases, the translocation mechanisms and energy sources that drive translocation. In this review, we will describe the different systems employed to translocate and insert proteins across or into the cytoplasmic membrane of archaea and bacteria.
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Affiliation(s)
- Jijun Yuan
- Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210 USA
| | - Jessica C. Zweers
- Department of Medical Microbiology, University Medical Center Groningen and University of Groningen, Hanzeplein 1, 30001, 9700 RB Groningen, The Netherlands
| | - Jan Maarten van Dijl
- Department of Medical Microbiology, University Medical Center Groningen and University of Groningen, Hanzeplein 1, 30001, 9700 RB Groningen, The Netherlands
| | - Ross E. Dalbey
- Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210 USA
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Tsukazaki T, Mori H, Fukai S, Ishitani R, Mori T, Dohmae N, Perederina A, Sugita Y, Vassylyev DG, Ito K, Nureki O. Conformational transition of Sec machinery inferred from bacterial SecYE structures. Nature 2008; 455:988-91. [PMID: 18923527 DOI: 10.1038/nature07421] [Citation(s) in RCA: 192] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2008] [Accepted: 09/10/2008] [Indexed: 11/09/2022]
Abstract
Over 30% of proteins are secreted across or integrated into membranes. Their newly synthesized forms contain either cleavable signal sequences or non-cleavable membrane anchor sequences, which direct them to the evolutionarily conserved Sec translocon (SecYEG in prokaryotes and Sec61, comprising alpha-, gamma- and beta-subunits, in eukaryotes). The translocon then functions as a protein-conducting channel. These processes of protein localization occur either at or after translation. In bacteria, the SecA ATPase drives post-translational translocation. The only high-resolution structure of a translocon available so far is that for SecYEbeta from the archaeon Methanococcus jannaschii, which lacks SecA. Here we present the 3.2-A-resolution crystal structure of the SecYE translocon from a SecA-containing organism, Thermus thermophilus. The structure, solved as a complex with an anti-SecY Fab fragment, revealed a 'pre-open' state of SecYE, in which several transmembrane helices are shifted, as compared to the previous SecYEbeta structure, to create a hydrophobic crack open to the cytoplasm. Fab and SecA bind to a common site at the tip of the cytoplasmic domain of SecY. Molecular dynamics and disulphide mapping analyses suggest that the pre-open state might represent a SecYE conformational transition that is inducible by SecA binding. Moreover, we identified a SecA-SecYE interface that comprises SecA residues originally buried inside the protein, indicating that both the channel and the motor components of the Sec machinery undergo cooperative conformational changes on formation of the functional complex.
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Affiliation(s)
- Tomoya Tsukazaki
- Department of Biological Information, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa 226-8501, Japan
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58
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Maximal efficiency of coupling between ATP hydrolysis and translocation of polypeptides mediated by SecB requires two protomers of SecA. J Bacteriol 2008; 191:978-84. [PMID: 18978043 DOI: 10.1128/jb.01321-08] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
SecA is the ATPase that provides energy for translocation of precursor polypeptides through the SecYEG translocon in Escherichia coli during protein export. We showed previously that when SecA receives the precursor from SecB, the ternary complex is fully active only when two protomers of SecA are bound. Here we used variants of SecA and of SecB that populate complexes containing two protomers of SecA to different degrees to examine both the hydrolysis of ATP and the translocation of polypeptides. We conclude that the low activity of the complexes with only one protomer is the result of a low efficiency of coupling between ATP hydrolysis and translocation.
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59
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Chen Y, Pan X, Tang Y, Quan S, Tai PC, Sui SF. Full-length Escherichia coli SecA dimerizes in a closed conformation in solution as determined by cryo-electron microscopy. J Biol Chem 2008; 283:28783-7. [PMID: 18772144 DOI: 10.1074/jbc.c800160200] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
SecA is an obligatory component of the Escherichia coli general secretion pathway. However, the oligomeric structure of SecA and SecA conformational changes during translocation processes are still unclear. Here we obtained the three-dimensional structure of E. coli wild-type full-length SecA in solution by single particle cryo-electron microscopy and determined its oligomeric organization. In this structure, SecA occurs as a dimer in which the two protomers are arranged in an antiparallel mode, with a novel electrostatic interface, and both protomers are in closed conformation. The system developed here may provide a promising technique for studying dynamic structural changes in SecA.
