1
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Mukherjee A, Kizziah JL, Hawkins NC, Nasef MO, Parker LK, Dokland T. Structure of the Portal Complex from Staphylococcus aureus Pathogenicity Island 1 Transducing Particles In Situ and In Isolation. J Mol Biol 2024; 436:168415. [PMID: 38135177 PMCID: PMC10923094 DOI: 10.1016/j.jmb.2023.168415] [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: 10/24/2023] [Revised: 12/15/2023] [Accepted: 12/18/2023] [Indexed: 12/24/2023]
Abstract
Staphylococcus aureus is an important human pathogen, and the prevalence of antibiotic resistance is a major public health concern. The evolution of pathogenicity and resistance in S. aureus often involves acquisition of mobile genetic elements (MGEs). Bacteriophages play an especially important role, since transduction represents the main mechanism for horizontal gene transfer. S. aureus pathogenicity islands (SaPIs), including SaPI1, are MGEs that carry genes encoding virulence factors, and are mobilized at high frequency through interactions with specific "helper" bacteriophages, such as 80α, leading to packaging of the SaPI genomes into virions made from structural proteins supplied by the helper. Among these structural proteins is the portal protein, which forms a ring-like portal at a fivefold vertex of the capsid, through which the DNA is packaged during virion assembly and ejected upon infection of the host. We have used high-resolution cryo-electron microscopy to determine structures of the S. aureus bacteriophage 80α portal itself, produced by overexpression, and in situ in the empty and full SaPI1 virions, and show how the portal interacts with the capsid. These structures provide a basis for understanding portal and capsid assembly and the conformational changes that occur upon DNA packaging and ejection.
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Affiliation(s)
- Amarshi Mukherjee
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - James L Kizziah
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - N'Toia C Hawkins
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Mohamed O Nasef
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Laura K Parker
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Terje Dokland
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA.
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2
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Mukherjee A, Kizziah JL, Hawkins NC, Nasef MO, Parker LK, Dokland T. Structure of the portal complex from Staphylococcus aureus pathogenicity island 1 transducing particles in situ and in solution. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.09.18.557803. [PMID: 37786723 PMCID: PMC10541612 DOI: 10.1101/2023.09.18.557803] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/04/2023]
Abstract
Staphylococcus aureus is an important human pathogen, and the prevalence of antibiotic resistance is a major public health concern. The evolution of pathogenicity and resistance in S. aureus often involves acquisition of mobile genetic elements (MGEs). Bacteriophages play an especially important role, since transduction represents the main mechanism for horizontal gene transfer. S. aureus pathogenicity islands (SaPIs), including SaPI1, are MGEs that carry genes encoding virulence factors, and are mobilized at high frequency through interactions with specific "helper" bacteriophages, such as 80α, leading to packaging of the SaPI genomes into virions made from structural proteins supplied by the helper. Among these structural proteins is the portal protein, which forms a ring-like portal at a fivefold vertex of the capsid, through which the DNA is packaged during virion assembly and ejected upon infection of the host. We have used high-resolution cryo-electron microscopy to determine structures of the S. aureus bacteriophage 80α portal in solution and in situ in the empty and full SaPI1 virions, and show how the portal interacts with the capsid. These structures provide a basis for understanding portal and capsid assembly and the conformational changes that occur upon DNA packaging and ejection.
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Affiliation(s)
| | | | | | - Mohamed O. Nasef
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Laura K. Parker
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Terje Dokland
- Department of Microbiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA
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3
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Mukherjee A, Kizziah J, Parker L, Dokland T. High-resolution Cryo-EM Structure of Staphylococcus aureus Bacteriophage 80α Portal Protein and SaPI1 Capsid. MICROSCOPY AND MICROANALYSIS : THE OFFICIAL JOURNAL OF MICROSCOPY SOCIETY OF AMERICA, MICROBEAM ANALYSIS SOCIETY, MICROSCOPICAL SOCIETY OF CANADA 2023; 29:925-926. [PMID: 37613470 DOI: 10.1093/micmic/ozad067.458] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/25/2023]
Affiliation(s)
- Amarshi Mukherjee
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL, United States
| | - James Kizziah
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL, United States
| | - Laura Parker
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL, United States
| | - Terje Dokland
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL, United States
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4
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Predicting the capsid architecture of phages from metagenomic data. Comput Struct Biotechnol J 2022; 20:721-732. [PMID: 35140890 PMCID: PMC8814770 DOI: 10.1016/j.csbj.2021.12.032] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2021] [Revised: 12/22/2021] [Accepted: 12/22/2021] [Indexed: 12/29/2022] Open
Abstract
Tailed phages are viruses that infect bacteria and are the most abundant biological entities on Earth. Their ecological, evolutionary, and biogeochemical roles in the planet stem from their genomic diversity. Known tailed phage genomes range from 10 to 735 kilobase pairs thanks to the size variability of the protective protein capsids that store them. However, the role of tailed phage capsids’ diversity in ecosystems is unclear. A fundamental gap is the difficulty of associating genomic information with viral capsids in the environment. To address this problem, here, we introduce a computational approach to predict the capsid architecture (T-number) of tailed phages using the sequence of a single gene—the major capsid protein. This approach relies on an allometric model that relates the genome length and capsid architecture of tailed phages. This allometric model was applied to isolated phage genomes to generate a library that associated major capsid proteins and putative capsid architectures. This library was used to train machine learning methods, and the most computationally scalable model investigated (random forest) was applied to human gut metagenomes. Compared to isolated phages, the analysis of gut data reveals a large abundance of mid-sized (T = 7) capsids, as expected, followed by a relatively large frequency of jumbo-like tailed phage capsids (T ≥ 25) and small capsids (T = 4) that have been under-sampled. We discussed how to increase the method’s accuracy and how to extend the approach to other viruses. The computational pipeline introduced here opens the doors to monitor the ongoing evolution and selection of viral capsids across ecosystems.
