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Sadiq A, Khan T, Bostan N, Yinda CK, Matthijnssens J. Antigenic epitope analysis of Pakistani G3 and G9 rotavirus strains compared to vaccine strains revealed multiple amino acid differences. Diagn Microbiol Infect Dis 2024; 109:116346. [PMID: 38759540 DOI: 10.1016/j.diagmicrobio.2024.116346] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2022] [Revised: 08/13/2023] [Accepted: 05/08/2024] [Indexed: 05/19/2024]
Abstract
Rotaviruses belong to genotype VP4-P[8] are a significant cause of severe loose diarrhea in infants and young children. In the present study, we characterised the complete genome of three of the Pakistani P[8]b RVA strains by Illumina HiSeq sequencing technology to determine the complete genotype constellation providing insight into the evolutionary dynamics of their genes using maximum likelihood analysis. The maximum genomic sequences of our study strains were similar to more recent human Wa-Like G1P[8]a, G3P[8]a, G4P[6], G4P[8], G9P[4], G9P[8]a, G11P[25],G12P[8]a and G12P[6] strains circulating around the world. Therefore, strains PAK274, PAK439 and PAK624 carry natively distinctive VP4 gene with universally common human Wa-Like genetic backbone. Comparing our study P[8]b strains with vaccines strains RotarixTM and RotaTeqTM, multiple amino acid differences were examined between vaccine virus antigenic epitopes and Pakistani isolates. Over time, these differences may result in the selection for strains that will escape the vaccine-induced RVA-neutralizing-antibody effect.
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Affiliation(s)
- Asma Sadiq
- Department of Microbiology, University of Jhang, Jhang, Pakistan
| | - Tariq Khan
- Department of Biosciences, COMSATS University (CUI), Park Road, Tarlai Kalan, Chak Shahzad, Islamabad,45550, Pakistan
| | - Nazish Bostan
- Department of Biosciences, COMSATS University (CUI), Park Road, Tarlai Kalan, Chak Shahzad, Islamabad,45550, Pakistan.
| | - Claude Kwe Yinda
- KU Leuven-University of Leuven, Department of Microbiology and Immunology, Rega Institute for Medical Research, Laboratory of Viral Metagenomics, Leuven, Belgium
| | - Jelle Matthijnssens
- KU Leuven-University of Leuven, Department of Microbiology and Immunology, Rega Institute for Medical Research, Laboratory of Viral Metagenomics, Leuven, Belgium
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2
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Nichols SL, Haller C, Borodavka A, Esstman SM. Rotavirus NSP2: A Master Orchestrator of Early Viral Particle Assembly. Viruses 2024; 16:814. [PMID: 38932107 PMCID: PMC11209291 DOI: 10.3390/v16060814] [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: 04/18/2024] [Revised: 05/06/2024] [Accepted: 05/16/2024] [Indexed: 06/28/2024] Open
Abstract
Rotaviruses (RVs) are 11-segmented, double-stranded (ds) RNA viruses and important causes of acute gastroenteritis in humans and other animal species. Early RV particle assembly is a multi-step process that includes the assortment, packaging and replication of the 11 genome segments in close connection with capsid morphogenesis. This process occurs inside virally induced, cytosolic, membrane-less organelles called viroplasms. While many viral and cellular proteins play roles during early RV assembly, the octameric nonstructural protein 2 (NSP2) has emerged as a master orchestrator of this key stage of the viral replication cycle. NSP2 is critical for viroplasm biogenesis as well as for the selective RNA-RNA interactions that underpin the assortment of 11 viral genome segments. Moreover, NSP2's associated enzymatic activities might serve to maintain nucleotide pools for use during viral genome replication, a process that is concurrent with early particle assembly. The goal of this review article is to summarize the available data about the structures, functions and interactions of RV NSP2 while also drawing attention to important unanswered questions in the field.