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Affiliation(s)
- Yong Chen
- Department of Biological Sciences and Biotechnology, the State Key Laboratory of Biomembrane and Membrane Biotechnology, Tsinghua University, Beijing 100084, China
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60
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Nithianantham S, Shilton BH. Analysis of the isolated SecA DEAD motor suggests a mechanism for chemical-mechanical coupling. J Mol Biol 2008; 383:380-9. [PMID: 18761349 DOI: 10.1016/j.jmb.2008.08.022] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2008] [Revised: 08/07/2008] [Accepted: 08/11/2008] [Indexed: 11/18/2022]
Abstract
The preprotein cross-linking domain and C-terminal domains of Escherichia coli SecA were removed to create a minimal DEAD motor, SecA-DM. SecA-DM hydrolyzes ATP and has the same affinity for ADP as full-length SecA. The crystal structure of SecA-DM in complex with ADP was solved and shows the DEAD motor in a closed conformation. Comparison with the structure of the E. coli DEAD motor in an open conformation (Protein Data Bank ID 2FSI) indicates main-chain conformational changes in two critical sequences corresponding to Motif III and Motif V of the DEAD helicase family. The structures that the Motif III and Motif V sequences adopt in the DEAD motor open conformation are incompatible with the closed conformation. Therefore, when the DEAD motor makes the transition from open to closed, Motif III and Motif V are forced to change their conformations, which likely functions to regulate passage through the transition state for ATP hydrolysis. The transition state for ATP hydrolysis for the SecA DEAD motor was modeled based on the conformation of the Vasa helicase in complex with adenylyl imidodiphosphate and RNA (Protein Data Bank ID 2DB3). A mechanism for chemical-mechanical coupling emerges, where passage through the transition state for ATP hydrolysis is hindered by the conformational changes required in Motif III and Motif V, and may be promoted by binding interactions with the preprotein substrate and/or other translocase domains and subunits.
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Affiliation(s)
- Stanley Nithianantham
- Department of Biochemistry, The University of Western Ontario, London, Ontario, Canada N6A 5C1
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61
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Clérico EM, Maki JL, Gierasch LM. Use of synthetic signal sequences to explore the protein export machinery. Biopolymers 2008; 90:307-19. [PMID: 17918185 DOI: 10.1002/bip.20856] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The information for correct localization of newly synthesized proteins in both prokaryotes and eukaryotes resides in self-contained, often transportable targeting sequences. Of these, signal sequences specify that a protein should be secreted from a cell or incorporated into the cytoplasmic membrane. A central puzzle is presented by the lack of primary structural homology among signal sequences, although they share common features in their sequences. Synthetic signal peptides have enabled a wide range of studies of how these "zipcodes" for protein secretion are decoded and used to target proteins to the protein machinery that facilitates their translocation across and integration into membranes. We review research on how the information in signal sequences enables their passenger proteins to be correctly and efficiently localized. Synthetic signal peptides have made possible binding and crosslinking studies to explore how selectivity is achieved in recognition by the signal sequence-binding receptors, signal recognition particle, or SRP, which functions in all organisms, and SecA, which functions in prokaryotes and some organelles of prokaryotic origins. While progress has been made, the absence of atomic resolution structures for complexes of signal peptides and their receptors has definitely left many questions to be answered in the future.
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Affiliation(s)
- Eugenia M Clérico
- Department of Biochemistry and Molecular Biology, University of Massachusetts-Amherst, Amherst, MA 01003, USA
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62
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SecA, the motor of the secretion machine, binds diverse partners on one interactive surface. J Mol Biol 2008; 382:74-87. [PMID: 18602400 DOI: 10.1016/j.jmb.2008.06.049] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2008] [Revised: 06/12/2008] [Accepted: 06/13/2008] [Indexed: 11/22/2022]
Abstract
In all living cells, regulated passage across membranes of specific proteins occurs through a universally conserved secretory channel. In bacteria and chloroplasts, the energy for the mechanical work of moving polypeptides through that channel is provided by SecA, a regulated ATPase. Here, we use site-directed spin labeling and electron paramagnetic resonance spectroscopy to identify the interactive surface used by SecA for each of the diverse binding partners encountered during the dynamic cycle of export. Although the binding sites overlap, resolution at the level of aminoacyl side chains allows us to identify contacts that are unique to each partner. Patterns of constraint and mobilization of residues on that interactive surface suggest a conformational change that may underlie the coupling of ATP hydrolysis to precursor translocation.