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5
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Luque A, Benler S, Lee DY, Brown C, White S. The Missing Tailed Phages: Prediction of Small Capsid Candidates. Microorganisms 2020; 8:E1944. [PMID: 33302408 PMCID: PMC7762592 DOI: 10.3390/microorganisms8121944] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2020] [Revised: 12/04/2020] [Accepted: 12/05/2020] [Indexed: 12/17/2022] Open
Abstract
Tailed phages are the most abundant and diverse group of viruses on the planet. Yet, the smallest tailed phages display relatively complex capsids and large genomes compared to other viruses. The lack of tailed phages forming the common icosahedral capsid architectures T = 1 and T = 3 is puzzling. Here, we extracted geometrical features from high-resolution tailed phage capsid reconstructions and built a statistical model based on physical principles to predict the capsid diameter and genome length of the missing small-tailed phage capsids. We applied the model to 3348 isolated tailed phage genomes and 1496 gut metagenome-assembled tailed phage genomes. Four isolated tailed phages were predicted to form T = 3 icosahedral capsids, and twenty-one metagenome-assembled tailed phages were predicted to form T < 3 capsids. The smallest capsid predicted was a T = 4/3 ≈ 1.33 architecture. No tailed phages were predicted to form the smallest icosahedral architecture, T = 1. We discuss the feasibility of the missing T = 1 tailed phage capsids and the implications of isolating and characterizing small-tailed phages for viral evolution and phage therapy.
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Affiliation(s)
- Antoni Luque
- Viral Information Institute, San Diego State University, San Diego, CA 92182, USA; (D.Y.L.); (C.B.)
- Computational Science Research Center, San Diego State University, San Diego, CA 92182, USA
- Department of Mathematics and Statistics, San Diego State University, San Diego, CA 92182, USA
| | - Sean Benler
- National Center for Biotechnology Information (NCBI), Bethesda, MD 20894, USA;
| | - Diana Y. Lee
- Viral Information Institute, San Diego State University, San Diego, CA 92182, USA; (D.Y.L.); (C.B.)
- Computational Science Research Center, San Diego State University, San Diego, CA 92182, USA
| | - Colin Brown
- Viral Information Institute, San Diego State University, San Diego, CA 92182, USA; (D.Y.L.); (C.B.)
- Department of Physics, San Diego State University, San Diego, CA 92182, USA
| | - Simon White
- Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269, USA;
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6
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Huang L, Xiang Y. Structures of the tailed bacteriophages that infect Gram-positive bacteria. Curr Opin Virol 2020; 45:65-74. [DOI: 10.1016/j.coviro.2020.09.002] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2020] [Revised: 07/07/2020] [Accepted: 09/06/2020] [Indexed: 01/04/2023]
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7
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Newcomer RL, Schrad JR, Gilcrease EB, Casjens SR, Feig M, Teschke CM, Alexandrescu AT, Parent KN. The phage L capsid decoration protein has a novel OB-fold and an unusual capsid binding strategy. eLife 2019; 8:e45345. [PMID: 30945633 PMCID: PMC6449081 DOI: 10.7554/elife.45345] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2019] [Accepted: 03/20/2019] [Indexed: 12/15/2022] Open
Abstract
The major coat proteins of dsDNA tailed phages (order Caudovirales) and herpesviruses form capsids by a mechanism that includes active packaging of the dsDNA genome into a precursor procapsid, followed by expansion and stabilization of the capsid. These viruses have evolved diverse strategies to fortify their capsids, such as non-covalent binding of auxiliary 'decoration' (Dec) proteins. The Dec protein from the P22-like phage L has a highly unusual binding strategy that distinguishes between nearly identical three-fold and quasi-three-fold sites of the icosahedral capsid. Cryo-electron microscopy and three-dimensional image reconstruction were employed to determine the structure of native phage L particles. NMR was used to determine the structure/dynamics of Dec in solution. The NMR structure and the cryo-EM density envelope were combined to build a model of the capsid-bound Dec trimer. Key regions that modulate the binding interface were verified by site-directed mutagenesis.
