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Lavieu G, Dunlop MH, Lerich A, Zheng H, Bottanelli F, Rothman JE. The Golgi ribbon structure facilitates anterograde transport of large cargoes. Mol Biol Cell 2014; 25:3028-36. [PMID: 25103235 PMCID: PMC4230591 DOI: 10.1091/mbc.e14-04-0931] [Citation(s) in RCA: 50] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Breaking down the ribbon of mammalian cells strongly inhibits intra-Golgi transport of large cargoes without altering the rate of transport of smaller cargoes. These results imply that the ribbon structure is an essential requirement for transport of large cargoes in mammalian cells. In mammalian cells, individual Golgi stacks fuse laterally to form the characteristic perinuclear ribbon structure. Yet the purpose of this remarkable structure has been an enigma. We report that breaking down the ribbon of mammalian cells strongly inhibits intra-Golgi transport of large cargoes without altering the rate of transport of smaller cargoes. In addition, insect cells that naturally harbor dispersed Golgi stacks have limited capacity to transport artificial oversized cargoes. These results imply that the ribbon structure is an essential requirement for transport of large cargoes in mammalian cells, and we suggest that this is because it enables the dilated rims of cisternae (containing the aggregates) to move across the stack as they transfer among adjacent stacks within the ribbon structure.
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77
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Li F, Kümmel D, Coleman J, Reinisch KM, Rothman JE, Pincet F. A half-zippered SNARE complex represents a functional intermediate in membrane fusion. J Am Chem Soc 2014; 136:3456-64. [PMID: 24533674 PMCID: PMC3985920 DOI: 10.1021/ja410690m] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
![]()
SNARE
(soluble N-ethylmaleimide-sensitive factor
attachment protein receptor) proteins mediate fusion by pulling biological
membranes together via a zippering mechanism. Recent biophysical studies
have shown that t- and v-SNAREs can assemble in multiple stages from
the N-termini toward the C-termini. Here we show that functionally,
membrane fusion requires a sequential, two-step folding pathway and
assign specific and distinct functions for each step. First, the N-terminal
domain (NTD) of the v-SNARE docks to the t-SNARE, which leads to a
conformational rearrangement into an activated half-zippered SNARE
complex. This partially assembled SNARE complex locks the C-terminal
(CTD) portion of the t-SNARE into the same structure as in the postfusion
4-helix bundle, thereby creating the binding site for the CTD of the
v-SNARE and enabling fusion. Then zippering of the remaining CTD,
the membrane-proximal linker (LD), and transmembrane (TMD) domains
is required and sufficient to trigger fusion. This intrinsic property
of the SNAREs fits well with the action of physiologically vital regulators
such as complexin. We also report that NTD assembly is the rate-limiting
step. Our findings provide a refined framework for delineating the
molecular mechanism of SNARE-mediated membrane fusion and action of
regulatory proteins.
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Pellett PA, Dietrich F, Bewersdorf J, Rothman JE, Lavieu G. Inter-Golgi transport mediated by COPI-containing vesicles carrying small cargoes. eLife 2013; 2:e01296. [PMID: 24137546 PMCID: PMC3787390 DOI: 10.7554/elife.01296] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2013] [Accepted: 08/21/2013] [Indexed: 01/15/2023] Open
Abstract
A core prediction of the vesicular transport model is that COPI vesicles are responsible for trafficking anterograde cargoes forward. In this study, we test this prediction by examining the properties and requirements of inter-Golgi transport within fused cells, which requires mobile carriers in order for exchange of constituents to occur. We report that both small soluble and membrane-bound secretory cargo and exogenous Golgi resident glycosyl-transferases are exchanged between separated Golgi. Large soluble aggregates, which traverse individual stacks, do not transfer between Golgi, implying that small cargoes (which can fit in a typical transport vesicle) are transported by a different mechanism. Super-resolution microscopy reveals that the carriers of both anterograde and retrograde cargoes are the size of COPI vesicles, contain coatomer, and functionally require ARF1 and coatomer for transport. The data suggest that COPI vesicles traffic both small secretory cargo and steady-state Golgi resident enzymes among stacked cisternae that are stationary. DOI:http://dx.doi.org/10.7554/eLife.01296.001 All eukaryotic cells contain an organelle called the Golgi apparatus, which consists of a series of four to six flattened structures called cisternae. Proteins that are intended for secretion from the cell, or proteins that go on to become part of the cell membrane, must pass through the Golgi, where they undergo modifications that ensure they are targeted to the correct place. There are two main models for how proteins are transported from the entry side of the Golgi, known as the cis face, to