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Lee JH, Jomaa A, Chung S, Hwang Fu YH, Qian R, Sun X, Hsieh HH, Chandrasekar S, Bi X, Mattei S, Boehringer D, Weiss S, Ban N, Shan SO. Receptor compaction and GTPase rearrangement drive SRP-mediated cotranslational protein translocation into the ER. SCIENCE ADVANCES 2021; 7:eabg0942. [PMID: 34020957 PMCID: PMC8139590 DOI: 10.1126/sciadv.abg0942] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/10/2020] [Accepted: 04/01/2021] [Indexed: 05/07/2023]
Abstract
The conserved signal recognition particle (SRP) cotranslationally delivers ~30% of the proteome to the eukaryotic endoplasmic reticulum (ER). The molecular mechanism by which eukaryotic SRP transitions from cargo recognition in the cytosol to protein translocation at the ER is not understood. Here, structural, biochemical, and single-molecule studies show that this transition requires multiple sequential conformational rearrangements in the targeting complex initiated by guanosine triphosphatase (GTPase)-driven compaction of the SRP receptor (SR). Disruption of these rearrangements, particularly in mutant SRP54G226E linked to severe congenital neutropenia, uncouples the SRP/SR GTPase cycle from protein translocation. Structures of targeting intermediates reveal the molecular basis of early SRP-SR recognition and emphasize the role of eukaryote-specific elements in regulating targeting. Our results provide a molecular model for the structural and functional transitions of SRP throughout the targeting cycle and show that these transitions provide important points for biological regulation that can be perturbed in genetic diseases.
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Affiliation(s)
- Jae Ho Lee
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Ahmad Jomaa
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
| | - SangYoon Chung
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA
| | - Yu-Hsien Hwang Fu
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Ruilin Qian
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Xuemeng Sun
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Hao-Hsuan Hsieh
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Sowmya Chandrasekar
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Xiaotian Bi
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
| | - Simone Mattei
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland
| | - Daniel Boehringer
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland
- Cryo-EM Knowledge Hub, ETH Zurich, 8093 Zurich, Switzerland
| | - Shimon Weiss
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA
- Department of Physics, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, 52900 Ramat-Gan, Israel
| | - Nenad Ban
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
| | - Shu-Ou Shan
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
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2
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Jian M, Zhang D, Wang X, Wei S, Zhao Y, Ding Q, Han Y, Ma L. Differential expression pattern of the proteome in response to cadmium stress based on proteomics analysis of wheat roots. BMC Genomics 2020; 21:343. [PMID: 32380942 PMCID: PMC7203821 DOI: 10.1186/s12864-020-6716-8] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2019] [Accepted: 04/05/2020] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND Heavy metal cadmium (Cd) is a common environmental pollutant in soils, which has an negative impacts on crop growth and development. At present, cadmium has become a major soil and water heavy metal pollutant, which not only causes permanent and irreversible health problems for humans, but also causes a significant reduction in crop yields. RESULTS This study examined the chemical forms of Cd in the roots of two wheat varieties (M1019 and Xinong20) by continuous extraction and analyzed differences in distribution characteristics of Cd in the root cell wall, cytoplasm, and organelles by elemental content determination and subcellular separation. Furthermore, we conducted proteomics analysis of the roots of the two varieties under Cd pollution using mass spectrometry quantitative proteomics techniques. A total of 11,651 proteins were identified, of which 10,532 proteins contained quantitative information. In addition, the differentially expressed proteins in the two varieties were related to DNA replication and repair, protein metabolism, and the glutathione metabolism pathway. CONCLUSION The results of this study improve our understanding of the mechanism of plant responses to Cd stress.