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Affiliation(s)
- Sarah L. Nichols
- Department of Biology, Wake Forest University, Wake Downtown, 455 Vine Street, Winston-Salem, NC 27106, USA;
| | - Cyril Haller
- Department of Chemical Engineering and Biotechnology, Cambridge University, Philippa Fawcett Drive, Cambridge CB3 0AS, UK;
| | - Alexander Borodavka
- Department of Chemical Engineering and Biotechnology, Cambridge University, Philippa Fawcett Drive, Cambridge CB3 0AS, UK;
| | - Sarah M. Esstman
- Department of Biology, Wake Forest University, Wake Downtown, 455 Vine Street, Winston-Salem, NC 27106, USA;
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3
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Diebold O, Gonzalez V, Venditti L, Sharp C, Blake RA, Tan WS, Stevens J, Caddy S, Digard P, Borodavka A, Gaunt E. Using Species a Rotavirus Reverse Genetics to Engineer Chimeric Viruses Expressing SARS-CoV-2 Spike Epitopes. J Virol 2022; 96:e0048822. [PMID: 35758692 PMCID: PMC9327695 DOI: 10.1128/jvi.00488-22] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Accepted: 05/31/2022] [Indexed: 02/02/2023] Open
Abstract
Species A rotavirus (RVA) vaccines based on live attenuated viruses are used worldwide in humans. The recent establishment of a reverse genetics system for rotoviruses (RVs) has opened the possibility of engineering chimeric viruses expressing heterologous peptides from other viral or microbial species in order to develop polyvalent vaccines. We tested the feasibility of this concept by two approaches. First, we inserted short SARS-CoV-2 spike peptides into the hypervariable region of the simian RV SA11 strain viral protein (VP) 4. Second, we fused the receptor binding domain (RBD) of the SARS-CoV-2 spike protein, or the shorter receptor binding motif (RBM) nested within the RBD, to the C terminus of nonstructural protein (NSP) 3 of the bovine RV RF strain, with or without an intervening Thosea asigna virus 2A (T2A) peptide. Mutating the hypervariable region of SA11 VP4 impeded viral replication, and for these mutants, no cross-reactivity with spike antibodies was detected. To rescue NSP3 mutants, we established a plasmid-based reverse genetics system for the bovine RV RF strain. Except for the RBD mutant that demonstrated a rescue defect, all NSP3 mutants delivered endpoint infectivity titers and exhibited replication kinetics comparable to that of the wild-type virus. In ELISAs, cell lysates of an NSP3 mutant expressing the RBD peptide showed cross-reactivity with a SARS-CoV-2 RBD antibody. 3D bovine gut enteroids were susceptible to infection by all NSP3 mutants, but cross-reactivity with SARS-CoV-2 RBD antibody was only detected for the RBM mutant. The tolerance of large SARS-CoV-2 peptide insertions at the C terminus of NSP3 in the presence of T2A element highlights the potential of this approach for the development of vaccine vectors targeting multiple enteric pathogens simultaneously. IMPORTANCE We explored the use of rotaviruses (RVs) to express heterologous peptides, using SARS-CoV-2 as an example. Small SARS-CoV-2 peptide insertions (<34 amino acids) into the hypervariable region of the viral protein 4 (VP4) of RV SA11 strain resulted in reduced viral titer and replication, demonstrating a limited tolerance for peptide insertions at this site. To test the RV RF strain for its tolerance for peptide insertions, we constructed a reverse genetics system. NSP3 was C-terminally tagged with SARS-CoV-2 spike peptides of up to 193 amino acids in length. With a T2A-separated 193 amino acid tag on NSP3, there was no significant effect on the viral rescue efficiency, endpoint titer, and replication kinetics. Tagged NSP3 elicited cross-reactivity with SARS-CoV-2 spike antibodies in ELISA. We highlight the potential for development of RV vaccine vectors targeting multiple enteric pathogens simultaneously.
