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Shi L, Yang C, Zhang M, Li K, Wang K, Jiao L, Liu R, Wang Y, Li M, Wang Y, Ma L, Hu S, Bian X. Dissecting the mechanism of atlastin-mediated homotypic membrane fusion at the single-molecule level. Nat Commun 2024; 15:2488. [PMID: 38509071 PMCID: PMC10954664 DOI: 10.1038/s41467-024-46919-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2023] [Accepted: 03/13/2024] [Indexed: 03/22/2024] Open
Abstract
Homotypic membrane fusion of the endoplasmic reticulum (ER) is mediated by dynamin-like GTPase atlastin (ATL). This fundamental process relies on GTP-dependent domain rearrangements in the N-terminal region of ATL (ATLcyto), including the GTPase domain and three-helix bundle (3HB). However, its conformational dynamics during the GTPase cycle remain elusive. Here, we combine single-molecule FRET imaging and molecular dynamics simulations to address this conundrum. Different from the prevailing model, ATLcyto can form a loose crossover dimer upon GTP binding, which is tightened by GTP hydrolysis for membrane fusion. Furthermore, the α-helical motif between the 3HB and transmembrane domain, which is embedded in the surface of the lipid bilayer and self-associates in the crossover dimer, is required for ATL function. To recycle the proteins, Pi release, which disassembles the dimer, activates frequent relative movements between the GTPase domain and 3HB, and subsequent GDP dissociation alters the conformational preference of the ATLcyto monomer for entering the next reaction cycle. Finally, we found that two disease-causing mutations affect human ATL1 activity by destabilizing GTP binding-induced loose crossover dimer formation and the membrane-embedded helix, respectively. These results provide insights into ATL-mediated homotypic membrane fusion and the pathological mechanisms of related disease.
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Affiliation(s)
- Lijun Shi
- State Key Laboratory of Medicinal Chemical Biology, College of Life Sciences, Frontiers Science Center for Cell Responses, Nankai University, Tianjin, 300071, China
| | - Chenguang Yang
- National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China
- University of Chinese Academy of Sciences, Beijing, 100049, China
| | - Mingyuan Zhang
- College of Life Sciences, Zhejiang University, Hangzhou, 310027, China
| | - Kangning Li
- State Key Laboratory of Medicinal Chemical Biology, College of Life Sciences, Frontiers Science Center for Cell Responses, Nankai University, Tianjin, 300071, China
| | - Keying Wang
- College of Life Sciences, Zhejiang University, Hangzhou, 310027, China
| | - Li Jiao
- College of Life Sciences, Nankai University, Tianjin, 300071, China
| | - Ruming Liu
- College of Life Sciences, Nankai University, Tianjin, 300071, China
| | - Yunyun Wang
- State Key Laboratory of Medicinal Chemical Biology, College of Life Sciences, Frontiers Science Center for Cell Responses, Nankai University, Tianjin, 300071, China
| | - Ming Li
- National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China
| | - Yong Wang
- College of Life Sciences, Zhejiang University, Hangzhou, 310027, China.
- The Provincial International Science and Technology Cooperation Base on Engineering Biology, International Campus of Zhejiang University, Haining, 314400, China.
| | - Lu Ma
- National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China.
| | - Shuxin Hu
- National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China.
| | - Xin Bian
- State Key Laboratory of Medicinal Chemical Biology, College of Life Sciences, Frontiers Science Center for Cell Responses, Nankai University, Tianjin, 300071, China.
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Jang E, Lee M, Yoon SY, Lee SS, Park J, Jin MS, Eom SH, Lee C, Jun Y. Yeast lunapark regulates the formation of trans-Sey1p complexes for homotypic ER membrane fusion. iScience 2023; 26:108386. [PMID: 38025788 PMCID: PMC10679814 DOI: 10.1016/j.isci.2023.108386] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2023] [Revised: 09/24/2023] [Accepted: 10/30/2023] [Indexed: 12/01/2023] Open
Abstract
The endoplasmic reticulum (ER) consists of the nuclear envelope and a connected peripheral network of tubules and interspersed sheets. The structure of ER tubules is generated and maintained by various proteins, including reticulons, DP1/Yop1p, atlastins, and lunapark. Reticulons and DP1/Yop1p stabilize the high membrane curvature of ER tubules, and atlastins mediate homotypic membrane fusion between ER tubules; however, the exact role of lunapark remains poorly characterized. Here, using isolated yeast ER microsomes and reconstituted proteoliposomes, we directly examined the function of the yeast lunapark Lnp1p for yeast atlastin Sey1p-mediated ER fusion and found that Lnp1p inhibits Sey1p-driven membrane fusion. Furthermore, by using a newly developed assay for monitoring trans-Sey1p complex assembly, a prerequisite for ER fusion, we found that assembly of trans-Sey1p complexes was increased by the deletion of LNP1 and decreased by the overexpression of Lnp1p, indicating that Lnp1p inhibits Sey1p-mediated fusion by interfering with assembly of trans-Sey1p complexes.
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Affiliation(s)
- Eunhong Jang
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Miriam Lee
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
- Department of Biochemistry and Cell Biology, Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA
| | - So Young Yoon
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Sang Soo Lee
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Jongseo Park
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Mi Sun Jin
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Soo Hyun Eom
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
| | - Changwook Lee
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea
| | - Youngsoo Jun
- School of Life Sciences, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
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Reynisdottir I, Arason A, Freysteinsdottir ES, Kristjansdottir SB, Hilmarsdottir B, Traustadottir GA, Johannsson OT, Agnarsson BA, Barkardottir RB. High Atlastin 2-2 (ATL2-2) Expression Associates with Worse Prognosis in Estrogen-Receptor-Positive Breast Cancer. Genes (Basel) 2023; 14:1559. [PMID: 37628611 PMCID: PMC10454310 DOI: 10.3390/genes14081559] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2023] [Revised: 07/21/2023] [Accepted: 07/26/2023] [Indexed: 08/27/2023] Open
Abstract
The disruption of endoplasmic reticulum (ER) homeostasis occurs in many human diseases. Atlastins (ATLs) maintain the branched network of the ER. The dysregulation of ATL2, located at ER network junctions, has been associated with cancer. ATL2 is necessary for lipid droplet formation in murine breast tissue. Thus, we analyzed whether ATL2 has a role in human breast cancer (BC) pathology. The expression of ATL2 variant ATL2-2 was analyzed in breast tumors from the BC cohorts of the TCGA, METABRIC, and two independent Icelandic cohorts, Cohort 1 and 2; its association with clinical, pathological, survival, and cellular pathways was explored. ATL2-2 mRNA and protein expression were higher in breast tumors than in normal tissue. ATL2-2 mRNA associated with tumor characteristics that indicate a worse prognosis. In METABRIC, high ATL2-2 mRNA levels were associated with shorter BC-specific survival (BCSS) in patients with estrogen-receptor-positive luminal breast tumors, which remained significant after correction for grade and tumor size (HR 1.334, CI 1.063-1.673). Tumors with high ATL2 mRNA showed an upregulation of hallmark pathways MYC targets v1, E2F targets, and G2M checkpoint genes. Taken together, the results suggest that high levels of ATL2-2 may support BC progression through key cancer driver pathways.