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Affiliation(s)
- Mingyang Jian
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Dazhong Zhang
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Xiaoying Wang
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Shuwei Wei
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Yue Zhao
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Qin Ding
- College of Horticulture, Northwest A&F University, Yangling, 712100 China
| | - Yucui Han
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
| | - Lingjian Ma
- College of Agronomy, Northwest A&F University, Yangling, 712100 China
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3
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Massenet S. In vivo assembly of eukaryotic signal recognition particle: A still enigmatic process involving the SMN complex. Biochimie 2019; 164:99-104. [PMID: 30978374 DOI: 10.1016/j.biochi.2019.04.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2019] [Accepted: 04/07/2019] [Indexed: 12/29/2022]
Abstract
The signal recognition particle (SRP) is a universally conserved non-coding ribonucleoprotein complex that is essential for targeting transmembrane and secretory proteins to the endoplasmic reticulum. Its composition and size varied during evolution. In mammals, SRP contains one RNA molecule, 7SL RNA, and six proteins: SRP9, 14, 19, 54, 68 and 72. Despite a very good understanding of the SRP structure and of the SRP assembly in vitro, how SRP is assembled in vivo remains largely enigmatic. Here we review current knowledge on how the 7SL RNA is assembled with core proteins to form functional RNP particles in cells. SRP biogenesis is believed to take place both in the nucleolus and in the cytoplasm and to rely on the survival of motor neuron complex, whose defect leads to spinal muscular atrophy.
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Affiliation(s)
- Séverine Massenet
- Ingénierie Moléculaire et Physiopathologie Articulaire, UMR 7365 CNRS-University of Lorraine, Biopôle de l'Université de Lorraine, Campus Brabois-Santé, 9 avenue de la forêt de Haye, BP 20199, 54505 Vandoeuvre-les-Nancy, France.
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4
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Mutations in the SRP54 gene cause severe congenital neutropenia as well as Shwachman-Diamond-like syndrome. Blood 2018; 132:1318-1331. [PMID: 29914977 DOI: 10.1182/blood-2017-12-820308] [Citation(s) in RCA: 73] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2017] [Accepted: 06/02/2018] [Indexed: 01/04/2023] Open
Abstract
Congenital neutropenias (CNs) are rare heterogeneous genetic disorders, with about 25% of patients without known genetic defects. Using whole-exome sequencing, we identified a heterozygous mutation in the SRP54 gene, encoding the signal recognition particle (SRP) 54 GTPase protein, in 3 sporadic cases and 1 autosomal dominant family. We subsequently sequenced the SRP54 gene in 66 probands from the French CN registry. In total, we identified 23 mutated cases (16 sporadic, 7 familial) with 7 distinct germ line SRP54 mutations including a recurrent in-frame deletion (Thr117del) in 14 cases. In nearly all patients, neutropenia was chronic and profound with promyelocytic maturation arrest, occurring within the first months of life, and required long-term granulocyte colony-stimulating factor therapy with a poor response. Neutropenia was sometimes associated with a severe neurodevelopmental delay (n = 5) and/or an exocrine pancreatic insufficiency requiring enzyme supplementation (n = 3). The SRP54 protein is a key component of the ribonucleoprotein complex that mediates the co-translational targeting of secretory and membrane proteins to the endoplasmic reticulum (ER). We showed that SRP54 was specifically upregulated during the in vitro granulocytic differentiation, and that SRP54 mutations or knockdown led to a drastically reduced proliferation of granulocytic cells associated with an enhanced P53-dependent apoptosis. Bone marrow examination of SRP54-mutated patients revealed a major dysgranulopoiesis and features of cellular ER stress and autophagy that were confirmed using SRP54-mutated primary cells and SRP54 knockdown cells. In conclusion, we characterized a pathological pathway, which represents the second most common cause of CN with maturation arrest in the French CN registry.