the exit side (trans face), through a process known as anterograde transport. One possibility is that the cargo protein matures within a single cisterna, which gradually moves from the cis to the trans face without the protein ever leaving it. Alternatively, the cisternae may remain fixed in position, while individual proteins are carried between them by specialized transport vesicles called COPI vesicles. Now, Pellett et al. have used modern molecular biology techniques to revisit this question, more than 25 years after members of the same group first obtained evidence suggesting the involvement of COPI vesicles. To do this, they labelled the proteins that reside within the Golgi of one cell green, and those within the Golgi of another cell, red. They then fused the two cells together, and traced the movement of labelled proteins between the two organelles. Proteins that are known to undergo anterograde transport were also transported between the two Golgi, whereas large protein aggregates were not. Super-resolution microscopy revealed that the transported proteins were carried in vesicles the size of COPI vesicles and surrounded by a coat protein that resembles COPI. Moreover, transport involved the adaptor protein ARF, which helps to load cargo into COPI vesicles. By providing evidence that Golgi resident proteins and proteins that normally undergo anterograde transport can be carried by COPI vesicles between two physically separate Golgi, Pellett et al. increase the weight of evidence that COPI vesicles may also be responsible for both retrograde and anterograde transport within the Golgi itself. DOI:http://dx.doi.org/10.7554/eLife.01296.002
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79
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Lavieu G, Zheng H, Rothman JE. Stapled Golgi cisternae remain in place as cargo passes through the stack. eLife 2013; 2:e00558. [PMID: 23755362 PMCID: PMC3673335 DOI: 10.7554/elife.00558] [Citation(s) in RCA: 76] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2013] [Accepted: 04/26/2013] [Indexed: 12/16/2022] Open
Abstract
We have designed a membrane ‘staple’, which consists of membrane-anchored repeats of the trans-aggregating FM domain that face the lumen of the secretory pathway. In the presence of the disaggregating drug these proteins transit the secretory pathway. When the drug is removed these proteins form electron-dense plaques which we term staples. Unexpectedly, when initially positioned within the cis-Golgi, staples remained at the cis face of the Golgi even after many hours. By contrast, soluble FM-aggregates transited the Golgi. Staples and soluble aggregates placed in cis-Golgi cisternae therefore have different fates. Whereas the membrane staples are located in the flattened, stacked central regions of the cisternae, the soluble aggregates are in the dilated rims. This suggests that while the cisternae are static on the time scale of protein traffic, the dilated rims are mobile and progress in the cis → trans direction via a mechanism that we term ‘Rim Progression’. DOI:http://dx.doi.org/10.7554/eLife.00558.001 Most plant and animal cells contain an organelle known as the Golgi apparatus, which consists of a series of four to six stacked cisternae. Almost all the proteins that are secreted from the cell, or targeted to its plasma membrane, transit through the Golgi. This process takes roughly 5–20 min. Although transport of proteins through the Golgi was first observed more than 50 years ago, it is still unclear exactly how this process occurs. One possibility is that proteins to be packaged move through the cisternae enclosed in vesicles, as if on a conveyor belt. Alternatively, the proteins themselves may remain stationary while the Golgi cisternae move over them. Now, Lavieu et al. provide evidence that the Golgi shows both mobile and static behaviour depending on the type and size of the cargo being processed. To distinguish between these two mechanisms, they created a new type of protein cargo—which they called a ‘staple’—that became fixed to the walls on each side of the cisternae and could not, therefore, move freely through the Golgi. They compared the processing of this protein to that of a more typical soluble protein cargo, which could move freely through the Golgi stack. Surprisingly, the Golgi processed these two types of cargo in very different ways. The staples remained embedded in the walls in the center of the cisternae, whereas the conventional soluble cargo was transported past the staples and collected at the edges of the cisternae, which are known as rims. These are wider than the center of the cisternae, and the staples are too narrow to span them. Lavieu et al. suggest that the Golgi cisternae can be divided into two functionally distinct domains: the centers of cisternae, which remain stationary, and the edges or rims, which can move. In addition to increasing our understanding of how proteins are prepared for transport inside cells, this new mechanism reconciles seemingly conflicting data by revealing that the Golgi can be both mobile and static. DOI:http://dx.doi.org/10.7554/eLife.00558.002
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80