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Affiliation(s)
- Inga Reynisdottir
- Cell Biology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland
- BMC (Biomedical Center), Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland; (A.A.); (B.H.); (G.A.T.); (R.B.B.)
| | - Adalgeir Arason
- BMC (Biomedical Center), Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland; (A.A.); (B.H.); (G.A.T.); (R.B.B.)
- Molecular Pathology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
| | - Edda S. Freysteinsdottir
- Molecular Pathology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
| | - Sigrun B. Kristjansdottir
- Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland; (S.B.K.); (B.A.A.)
| | - Bylgja Hilmarsdottir
- BMC (Biomedical Center), Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland; (A.A.); (B.H.); (G.A.T.); (R.B.B.)
- Molecular Pathology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
| | - Gunnhildur A. Traustadottir
- BMC (Biomedical Center), Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland; (A.A.); (B.H.); (G.A.T.); (R.B.B.)
- Molecular Pathology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
| | - Oskar T. Johannsson
- Department of Oncology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
| | - Bjarni A. Agnarsson
- Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland; (S.B.K.); (B.A.A.)
- Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland
| | - Rosa B. Barkardottir
- BMC (Biomedical Center), Faculty of Medicine, University of Iceland, 101 Reykjavik, Iceland; (A.A.); (B.H.); (G.A.T.); (R.B.B.)
- Molecular Pathology Unit, Department of Pathology, Landspitali—The National University Hospital of Iceland, 101 Reykjavik, Iceland;
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Bryce S, Stolzer M, Crosby D, Yang R, Durand D, Lee TH. Human atlastin-3 is a constitutive ER membrane fusion catalyst. J Cell Biol 2023; 222:e202211021. [PMID: 37102997 PMCID: PMC10140384 DOI: 10.1083/jcb.202211021] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2022] [Revised: 02/28/2023] [Accepted: 04/04/2023] [Indexed: 04/28/2023] Open
Abstract
Homotypic membrane fusion catalyzed by the atlastin (ATL) GTPase sustains the branched endoplasmic reticulum (ER) network in metazoans. Our recent discovery that two of the three human ATL paralogs (ATL1/2) are C-terminally autoinhibited implied that relief of autoinhibition would be integral to the ATL fusion mechanism. An alternative hypothesis is that the third paralog ATL3 promotes constitutive ER fusion with relief of ATL1/2 autoinhibition used conditionally. However, published studies suggest ATL3 is a weak fusogen at best. Contrary to expectations, we demonstrate here that purified human ATL3 catalyzes efficient membrane fusion in vitro and is sufficient to sustain the ER network in triple knockout cells. Strikingly, ATL3 lacks any detectable C-terminal autoinhibition, like the invertebrate Drosophila ATL ortholog. Phylogenetic analysis of ATL C-termini indicates that C-terminal autoinhibition is a recent evolutionary innovation. We suggest that ATL3 is a constitutive ER fusion catalyst and that ATL1/2 autoinhibition likely evolved in vertebrates as a means of upregulating ER fusion activity on demand.
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Affiliation(s)
- Samantha Bryce
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA, USA
| | - Maureen Stolzer
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA, USA
| | - Daniel Crosby
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA, USA
| | - Ruijin Yang
- Department of Computational Biology, Carnegie Mellon University, Pittsburgh, PA, USA
| | - Dannie Durand
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA, USA
- Department of Computational Biology, Carnegie Mellon University, Pittsburgh, PA, USA
| | - Tina H. Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA, USA
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5
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Jang E, Moon Y, Yoon SY, Diaz JAR, Lee M, Ko N, Park J, Eom SH, Lee C, Jun Y. Human atlastins are sufficient to drive the fusion of liposomes with a physiological lipid composition. J Cell Biol 2023; 222:e202109090. [PMID: 36757370 PMCID: PMC9949273 DOI: 10.1083/jcb.202109090] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2021] [Revised: 10/20/2022] [Accepted: 01/17/2023] [Indexed: 02/10/2023] Open
Abstract
The dynamin-like GTPase atlastin is believed to be the minimal machinery required for homotypic endoplasmic reticulum (ER) membrane fusion, mainly because Drosophila atlastin is sufficient to drive liposome fusion. However, it remains unclear whether mammalian atlastins, including the three human atlastins, are sufficient to induce liposome fusion, raising doubts about their major roles in mammalian cells. Here, we show that all human atlastins are sufficient to induce fusion when reconstituted into liposomes with a lipid composition mimicking that of the ER. Although the fusogenic activity of ATL1, which is predominantly expressed in neuronal cells, was weaker than that of ATL2 or ATL3, the addition of M1-spastin, a neuron-specific factor, markedly increased ATL1-mediated liposome fusion. Although we observed efficient fusion between ER microsomes isolated from cultured, non-neuronal cells that predominantly express ATL2-1, an autoinhibited isoform of ATL2, ATL2-1 failed to support liposome fusion by itself as reported previously, indicating that cellular factors enable ATL2-1 to mediate ER fusion in vivo.
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Affiliation(s)
- Eunhong Jang
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Yeojin Moon
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - So Young Yoon
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Joyce Anne R. Diaz
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Miriam Lee
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Naho Ko
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Jongseo Park
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Soo Hyun Eom
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Changwook Lee
- Department of Biological Sciences, School of Life Sciences, Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
| | - Youngsoo Jun
- School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- Cell Logistics Research Center, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
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6
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Abstract
Atlastin (ATL) GTPases undergo trans dimerization and a power strokelike crossover conformational rearrangement to drive endoplasmic reticulum membrane fusion. Fusion depends on GTP, but the role of nucleotide hydrolysis has remained controversial. For instance, nonhydrolyzable GTP analogs block fusion altogether, suggesting a requirement for GTP hydrolysis in ATL dimerization and crossover, but this leaves unanswered the question of how the ATL dimer is disassembled after fusion. We recently used the truncated cytoplasmic domain of wild-type Drosophila ATL (DATL) and a novel hydrolysis-deficient D127N variant in single turnover assays to reveal that dimerization and crossover consistently precede GTP hydrolysis, with hydrolysis coinciding more closely with dimer disassembly. Moreover, while nonhydrolyzable analogs can bind the DATL G domain, they fail to fully recapitulate the GTP-bound state. This predicted that nucleotide hydrolysis would be dispensable for fusion. Here we report that the D127N variant of full-length DATL drives both outer and inner leaflet membrane fusion with little to no detectable hydrolysis of GTP. However, the trans dimer fails to disassemble and subsequent rounds of fusion fail to occur. Our findings confirm that ATL mediated fusion is driven in the GTP-bound state, with nucleotide hydrolysis serving to reset the fusion machinery for recycling.