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5
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Piazzon N, Schlotter F, Lefebvre S, Dodré M, Méreau A, Soret J, Besse A, Barkats M, Bordonné R, Branlant C, Massenet S. Implication of the SMN complex in the biogenesis and steady state level of the signal recognition particle. Nucleic Acids Res 2012; 41:1255-72. [PMID: 23221635 PMCID: PMC3553995 DOI: 10.1093/nar/gks1224] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Spinal muscular atrophy is a severe motor neuron disease caused by reduced levels of the ubiquitous Survival of MotoNeurons (SMN) protein. SMN is part of a complex that is essential for spliceosomal UsnRNP biogenesis. Signal recognition particle (SRP) is a ribonucleoprotein particle crucial for co-translational targeting of secretory and membrane proteins to the endoplasmic reticulum. SRP biogenesis is a nucleo-cytoplasmic multistep process in which the protein components, except SRP54, assemble with 7S RNA in the nucleolus. Then, SRP54 is incorporated after export of the pre-particle into the cytoplasm. The assembly factors necessary for SRP biogenesis remain to be identified. Here, we show that 7S RNA binds to purified SMN complexes in vitro and that SMN complexes associate with SRP in cellular extracts. We identified the RNA determinants required. Moreover, we report a specific reduction of 7S RNA levels in the spinal cord of SMN-deficient mice, and in a Schizosaccharomyces pombe strain carrying a temperature-degron allele of SMN. Additionally, microinjected antibodies directed against SMN or Gemin2 interfere with the association of SRP54 with 7S RNA in Xenopus laevis oocytes. Our data show that reduced levels of the SMN protein lead to defect in SRP steady-state level and describe the SMN complex as the first identified cellular factor required for SRP biogenesis.
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Affiliation(s)
- Nathalie Piazzon
- Laboratoire ARN-RNP structure-fonction-maturation, Enzymologie Moléculaire et Structurale (AREMS), Nancy Université-CNRS, UMR 7214, FR 3209, Faculté de Médecine de Nancy, BP 184, 9 avenue de la forêt de Haye, 54506 Vandoeuvre-les-Nancy Cedex, France
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6
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Gilmore R, Mandon EC. Understanding integration of α-helical membrane proteins: the next steps. Trends Biochem Sci 2012; 37:303-8. [PMID: 22748693 DOI: 10.1016/j.tibs.2012.05.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2012] [Revised: 05/16/2012] [Accepted: 05/23/2012] [Indexed: 11/19/2022]
Abstract
Integration of a protein into the endoplasmic reticulum (ER) membrane occurs through a series of multistep reactions that include targeting of ribosome-nascent polypeptide complexes to the ER, attachment of the ribosome to the protein translocation channel, lateral partitioning of α-helical transmembrane spans into the lipid bilayer, and folding of the lumenal, cytosolic and membrane-embedded domains of the protein. However, the molecular mechanisms and kinetics of these steps are still not entirely clear. To obtain a better understanding of the mechanism of membrane protein integration, we propose that it will be important to utilize in vivo experiments to examine the kinetics of membrane protein integration and in vitro experiments to characterize interactions between nascent membrane proteins, protein translocation factors and molecular chaperones.
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Affiliation(s)
- Reid Gilmore
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605-2324, USA.
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7
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Ma Y, Sugiura R, Zhang L, Zhou X, Takeuchi M, He Y, Kuno T. Isolation of a fission yeast mutant that is sensitive to valproic acid and defective in the gene encoding Ric1, a putative component of Ypt/Rab-specific GEF for Ryh1 GTPase. Mol Genet Genomics 2010; 284:161-71. [PMID: 20623139 DOI: 10.1007/s00438-010-0550-7] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2010] [Accepted: 06/10/2010] [Indexed: 01/08/2023]
Abstract
Valproic acid (VPA) causes various therapeutic and biological effects, but the exact mechanisms underlying these effects, however, remain elusive. To gain insights into the molecular mechanisms of VPA action, we performed in fission yeast a genetic screen for mutants that show VPA hypersensitivity and have identified several membrane-trafficking mutants including vas1-1/vps45 and vas2-1/aps1. Here, we describe the isolation and characterization of vas3-1/ric1-v3, a mutant allele of the ric1 (+) gene encoding a fission yeast homolog of the budding yeast Ric1p, a component of Ypt/Rab-specific guanyl-nucleotide exchange factor (GEF). The Rab GTPase Ryh1 knockout (Deltaryh1) cells and Deltaric1 cells exhibited similar phenotypes. The double knockout Deltaric1Deltaryh1 cells did not display synthetic growth defects. These results are consistent with the notion that Ric1 may be a component of the GEF complex for Ryh1. Overexpression of wild-type Ryh1 and the constitutively active Ryh1Q70L only partially suppressed the phenotypes of ric1-v3 and Deltaric1 cells, and they failed to localize to the Golgi/endosomes in ric1-v3 and Deltaric1 cells. Furthermore, we isolated vps15 (+) gene, encoding a serine/threonine protein kinase, as a dosage-dependent suppressor of the temperature-sensitive phenotype of ric1-v3 mutant, but not that of Deltaric1 cells. Our results showed that the ric1-v3 mutant allele has some residual functional activity and suggest that Vps15 plays a role in the regulation of Ric1 function. In conclusion, Ric1 is a putative component of GEF for Ryh1 and might be regulated by Vps15. Further studies are needed to reveal the mechanism underlying the regulation.