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63
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Affiliation(s)
- Arnold J.M. Driessen
- Department of Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute and the Zernike Institute for Advanced Materials, University of Groningen, 9751 NN, Haren, The Netherlands; ,
| | - Nico Nouwen
- Department of Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute and the Zernike Institute for Advanced Materials, University of Groningen, 9751 NN, Haren, The Netherlands; ,
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64
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Structural basis for signal-sequence recognition by the translocase motor SecA as determined by NMR. Cell 2008; 131:756-69. [PMID: 18022369 DOI: 10.1016/j.cell.2007.09.039] [Citation(s) in RCA: 341] [Impact Index Per Article: 21.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2007] [Revised: 07/30/2007] [Accepted: 09/28/2007] [Indexed: 11/21/2022]
Abstract
Recognition of signal sequences by cognate receptors controls the entry of virtually all proteins to export pathways. Despite its importance, this process remains poorly understood. Here, we present the solution structure of a signal peptide bound to SecA, the 204 kDa ATPase motor of the Sec translocase. Upon encounter, the signal peptide forms an alpha-helix that inserts into a flexible and elongated groove in SecA. The mode of binding is bimodal, with both hydrophobic and electrostatic interactions mediating recognition. The same groove is used by SecA to recognize a diverse set of signal sequences. Impairment of the signal-peptide binding to SecA results in significant translocation defects. The C-terminal tail of SecA occludes the groove and inhibits signal-peptide binding, but autoinhibition is relieved by the SecB chaperone. Finally, it is shown that SecA interconverts between two conformations in solution, suggesting a simple mechanism for polypeptide translocation.
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65
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Abstract
Bacteria and archaea possess a protein complex in the plasma membrane that governs protein secretion and membrane protein insertion. Eukaryotes carry homologues in the endoplasmic reticulum (ER) where they direct the same reaction. A combination of experiments conducted on the systems found in all three domains of life has revealed a great deal about protein translocation. The channel provides a route for proteins to pass through the hydrophobic barrier of the membrane, assisted by various partner proteins which maintain an unfolded state of the substrate, target it to the channel and provide the energy and mechanical drive required for transport. In bacteria, the post-translational reaction utilizes an ATPase that couples the free energy of ATP binding and hydrolysis to move the substrate through the protein pore. This review will draw on genetic, biochemical and structural findings in an account of our current understanding of this mechanism.
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Affiliation(s)
- Vicki A M Gold
- Department of Biochemistry, University of Bristol, Bristol, UK
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66
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67
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Robson A, Booth AEG, Gold VAM, Clarke AR, Collinson I. A large conformational change couples the ATP binding site of SecA to the SecY protein channel. J Mol Biol 2007; 374:965-76. [PMID: 17964601 DOI: 10.1016/j.jmb.2007.09.086] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2007] [Revised: 09/26/2007] [Accepted: 09/27/2007] [Indexed: 11/30/2022]
Abstract
In bacteria, the SecYEG protein translocation complex employs the cytosolic ATPase SecA to couple the energy of ATP binding and hydrolysis to the mechanical force required to push polypeptides through the membrane. The molecular basis of this energy transducing reaction is not well understood. A peptide-binding array has been employed to identify sites on SecYEG that interact with SecA. These results along with fluorescence spectroscopy have been exploited to characterise a long-distance conformational change that connects the nucleotide-binding fold of SecA to the transmembrane polypeptide channel in SecY. These movements are driven by binding of non-hydrolysable ATP analogues to a monomer of SecA in association with the SecYEG complex. We also determine that interaction with SecYEG simultaneously decreases the affinity of SecA for ATP and inhibitory magnesium, favouring a previously identified active state of the ATPase. Mutants of SecA capable of binding but not hydrolysing ATP do not elicit this conformationally active state, implicating residues of the Walker B motif in the early chain of events that couple ATP binding to the mobility of the channel.
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Affiliation(s)
- Alice Robson
- Department of Biochemistry, University of Bristol, University Walk, Bristol, BS8 1TD, UK
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68
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Sec- and Tat-mediated protein secretion across the bacterial cytoplasmic membrane--distinct translocases and mechanisms. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2007; 1778:1735-56. [PMID: 17935691 DOI: 10.1016/j.bbamem.2007.07.015] [Citation(s) in RCA: 343] [Impact Index Per Article: 20.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/23/2007] [Revised: 07/23/2007] [Accepted: 07/24/2007] [Indexed: 11/20/2022]
Abstract
In bacteria, two major pathways exist to secrete proteins across the cytoplasmic membrane. The general Secretion route, termed Sec-pathway, catalyzes the transmembrane translocation of proteins in their unfolded conformation, whereupon they fold into their native structure at the trans-side of the membrane. The Twin-arginine translocation pathway, termed Tat-pathway, catalyses the translocation of secretory proteins in their folded state. Although the targeting signals that direct secretory proteins to these pathways show a high degree of similarity, the translocation mechanisms and translocases involved are vastly different.