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Gao Y, Zorman S, Gundersen G, Xi Z, Ma L, Sirinakis G, Rothman JE, Zhang Y. Single Neuronal Snare Complexes Zipper in Three Distinct Stages. Biophys J 2013. [DOI: 10.1016/j.bpj.2012.11.531] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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81
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Gao Y, Zorman S, Gundersen G, Xi Z, Ma L, Sirinakis G, Rothman JE, Zhang Y. Single reconstituted neuronal SNARE complexes zipper in three distinct stages. Science 2012; 337:1340-3. [PMID: 22903523 DOI: 10.1126/science.1224492] [Citation(s) in RCA: 298] [Impact Index Per Article: 24.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins drive membrane fusion by assembling into a four-helix bundle in a zippering process. Here, we used optical tweezers to observe in a cell-free reconstitution experiment in real time a long-sought SNARE assembly intermediate in which only the membrane-distal amino-terminal half of the bundle is assembled. Our findings support the zippering hypothesis, but suggest that zippering proceeds through three sequential binary switches, not continuously, in the amino- and carboxyl-terminal halves of the bundle and the linker domain. The half-zippered intermediate was stabilized by externally applied force that mimicked the repulsion between apposed membranes being forced to fuse. This intermediate then rapidly and forcefully zippered, delivering free energy of 36 k(B)T (where k(B) is Boltzmann's constant and T is temperature) to mediate fusion.
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82
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Shi L, Shen QT, Kiel A, Wang J, Wang HW, Melia TJ, Rothman JE, Pincet F. SNARE proteins: one to fuse and three to keep the nascent fusion pore open. Science 2012; 335:1355-9. [PMID: 22422984 PMCID: PMC3736847 DOI: 10.1126/science.1214984] [Citation(s) in RCA: 201] [Impact Index Per Article: 16.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Neurotransmitters are released through nascent fusion pores, which ordinarily dilate after bilayer fusion, preventing consistent biochemical studies. We used lipid bilayer nanodiscs as fusion partners; their rigid protein framework prevents dilation and reveals properties of the fusion pore induced by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). We found that although only one SNARE per nanodisc is required for maximum rates of bilayer fusion, efficient release of content on the physiologically relevant time scale of synaptic transmission apparently requires three or more SNARE complexes (SNAREpins) and the native transmembrane domain of vesicle-associated membrane protein 2 (VAMP2). We suggest that several SNAREpins simultaneously zippering their SNARE transmembrane helices within the freshly fused bilayers provide a radial force that prevents the nascent pore from resealing during synchronous neurotransmitter release.
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83
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Cremona ML, Matthies HJG, Pau K, Bowton E, Speed N, Lute BJ, Anderson M, Sen N, Robertson SD, Vaughan RA, Rothman JE, Galli A, Javitch JA, Yamamoto A. Erratum: Corrigendum: Flotillin-1 is essential for PKC-triggered endocytosis and membrane microdomain localization of DAT. Nat Neurosci 2011. [DOI: 10.1038/nn1211-1617a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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84
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Abstract
F.-Ulrich Hartl and Arthur Horwich will share this year's Lasker Basic Medical Science Award for the discovery of the cell's protein-folding machinery, exemplified by cage-like structures that convert newly synthesized proteins into their biologically active forms. Their fundamental findings reveal mechanisms that operate in normal physiologic processes and help to explain the problems that arise in diseases of protein folding.
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85
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Pellett PA, Sun X, Gould TJ, Rothman JE, Xu MQ, Corrêa IR, Bewersdorf J. Two-color STED microscopy in living cells. BIOMEDICAL OPTICS EXPRESS 2011; 2:2364-71. [PMID: 21833373 PMCID: PMC3149534 DOI: 10.1364/boe.2.002364] [Citation(s) in RCA: 82] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/15/2011] [Revised: 07/21/2011] [Accepted: 07/21/2011] [Indexed: 05/15/2023]
Abstract
Diffraction-unlimited resolution provided by Stimulated Emission Depletion (STED) microscopy allows for imaging cellular processes in living cells that are not visible by conventional microscopy. However, it has so far not been possible to study dynamic nanoscale interactions because multicolor live cell STED microscopy has yet to be demonstrated and suitable labeling technologies and protocols are lacking. Here we report the first realization of two-color STED imaging in living cells. Using improved SNAP(f) and CLIP(f) technologies to label epidermal growth factor (EGF) and EGF receptor (EGFR), we report resolutions of 78 nm and 82 nm for 22 sequential two-color scans in living cells.