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Affiliation(s)
- Yan Ma
- Division of Molecular Pharmacology and Pharmacogenomics, Department of Biochemistry and Molecular Biology, Kobe University Graduate School of Medicine, Kobe, Hyogo, Japan.
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8
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Lakkaraju AKK, Luyet PP, Parone P, Falguières T, Strub K. Inefficient targeting to the endoplasmic reticulum by the signal recognition particle elicits selective defects in post-ER membrane trafficking. Exp Cell Res 2006; 313:834-47. [PMID: 17239854 DOI: 10.1016/j.yexcr.2006.12.003] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2006] [Revised: 12/07/2006] [Accepted: 12/11/2006] [Indexed: 11/29/2022]
Abstract
The signal recognition particle (SRP) is required for protein translocation into the endoplasmic reticulum (ER). With RNA interference we reduced its level about ten-fold in mammalian cells to study its cellular functions. Such low levels proved insufficient for efficient ER-targeting, since the accumulation of several proteins in the secretory pathway was specifically diminished. Although the cells looked unaffected, they displayed noticeable and selective defects in post-ER membrane trafficking. Specifically, the anterograde transport of VSV-G and the retrograde transport of the Shiga toxin B-subunit were stalled at the level of the Golgi whereas the endocytosed transferrin receptor failed to recycle to the plasma membrane. Endocytic membrane trafficking from the plasma membrane to lysosomes or Golgi was undisturbed and major morphological changes in the ER and the Golgi were undetectable at low resolution. Selective membrane trafficking defects were specifically suppressed under conditions when low levels of SRP became sufficient for efficient ER-targeting and are therefore a direct consequence of the lower targeting capacity of cells with reduced SRP levels. Selective post-ER membrane trafficking defects occur at SRP levels sufficient for survival suggesting that changes in SRP levels and their effects on post-ER membrane trafficking might serve as a mechanism to alter temporarily the localization of selected proteins.
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Affiliation(s)
- Asvin K K Lakkaraju
- Department of Cell Biology, University of Geneva, Sciences III, CH-1211 Geneva 4, Switzerland
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9
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Lustig Y, Goldshmidt H, Uliel S, Michaeli S. The Trypanosoma brucei signal recognition particle lacks the Alu-domain-binding proteins: purification and functional analysis of its binding proteins by RNAi. J Cell Sci 2006; 118:4551-62. [PMID: 16179612 DOI: 10.1242/jcs.02578] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Trypanosomes are protozoan parasites that have a major impact on human health and that of livestock. These parasites represent a very early branch in the eukaryotic lineage, and possess unique RNA processing mechanisms. The trypanosome signal recognition particle (SRP) is also unusual in being the first signal recognition particle described in nature to be comprised of two RNA molecules, the 7SL RNA and a tRNA-like molecule. In this study, we further elucidated the unique properties of this particle. The genes encoding three SRP proteins (SRP19, SRP72 and SRP68) were identified by bioinformatics analysis. Silencing of these genes by RNAi suggests that the SRP-mediated protein translocation pathway is essential for growth. The depletion of SRP72 and SRP68 induced sudden death, most probably as a result of toxicity due to the accumulation of the pre-SRP in the nucleolus. Purification of the trypanosome particle to homogeneity, by TAP-tagging, identified four SRP proteins (SRP72, SRP68, SRP54 and SRP19), but no Alu-domain-binding protein homologs. This study highlights the unique features of the trypanosome SRP complex and further supports the hypothesis that the tRNA-like molecule present in this particle may replace the function of the Alu-domain-binding proteins present in many eukaryotic SRP complexes.