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Affiliation(s)
- Daniel Crosby
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - Tina H. Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213,*Address correspondence to: Tina H. Lee ()
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7
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Crosby D, Mikolaj MR, Nyenhuis SB, Bryce S, Hinshaw JE, Lee TH. Reconstitution of human atlastin fusion activity reveals autoinhibition by the C terminus. J Cell Biol 2022; 221:212879. [PMID: 34817557 PMCID: PMC8624677 DOI: 10.1083/jcb.202107070] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2021] [Revised: 10/13/2021] [Accepted: 11/12/2021] [Indexed: 01/31/2023] Open
Abstract
ER network formation depends on membrane fusion by the atlastin (ATL) GTPase. In humans, three paralogs are differentially expressed with divergent N- and C-terminal extensions, but their respective roles remain unknown. This is partly because, unlike Drosophila ATL, the fusion activity of human ATLs has not been reconstituted. Here, we report successful reconstitution of fusion activity by the human ATLs. Unexpectedly, the major splice isoforms of ATL1 and ATL2 are each autoinhibited, albeit to differing degrees. For the more strongly inhibited ATL2, autoinhibition mapped to a C-terminal α-helix is predicted to be continuous with an amphipathic helix required for fusion. Charge reversal of residues in the inhibitory domain strongly activated its fusion activity, and overexpression of this disinhibited version caused ER collapse. Neurons express an ATL2 splice isoform whose sequence differs in the inhibitory domain, and this form showed full fusion activity. These findings reveal autoinhibition and alternate splicing as regulators of atlastin-mediated ER fusion.
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Affiliation(s)
- Daniel Crosby
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - Melissa R Mikolaj
- Laboratory of Cell and Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD
| | - Sarah B Nyenhuis
- Laboratory of Cell and Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD
| | - Samantha Bryce
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - Jenny E Hinshaw
- Laboratory of Cell and Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD
| | - Tina H Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
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8
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Amaya C, Cameron CJF, Devarkar SC, Seager SJH, Gerstein MB, Xiong Y, Schlieker C. Nodal modulator (NOMO) is required to sustain endoplasmic reticulum morphology. J Biol Chem 2021; 297:100937. [PMID: 34224731 PMCID: PMC8327139 DOI: 10.1016/j.jbc.2021.100937] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2021] [Revised: 06/24/2021] [Accepted: 07/01/2021] [Indexed: 12/03/2022] Open
Abstract
The endoplasmic reticulum (ER) is a membrane-bound organelle responsible for protein folding, lipid synthesis, and calcium homeostasis. Maintenance of ER structural integrity is crucial for proper function, but much remains to be learned about the molecular players involved. To identify proteins that support the structure of the ER, we performed a proteomic screen and identified nodal modulator (NOMO), a widely conserved type I transmembrane protein of unknown function, with three nearly identical orthologs specified in the human genome. We found that overexpression of NOMO1 imposes a sheet morphology on the ER, whereas depletion of NOMO1 and its orthologs causes a collapse of ER morphology concomitant with the formation of membrane-delineated holes in the ER network positive for the lysosomal marker lysosomal-associated protein 1. In addition, the levels of key players of autophagy including microtubule-associated protein light chain 3 and autophagy cargo receptor p62/sequestosome 1 strongly increase upon NOMO depletion. In vitro reconstitution of NOMO1 revealed a "beads on a string" structure likely representing consecutive immunoglobulin-like domains. Extending NOMO1 by insertion of additional immunoglobulin folds results in a correlative increase in the ER intermembrane distance. Based on these observations and a genetic epistasis analysis including the known ER-shaping proteins Atlastin2 and Climp63, we propose a role for NOMO1 in the functional network of ER-shaping proteins.
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Affiliation(s)
- Catherine Amaya
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA
| | - Christopher J F Cameron
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA
| | - Swapnil C Devarkar
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA
| | - Sebastian J H Seager
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA
| | - Mark B Gerstein
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA; Program in Computational Biology and Bioinformatics, Yale University, New Haven, Connecticut, USA; Department of Computer Science, Yale University, New Haven, Connecticut, USA; Department of Statistics and Data Science, Yale University, New Haven, Connecticut, USA
| | - Yong Xiong
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA
| | - Christian Schlieker
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA; Department of Cell Biology, Yale School of Medicine, New Haven, Connecticut, USA.
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9
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Ryanodine receptor-mediated Ca 2+ release and atlastin-2 GTPase activity contribute to IP 3-induced dendritic Ca 2+ signals in primary hippocampal neurons. Cell Calcium 2021; 96:102399. [PMID: 33812310 DOI: 10.1016/j.ceca.2021.102399] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2020] [Revised: 03/17/2021] [Accepted: 03/18/2021] [Indexed: 12/19/2022]
Abstract
Neuronal Ca2+ signals are fundamental for synaptic transmission and activity-dependent changes in gene expression. Voltage-gated Ca2+ channels and N-methyl-d-aspartate receptors play major roles in mediating external Ca2+ entry during action potential firing and glutamatergic activity. Additionally, the inositol-1,4,5-trisphosphate receptor (IP3R) and the ryanodine receptor (RyR) channels expressed in the endoplasmic reticulum (ER) also contribute to the generation of Ca2+ signals in response to neuronal activity. The ER forms a network that pervades the entire neuronal volume, allowing intracellular Ca2+ release in dendrites, soma and presynaptic boutons. Despite its unique morphological features, the contributions of ER structure and of ER-shaping proteins such as atlastin - an ER enriched GTPase that mediates homotypic ER tubule fusion - to the generation of Ca2+ signals in dendrites remains unreported. Here, we investigated the contribution of RyR-mediated Ca2+ release to IP3-generated Ca2+ signals in dendrites of cultured hippocampal neurons. We also employed GTPase activity-deficient atlastin-2 (ATL2) mutants to evaluate the potential role of atlastin on Ca2+ signaling and ER-resident Ca2+ channel distribution. We found that pharmacological suppression of RyR channel activity increased the rising time and reduced the magnitude and propagation of IP3-induced Ca2+ signals. Additionally, ATL2 mutants induced specific ER morphological alterations, delayed the onset and increased the rising time of IP3-evoked Ca2+ signals, and caused RyR2 and IP3R1 aggregation and RyR2 redistribution. These results indicate that both RyR and ATL2 activity regulate IP3-induced Ca2+ signal dynamics through RyR-mediated Ca2+-induced Ca2+ release, ER shaping and RyR2 distribution.