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86
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Krishnakumar SS, Radoff DT, Kümmel D, Giraudo CG, Li F, Khandan L, Baguley SW, Coleman J, Reinisch KM, Pincet F, Rothman JE. A conformational switch in complexin is required for synaptotagmin to trigger synaptic fusion. Nat Struct Mol Biol 2011; 18:934-40. [PMID: 21785412 DOI: 10.1038/nsmb.2103] [Citation(s) in RCA: 81] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2011] [Accepted: 05/19/2011] [Indexed: 11/09/2022]
Abstract
The crystal structure of complexin bound to a prefusion SNAREpin mimetic shows that the accessory helix extends away from the SNAREpin in an 'open' conformation, binding another SNAREpin and inhibiting its assembly, to clamp fusion. In contrast, the accessory helix in the postfusion complex parallels the SNARE complex in a 'closed' conformation. Here we use targeted mutations, FRET spectroscopy and a functional assay that reconstitutes Ca(2+)-triggered exocytosis to show that the conformational switch from open to closed in complexin is needed for synaptotagmin-Ca(2+) to trigger fusion. Triggering fusion requires the zippering of three crucial aspartate residues in the switch region (residues 64-68) of v-SNARE. Conformational switching in complexin is integral to clamp release and is probably triggered when its accessory helix is released from its trans-binding to the neighboring SNAREpin, allowing the v-SNARE to complete zippering and open a fusion pore.
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87
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Abstract
This essay looks backward on the past three decades of research toward understanding the mechanism of macromolecular traffic through and within the Golgi apparatus with an eye to the future. I also explain why I feel the Golgi should continue to hold the attention of molecular cell biologists.
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88
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Karatekin E, Gohlke A, Smith M, Vavylonis D, Rothman JE. Visualizing Release of Single Fluorophores at Membrane Fusion Sites. Biophys J 2011. [DOI: 10.1016/j.bpj.2010.12.1230] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022] Open
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89
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Ji H, Coleman J, Yang R, Melia TJ, Rothman JE, Tareste D. Protein determinants of SNARE-mediated lipid mixing. Biophys J 2010; 99:553-60. [PMID: 20643074 DOI: 10.1016/j.bpj.2010.04.060] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2010] [Revised: 04/19/2010] [Accepted: 04/26/2010] [Indexed: 01/14/2023] Open
Abstract
Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE)-mediated lipid mixing can be efficiently recapitulated in vitro by the incorporation of purified vesicle membrane (-v) SNARE and target membrane (t-) SNARE proteins into separate liposome populations. Despite the strong correlation between the observed activities in this system and the known SNARE physiology, some recent works have suggested that SNARE-mediated lipid mixing may be limited to circumstances where membrane defects arise from artifactual reconstitution conditions (such as nonphysiological high-protein concentrations or unrealistically small liposome populations). Here, we show that the previously published strategies used to reconstitute SNAREs into liposomes do not significantly affect either the physical parameters of the proteoliposomes or the ability of SNAREs to drive lipid mixing in vitro. The surface density of SNARE proteins turns out to be the most critical parameter, which controls both the rate and the extent of SNARE-mediated liposome fusion. In addition, the specific activity of the t-SNARE complex is significantly influenced by expression and reconstitution protocols, such that we only observe optimal lipid mixing when the t-SNARE proteins are coexpressed before purification.
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90
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Shen J, Rathore SS, Khandan L, Rothman JE. SNARE bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of membrane fusion. ACTA ACUST UNITED AC 2010; 190:55-63. [PMID: 20603329 PMCID: PMC2911676 DOI: 10.1083/jcb.201003148] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Whittling away SNARE complex components reveals essential domains for Munc18-1–mediated membrane fusion. Sec1/Munc18 (SM) proteins activate intracellular membrane fusion through binding to cognate SNAP receptor (SNARE) complexes. The synaptic target membrane SNARE syntaxin 1 contains a highly conserved Habc domain, which connects an N-peptide motif to the SNARE core domain and is thought to participate in the binding of Munc18-1 (the neuronal SM protein) to the SNARE complex. Unexpectedly, we found that mutation or complete removal of the Habc domain had no effect on Munc18-1 stimulation of fusion. The central cavity region of Munc18-1 is required to stimulate fusion but not through its binding to the syntaxin Habc domain. SNAP-25, another synaptic SNARE subunit, contains a flexible linker and exhibits an atypical conjoined Qbc configuration. We found that neither the linker nor the Qbc configuration is necessary for Munc18-1 promotion of fusion. As a result, Munc18-1 activates a SNARE complex with the typical configuration, in which each of the SNARE core domains is individually rooted in the membrane bilayer. Thus, the SNARE four-helix bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of fusion.