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Affiliation(s)
- Yaniv Lustig
- Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel
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10
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Johnson AE, Chen JC, Flanagan JJ, Miao Y, Shao Y, Lin J, Bock PE. Structure, function, and regulation of free and membrane-bound ribosomes: the view from their substrates and products. COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY 2003; 66:531-41. [PMID: 12762055 DOI: 10.1101/sqb.2001.66.531] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Affiliation(s)
- A E Johnson
- Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, Departments of Chemistry and of Biochemistry and Biophysics, Texas A&M University, College Station, Texas, USA
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11
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Smith MD, Hiltbrunner A, Kessler F, Schnell DJ. The targeting of the atToc159 preprotein receptor to the chloroplast outer membrane is mediated by its GTPase domain and is regulated by GTP. J Cell Biol 2002; 159:833-43. [PMID: 12473690 PMCID: PMC2173378 DOI: 10.1083/jcb.200208017] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
The multimeric translocon at the outer envelope membrane of chloroplasts (Toc) initiates the recognition and import of nuclear-encoded preproteins into chloroplasts. Two Toc GTPases, Toc159 and Toc33/34, mediate preprotein recognition and regulate preprotein translocation. Although these two proteins account for the requirement of GTP hydrolysis for import, the functional significance of GTP binding and hydrolysis by either GTPase has not been defined. A recent study indicates that Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, raising the possibility that it might cycle between the cytoplasm and chloroplast as a soluble preprotein receptor. In the present study, we examined the mechanism of targeting and insertion of the Arabidopsis thaliana orthologue of Toc159, atToc159, to chloroplasts. Targeting of atToc159 to the outer envelope membrane is strictly dependent only on guanine nucleotides. Although GTP is not required for initial binding, the productive insertion and assembly of atToc159 into the Toc complex requires its intrinsic GTPase activity. Targeting is mediated by direct binding between the GTPase domain of atToc159 and the homologous GTPase domain of atToc33, the Arabidopsis Toc33/34 orthologue. Our findings demonstrate a role for the coordinate action of the Toc GTPases in assembly of the functional Toc complex at the chloroplast outer envelope membrane.
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Affiliation(s)
- Matthew D Smith
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA 01003, USA
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12
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Liu L, Liang XH, Uliel S, Unger R, Ullu E, Michaeli S. RNA interference of signal peptide-binding protein SRP54 elicits deleterious effects and protein sorting defects in trypanosomes. J Biol Chem 2002; 277:47348-57. [PMID: 12244113 DOI: 10.1074/jbc.m207736200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Trypanosomes are protozoan parasites that have a major impact on health. This family diverged very early from the eukaryotic lineage and possesses unique RNA processing mechanisms such as trans-splicing and RNA editing. The trypanosome signal recognition particle (SRP) has a unique composition compared with all known SRP complexes, because it contains two RNA molecules, the 7SL RNA and a tRNA-like molecule. RNA interference was utilized to elucidate the essentiality of the SRP pathway and its role in protein translocation in Trypanosoma brucei. The production of double stranded RNA specific for the signal peptide-binding protein SRP54 induced the degradation of the mRNA and a loss of the SRP54 protein. SRP54 depletion elicited inhibition in growth and cytokinesis, suggesting that the SRP pathway is essential. The translocation of four signal peptide-containing proteins was examined. Surprisingly, the proteins were translocated to the endoplasmic reticulum and properly processed. However, the surface EP procyclin, the lysosomal protein p67, and the flagellar pocket protein CRAM were mislocalized and accumulated in megavesicles, most likely because of a secondary effect on protein sorting. The translocation of these proteins to the endoplasmic reticulum under SRP54 depletion suggests that an alternative pathway for protein translocation exists in trypanosomes.