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69
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Osborne AR, Rapoport TA. Protein translocation is mediated by oligomers of the SecY complex with one SecY copy forming the channel. Cell 2007; 129:97-110. [PMID: 17418789 DOI: 10.1016/j.cell.2007.02.036] [Citation(s) in RCA: 112] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2006] [Revised: 01/11/2007] [Accepted: 02/13/2007] [Indexed: 10/23/2022]
Abstract
Many proteins are translocated across the bacterial plasma membrane by the interplay of the cytoplasmic ATPase SecA with a protein-conducting channel, formed from the evolutionarily conserved heterotrimeric SecY complex. Here, we have used purified E. coli components to address the mechanism of translocation. Disulfide bridge crosslinking demonstrates that SecA transfers both the signal sequence and the mature region of a secreted substrate into a single SecY molecule. However, protein translocation involves oligomers of the SecY complex, because a SecY molecule defective in translocation can be rescued by linking it covalently with a wild-type SecY copy. SecA interacts through one of its domains with a nontranslocating SecY copy and moves the polypeptide chain through a neighboring SecY copy. Oligomeric channels with only one active pore likely mediate protein translocation in all organisms.
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Affiliation(s)
- Andrew R Osborne
- Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA
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70
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Or E, Rapoport T. Cross-linked SecA dimers are not functional in protein translocation. FEBS Lett 2007; 581:2616-20. [PMID: 17511989 PMCID: PMC2755086 DOI: 10.1016/j.febslet.2007.04.083] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2006] [Revised: 03/17/2007] [Accepted: 04/27/2007] [Indexed: 11/29/2022]
Abstract
The ATPase SecA is involved in post-translational protein translocation through the SecY channel across the bacterial inner membrane. SecA is a dimer that can dissociate into monomers with translocation activity. Here, we have addressed whether dissociation of the SecA dimer is required for translocation. We show that a dimer in which the two subunits are cross-linked by disulfide bridges is inactive in protein translocation, translocation ATPase, and binding to a lipid bilayer. In contrast, upon reduction of the disulfide bridges, the resulting monomers regain these activities. These data support the notion that dissociation of SecA dimers into monomers occurs during protein translocation.
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Affiliation(s)
- Eran Or
- Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115
| | - Tom Rapoport
- Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115
- Corresponding author. Fax: (1)-617-4321190. E-Mail:
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71
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Tomkiewicz D, Nouwen N, Driessen AJM. Pushing, pulling and trapping--modes of motor protein supported protein translocation. FEBS Lett 2007; 581:2820-8. [PMID: 17466297 DOI: 10.1016/j.febslet.2007.04.015] [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] [Received: 02/26/2007] [Revised: 04/01/2007] [Accepted: 04/11/2007] [Indexed: 11/19/2022]
Abstract
Protein translocation across the cellular membranes is an ubiquitous and crucial activity of cells. This process is mediated by translocases that consist of a protein conducting channel and an associated motor protein. Motor proteins interact with protein substrates and utilize the free energy of ATP binding and hydrolysis for protein unfolding, translocation and unbinding. Since motor proteins are found either at the cis- or trans-side of the membrane, different mechanisms for translocation have been proposed. In the Power stroke model, cis-acting motors are thought to push, while trans-motors pull on the substrate protein during translocation. In the Brownian ratchet model, translocation occurs by diffusion of the unfolded polypeptide through the translocation pore while directionality is achieved by trapping and refolding. Recent insights in the structure and function of the molecular motors suggest that different mechanisms can be employed simultaneously.
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Affiliation(s)
- Danuta Tomkiewicz
- Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands
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72
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Gold VAM, Robson A, Clarke AR, Collinson I. Allosteric regulation of SecA: magnesium-mediated control of conformation and activity. J Biol Chem 2007; 282:17424-32. [PMID: 17416585 DOI: 10.1074/jbc.m702066200] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In bacteria, the SecA protein associates with a ubiquitous protein channel SecYEG where it drives the post-translational secretion of pre-proteins across the plasma membrane. The high-resolution structures of both proteins have been determined in their resting states; however, the mechanism that couples ATP hydrolysis to active transport of substrate proteins through the membrane is not well understood. An analysis of the steady-state ATPase activity of the enzyme reveals that there is an allosteric binding site for magnesium distinct from that associated with hydrolysis of ATP. We have demonstrated that this regulation involves a large conformational change to the SecA dimer, which exerts a strong influence on the turnover and affinity for ATP, as well as the affinity for ADP. The strong inhibitory influence of magnesium on the ATPase activity can be countered by cardiolipin and conditions that promote protein translocation.