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91
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Rothman JE. The Power of One. Annu Rev Biochem 2010. [DOI: 10.1146/annurev.bi.79.052110.100001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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92
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Karatekin E, Di Giovanni J, Iborra C, Coleman J, O'Shaughnessy B, Seagar M, Rothman JE. A Fast, Single-Vesicle Fusion Assay Mimics Physiological SNARE Requirements. Biophys J 2010. [DOI: 10.1016/j.bpj.2009.12.3362] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022] Open
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93
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Emr S, Glick BS, Linstedt AD, Lippincott-Schwartz J, Luini A, Malhotra V, Marsh BJ, Nakano A, Pfeffer SR, Rabouille C, Rothman JE, Warren G, Wieland FT. Journeys through the Golgi--taking stock in a new era. ACTA ACUST UNITED AC 2009; 187:449-53. [PMID: 19948493 PMCID: PMC2779233 DOI: 10.1083/jcb.200909011] [Citation(s) in RCA: 133] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The Golgi apparatus is essential for protein sorting and transport. Many researchers have long been fascinated with the form and function of this organelle. Yet, despite decades of scrutiny, the mechanisms by which proteins are transported across the Golgi remain controversial. At a recent meeting, many prominent Golgi researchers assembled to critically evaluate the core issues in the field. This report presents the outcome of their discussions and highlights the key open questions that will help guide the field into a new era.
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94
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Paumet F, Wesolowski J, Garcia-Diaz A, Delevoye C, Aulner N, Shuman HA, Subtil A, Rothman JE. Intracellular bacteria encode inhibitory SNARE-like proteins. PLoS One 2009; 4:e7375. [PMID: 19823575 PMCID: PMC2756591 DOI: 10.1371/journal.pone.0007375] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2009] [Accepted: 09/15/2009] [Indexed: 11/24/2022] Open
Abstract
Pathogens use diverse molecular machines to penetrate host cells and manipulate intracellular vesicular trafficking. Viruses employ glycoproteins, functionally and structurally similar to the SNARE proteins, to induce eukaryotic membrane fusion. Intracellular pathogens, on the other hand, need to block fusion of their infectious phagosomes with various endocytic compartments to escape from the degradative pathway. The molecular details concerning the mechanisms underlying this process are lacking. Using both an in vitro liposome fusion assay and a cellular assay, we showed that SNARE-like bacterial proteins block membrane fusion in eukaryotic cells by directly inhibiting SNARE-mediated membrane fusion. More specifically, we showed that IncA and IcmG/DotF, two SNARE-like proteins respectively expressed by Chlamydia and Legionella, inhibit the endocytic SNARE machinery. Furthermore, we identified that the SNARE-like motif present in these bacterial proteins encodes the inhibitory function. This finding suggests that SNARE-like motifs are capable of specifically manipulating membrane fusion in a wide variety of biological environments. Ultimately, this motif may have been selected during evolution because it is an efficient structural motif for modifying eukaryotic membrane fusion and thus contribute to pathogen survival.
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95
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Giraudo CG, Garcia-Diaz A, Eng WS, Chen Y, Hendrickson WA, Melia TJ, Rothman JE. Alternative zippering as an on-off switch for SNARE-mediated fusion. Science 2009; 323:512-6. [PMID: 19164750 DOI: 10.1126/science.1166500] [Citation(s) in RCA: 131] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Membrane fusion between vesicles and target membranes involves the zippering of a four-helix bundle generated by constituent helices derived from target- and vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). In neurons, the protein complexin clamps otherwise spontaneous fusion by SNARE proteins, allowing neurotransmitters and other mediators to be secreted when and where they are needed as this clamp is released. The membrane-proximal accessory helix of complexin is necessary for clamping, but its mechanism of action is unknown. Here, we present experiments using a reconstituted fusion system that suggest a simple model in which the complexin accessory helix forms an alternative four-helix bundle with the target-SNARE near the membrane, preventing the vesicle-SNARE from completing its zippering.
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96
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Abstract
The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.