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Affiliation(s)
- Li Liu
- Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel
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13
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Romfo CM, Lakhe-Reddy S, Wise JA. Molecular genetic analysis of U2AF59 in Schizosaccharomyces pombe: differential sensitivity of introns to mutational inactivation. RNA (NEW YORK, N.Y.) 1999; 5:49-65. [PMID: 9917066 PMCID: PMC1369739 DOI: 10.1017/s1355838299981323] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
The large subunit of the mammalian U2AF heterodimer (U2AF65) is essential for splicing in vitro. To expand our understanding of how this protein functions in vivo, we have created a null allele of the gene encoding the Schizosaccharomyces pombe ortholog, U2AF59, and employed it in a variety of genetic complementation assays. First, analysis of an extensive series of double amino acid substitutions indicates that this splicing factor is surprisingly refractory to mutations. Second, despite extensive structural conservation, we find that metazoan large subunit orthologs cannot substitute in vivo for fission yeast U2AF59. Third, because the activity of U2AF65 in vitro involves binding to the 3' polypyrimidine tract, we examined the splicing of introns containing or lacking this feature in a U2AF59 mutant described here as well as a previously isolated temperature-sensitive mutant (Potashkin et al., 1993, Science 262:573-575). Our data indicate that all four introns tested, including two that lack extensive runs of pyrimidines between the branchpoint and 3' splice site, show splicing defects upon shifting to the nonpermissive condition. In all cases, splicing is blocked prior to the first transesterification reaction in the mutants, consistent with the role inferred for human U2AF65 based on in vitro experiments.
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Affiliation(s)
- C M Romfo
- Case Western Reserve University, School of Medicine, Department of Molecular Biology and Microbiology, Cleveland, Ohio 44106-4960, USA
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14
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Regnacq M, Hewitt E, Allen J, Rosamond J, Stirling CJ. Deletion analysis of yeast Sec65p reveals a central domain that is sufficient for function in vivo. Mol Microbiol 1998; 29:753-62. [PMID: 9723915 DOI: 10.1046/j.1365-2958.1998.00969.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The Saccharomyces cerevisiae SEC65 gene encodes a 32 kDa subunit of yeast signal recognition particle that is homologous to human SRP19. Sequence comparisons suggest that the yeast protein comprises three distinct domains. The central domain (residues 98-171) exhibits substantial sequence similarity to the 144 residue SRP19. In contrast, the N-terminal and C-terminal domains (residues 1-97 and 172-273 respectively) share no similarity to SRP19, with the exception of a cluster of positively charged residues at the extreme C-terminus of both proteins. Here, we report the cloning of a Sec65p homologue from the yeast Candida albicans that shares the same extended domain structure as its S. cerevisiae counterpart. This conservation of sequence is reflected at the functional level, as the C. albicans gene can complement the conditional lethal sec65-1 mutation in S. cerevisiae. In order to examine the role of the N- and C- terminal domains in Sec65p function, we have engineered truncation mutants of S. cerevisiae SEC65 and tested these for complementing activity in vivo and for SRP integrity in vitro. These studies indicate that a minimal Sec65p comprising residues 76-209, which includes the entire central SRP19-like domain, is sufficient for SRP function in yeast.
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Affiliation(s)
- M Regnacq
- School of Biological Sciences, University of Manchester, UK
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15
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Rapiejko PJ, Gilmore R. Empty site forms of the SRP54 and SR alpha GTPases mediate targeting of ribosome-nascent chain complexes to the endoplasmic reticulum. Cell 1997; 89:703-13. [PMID: 9182758 DOI: 10.1016/s0092-8674(00)80253-6] [Citation(s) in RCA: 104] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The SRP54 and SR alpha subunits of the signal recognition particle (SRP) and the SRP receptor (SR) undergo a tightly coupled GTPase cycle that mediates the signal sequence-dependent attachment of ribosomes to the Sec61 complex. Here, we show that SRP54 and SR alpha are in the empty site conformation prior to contact between the SRP-ribosome complex and the membrane-bound SR. Cooperative binding of GTP to SRP54 and SR alpha stabilizes the SRP-SR complex and initiates signal sequence transfer from SRP54 to Sec61 alpha. The GTP-bound conformations of SR alpha and SRP54 perform distinct roles, with SR alpha performing a predominant role in complex stabilization. Hydrolysis by both SRP54 and SR alpha is a prerequisite for dissociation of the SRP-SR complex.