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Affiliation(s)
- Rebecca L Newcomer
- Department of Molecular and Cell BiologyUniversity of ConnecticutStorrsUnited States
| | - Jason R Schrad
- Department of Biochemistry and Molecular BiologyMichigan State UniversityEast LansingUnited States
| | - Eddie B Gilcrease
- Division of Microbiology and Immunology, Department of PathologyUniversity of Utah School of MedicineSalt Lake CityUnited States
| | - Sherwood R Casjens
- Division of Microbiology and Immunology, Department of PathologyUniversity of Utah School of MedicineSalt Lake CityUnited States
| | - Michael Feig
- Department of Biochemistry and Molecular BiologyMichigan State UniversityEast LansingUnited States
| | - Carolyn M Teschke
- Department of Molecular and Cell BiologyUniversity of ConnecticutStorrsUnited States
| | - Andrei T Alexandrescu
- Department of Molecular and Cell BiologyUniversity of ConnecticutStorrsUnited States
| | - Kristin N Parent
- Department of Biochemistry and Molecular BiologyMichigan State UniversityEast LansingUnited States
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8
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Manning KA, Quiles-Puchalt N, Penadés JR, Dokland T. A novel ejection protein from bacteriophage 80α that promotes lytic growth. Virology 2018; 525:237-247. [PMID: 30308422 DOI: 10.1016/j.virol.2018.09.025] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2018] [Revised: 09/28/2018] [Accepted: 09/28/2018] [Indexed: 12/30/2022]
Abstract
Many staphylococcal bacteriophages encode a minor capsid protein between the genes for the portal and scaffolding proteins. In Staphylococcus aureus bacteriophage 80α, this protein, called gp44, is essential for the production of viable phage, but dispensable for the phage-mediated mobilization of S. aureus pathogenicity islands. We show here that gp44 is not required for capsid assembly, DNA packaging or ejection of the DNA, nor for generalized transduction of plasmids. An 80α Δ44 mutant could be complemented in trans by gp44 expressed from a plasmid, indicating that gp44 plays a post-injection role in the host. Our results show that gp44 is an ejection (pilot) protein that is involved in deciding the fate of the phage DNA after injection. Our data are consistent with a model in which gp44 acts as a regulatory protein that promotes progression to the lytic cycle.
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Affiliation(s)
- Keith A Manning
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Nuria Quiles-Puchalt
- Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK
| | - José R Penadés
- Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK
| | - Terje Dokland
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
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9
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Kizziah JL, Manning KA, Dearborn AD, Wall EA, Klenow L, Hill RLL, Spilman MS, Stagg SM, Christie GE, Dokland T. Cleavage and Structural Transitions during Maturation of Staphylococcus aureus Bacteriophage 80α and SaPI1 Capsids. Viruses 2017; 9:v9120384. [PMID: 29258203 PMCID: PMC5744158 DOI: 10.3390/v9120384] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2017] [Revised: 12/08/2017] [Accepted: 12/15/2017] [Indexed: 12/21/2022] Open
Abstract
In the tailed bacteriophages, DNA is packaged into spherical procapsids, leading to expansion into angular, thin-walled mature capsids. In many cases, this maturation is accompanied by cleavage of the major capsid protein (CP) and other capsid-associated proteins, including the scaffolding protein (SP) that serves as a chaperone for the assembly process. Staphylococcus aureus bacteriophage 80α is capable of high frequency mobilization of mobile genetic elements called S. aureus pathogenicity islands (SaPIs), such as SaPI1. SaPI1 redirects the assembly pathway of 80α to form capsids that are smaller than those normally made by the phage alone. Both CP and SP of 80α are N-terminally processed by a host-encoded protease, Prp. We have analyzed phage mutants that express pre-cleaved or uncleavable versions of CP or SP, and show that the N-terminal sequence in SP is absolutely required for assembly, but does not need to be cleaved in order to produce viable capsids. Mutants with pre-cleaved or uncleavable CP display normal viability. We have used cryo-EM to solve the structures of mature capsids from an 80α mutant expressing uncleavable CP, and from wildtype SaPI1. Comparisons with structures of 80α and SaPI1 procapsids show that capsid maturation involves major conformational changes in CP, consistent with a release of the CP N-arm by SP. The hexamers reorganize during maturation to accommodate the different environments in the 80α and SaPI1 capsids.