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97
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Giraudo CG, Garcia-Diaz A, Eng WS, Yamamoto A, Melia TJ, Rothman JE. Distinct domains of complexins bind SNARE complexes and clamp fusion in vitro. J Biol Chem 2008; 283:21211-9. [PMID: 18499660 PMCID: PMC2475712 DOI: 10.1074/jbc.m803478200] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2008] [Indexed: 12/29/2022] Open
Abstract
In regulated exocytosis, the core membrane fusion machinery proteins, the SNARE proteins, are assisted by a group of regulatory factors in order to couple membrane fusion to an increase of intracellular calcium ion (Ca(2+)) concentration. Complexin-I and synaptotagmin-I have been shown to be key elements for this tightly regulated process. Many studies suggest that complexin-I can arrest the fusion reaction and that synaptotagmin-I can release the complexin-I blockage in a calcium-dependent manner. Although the actual molecular mechanism by which they exert their function is still unknown, recent in vivo experiments postulate that domains of complexin-I produce different effects on neurotransmitter release. Herein, by using an in vitro flipped SNARE cell fusion assay, we have identified and characterized the minimal functional domains of complexin-I necessary to couple calcium and synaptotagmin-I to membrane fusion. Moreover, we provide evidence that other isoforms of complexin, complexin-II, -III, and -IV, can also be functionally coupled to synaptotagmin-I and calcium. These correspond closely to results from in vivo experiments, providing further validation of the physiological relevance of the flipped SNARE system.
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98
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Rothman JE. The Golgi apparatus: roles for distinct 'cis' and 'trans' compartments. CIBA FOUNDATION SYMPOSIUM 2008:120-37. [PMID: 6295718 DOI: 10.1002/9780470720745.ch7] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The Golgi apparatus seems to consist of distinct cis and trans compartments that are proposed to act sequentially to refine the protein export of the endoplasmic reticulum by removing escaped endoplasmic reticulum proteins. Refinement may be a multi-stage process that employs a principle akin to fractional distillation; the stack of cisternae comprising the cis Golgi may be the plates in this distillation tower. The trans Golgi, consisting of the last one or two cisternae, may be the receiver that collects from the cis Golgi only its most refined fraction for later distribution to specific locations throughout the cell.
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99
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Li F, Pincet F, Perez E, Eng WS, Melia TJ, Rothman JE, Tareste D. Energetics and dynamics of SNAREpin folding across lipid bilayers. Nat Struct Mol Biol 2007; 14:890-6. [PMID: 17906638 DOI: 10.1038/nsmb1310] [Citation(s) in RCA: 201] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2007] [Accepted: 09/05/2007] [Indexed: 11/09/2022]
Abstract
Membrane fusion occurs when SNAREpins fold up between lipid bilayers. How much energy is generated during SNAREpin folding and how this energy is coupled to the fusion of apposing membranes is unknown. We have used a surface forces apparatus to determine the energetics and dynamics of SNAREpin formation and characterize the different intermediate structures sampled by cognate SNAREs in the course of their assembly. The interaction energy-versus-distance profiles of assembling SNAREpins reveal that SNARE motifs begin to interact when the membranes are 8 nm apart. Even after very close approach of the bilayers (approximately 2-4 nm), the SNAREpins remain partly unstructured in their membrane-proximal region. The energy stabilizing a single SNAREpin in this configuration (35 k(B)T) corresponds closely with the energy needed to fuse outer but not inner leaflets (hemifusion) of pure lipid bilayers (40-50 k(B)T).
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Shen J, Tareste DC, Paumet F, Rothman JE, Melia TJ. Selective Activation of Cognate SNAREpins by Sec1/Munc18 Proteins. Cell 2007; 128:183-95. [PMID: 17218264 DOI: 10.1016/j.cell.2006.12.016] [Citation(s) in RCA: 379] [Impact Index Per Article: 22.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2006] [Revised: 10/31/2006] [Accepted: 12/05/2006] [Indexed: 01/11/2023]
Abstract
Sec1/Munc18 (SM) proteins are required for every step of intracellular membrane fusion, but their molecular mechanism of action has been unclear. In this work, we demonstrate a fundamental role of the SM protein: to act as a stimulatory subunit of its cognate SNARE fusion machinery. In a reconstituted system, mammalian SNARE pairs assemble between bilayers to drive a basal fusion reaction. Munc18-1/nSec1, a synaptic SM protein required for neurotransmitter release, strongly accelerates this reaction through direct contact with both t- and v-SNAREs. Munc18-1 accelerates fusion only for the cognate SNAREs for exocytosis, therefore enhancing fusion specificity.
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