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Affiliation(s)
- P J Rapiejko
- Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester 01655-0103, USA
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16
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Abstract
To investigate the role of Srp54p in protein translocation, the Yarrowia lipolytica SRP54 homolog was cloned. Sequencing revealed an open reading frame of 536 amino acids coding for a 57.2 kilodalton polypeptide with 55 to 57% sequence identity to Srp54ps of Saccharomyces cerevisiae, Schizosaccharomyces pombe, and mouse. Like these Srp54ps, Y. lipolytica Srp54p has an N-terminal domain with a highly conserved GTP-binding site and a methionine-rich C-terminal domain. Differing results regarding the essentiality of SRP subunits were obtained. SRP54 is important but not essential for growth, but it was reconfirmed that at least one SRP RNA gene is essential. Cells with SRP54 deleted grow about six times more slowly than wild type; faster-growing colonies, still growing much slower than wild type, appeared quite frequently. In srp54 delta cells, no untranslocated alkaline extracellular protease precursor was detected. Therefore, to develop another reporter molecule the Y. lipolytica KAR2 homolog was cloned and Kar2p antibodies were produced. For Kar2p an untranslocated precursor was detected in srp54 delta but not in wild-type cells, suggesting that its translocation was defective in the srp54 delta cells. These results confirm an in vivo rule for SRP in protein translocation in Y. lipolytica, suggest that SRP RNA or an SRP core-particle has functions not shared by Srp54p, and show that, as in S. cerevisiae and Sz. pombe, reporter molecules differ in their dependency on SRP for translocation.
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Affiliation(s)
- I H Lee
- Institute of Marine Resources, University of California, Davis 95616, USA
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17
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Chu B, Brodl MR, Belanger FC. Heat shock inhibits release of the signal recognition particle from the endoplasmic reticulum in barley aleurone layers. J Biol Chem 1997; 272:7306-13. [PMID: 9054428 DOI: 10.1074/jbc.272.11.7306] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
When barley (Hordeum vulgare) aleurone layers are subjected to heat shock there is a selective degradation of the normally stable mRNAs encoding secreted proteins. Messages for nonsecreted proteins are not degraded. The synthesis of heat shock proteins is not required for this selective message degradation. Our hypothesis explaining this phenomenon is that a component of the early steps in the synthesis of secreted proteins is damaged by heat shock, resulting in a selective halt in translation on secretory mRNAs, which may in turn lead to degradation of those messages. The first committed step in the synthesis of secreted proteins is the binding of the nascent signal sequence to the signal recognition particle. We have obtained cDNA clones and antibodies for the barley 54-kDa subunit of the signal recognition particle. In cell fractionation experiments, more signal recognition particle was bound to the endoplasmic reticulum membranes and less was in the free particle fraction following a heat shock. The results suggest that heat shock inhibits the release of the signal recognition particle from the endoplasmic reticulum. This would, in turn, inhibit the resumption of translation and may be the underlying cause of the secretory message degradation.
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Affiliation(s)
- B Chu
- Dana Farber Cancer Institute, Boston, Massachusetts 02115, USA
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18
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Abstract
Saccharomyces cerevisiae is an excellent model organism for addressing questions in cell biology, but other yeast systems are also providing new insights into several fundamental cellular processes.
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Affiliation(s)
- B S Glick
- Department of Molecular Genetics and Cell Biology, University of Chicago, Illinois 60637, USA.