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Affiliation(s)
- James L Kizziah
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
| | - Keith A Manning
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
| | - Altaira D Dearborn
- Protein Expression Laboratory, National Institute of Arthritis and Musculoskeletal and Skin Diseases, The National Institutes of Health, Bethesda, MD 20892, USA.
| | - Erin A Wall
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, VA 23298, USA.
| | - Laura Klenow
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, VA 23298, USA.
| | - Rosanne L L Hill
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
| | - Michael S Spilman
- Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306, USA.
| | - Scott M Stagg
- Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306, USA.
| | - Gail E Christie
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, VA 23298, USA.
| | - Terje Dokland
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
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10
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Carpena N, Manning KA, Dokland T, Marina A, Penadés JR. Convergent evolution of pathogenicity islands in helper cos phage interference. Philos Trans R Soc Lond B Biol Sci 2017; 371:rstb.2015.0505. [PMID: 27672154 PMCID: PMC5052747 DOI: 10.1098/rstb.2015.0505] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/28/2016] [Indexed: 12/21/2022] Open
Abstract
Staphylococcus aureus pathogenicity islands (SaPIs) are phage satellites that exploit the life cycle of their helper phages for their own benefit. Most SaPIs are packaged by their helper phages using a headful (pac) packaging mechanism. These SaPIs interfere with pac phage reproduction through a variety of strategies, including the redirection of phage capsid assembly to form small capsids, a process that depends on the expression of the SaPI-encoded cpmA and cpmB genes. Another SaPI subfamily is induced and packaged by cos-type phages, and although these cos SaPIs also block the life cycle of their inducing phages, the basis for this mechanism of interference remains to be deciphered. Here we have identified and characterized one mechanism by which the SaPIs interfere with cos phage reproduction. This mechanism depends on a SaPI-encoded gene, ccm, which encodes a protein involved in the production of small isometric capsids, compared with the prolate helper phage capsids. As the Ccm and CpmAB proteins are completely unrelated in sequence, this strategy represents a fascinating example of convergent evolution. Moreover, this result also indicates that the production of SaPI-sized particles is a widespread strategy of phage interference conserved during SaPI evolution. This article is part of the themed issue ‘The new bacteriology’.
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Affiliation(s)
- Nuria Carpena
- Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8TA, UK Departamento de Ciencias Biomédicas, Facultad de Ciencias de la Salud, Universidad CEU Cardenal Herrera, 46113 Moncada, Valencia, Spain
| | - Keith A Manning
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Terje Dokland
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
| | - Alberto Marina
- Instituto de Biomedicina de Valencia (IBV-CSIC) and CIBER de Enfermedades Raras (CIBERER), 46010 Valencia, Spain
| | - José R Penadés
- Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8TA, UK
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11
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Dearborn AD, Wall EA, Kizziah JL, Klenow L, Parker LK, Manning KA, Spilman MS, Spear JM, Christie GE, Dokland T. Competing scaffolding proteins determine capsid size during mobilization of Staphylococcus aureus pathogenicity islands. eLife 2017; 6:30822. [PMID: 28984245 PMCID: PMC5644958 DOI: 10.7554/elife.30822] [Citation(s) in RCA: 36] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2017] [Accepted: 10/02/2017] [Indexed: 01/03/2023] Open
Abstract
Staphylococcus aureus pathogenicity islands (SaPIs), such as SaPI1, exploit specific helper bacteriophages, like 80α, for their high frequency mobilization, a process termed 'molecular piracy'. SaPI1 redirects the helper's assembly pathway to form small capsids that can only accommodate the smaller SaPI1 genome, but not a complete phage genome. SaPI1 encodes two proteins, CpmA and CpmB, that are responsible for this size redirection. We have determined the structures of the 80α and SaPI1 procapsids to near-atomic resolution by cryo-electron microscopy, and show that CpmB competes with the 80α scaffolding protein (SP) for a binding site on the capsid protein (CP), and works by altering the angle between capsomers. We probed these interactions genetically and identified second-site suppressors of lethal mutations in SP. Our structures show, for the first time, the detailed interactions between SP and CP in a bacteriophage, providing unique insights into macromolecular assembly processes.
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Affiliation(s)
- Altaira D Dearborn
- Protein Expression Laboratory, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, United States
| | - Erin A Wall
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, United States
| | - James L Kizziah
- Department of Microbiology, University of Alabama, Birmingham, United States
| | - Laura Klenow
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, United States
| | - Laura K Parker
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, United States.,Department of Microbiology, University of Alabama, Birmingham, United States
| | - Keith A Manning
- Department of Microbiology, University of Alabama, Birmingham, United States
| | | | - John M Spear
- Biological Science Imaging Resource, Florida State University, Tallahassee, United States
| | - Gail E Christie
- Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, United States
| | - Terje Dokland
- Department of Microbiology, University of Alabama, Birmingham, United States
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12
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Penadés JR, Christie GE. The Phage-Inducible Chromosomal Islands: A Family of Highly Evolved Molecular Parasites. Annu Rev Virol 2016; 2:181-201. [PMID: 26958912 DOI: 10.1146/annurev-virology-031413-085446] [Citation(s) in RCA: 109] [Impact Index Per Article: 13.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The phage-inducible chromosomal islands (PICIs) are a family of highly mobile genetic elements that contribute substantively to horizontal gene transfer, host adaptation, and virulence. Initially identified in Staphylococcus aureus, these elements are now thought to occur widely in gram-positive bacteria. They are molecular parasites that exploit certain temperate phages as helpers, using a variety of elegant strategies to manipulate the phage life cycle and promote their own spread, both intra- and intergenerically. At the same time, these PICI-encoded mechanisms severely interfere with helper phage reproduction, thereby enhancing survival of the bacterial population. In this review we discuss the genetics and the life cycle of these elements, with special emphasis on how they interact and interfere with the helper phage machinery for their own benefit. We also analyze the role that these elements play in driving bacterial and viral evolution.