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Affiliation(s)
- Vicki A M Gold
- Department of Biochemistry, University of Bristol, Bristol BS8 1TD, United Kingdom
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73
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Alami M, Dalal K, Lelj-Garolla B, Sligar SG, Duong F. Nanodiscs unravel the interaction between the SecYEG channel and its cytosolic partner SecA. EMBO J 2007; 26:1995-2004. [PMID: 17396152 PMCID: PMC1852787 DOI: 10.1038/sj.emboj.7601661] [Citation(s) in RCA: 119] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2005] [Accepted: 03/02/2007] [Indexed: 11/08/2022] Open
Abstract
The translocon is a membrane-embedded protein assembly that catalyzes protein movement across membranes. The core translocon, the SecYEG complex, forms oligomers, but the protein-conducting channel is at the center of the monomer. Defining the properties of the SecYEG protomer is thus crucial to understand the underlying function of oligomerization. We report here the reconstitution of a single SecYEG complex into nano-scale lipid bilayers, termed Nanodiscs. These water-soluble particles allow one to probe the interactions of the SecYEG complex with its cytosolic partner, the SecA dimer, in a membrane-like environment. The results show that the SecYEG complex triggers dissociation of the SecA dimer, associates only with the SecA monomer and suffices to (pre)-activate the SecA ATPase. Acidic lipids surrounding the SecYEG complex also contribute to the binding affinity and activation of SecA, whereas mutations in the largest cytosolic loop of the SecY subunit, known to abolish the translocation reaction, disrupt both the binding and activation of SecA. Altogether, the results define the fundamental contribution of the SecYEG protomer in the translocation subreactions and illustrate the power of nanoscale lipid bilayers in analyzing the dynamics occurring at the membrane.
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Affiliation(s)
- Meriem Alami
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, Faculty of Medicine, University of British Columbia, British Columbia, Canada
| | - Kush Dalal
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, Faculty of Medicine, University of British Columbia, British Columbia, Canada
| | - Barbara Lelj-Garolla
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, Faculty of Medicine, University of British Columbia, British Columbia, Canada
| | - Stephen G Sligar
- Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA
| | - Franck Duong
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, Faculty of Medicine, University of British Columbia, British Columbia, Canada
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver, British Columbia, Canada V6T 1Z3. Tel.: +1 604 822 5975; Fax: +1 604 822 5227; E-mail:
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Chen Y, Tai PC, Sui SF. The active ring-like structure of SecA revealed by electron crystallography: conformational change upon interaction with SecB. J Struct Biol 2007; 159:149-53. [PMID: 17419072 PMCID: PMC2691388 DOI: 10.1016/j.jsb.2007.01.015] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2006] [Revised: 01/02/2007] [Accepted: 01/27/2007] [Indexed: 10/23/2022]
Abstract
SecA is a multifunctional protein involved in protein translocation in bacteria. The structure of SecA on membrane is dramatically altered compared with that in solution, accompanying with functional changes. We previously reported the formation of a novel ring-like structure of SecA on lipid layers, which may constitute part of the preprotein translocation channel. In the present work, two-dimensional crystallization of Escherichia coli SecA on lipid monolayers was performed to reveal the structural details of SecA on lipid layers and to investigate its function. The 2D crystals composed of ring-like structures were obtained by specific interaction between SecA and negatively charged lipid. The 2D projection map and 3D reconstruction from negative stained 2D crystals exhibited a distinct open channel-like structure of SecA, with an outer diameter of 7 nm and an inner diameter of 2 nm, providing the structural evidence for SecA importance in forming the part of the translocation channel. This pore structure is altered after transferring crystals to the SecB solution, indicating that the lipid-specific SecA structure has the SecB binding activity. The strategy developed here provides a promising technique for studying structure of SecA complex with its ligand on membrane.
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Affiliation(s)
- Yong Chen
- Department of Biological Sciences & Biotechnology, State-Key Laboratory of Biomembranes and Membrane Biotechnology, Tsinghua University, Beijing 100084, China
| | - Phang C. Tai
- Department of Biology, Georgia State University, Atlanta, GA 30303
| | - Sen-Fang Sui
- Department of Biological Sciences & Biotechnology, State-Key Laboratory of Biomembranes and Membrane Biotechnology, Tsinghua University, Beijing 100084, China
- Corresponding Author: Sen-Fang Sui, Department of Biological Sciences & Biotechnology, State-Key Laboratory of Biomembranes and Membrane Biotechnology, Tsinghua University, Beijing 100084, China, Telephone: +8610-62784768, Fax: +8610-62793367, E-mail:
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