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Affiliation(s)
- Ola Diebold
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Victoria Gonzalez
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Luca Venditti
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Colin Sharp
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Rosemary A. Blake
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Wenfang S. Tan
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Joanne Stevens
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Sarah Caddy
- Cambridge Institute of Therapeutic Immunology and Infectious Disease, Department of Medicine, University of Cambridge, Cambridge, United Kingdom
| | - Paul Digard
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
| | - Alexander Borodavka
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Eleanor Gaunt
- Infection and Immunity Division, Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, United Kingdom
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4
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Rotavirus spike protein ΔVP8* as a novel carrier protein for conjugate vaccine platform with demonstrated antigenic potential for use as bivalent vaccine. Sci Rep 2021; 11:22037. [PMID: 34764353 PMCID: PMC8586335 DOI: 10.1038/s41598-021-01549-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2021] [Accepted: 10/29/2021] [Indexed: 11/18/2022] Open
Abstract
Conjugate vaccine platform is a promising strategy to overcome the poor immunogenicity of bacterial polysaccharide antigens in infants and children. A carrier protein in conjugate vaccines works not only as an immune stimulator to polysaccharide, but also as an immunogen; with the latter generally not considered as a measured outcome in real world. Here, we probed the potential of a conjugate vaccine platform to induce enhanced immunogenicity of a truncated rotavirus spike protein ΔVP8*. ΔVP8* was covalently conjugated to Vi capsular polysaccharide (Vi) of Salmonella Typhi to develop a bivalent vaccine, termed Vi-ΔVP8*. Our results demonstrated that the Vi-ΔVP8* vaccine can induce specific immune responses against both antigens in immunized mice. The conjugate vaccine elicits high antibody titers and functional antibodies against S. Typhi and Rotavirus (RV) when compared to immunization with a single antigen. Together, these results indicate that Vi-ΔVP8* is a potent and immunogenic vaccine candidate, thus strengthening the potential of conjugate vaccine platform with enhanced immune responses to carrier protein, including ΔVP8*.
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5
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Abid N, Chillemi G, Salemi M. Coding-Gene Coevolution Analysis of Rotavirus Proteins: A Bioinformatics and Statistical Approach. Genes (Basel) 2019; 11:genes11010028. [PMID: 31878331 PMCID: PMC7016848 DOI: 10.3390/genes11010028] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2019] [Revised: 12/10/2019] [Accepted: 12/19/2019] [Indexed: 01/12/2023] Open
Abstract
Rotavirus remains a major cause of diarrhea in infants and young children worldwide. The permanent emergence of new genotypes puts the potential effectiveness of vaccines under serious question. The distribution of unusual genotypes subject to viral fitness is influenced by interactions among viral proteins. The present work aimed at analyzing the genetic constellation and the coevolution of rotavirus coding genes for the available rotavirus genotypes. Seventy-two full genome sequences of different genetic constellations were analyzed using a genetic algorithm. The results revealed an extensive genome-wide covariance network among the 12 viral proteins. Altogether, the emergence of new genotypes represents a challenge to the outcome and success of vaccination and the coevolutionary analysis of rotavirus proteins may boost efforts to better understand the interaction networks of proteins during viral replication/transcription.
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Affiliation(s)
- Nabil Abid
- Laboratory of Transmissible Diseases and Biological Active Substances LR99ES27, Faculty of Pharmacy, University of Monastir, Rue Ibn Sina, Monastir 5000, Tunisia
- High Institute of Biotechnology of Sidi Thabet, Department of Biotechnology, University Manouba, BP-66, Ariana-Tunis 2020, Tunisia
- Correspondence: or ; Tel.: +216-92–974-000
| | - Giovanni Chillemi
- Department for Innovation in Biological, Agro-food and Forest systems, DIBAF, University of Tuscia, via S. Camillo de Lellis s.n.c., 01100 Viterbo, Italy;