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Affiliation(s)
- José R Penadés
- Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8TA Glasgow, United Kingdom;
| | - Gail E Christie
- Department of Microbiology and Immunology, Virginia Commonwealth University School of Medicine, Richmond, Virginia 23298;
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13
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Suhanovsky MM, Teschke CM. Nature's favorite building block: Deciphering folding and capsid assembly of proteins with the HK97-fold. Virology 2015; 479-480:487-97. [PMID: 25864106 PMCID: PMC4424165 DOI: 10.1016/j.virol.2015.02.055] [Citation(s) in RCA: 80] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2015] [Revised: 02/24/2015] [Accepted: 02/27/2015] [Indexed: 01/08/2023]
Abstract
For many (if not all) bacterial and archaeal tailed viruses and eukaryotic Herpesvirdae the HK97-fold serves as the major architectural element in icosahedral capsid formation while still enabling the conformational flexibility required during assembly and maturation. Auxiliary proteins or Δ-domains strictly control assembly of multiple, identical, HK97-like subunits into procapsids with specific icosahedral symmetries, rather than aberrant non-icosahedral structures. Procapsids are precursor structures that mature into capsids in a process involving release of auxiliary proteins (or cleavage of Δ-domains), dsDNA packaging, and conformational rearrangement of the HK97-like subunits. Some coat proteins built on the ubiquitous HK97-fold also have accessory domains or loops that impart specific functions, such as increased monomer, procapsid, or capsid stability. In this review, we analyze the numerous HK97-like coat protein structures that are emerging in the literature (over 40 at time of writing) by comparing their topology, additional domains, and their assembly and misassembly reactions.
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Affiliation(s)
- Margaret M Suhanovsky
- Department of Molecular and Cell Biology, University of Connecticut, 91N. Eagleville Rd. Storrs, CT 06269-3125, USA.
| | - Carolyn M Teschke
- Department of Molecular and Cell Biology, University of Connecticut, 91N. Eagleville Rd. Storrs, CT 06269-3125, USA; Department of Chemistry, University of Connecticut, 91N. Eagleville Rd. Storrs, CT 06269-3125, USA.
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14
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Dearborn AD, Dokland T. Mobilization of pathogenicity islands by Staphylococcus aureus strain Newman bacteriophages. BACTERIOPHAGE 2014; 2:70-78. [PMID: 23050217 PMCID: PMC3442828 DOI: 10.4161/bact.20632] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Staphylococcus aureus pathogenicity islands (SaPIs) are mobile genetic elements that encode virulence factors and depend on helper phages for their mobilization. Such mobilization is specific and depends on the ability of a phage protein to inactivate the SaPI repressor Stl. Phage 80α can mobilize several SaPIs, including SaPI1 and SaPIbov1, via its Sri and Dut proteins, respectively. In many cases, the capsids formed in the presence of the SaPI are smaller than those normally produced by the phage. Two SaPI-encoded proteins, CpmA and CpmB, are involved in this size determination process. S. aureus strain Newman contains four prophages, named φNM1 through φNM4. Phages φNM1 and φNM2 are very similar to phage 80α in the structural genes, and encode almost identical Sri proteins, while their Dut proteins are highly divergent. We show that φNM1 and φNM2 are able to mobilize both SaPI1 and SaPIbov1 and yield infectious transducing particles. The majority of the capsids formed in all cases are small, showing that both SaPIs can redirect the capsid size of both φNM1 and φNM2.
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Affiliation(s)
- Altaira D Dearborn
- Department of Microbiology; University of Alabama at Birmingham; Birmingham, AL USA
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15
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Parent KN, Tang J, Cardone G, Gilcrease EB, Janssen ME, Olson NH, Casjens SR, Baker TS. Three-dimensional reconstructions of the bacteriophage CUS-3 virion reveal a conserved coat protein I-domain but a distinct tailspike receptor-binding domain. Virology 2014; 464-465:55-66. [PMID: 25043589 DOI: 10.1016/j.virol.2014.06.017] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2014] [Revised: 06/12/2014] [Accepted: 06/15/2014] [Indexed: 01/21/2023]
Abstract
CUS-3 is a short-tailed, dsDNA bacteriophage that infects serotype K1 Escherichia coli. We report icosahedrally averaged and asymmetric, three-dimensional, cryo-electron microscopic reconstructions of the CUS-3 virion. Its coat protein structure adopts the "HK97-fold" shared by other tailed phages and is quite similar to that in phages P22 and Sf6 despite only weak amino acid sequence similarity. In addition, these coat proteins share a unique extra external domain ("I-domain"), suggesting that the group of P22-like phages has evolved over a very long time period without acquiring a new coat protein gene from another phage group. On the other hand, the morphology of the CUS-3 tailspike differs significantly from that of P22 or Sf6, but is similar to the tailspike of phage K1F, a member of the extremely distantly related T7 group of phages. We conclude that CUS-3 obtained its tailspike gene from a distantly related phage quite recently.