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10
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Sensory-Neuropathy-Causing Mutations in ATL3 Cause Aberrant ER Membrane Tethering. Cell Rep 2019; 23:2026-2038. [PMID: 29768202 DOI: 10.1016/j.celrep.2018.04.071] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2017] [Revised: 02/28/2018] [Accepted: 04/13/2018] [Indexed: 12/31/2022] Open
Abstract
The endoplasmic reticulum (ER) is a complex network of sheets and tubules that is continuously remodeled. The relevance of this membrane dynamics is underscored by the fact that mutations in atlastins (ATLs), the ER fusion proteins in mammals, cause neurodegeneration. How defects in this process disrupt neuronal homeostasis is unclear. Using electron microscopy (EM) volume reconstruction of transfected cells, neurons, and patient fibroblasts, we show that hereditary sensory and autonomic neuropathy (HSAN)-causing ATL3 mutants promote aberrant ER tethering hallmarked by bundles of laterally attached ER tubules. In vitro, these mutants cause excessive liposome tethering, recapitulating the results in cells. Moreover, ATL3 variants retain their dimerization-dependent GTPase activity but are unable to promote membrane fusion, suggesting a defect in an intermediate step of the ATL3 functional cycle. Our data show that the effects of ATL3 mutations on ER network organization go beyond a loss of fusion and shed light on neuropathies caused by atlastin defects.
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11
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Betancourt-Solis MA, Desai T, McNew JA. The atlastin membrane anchor forms an intramembrane hairpin that does not span the phospholipid bilayer. J Biol Chem 2018; 293:18514-18524. [PMID: 30287684 DOI: 10.1074/jbc.ra118.003812] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Revised: 09/27/2018] [Indexed: 12/27/2022] Open
Abstract
The endoplasmic reticulum (ER) is composed of flattened sheets and interconnected tubules that extend throughout the cytosol and makes physical contact with all other cytoplasmic organelles. This cytoplasmic distribution requires continuous remodeling. These discrete ER morphologies require specialized proteins that drive and maintain membrane curvature. The GTPase atlastin is required for homotypic fusion of ER tubules. All atlastin homologs possess a conserved domain architecture consisting of a GTPase domain, a three-helix bundle middle domain, a hydrophobic membrane anchor, and a C-terminal cytosolic tail. Here, we examined several Drosophila-human atlastin chimeras to identify functional domains of human atlastin-1 in vitro Although all chimeras could hydrolyze GTP, only chimeras containing the human C-terminal tail, hydrophobic segments, or both could fuse membranes in vitro We also determined that co-reconstitution of atlastin with reticulon does not influence GTPase activity or membrane fusion. Finally, we found that both human and Drosophila atlastin hydrophobic membrane anchors do not span the membrane, but rather form two intramembrane hairpin loops. The topology of these hairpins remains static during membrane fusion and does not appear to play an active role in lipid mixing.
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Affiliation(s)
| | - Tanvi Desai
- From the Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005
| | - James A McNew
- From the Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005
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12
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Winsor J, Machi U, Han Q, Hackney DD, Lee TH. GTP hydrolysis promotes disassembly of the atlastin crossover dimer during ER fusion. J Cell Biol 2018; 217:4184-4198. [PMID: 30249723 PMCID: PMC6279388 DOI: 10.1083/jcb.201805039] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2018] [Revised: 08/15/2018] [Accepted: 09/17/2018] [Indexed: 02/06/2023] Open
Abstract
The GTPase atlastin mediates homotypic ER fusion through trans-crossover dimerization, but how dimerization is coupled to the GTPase cycle has remained unclear. Winsor et al. show that GTP binding causes crossover dimerization for fusion, whereas GTP hydrolysis promotes disassembly of the crossover dimer for subunit recycling. Membrane fusion of the ER is catalyzed when atlastin GTPases anchored in opposing membranes dimerize and undergo a crossed over conformational rearrangement that draws the bilayers together. Previous studies have suggested that GTP hydrolysis triggers crossover dimerization, thus directly driving fusion. In this study, we make the surprising observations that WT atlastin undergoes crossover dimerization before hydrolyzing GTP and that nucleotide hydrolysis and Pi release coincide more closely with dimer disassembly. These findings suggest that GTP binding, rather than its hydrolysis, triggers crossover dimerization for fusion. In support, a new hydrolysis-deficient atlastin variant undergoes rapid GTP-dependent crossover dimerization and catalyzes fusion at an initial rate similar to WT atlastin. However, the variant cannot sustain fusion activity over time, implying a defect in subunit recycling. We suggest that GTP binding induces an atlastin conformational change that favors crossover dimerization for fusion and that the input of energy from nucleotide hydrolysis promotes complex disassembly for subunit recycling.
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Affiliation(s)
- James Winsor
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - Ursula Machi
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - Qixiu Han
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - David D Hackney
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
| | - Tina H Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA
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13
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O'Donnell JP, Byrnes LJ, Cooley RB, Sondermann H. A hereditary spastic paraplegia-associated atlastin variant exhibits defective allosteric coupling in the catalytic core. J Biol Chem 2017; 293:687-700. [PMID: 29180453 DOI: 10.1074/jbc.ra117.000380] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2017] [Revised: 11/17/2017] [Indexed: 11/06/2022] Open
Abstract
The dynamin-related GTPase atlastin (ATL) catalyzes membrane fusion of the endoplasmic reticulum and thus establishes a network of branched membrane tubules. When ATL function is compromised, the morphology of the endoplasmic reticulum deteriorates, and these defects can result in neurological disorders such as hereditary spastic paraplegia and hereditary sensory neuropathy. ATLs harness the energy of GTP hydrolysis to initiate a series of conformational changes that enable homodimerization and subsequent membrane fusion. Disease-associated amino acid substitutions cluster in regions adjacent to ATL's catalytic site, but the consequences for the GTPase's molecular mechanism are often poorly understood. Here, we elucidate structural and functional defects of an atypical hereditary spastic paraplegia mutant, ATL1-F151S, that is impaired in its nucleotide-hydrolysis cycle but can still adopt a high-affinity homodimer when bound to a transition-state analog. Crystal structures of mutant proteins yielded models of the monomeric pre- and post-hydrolysis states of ATL. Together, these findings define a mechanism for allosteric coupling in which Phe151 is the central residue in a hydrophobic interaction network connecting the active site to an interdomain interface responsible for nucleotide loading.