- Institute of Biomembranes, Bioenergetics and Molecular Biotechnologies, IBIOM, CNR, Via Giovanni Amendola, 122/O, 70126 Bari, Italy
| | - Marco Salemi
- Department of Pathology, Immunology and Laboratory Medicine, University of Florida College of Medicine, Emerging Pathogens Institute, P.O. Box 100009, Gainesville, FL 32610-3633, USA;
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6
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Li Y, Xue M, Yu L, Luo G, Yang H, Jia L, Zeng Y, Li T, Ge S, Xia N. Expression and characterization of a novel truncated rotavirus VP4 for the development of a recombinant rotavirus vaccine. Vaccine 2018; 36:2086-2092. [DOI: 10.1016/j.vaccine.2018.03.011] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2017] [Revised: 02/27/2018] [Accepted: 03/05/2018] [Indexed: 12/28/2022]
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8
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Abdelhakim AH, Salgado EN, Fu X, Pasham M, Nicastro D, Kirchhausen T, Harrison SC. Structural correlates of rotavirus cell entry. PLoS Pathog 2014; 10:e1004355. [PMID: 25211455 PMCID: PMC4161437 DOI: 10.1371/journal.ppat.1004355] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2013] [Accepted: 07/24/2014] [Indexed: 01/06/2023] Open
Abstract
Cell entry by non-enveloped viruses requires translocation into the cytosol of a macromolecular complex--for double-strand RNA viruses, a complete subviral particle. We have used live-cell fluorescence imaging to follow rotavirus entry and penetration into the cytosol of its ∼ 700 Å inner capsid particle ("double-layered particle", DLP). We label with distinct fluorescent tags the DLP and each of the two outer-layer proteins and track the fates of each species as the particles bind and enter BSC-1 cells. Virions attach to their glycolipid receptors in the host cell membrane and rapidly become inaccessible to externally added agents; most particles that release their DLP into the cytosol have done so by ∼ 10 minutes, as detected by rapid diffusional motion of the DLP away from residual outer-layer proteins. Electron microscopy shows images of particles at various stages of engulfment into tightly fitting membrane invaginations, consistent with the interpretation that rotavirus particles drive their own uptake. Electron cryotomography of membrane-bound virions also shows closely wrapped membrane. Combined with high resolution structural information about the viral components, these observations suggest a molecular model for membrane disruption and DLP penetration.
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Affiliation(s)
- Aliaa H. Abdelhakim
- Laboratory of Molecular Medicine, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Eric N. Salgado
- Laboratory of Molecular Medicine, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Xiaofeng Fu
- Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts, United States of America
| | - Mithun Pasham
- Program in Cellular and Molecular Medicine, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Daniela Nicastro
- Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts, United States of America
| | - Tomas Kirchhausen
- Program in Cellular and Molecular Medicine, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America
| | - Stephen C. Harrison
- Laboratory of Molecular Medicine, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America
- Howard Hughes Medical Institute, Boston, Massachusetts, United States of America
- * E-mail:
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9
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Abstract
A large number of viruses, including many human pathogens, bind cell-surface glycans during the initial steps of infection. Viral glycan receptors such as glycosaminoglycans and sialic acid-containing carbohydrates are often negatively charged, but neutral glycans such as histo-blood group antigens can also function as receptors. The engagement of glycans facilitates attachment and entry and, consequently, is often a key determinant of the host range, tissue tropism, pathogenicity, and transmissibility of viruses. Here, we review current knowledge about virus-glycan interactions using representative crystal structures of viral attachment proteins in complex with glycans. We illuminate the determinants of specificity utilized by different glycan-binding viruses and explore the potential of these interactions for switching receptor specificities within or even between glycan classes. A detailed understanding of these parameters is important for the prediction of binding sites where structural information is not available, and is invaluable for the development of antiviral therapeutics.