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Affiliation(s)
- Kristin N Parent
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States.
| | - Jinghua Tang
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States
| | - Giovanni Cardone
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States
| | - Eddie B Gilcrease
- University of Utah School of Medicine, Division of Microbiology and Immunology, Department of Pathology, Salt Lake City, UT 84112, United States
| | - Mandy E Janssen
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States
| | - Norman H Olson
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States
| | - Sherwood R Casjens
- University of Utah School of Medicine, Division of Microbiology and Immunology, Department of Pathology, Salt Lake City, UT 84112, United States.
| | - Timothy S Baker
- Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0378, United States; University of California, San Diego, Division of Biological Sciences, La Jolla, CA, 92093, United States.
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16
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McHugh CA, Fontana J, Nemecek D, Cheng N, Aksyuk AA, Heymann JB, Winkler DC, Lam AS, Wall JS, Steven AC, Hoiczyk E. A virus capsid-like nanocompartment that stores iron and protects bacteria from oxidative stress. EMBO J 2014; 33:1896-911. [PMID: 25024436 DOI: 10.15252/embj.201488566] [Citation(s) in RCA: 129] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Living cells compartmentalize materials and enzymatic reactions to increase metabolic efficiency. While eukaryotes use membrane-bound organelles, bacteria and archaea rely primarily on protein-bound nanocompartments. Encapsulins constitute a class of nanocompartments widespread in bacteria and archaea whose functions have hitherto been unclear. Here, we characterize the encapsulin nanocompartment from Myxococcus xanthus, which consists of a shell protein (EncA, 32.5 kDa) and three internal proteins (EncB, 17 kDa; EncC, 13 kDa; EncD, 11 kDa). Using cryo-electron microscopy, we determined that EncA self-assembles into an icosahedral shell 32 nm in diameter (26 nm internal diameter), built from 180 subunits with the fold first observed in bacteriophage HK97 capsid. The internal proteins, of which EncB and EncC have ferritin-like domains, attach to its inner surface. Native nanocompartments have dense iron-rich cores. Functionally, they resemble ferritins, cage-like iron storage proteins, but with a massively greater capacity (~30,000 iron atoms versus ~3,000 in ferritin). Physiological data reveal that few nanocompartments are assembled during vegetative growth, but they increase fivefold upon starvation, protecting cells from oxidative stress through iron sequestration.
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Affiliation(s)
- Colleen A McHugh
- W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
| | - Juan Fontana
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Daniel Nemecek
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Naiqian Cheng
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Anastasia A Aksyuk
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - J Bernard Heymann
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Dennis C Winkler
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Alan S Lam
- W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
| | - Joseph S Wall
- Department of Biology, Brookhaven National Laboratory, Upton, NY, USA
| | - Alasdair C Steven
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - Egbert Hoiczyk
- W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA
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17
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Stroupe ME, Brewer TE, Sousa DR, Jones KM. The structure of Sinorhizobium meliloti phage ΦM12, which has a novel T=19l triangulation number and is the founder of a new group of T4-superfamily phages. Virology 2014; 450-451:205-12. [PMID: 24503083 DOI: 10.1016/j.virol.2013.11.019] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2013] [Revised: 09/23/2013] [Accepted: 11/10/2013] [Indexed: 11/16/2022]
Abstract
ΦM12 is the first example of a T=19l geometry capsid, encapsulating the recently sequenced genome. Here, we present structures determined by cryo-EM of full and empty capsids. The structure reveals the pattern for assembly of 1140 HK97-like capsid proteins, pointing to interactions at the pseudo 3-fold symmetry axes that hold together the asymmetric unit. The particular smooth surface of the capsid, along with a lack of accessory coat proteins encoded by the genome, suggest that this interface is the primary mechanism for capsid assembly. Two-dimensional averages of the tail, including the neck and baseplate, reveal that ΦM12 has a relatively narrow neck that attaches the tail to the capsid, as well as a three-layer baseplate. When free from DNA, the icosahedral edges expand by about 5nm, while the vertices stay at the same position, forming a similarly smooth, but bowed, T=19l icosahedral capsid.
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Affiliation(s)
- M Elizabeth Stroupe
- Department of Biological Science, Florida State University, Biology Unit I, 230A, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States; Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States.
| | - Tess E Brewer
- Department of Biological Science, Florida State University, Biology Unit I, 230A, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States
| | - Duncan R Sousa
- Department of Biological Science, Florida State University, Biology Unit I, 230A, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States; Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States
| | - Kathryn M Jones
- Department of Biological Science, Florida State University, Biology Unit I, 230A, 91 Chieftan Way, Tallahassee, FL, 32306-4370, United States
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18
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Abstract
Molecular piracy is a biological phenomenon in which one replicon (the pirate) uses the structural proteins encoded by another replicon (the helper) to package its own genome and thus allow its propagation and spread. Such piracy is dependent on a complex web of interactions between the helper and the pirate that occur at several levels, from transcriptional control to macromolecular assembly. The best characterized examples of molecular piracy are from the E. coli P2/P4 system and the S. aureus SaPI pathogenicity island/helper system. In both of these cases, the pirate element is mobilized and packaged into phage-like transducing particles assembled from proteins supplied by a helper phage that belongs to the Caudovirales order of viruses (tailed, dsDNA bacteriophages). In this review we will summarize and compare the processes that are involved in molecular piracy in these two systems.