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Affiliation(s)
- John P O'Donnell
- From the Department of Molecular Medicine, Cornell University, Ithaca, New York 14853
| | - Laura J Byrnes
- From the Department of Molecular Medicine, Cornell University, Ithaca, New York 14853
| | - Richard B Cooley
- From the Department of Molecular Medicine, Cornell University, Ithaca, New York 14853
| | - Holger Sondermann
- From the Department of Molecular Medicine, Cornell University, Ithaca, New York 14853
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14
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Ferencz C, Guigas G, Veres A, Neumann B, Stemmann O, Weiss M. In Vitro Reconstitution of the Endoplasmic Reticulum. ACTA ACUST UNITED AC 2017; 76:11.22.1-11.22.16. [DOI: 10.1002/cpcb.30] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Csilla‐Maria Ferencz
- Department of Experimental Physics I, University of Bayreuth Bayreuth Germany
- Current address: Max Planck Institute of Colloids and Interfaces, Department of Theory and Bio‐Systems Potsdam Germany
| | - Gernot Guigas
- Department of Experimental Physics I, University of Bayreuth Bayreuth Germany
- Current address: Institute of Applied Physics, Karlsruhe Institute of Technology (KIT) Karlsruhe Germany
| | - Andreas Veres
- Department of Experimental Physics I, University of Bayreuth Bayreuth Germany
| | | | - Olaf Stemmann
- Department of Genetics, University of Bayreuth Bayreuth Germany
| | - Matthias Weiss
- Department of Experimental Physics I, University of Bayreuth Bayreuth Germany
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15
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Winsor J, Hackney DD, Lee TH. The crossover conformational shift of the GTPase atlastin provides the energy driving ER fusion. J Cell Biol 2017; 216:1321-1335. [PMID: 28356327 PMCID: PMC5412568 DOI: 10.1083/jcb.201609071] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2016] [Revised: 01/17/2017] [Accepted: 02/17/2017] [Indexed: 01/04/2023] Open
Abstract
The GTPase atlastin mediates homotypic membrane ER fusion through trans-dimerization between GTPase heads. Winsor et al. use a mutagenesis approach to show that, upon contact between atlastin heads, the proteins concurrently display GTP hydrolysis-catalyzed head-to-head dimerization and a crossover conformational shift, and these changes energize fusion. The homotypic fusion of endoplasmic reticulum membranes is catalyzed by the atlastin GTPase. The mechanism involves trans-dimerization between GTPase heads and a favorable crossover conformational shift, catalyzed by GTP hydrolysis, that converts the dimer from a “prefusion” to “postfusion” state. However, whether crossover formation actually energizes fusion remains unclear, as do the sequence of events surrounding it. Here, we made mutations in atlastin to selectively destabilize the crossover conformation and used fluorescence-based kinetic assays to analyze the variants. All variants underwent dimerization and crossover concurrently, and at wild-type rates. However, certain variants were unstable once in the crossover dimer conformation, and crossover dimer stability closely paralleled lipid-mixing activity. Tethering, however, appeared to be unimpaired in all mutant variants. The results suggest that tethering and lipid mixing are catalyzed concurrently by GTP hydrolysis but that the energy requirement for lipid mixing exceeds that for tethering, and the full energy released through crossover formation is necessary for fusion.
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Affiliation(s)
- James Winsor
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - David D Hackney
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - Tina H Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
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16
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Triyatni M, Berger EA, Saunier B. Assembly and release of infectious hepatitis C virus involving unusual organization of the secretory pathway. World J Hepatol 2016; 8:796-814. [PMID: 27429716 PMCID: PMC4937168 DOI: 10.4254/wjh.v8.i19.796] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/31/2016] [Revised: 04/30/2016] [Accepted: 06/03/2016] [Indexed: 02/06/2023] Open
Abstract
AIM: To determine if calnexin (CANX), RAB1 and alpha-tubulin were involved in the production of hepatitis C virus (HCV) particles by baby hamster kidney-West Nile virus (BHK-WNV) cells.
METHODS: Using a siRNA-based approach complemented with immuno-fluorescence confocal microscope and Western blot studies, we examined the roles of CANX, RAB1 and alpha-tubulin in the production of HCV particles by permissive BHK-WNV cells expressing HCV structural proteins or the full-length genome of HCV genotype 1a. Immuno-fluorescence studies in producer cells were performed with monoclonal antibodies against HCV structural proteins, as well as immunoglobulin from the serum of a patient recently cured from an HCV infection of same genotype. The cellular compartment stained by the serum immunoglobulin was also observed in thin section transmission electron microscopy. These findings were compared with the JFH-1 strain/Huh-7.5 cell model.
RESULTS: We found that CANX was necessary for the production of HCV particles by BHK-WNV cells. This process involved the recruitment of a subset of HCV proteins, detected by immunoglobulin of an HCV-cured patient, in a compartment of rearranged membranes bypassing the endoplasmic reticulum-Golgi intermediary compartment and surrounded by mitochondria. It also involved the maturation of N-linked glycans on HCV envelope proteins, which was required for assembly and/or secretion of HCV particles. The formation of this specialized compartment required RAB1; upon expression of HCV structural genes, this compartment developed large vesicles with viral particles. RAB1 and alpha-tubulin were required for the release of HCV particles. These cellular factors were also involved in the production of HCVcc in the JFH-1 strain/Huh-7.5 cell system, which involves HCV RNA replication. The secretion of HCV particles by BHK-WNV cells presents similarities with a pathway involving caspase-1; a caspase-1 inhibitor was found to suppress the production of HCV particles from a full-length genome.
CONCLUSION: Prior activity of the WNV subgenomic replicon in BHK-21 cells promoted re-wiring of host factors for the assembly and release of infectious HCV in a caspase-1-dependent mechanism.
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17
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Shaping the endoplasmic reticulum in vitro. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:2035-2040. [PMID: 27287725 DOI: 10.1016/j.bbamem.2016.06.004] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/17/2016] [Revised: 06/02/2016] [Accepted: 06/05/2016] [Indexed: 11/22/2022]
Abstract
Organelles in eukaryotic cells often have complex shapes that deviate significantly from simple spheres. A prime example is the endoplasmic reticulum (ER) that forms an extensive network of membrane tubules in many mammalian cell types and in reconstitution assays in vitro. Despite the successful hunt for molecular determinants of ER shape we are still far from having a comprehensive understanding of ER network morphogenesis. Here, we have studied the hitherto neglected influence of the host substrate when reconstituting ER networks in vitro as compared to ER networks in vivo. In culture cells we observed cytoplasm-spanning ER networks with tubules being connected almost exclusively by three-way junctions and segment lengths being narrowly distributed around a mean length of about 1μm. In contrast, networks reconstituted from purified ER microsomes on flat glass or gel substrates of varying stiffness showed significantly broader length distributions with an up to fourfold larger mean length. Self-assembly of ER microsomes on small oil droplets, however, yielded networks that resembled more closely the native ER network of mammalian cells. We conclude from these observations that the ER microsomes' inherent self-assembly capacity is sufficient to support network formation with a native geometry if the influence of the host substrate's surface chemistry becomes negligible. We hypothesize that under these conditions the networks' preference for three-way junctions follows from creating 'starfish-shaped' vesicles when ER microsomes with a protein-induced spontaneous curvature undergo fusion.
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18
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Fusion of the endoplasmic reticulum by membrane-bound GTPases. Semin Cell Dev Biol 2016; 60:105-111. [PMID: 27269373 DOI: 10.1016/j.semcdb.2016.06.001] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2016] [Revised: 05/31/2016] [Accepted: 06/02/2016] [Indexed: 12/14/2022]
Abstract
The endoplasmic reticulum (ER) membrane forms an elaborate network of tubules and sheets that is continuously remodeled. This dynamic behavior requires membrane fusion that is mediated by dynamin-like GTPases: the atlastins in metazoans and Sey1p and related proteins in yeast and plants. Crystal structures of the cytosolic domains of these membrane proteins and biochemical experiments can now be integrated into a model that explains many aspects of the molecular mechanism by which these membrane-bound GTPases mediate membrane fusion.