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19
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Abstract
The last few years has seen enormous progress in understanding of protein targeting and translocation across biological membranes. Many of the key molecules involved have been identified, isolated, and the corresponding genes cloned, opening up the way for detailed analysis of the structure and function of these molecular machines. It has become clear that the protein translocation machinery of the endoplasmic reticulum is very closely related to that of bacteria, and probably represents an ancient solution to the problem of how to get a protein across a membrane. One of the thylakoid translocation systems looks as if it will also be very similar, and probably represents a pathway inherited from the ancestral endosymbiont. It is interesting that, so far, there is a perfect correlation between thylakoid proteins which are present in photosynthetic prokaryotes and those which use the sec pathway in chloroplasts; conversely, OE16 and 23 which use the delta pH pathway are not found in cyanobacteria. To date, no Sec-related proteins have been found in mitochondria, although these organelles also arose as a result of endosymbiotic events. However, virtually nothing is known about the insertion of mitochondrially encoded proteins into the inner membrane. Is the inner membrane machinery which translocates cytoplasmically synthesized proteins capable of operating in reverse to export proteins from the matrix, or is there a separate system? Alternatively, do membrane proteins encoded by mitochondrial DNA insert independently of accessory proteins? Unlike nuclear-encoded proteins, proteins encoded by mtDNA are not faced with a choice of membrane and, in principle, could simply partition into the inner membrane. The ancestors of mitochondria almost certainly had a Sec system; has this been lost along with many of the proteins once encoded in the endosymbiont genome, or is there still such a system waiting to be discovered? The answer to this question may also shed light on the controversy concerning the sorting of the inter-membrane space proteins cytochrome c1 and cytochrome b2, as the conservative-sorting hypothesis would predict re-export of matrix intermediates via an ancestral (possibly Sec-type) pathway. Whereas the ER and bacterial systems clearly share homologous proteins, the protein import machineries of mitochondria and chloroplasts appear to be analogous rather than homologous. In both cases, import occurs through contact sites and there are separate translocation complexes in each membrane, however, with the exception of some of the chaperone molecules, the individual protein components do not appear to be related. Their similarities may be a case of convergent rather than divergent evolution, and may reflect what appear to be common requirements for translocation, namely unfolding, a receptor, a pore complex and refolding. There are also important differences. Translocation across the mitochondrial inner membrane is absolutely dependent upon delta psi, but no GTP requirement has been identified. In chloroplasts the reverse is the case. The roles of delta psi and GTP, respectively, remain uncertain, but it is tempting to speculate that they may play a role in regulating the import process, perhaps by controlling the assembly of a functional translocation complex. In the case of peroxisomes, much still remains to be learned. Many genes involved in peroxisome biogenesis have been identified but, in most cases, the biochemical function remains to be elucidated. In this respect, understanding of peroxisome biogenesis is at a similar stage to that of the ER 10 years ago. The coming together of genetic and biochemical approaches, as with the other organelles, should provide many of the answers.
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Affiliation(s)
- A Baker
- Department of Biochemistry, University of Cambridge, UK
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20
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Taylor GA, Jeffers M, Largaespada DA, Jenkins NA, Copeland NG, Vande Woude GF. Identification of a novel GTPase, the inducibly expressed GTPase, that accumulates in response to interferon gamma. J Biol Chem 1996; 271:20399-405. [PMID: 8702776 DOI: 10.1074/jbc.271.34.20399] [Citation(s) in RCA: 95] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Interferon gamma is a pleiotropic cytokine that regulates many immune functions. We have identified a novel protein, inducibly expressed GTPase (IGTP), whose expression was regulated by interferon gamma in macrophages. In mouse RAW 264.7 macrophages, IGTP mRNA levels were almost undetectable but increased within 1 h of exposure to interferon gamma, peaked at very high levels within 3 h, and remained at high levels to at least 48 h; pretreatment of the cells with cycloheximide blocked the majority of mRNA accumulation. In the mouse, the mRNA was highly expressed in thymus, spleen, lung, and small intestine. Using interspecific backcross analysis, the Igtp gene was mapped to mouse chromosome 11. The IGTP cDNA encoded a putative polypeptide of Mr 48,507 and pI 7.79 that contained three consensus GTP binding motifs, GXXXXGK(S/T), DXXG, and NTKXD. Both IGTP that had been immunoprecipitated from RAW cells and a glutathione S-transferase IGTP fusion protein were able to convert GTP to GDP in vitro. Subcellular protein fractionation and Western blotting localized IGTP to the cytosol of RAW cells. In addition, the protein was homologous to proteins encoded by three previously cloned cDNAs, IRG-47, TGTP/Mg21, and LRG-47, and thus may be representative of a new family of interferon gamma-regulated GTPases.
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Affiliation(s)
- G A Taylor
- ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland 21702, USA
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