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Affiliation(s)
- Gail E. Christie
- Department of Microbiology and Immunology, Virginia Commonwealth University School of Medicine, PO Box 980678, Richmond, VA 23298-0678, USA
| | - Terje Dokland
- Department of Microbiology, University of Alabama at Birmingham, 845 19th St South BBRB 311, Birmingham, AL 35294 USA
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19
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Spilman MS, Damle PK, Dearborn AD, Rodenburg CM, Chang JR, Wall EA, Christie GE, Dokland T. Assembly of bacteriophage 80α capsids in a Staphylococcus aureus expression system. Virology 2012; 434:242-50. [PMID: 22980502 DOI: 10.1016/j.virol.2012.08.031] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2012] [Revised: 08/20/2012] [Accepted: 08/22/2012] [Indexed: 11/26/2022]
Abstract
80α is a temperate, double-stranded DNA bacteriophage of Staphylococcus aureus that can act as a "helper" for the mobilization of S. aureus pathogenicity islands (SaPIs), including SaPI1. When SaPI1 is mobilized by 80α, the SaPI genomes are packaged into capsids that are composed of phage proteins, but that are smaller than those normally formed by the phage. This size determination is dependent on SaPI1 proteins CpmA and CpmB. Here, we show that co-expression of the 80α capsid and scaffolding proteins in S. aureus, but not in E. coli, leads to the formation of procapsid-related structures, suggesting that a host co-factor is required for assembly. The capsid and scaffolding proteins also undergo normal N-terminal processing upon expression in S. aureus, implicating a host protease. We also find that SaPI1 proteins CpmA and CpmB promote the formation of small capsids upon co-expression with 80α capsid and scaffolding proteins in S. aureus.
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Affiliation(s)
- Michael S Spilman
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
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20
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Conformational switch-defective X174 internal scaffolding proteins kinetically trap assembly intermediates before procapsid formation. J Virol 2012; 86:9911-8. [PMID: 22761377 DOI: 10.1128/jvi.01120-12] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Conformational switching is an overarching paradigm in which to describe scaffolding protein-mediated virus assembly. However, rapid morphogenesis with small assembly subunits hinders the isolation of early morphogenetic intermediates in most model systems. Consequently, conformational switches are often defined by comparing the structures of virions, procapsids and aberrantly assembled particles. In contrast, X174 morphogenesis proceeds through at least three preprocapsid intermediates, which can be biochemically isolated. This affords a detailed analysis of early morphogenesis and internal scaffolding protein function. Amino acid substitutions were generated for the six C-terminal, aromatic amino acids that mediate most coat-internal scaffolding protein contacts. The biochemical characterization of mutant assembly pathways revealed two classes of molecular defects, protein binding and conformational switching, a novel phenotype. The conformational switch mutations kinetically trapped assembly intermediates before procapsid formation. Although mutations trapped different particles, they shared common second-site suppressors located in the viral coat protein. This suggests a fluid assembly pathway, one in which the scaffolding protein induces a single, coat protein conformational switch and not a series of sequential reactions. In this model, an incomplete or improper switch would kinetically trap intermediates.
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21
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Damle PK, Wall EA, Spilman MS, Dearborn AD, Ram G, Novick RP, Dokland T, Christie GE. The roles of SaPI1 proteins gp7 (CpmA) and gp6 (CpmB) in capsid size determination and helper phage interference. Virology 2012; 432:277-82. [PMID: 22709958 DOI: 10.1016/j.virol.2012.05.026] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2012] [Revised: 03/28/2012] [Accepted: 05/24/2012] [Indexed: 11/28/2022]
Abstract
SaPIs are molecular pirates that exploit helper bacteriophages for their own high frequency mobilization. One striking feature of helper exploitation by SaPIs is redirection of the phage capsid assembly pathway to produce smaller phage-like particles with T=4 icosahedral symmetry rather than T=7 bacteriophage capsids. Small capsids can accommodate the SaPI genome but not that of the helper phage, leading to interference with helper propagation. Previous studies identified two proteins encoded by the prototype element SaPI1, gp6 and gp7, in SaPI1 procapsids but not in mature SaPI1 particles. Dimers of gp6 form an internal scaffold, aiding fidelity of small capsid assembly. Here we show that both SaPI1 gp6 (CpmB) and gp7 (CpmA) are necessary and sufficient to direct small capsid formation. Surprisingly, failure to form small capsids did not restore wild-type levels of helper phage growth, suggesting an additional role for these SaPI1 proteins in phage interference.