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19
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Ueda H, Yokota E, Kuwata K, Kutsuna N, Mano S, Shimada T, Tamura K, Stefano G, Fukao Y, Brandizzi F, Shimmen T, Nishimura M, Hara-Nishimura I. Phosphorylation of the C Terminus of RHD3 Has a Critical Role in Homotypic ER Membrane Fusion in Arabidopsis. PLANT PHYSIOLOGY 2016; 170:867-80. [PMID: 26684656 PMCID: PMC4734555 DOI: 10.1104/pp.15.01172] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/28/2015] [Accepted: 12/16/2015] [Indexed: 05/19/2023]
Abstract
The endoplasmic reticulum (ER) consists of dynamically changing tubules and cisternae. In animals and yeast, homotypic ER membrane fusion is mediated by fusogens (atlastin and Sey1p, respectively) that are membrane-associated dynamin-like GTPases. In Arabidopsis (Arabidopsis thaliana), another dynamin-like GTPase, ROOT HAIR DEFECTIVE3 (RHD3), has been proposed as an ER membrane fusogen, but direct evidence is lacking. Here, we show that RHD3 has an ER membrane fusion activity that is enhanced by phosphorylation of its C terminus. The ER network was RHD3-dependently reconstituted from the cytosol and microsome fraction of tobacco (Nicotiana tabacum) cultured cells by exogenously adding GTP, ATP, and F-actin. We next established an in vitro assay system of ER tubule formation with Arabidopsis ER vesicles, in which addition of GTP caused ER sac formation from the ER vesicles. Subsequent application of a shearing force to this system triggered the formation of tubules from the ER sacs in an RHD-dependent manner. Unexpectedly, in the absence of a shearing force, Ser/Thr kinase treatment triggered RHD3-dependent tubule formation. Mass spectrometry showed that RHD3 was phosphorylated at multiple Ser and Thr residues in the C terminus. An antibody against the RHD3 C-terminal peptide abolished kinase-triggered tubule formation. When the Ser cluster was deleted or when the Ser residues were replaced with Ala residues, kinase treatment had no effect on tubule formation. Kinase treatment induced the oligomerization of RHD3. Neither phosphorylation-dependent modulation of membrane fusion nor oligomerization has been reported for atlastin or Sey1p. Taken together, we propose that phosphorylation-stimulated oligomerization of RHD3 enhances ER membrane fusion to form the ER network.
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Affiliation(s)
- Haruko Ueda
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Etsuo Yokota
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Keiko Kuwata
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Natsumaro Kutsuna
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Shoji Mano
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Tomoo Shimada
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Kentaro Tamura
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Giovanni Stefano
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Yoichiro Fukao
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Federica Brandizzi
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Teruo Shimmen
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Mikio Nishimura
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
| | - Ikuko Hara-Nishimura
- Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan (H.U., To.S., K.T., I.H.-N.); Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan (E.Y., Te.S.); Institute of Transformative Bio-Molecules, Nagoya University, Nagoya 464-8601, Japan (K.K.); Graduate School of Frontier Sciences, University of Tokyo, Chiba 277-8562, Japan (N.K.); Department of Cell Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan (S.M., M.N.); MSU-DOE Plant Research Laboratory and Department of Plant Biology, Michigan State University, Michigan 48824 (G.S., F.B.); and Department of Bioinformatics, Ritsumeikan University, Kusatsu 525-8577, Japan (Y.F.)
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Cis and trans interactions between atlastin molecules during membrane fusion. Proc Natl Acad Sci U S A 2015; 112:E1851-60. [PMID: 25825753 DOI: 10.1073/pnas.1504368112] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Atlastin (ATL), a membrane-anchored GTPase that mediates homotypic fusion of endoplasmic reticulum (ER) membranes, is required for formation of the tubular network of the peripheral ER. How exactly ATL mediates membrane fusion is only poorly understood. Here we show that fusion is preceded by the transient tethering of ATL-containing vesicles caused by the dimerization of ATL molecules in opposing membranes. Tethering requires GTP hydrolysis, not just GTP binding, because the two ATL molecules are pulled together most strongly in the transition state of GTP hydrolysis. Most tethering events are futile, so that multiple rounds of GTP hydrolysis are required for successful fusion. Supported lipid bilayer experiments show that ATL molecules sitting on the same (cis) membrane can also undergo nucleotide-dependent dimerization. These results suggest that GTP hydrolysis is required to dissociate cis dimers, generating a pool of ATL monomers that can dimerize with molecules on a different (trans) membrane. In addition, tethering and fusion require the cooperation of multiple ATL molecules in each membrane. We propose a comprehensive model for ATL-mediated fusion that takes into account futile tethering and competition between cis and trans interactions.
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21
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Ulengin I, Park JJ, Lee TH. ER network formation and membrane fusion by atlastin1/SPG3A disease variants. Mol Biol Cell 2015; 26:1616-28. [PMID: 25761634 PMCID: PMC4436774 DOI: 10.1091/mbc.e14-10-1447] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2014] [Accepted: 03/03/2015] [Indexed: 12/20/2022] Open
Abstract
Atlastin catalyzes GTP-dependent membrane fusion to form the ER network. Mutations in atlastin1 cause the disease hereditary spastic paraplegia (HSP), implying that defects in ER membrane fusion cause HSP. Surprisingly, several disease variants are functional in assays for ER network formation and membrane fusion, warranting rethinking of HSP causation by atlastin1 mutations. At least 38 distinct missense mutations in the neuronal atlastin1/SPG3A GTPase are implicated in an autosomal dominant form of hereditary spastic paraplegia (HSP), a motor-neurological disorder manifested by lower limb weakness and spasticity and length-dependent axonopathy of corticospinal motor neurons. Because the atlastin GTPase is sufficient to catalyze membrane fusion and required to form the ER network, at least in nonneuronal cells, it is logically assumed that defects in ER membrane morphogenesis due to impaired fusion activity are the primary drivers of SPG3A-associated HSP. Here we analyzed a subset of established atlastin1/SPG3A disease variants using cell-based assays for atlastin-mediated ER network formation and biochemical assays for atlastin-catalyzed GTP hydrolysis, dimer formation, and membrane fusion. As anticipated, some variants exhibited clear deficits. Surprisingly however, at least two disease variants, one of which represents that most frequently identified in SPG3A HSP patients, displayed wild-type levels of activity in all assays. The same variants were also capable of co-redistributing ER-localized REEP1, a recently identified function of atlastins that requires its catalytic activity. Taken together, these findings indicate that a deficit in the membrane fusion activity of atlastin1 may be a key contributor, but is not required, for HSP causation.