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Affiliation(s)
- Luisa J Ströh
- Interfaculty Institute of Biochemistry, University of Tübingen, D-72076 Tübingen, Germany;
| | - Thilo Stehle
- Interfaculty Institute of Biochemistry, University of Tübingen, D-72076 Tübingen, Germany; .,Department of Pediatrics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
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10
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New insights into rotavirus entry machinery: stabilization of rotavirus spike conformation is independent of trypsin cleavage. PLoS Pathog 2014; 10:e1004157. [PMID: 24873828 PMCID: PMC4038622 DOI: 10.1371/journal.ppat.1004157] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2014] [Accepted: 04/16/2014] [Indexed: 12/31/2022] Open
Abstract
The infectivity of rotavirus, the main causative agent of childhood diarrhea, is dependent on activation of the extracellular viral particles by trypsin-like proteases in the host intestinal lumen. This step entails proteolytic cleavage of the VP4 spike protein into its mature products, VP8* and VP5*. Previous cryo-electron microscopy (cryo-EM) analysis of trypsin-activated particles showed well-resolved spikes, although no density was identified for the spikes in uncleaved particles; these data suggested that trypsin activation triggers important conformational changes that give rise to the rigid, entry-competent spike. The nature of these structural changes is not well understood, due to lack of data relative to the uncleaved spike structure. Here we used cryo-EM and cryo-electron tomography (cryo-ET) to characterize the structure of the uncleaved virion in two model rotavirus strains. Cryo-EM three-dimensional reconstruction of uncleaved virions showed spikes with a structure compatible with the atomic model of the cleaved spike, and indistinguishable from that of digested particles. Cryo-ET and subvolume average, combined with classification methods, resolved the presence of non-icosahedral structures, providing a model for the complete structure of the uncleaved spike. Despite the similar rigid structure observed for uncleaved and cleaved particles, trypsin activation is necessary for successful infection. These observations suggest that the spike precursor protein must be proteolytically processed, not to achieve a rigid conformation, but to allow the conformational changes that drive virus entry. Rotavirus is responsible for more than 400,000 annual infant deaths worldwide. Its viral particle bears 60 protuberant spikes that constitute the machinery responsible for virus binding to and entry into the host cell. For efficient infection, the protein molecules that build the spike must be cleaved. Despite the importance of this activation step, the nature of the changes induced in the spike structure is unknown. According to the current hypothesis, the uncleaved spike is very flexible, and activation stabilizes the spike in an entry-competent conformation. Here we used distinct electron microscopy techniques to determine the structure of the uncleaved particle in two model rotavirus strains. Our results provide a complete structure of the uncleaved spike and demonstrate that cleaved and uncleaved spikes have similar conformations, indicating that proteolytic processing is not involved in stabilization of the spike. We suggest that spike processing is important for infection since it is necessary to allow the spike domain movements involved in rotavirus entry.
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11
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Carbohydrate recognition by rotaviruses. ACTA ACUST UNITED AC 2013; 15:101-6. [DOI: 10.1007/s10969-013-9167-5] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2013] [Accepted: 11/13/2013] [Indexed: 02/04/2023]
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12
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Trask SD, Wetzel JD, Dermody TS, Patton JT. Mutations in the rotavirus spike protein VP4 reduce trypsin sensitivity but not viral spread. J Gen Virol 2013; 94:1296-1300. [PMID: 23426355 DOI: 10.1099/vir.0.050674-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Infectious entry of the nonenveloped rotavirus virion requires proteolysis of the spike protein VP4 to mediate conformational changes associated with membrane penetration. We sequenced and characterized an isolate that was cultured in the absence of trypsin and found that it is more resistant to proteolysis than WT virus. A substitution mutation abrogates one of the defined trypsin-cleavage sites, suggesting that blocking proteolysis at this site reduces the overall kinetics of proteolysis. Kinetic analysis of the membrane penetration-associated conformational change indicated that the 'fold-back' of the mutant spike protein is slower than that of WT. Despite these apparent biochemical defects, the mutant virus replicates in an identical manner to the WT virus. These findings enhance an understanding of VP4 functions and establish new strategies to interrogate rotavirus cell entry.
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Affiliation(s)
- Shane D Trask
- Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-8026, USA
| | - J Denise Wetzel
- Department of Pediatrics and the Elizabeth B. Lamb Center for Pediatric Research, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2581, USA
| | - Terence S Dermody
- Departments of Pediatrics and Pathology, Microbiology and Immunology and the Elizabeth B. Lamb Center for Pediatric Research, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2581, USA
| | - John T Patton
- Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-8026, USA
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13
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Rhesus rotavirus trafficking during entry into MA104 cells is restricted to the early endosome compartment. J Virol 2012; 86:4009-13. [PMID: 22278225 DOI: 10.1128/jvi.06667-11] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Endocytosis has recently been implicated in rotavirus (RV) entry. We examined the role of Rabs, which regulate endosomal trafficking, during RV entry. Several structural proteins of neuraminidase-sensitive and -insensitive RVs colocalized with Rab5, an early endosome marker, but not Rab7, a late endosome marker. Dominant-negative and constitutively active mutants demonstrated that Rab5 but not Rab4 or Rab7 affects rhesus RV (RRV) infectivity. These data suggest that early RRV trafficking is confined to the early endosome compartment and requires Rab5.