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Affiliation(s)
- Priyadarshan K Damle
- Department of Microbiology and Immunology, Virginia Commonwealth University School of Medicine, Richmond, VA 23298-0678, USA.
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22
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Dearborn AD, Laurinmaki P, Chandramouli P, Rodenburg CM, Wang S, Butcher SJ, Dokland T. Structure and size determination of bacteriophage P2 and P4 procapsids: function of size responsiveness mutations. J Struct Biol 2012; 178:215-24. [PMID: 22508104 DOI: 10.1016/j.jsb.2012.04.002] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2010] [Revised: 02/22/2012] [Accepted: 04/02/2012] [Indexed: 02/02/2023]
Abstract
Bacteriophage P4 is dependent on structural proteins supplied by a helper phage, P2, to assemble infectious virions. Bacteriophage P2 normally forms an icosahedral capsid with T=7 symmetry from the gpN capsid protein, the gpO scaffolding protein and the gpQ portal protein. In the presence of P4, however, the same structural proteins are assembled into a smaller capsid with T=4 symmetry. This size determination is effected by the P4-encoded protein Sid, which forms an external scaffold around the small P4 procapsids. Size responsiveness (sir) mutants in gpN fail to assemble small capsids even in the presence of Sid. We have produced large and small procapsids by co-expression of gpN with gpO and Sid, respectively, and applied cryo-electron microscopy and three-dimensional reconstruction methods to visualize these procapsids. gpN has an HK97-like fold and interacts with Sid in an exposed loop where the sir mutations are clustered. The T=7 lattice of P2 has dextro handedness, unlike the laevo lattices of other phages with this fold observed so far.
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Affiliation(s)
- Altaira D Dearborn
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
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23
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Parent KN, Gilcrease EB, Casjens SR, Baker TS. Structural evolution of the P22-like phages: comparison of Sf6 and P22 procapsid and virion architectures. Virology 2012; 427:177-88. [PMID: 22386055 DOI: 10.1016/j.virol.2012.01.040] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2011] [Revised: 01/10/2012] [Accepted: 01/30/2012] [Indexed: 01/07/2023]
Abstract
Coat proteins of tailed, dsDNA phages and in herpesviruses include a conserved core similar to the bacteriophage HK97 subunit. This core is often embellished with other domains such as the telokin Ig-like domain of phage P22. Eighty-six P22-like phages and prophages with sequenced genomes share a similar set of virion assembly genes and, based on comparisons of twelve viral assembly proteins (structural and assembly/packaging chaperones), these phages are classified into three groups (P22-like, Sf6-like, and CUS-3-like). We used cryo-electron microscopy and 3D image reconstruction to determine the structures of Sf6 procapsids and virions (~7Å resolution), and the structure of the entire, asymmetric Sf6 virion (16-Å resolution). The Sf6 coat protein is similar to that of P22 yet it has differences in the telokin domain and in its overall quaternary organization. Thermal stability and agarose gel experiments show that Sf6 virions are slightly less stable than those of P22. Finally, bacterial host outer membrane proteins A and C were identified in lipid vesicles that co-purify with Sf6 particles, but are not components of the capsid.
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Affiliation(s)
- Kristin N Parent
- University of California, San Diego, Department of Chemistry & Biochemistry, La Jolla, CA 92093, USA
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24
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Building the Machines: Scaffolding Protein Functions During Bacteriophage Morphogenesis. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2012; 726:325-50. [DOI: 10.1007/978-1-4614-0980-9_14] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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25
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Dearborn AD, Spilman MS, Damle PK, Chang JR, Monroe EB, Saad JS, Christie GE, Dokland T. The Staphylococcus aureus pathogenicity island 1 protein gp6 functions as an internal scaffold during capsid size determination. J Mol Biol 2011; 412:710-22. [PMID: 21821042 DOI: 10.1016/j.jmb.2011.07.036] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2011] [Revised: 07/18/2011] [Accepted: 07/18/2011] [Indexed: 10/17/2022]
Abstract
Staphylococcus aureus pathogenicity island 1 (SaPI1) is a mobile genetic element that carries genes for several superantigen toxins. SaPI1 is normally stably integrated into the host genome but can become mobilized by "helper" bacteriophage 80α, leading to the packaging of SaPI1 genomes into phage-like transducing particles that are composed of structural proteins supplied by the helper phage but having smaller capsids. We show that the SaPI1-encoded protein gp6 is necessary for efficient formation of small capsids. The NMR structure of gp6 reveals a dimeric protein with a helix-loop-helix motif similar to that of bacteriophage scaffolding proteins. The gp6 dimer matches internal densities that bridge capsid subunits in cryo-electron microscopy reconstructions of SaPI1 procapsids, suggesting that gp6 acts as an internal scaffolding protein in capsid size determination.
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Affiliation(s)
- Altaira D Dearborn
- Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA
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