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Affiliation(s)
- Idil Ulengin
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - John J Park
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - Tina H Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
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22
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Faust JE, Desai T, Verma A, Ulengin I, Sun TL, Moss TJ, Betancourt-Solis MA, Huang HW, Lee T, McNew JA. The Atlastin C-terminal tail is an amphipathic helix that perturbs the bilayer structure during endoplasmic reticulum homotypic fusion. J Biol Chem 2015; 290:4772-4783. [PMID: 25555915 DOI: 10.1074/jbc.m114.601823] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Fusion of tubular membranes is required to form three-way junctions found in reticular subdomains of the endoplasmic reticulum. The large GTPase Atlastin has recently been shown to drive endoplasmic reticulum membrane fusion and three-way junction formation. The mechanism of Atlastin-mediated membrane fusion is distinct from SNARE-mediated membrane fusion, and many details remain unclear. In particular, the role of the amphipathic C-terminal tail of Atlastin is still unknown. We found that a peptide corresponding to the Atlastin C-terminal tail binds to membranes as a parallel α helix, induces bilayer thinning, and increases acyl chain disorder. The function of the C-terminal tail is conserved in human Atlastin. Mutations in the C-terminal tail decrease fusion activity in vitro, but not GTPase activity, and impair Atlastin function in vivo. In the context of unstable lipid bilayers, the requirement for the C-terminal tail is abrogated. These data suggest that the C-terminal tail of Atlastin locally destabilizes bilayers to facilitate membrane fusion.
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Affiliation(s)
- Joseph E Faust
- Department of Biochemistry and Cell Biology, Rice University, Houston Texas 77005
| | - Tanvi Desai
- Department of Biochemistry and Cell Biology, Rice University, Houston Texas 77005
| | - Avani Verma
- Department of Biochemistry and Cell Biology, Rice University, Houston Texas 77005
| | - Idil Ulengin
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
| | - Tzu-Lin Sun
- Department of Physics and Astronomy, Rice University, Houston, Texas 77005
| | - Tyler J Moss
- Department of Biochemistry and Cell Biology, Rice University, Houston Texas 77005
| | | | - Huey W Huang
- Department of Physics and Astronomy, Rice University, Houston, Texas 77005
| | - Tina Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
| | - James A McNew
- Department of Biochemistry and Cell Biology, Rice University, Houston Texas 77005,.
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Affiliation(s)
- Katherine Labbé
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616; , ,
| | - Andrew Murley
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616; , ,
| | - Jodi Nunnari
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616; , ,
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24
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Saini SG, Liu C, Zhang P, Lee TH. Membrane tethering by the atlastin GTPase depends on GTP hydrolysis but not on forming the cross-over configuration. Mol Biol Cell 2014; 25:3942-53. [PMID: 25253720 PMCID: PMC4244202 DOI: 10.1091/mbc.e14-08-1284] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
The atlastin GTPase couples nucleotide hydrolysis to formation of a trans cross-over dimer to catalyze homotypic endoplasmic reticulum membrane fusion. Assays that separate tethering from fusion reveal that stable trans contact for tethering depends on GTP hydrolysis. In contrast, cross-over formation is required only for the fusion step. The membrane-anchored atlastin GTPase couples nucleotide hydrolysis to the catalysis of homotypic membrane fusion to form a branched endoplasmic reticulum network. Trans dimerization between atlastins anchored in opposing membranes, accompanied by a cross-over conformational change, is thought to draw the membranes together for fusion. Previous studies on the conformational coupling of atlastin to its GTP hydrolysis cycle have been carried out largely on atlastins lacking a membrane anchor. Consequently, whether fusion involves a discrete tethering step and, if so, the potential role of GTP hydrolysis and cross-over in tethering remain unknown. In this study, we used membrane-anchored atlastins in assays that separate tethering from fusion to dissect the requirements for each. We found that tethering depended on GTP hydrolysis, but, unlike fusion, it did not depend on cross-over. Thus GTP hydrolysis initiates stable head-domain contact in trans to tether opposing membranes, whereas cross-over formation plays a more pivotal role in powering the lipid rearrangements for fusion.
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Affiliation(s)
- Simran G Saini
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
| | - Chuang Liu
- Department of Structural Biology, University of Pittsburgh, Pittsburgh, PA 15260
| | - Peijun Zhang
- Department of Structural Biology, University of Pittsburgh, Pittsburgh, PA 15260
| | - Tina H Lee
- Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
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25
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Sackmann E. Endoplasmatic reticulum shaping by generic mechanisms and protein-induced spontaneous curvature. Adv Colloid Interface Sci 2014; 208:153-60. [PMID: 24607001 DOI: 10.1016/j.cis.2014.02.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2013] [Revised: 02/07/2014] [Accepted: 02/07/2014] [Indexed: 01/13/2023]
Abstract
The endoplasmatic reticulum (ER) comprises flattened vesicles (cisternae) with worm holes dubbed with ribosomes coexisting with a network of interconnected tubes which can extend to the cell periphery or even penetrate nerve axons. The coexisting topologies enclose a continuous luminal space. The complex ER topology is specifically controlled by a group of ER-shaping proteins often called reticulons (discovered by the group of Tom Rapoport). They include atlastin, reticulon, REEP and the MT severing protein spastin. A generic ER shape controlling factor is the necessity to maximize the area-to-volume ratio of ER membranes in the highly crowded cytoplasmic space. I present a model of the ER-shaping function of the reticulons based on the Helfrich bending elasticity concept of soft shell shape changes. Common structural motifs of the reticulons are hydrophobic sequences forming wedge shaped hairpins which penetrate the lipid bilayer of the cell membranes. The wedge-like hydrophobic anchors can both induce the high curvature of the tubular ER fraction and ensure the preferred distribution of the reticulons along the tubules. Tubular junctions may be stabilized by the reticulons forming two forceps twisted by 90°. The ER extensions to the cell periphery and the axons are mediated by coupling of the tubes to the microtubules which is mediated by REEP and spastin. At the end I present a model of the tension driven homotype fusion of ER-membranes by atlastin, based on analogies to the SNARE-complexin-SNARE driven heterotype fusion process.
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Affiliation(s)
- Erich Sackmann
- Physics Department, Technical University of Munich, James Franck Str.1, D85747 Garching, Germany.
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26
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Chiurchiù V, Maccarrone M, Orlacchio A. The role of reticulons in neurodegenerative diseases. Neuromolecular Med 2013; 16:3-15. [PMID: 24218324 PMCID: PMC3918113 DOI: 10.1007/s12017-013-8271-9] [Citation(s) in RCA: 55] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2013] [Accepted: 10/23/2013] [Indexed: 01/08/2023]
Abstract
Reticulons (RTNs) are a group of membrane-associated proteins mainly responsible for shaping the tubular endoplasmic reticulum network, membrane trafficking, inhibition of axonal growth, and apoptosis. These proteins share a common sequence feature, the reticulon homology domain, which consists of paired hydrophobic stretches that are believed to induce membrane curvature by acting as a wedge in bilayer membranes. RTNs are ubiquitously expressed in all tissues, but each RTN member exhibits a unique expression pattern that prefers certain tissues or even cell types. Recently, accumulated evidence has suggested additional and unexpected roles for RTNs, including those on DNA binding, autophagy, and several inflammatory-related functions. These manifold actions of RTNs account for their ever-growing recognition of their involvement in neurodegenerative diseases like Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, as well as hereditary spastic paraplegia. This review summarizes the latest discoveries on RTNs in human pathophysiology, and the engagement of these in neurodegeneration, along with the implications of these findings for a better understanding of the molecular events triggered by RTNs and their potential exploitation as next-generation therapeutics.
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Affiliation(s)
- Valerio Chiurchiù
- Laboratorio di Neurochimica dei Lipidi, Centro Europeo di Ricerca sul Cervello (CERC) - Istituto di Ricovero e Cura a Carattere Scientifico (IRCCS) Santa Lucia, Rome, Italy
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Abstract
Shape changes and topological remodeling of membranes are essential for the identity of organelles and membrane trafficking. Although all cellular membranes have common features, membranes of different organelles create unique environments that support specialized biological functions. The endoplasmic reticulum (ER) is a prime example of this specialization, as its lipid bilayer forms an interconnected system of cisternae, vesicles, and tubules, providing a highly compartmentalized structure for a multitude of biochemical processes. A variety of peripheral and integral membrane proteins that facilitate membrane curvature generation, fission, and/or fusion have been identified over the past two decades. Among these, the dynamin-related proteins (DRPs) have emerged as key players. Here, we review recent advances in our functional and molecular understanding of fusion DRPs, exemplified by atlastin, an ER-resident DRP that controls ER structure, function, and signaling.
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Affiliation(s)
- James A McNew
- Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005;
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28
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Structural basis for conformational switching and GTP loading of the large G protein atlastin. EMBO J 2013; 32:369-84. [PMID: 23334294 DOI: 10.1038/emboj.2012.353] [Citation(s) in RCA: 71] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2012] [Accepted: 12/18/2012] [Indexed: 12/22/2022] Open
Abstract
Atlastin, a member of the dynamin superfamily, is known to catalyse homotypic membrane fusion in the smooth endoplasmic reticulum (ER). Recent studies of atlastin have elucidated key features about its structure and function; however, several mechanistic details, including the catalytic mechanism and GTP hydrolysis-driven conformational changes, are yet to be determined. Here, we present the crystal structures of atlastin-1 bound to GDP·AlF(4)(-) and GppNHp, uncovering an intramolecular arginine finger that stimulates GTP hydrolysis when correctly oriented through rearrangements within the G domain. Utilizing Förster Resonance Energy Transfer, we describe nucleotide binding and hydrolysis-driven conformational changes in atlastin and their sequence. Furthermore, we discovered a nucleotide exchange mechanism that is intrinsic to atlastin's N-terminal domains. Our results indicate that the cytoplasmic domain of atlastin acts as a tether and homotypic interactions are timed by GTP binding and hydrolysis. Perturbation of these mechanisms may be implicated in a group of atlastin-associated hereditary neurodegenerative diseases.
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Escobar-Henriques M, Anton F. Mechanistic perspective of mitochondrial fusion: tubulation vs. fragmentation. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2012; 1833:162-75. [PMID: 22884630 DOI: 10.1016/j.bbamcr.2012.07.016] [Citation(s) in RCA: 67] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2012] [Revised: 07/28/2012] [Accepted: 07/30/2012] [Indexed: 01/02/2023]
Abstract
Mitochondrial fusion is a fundamental process driven by dynamin related GTPase proteins (DRPs), in contrast to the general SNARE-dependence of most cellular fusion events. The DRPs Mfn1/Mfn2/Fzo1 and OPA1/Mgm1 are the key effectors for fusion of the mitochondrial outer and inner membranes, respectively. In order to promote fusion, these two DRPs require post-translational modifications and proteolysis. OPA1/Mgm1 undergoes partial proteolytic processing, which results in a combination between short and long isoforms. In turn, ubiquitylation of mitofusins, after oligomerization and GTP hydrolysis, promotes and positively regulates mitochondrial fusion. In contrast, under conditions of mitochondrial dysfunction, negative regulation by proteolysis on these DRPs results in mitochondrial fragmentation. This occurs by complete processing of OPA1 and via ubiquitylation and degradation of mitofusins. Mitochondrial fragmentation contributes to the elimination of damaged mitochondria by mitophagy, and may play a protective role against Parkinson's disease. Moreover, a link of Mfn2 to Alzheimer's disease is emerging and mutations in Mfn2 or OPA1 cause Charcot-Marie-Tooth type 2A neuropathy or autosomal-dominant optic atrophy. Here, we summarize our current understanding on the molecular mechanisms promoting or inhibiting fusion of mitochondrial membranes, which is essential for cellular survival and disease control. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.
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Anwar K, Klemm RW, Condon A, Severin KN, Zhang M, Ghirlando R, Hu J, Rapoport TA, Prinz WA. The dynamin-like GTPase Sey1p mediates homotypic ER fusion in S. cerevisiae. ACTA ACUST UNITED AC 2012; 197:209-17. [PMID: 22508509 PMCID: PMC3328390 DOI: 10.1083/jcb.201111115] [Citation(s) in RCA: 90] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Budding yeast Sey1p functions analogously to mammalian atlastins in mediating ER fusion through a mechanism that is redundant with a second, ER SNARE-mediated fusion mechanism. The endoplasmic reticulum (ER) forms a network of tubules and sheets that requires homotypic membrane fusion to be maintained. In metazoans, this process is mediated by dynamin-like guanosine triphosphatases (GTPases) called atlastins (ATLs), which are also required to maintain ER morphology. Previous work suggested that the dynamin-like GTPase Sey1p was needed to maintain ER morphology in Saccharomyces cerevisiae. In this paper, we demonstrate that Sey1p, like ATLs, mediates homotypic ER fusion. The absence of Sey1p resulted in the ER undergoing delayed fusion in vivo and proteoliposomes containing purified Sey1p fused in a GTP-dependent manner in vitro. Sey1p could be partially replaced by ATL1 in vivo. Like ATL1, Sey1p underwent GTP-dependent dimerization. We found that the residual ER–ER fusion that occurred in cells lacking Sey1p required the ER SNARE Ufe1p. Collectively, our results show that Sey1p and its homologues function analogously to ATLs in mediating ER fusion. They also indicate that S. cerevisiae has an alternative fusion mechanism that requires ER SNAREs.
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Affiliation(s)
- Kamran Anwar
- Laboratory of Molecular and Cell Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA
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31
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Short B. Bridging the gap between atlastin conformations. J Biophys Biochem Cytol 2011. [PMCID: PMC3257532 DOI: 10.1083/jcb.1954iti1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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