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14
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Structural insights into the coupling of virion assembly and rotavirus replication. Nat Rev Microbiol 2012; 10:165-77. [PMID: 22266782 DOI: 10.1038/nrmicro2673] [Citation(s) in RCA: 148] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Viral replication is rapid and robust, but it is far from a chaotic process. Instead, successful production of infectious progeny requires that events occur in the correct place and at the correct time. Rotaviruses (segmented double-stranded RNA viruses of the Reoviridae family) seem to govern their replication through ordered disassembly and assembly of a triple-layered icosahedral capsid. In recent years, high-resolution structural data have provided unprecedented insight into these events. In this Review, we explore the current understanding of rotavirus replication and how it compares to replication of other Reoviridae family members.
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15
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Lou F, Neetoo H, Li J, Chen H, Li J. Lack of correlation between virus barosensitivity and the presence of a viral envelope during inactivation of human rotavirus, vesicular stomatitis virus, and avian metapneumovirus by high-pressure processing. Appl Environ Microbiol 2011; 77:8538-47. [PMID: 22003028 PMCID: PMC3233080 DOI: 10.1128/aem.06711-11] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2011] [Accepted: 10/08/2011] [Indexed: 11/20/2022] Open
Abstract
High-pressure processing (HPP) is a nonthermal technology that has been shown to effectively inactivate a wide range of microorganisms. However, the effectiveness of HPP on inactivation of viruses is relatively less well understood. We systematically investigated the effects of intrinsic (pH) and processing (pressure, time, and temperature) parameters on the pressure inactivation of a nonenveloped virus (human rotavirus [HRV]) and two enveloped viruses (vesicular stomatitis virus [VSV] and avian metapneumovirus [aMPV]). We demonstrated that HPP can efficiently inactivate all tested viruses under optimal conditions, although the pressure susceptibilities and the roles of temperature and pH substantially varied among these viruses regardless of the presence of a viral envelope. We found that VSV was much more stable than most food-borne viruses, whereas aMPV was highly susceptible to HPP. When viruses were held for 2 min under 350 MPa at 4°C, 1.1-log, 3.9-log, and 5.0-log virus reductions were achieved for VSV, HRV, and aMPV, respectively. Both VSV and aMPV were more susceptible to HPP at higher temperature and lower pH. In contrast, HRV was more easily inactivated at higher pH, although temperature did not have a significant impact on inactivation. Furthermore, we demonstrated that the damage of virion structure by disruption of the viral envelope and/or capsid is the primary mechanism underlying HPP-induced viral inactivation. In addition, VSV glycoprotein remained antigenic although VSV was completely inactivated. Taken together, our findings suggest that HPP is a promising technology to eliminate viral contaminants in high-risk foods, water, and other fomites.
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Affiliation(s)
- Fangfei Lou
- Department of Food Science and Technology, College of Food, Agricultural and Environmental Sciences
| | - Hudaa Neetoo
- Department of Animal and Food Sciences, University of Delaware, Newark, Delaware 19716
| | - Junan Li
- Division of Environmental Health Sciences, College of Public Health
| | - Haiqiang Chen
- Department of Animal and Food Sciences, University of Delaware, Newark, Delaware 19716
| | - Jianrong Li
- Department of Food Science and Technology, College of Food, Agricultural and Environmental Sciences
- Division of Environmental Health Sciences, College of Public Health
- Center for RNA Biology, The Ohio State University, Columbus, Ohio 43210
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16
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Lentz EM, Mozgovoj MV, Bellido D, Dus Santos MJ, Wigdorovitz A, Bravo-Almonacid FF. VP8* antigen produced in tobacco transplastomic plants confers protection against bovine rotavirus infection in a suckling mouse model. J Biotechnol 2011; 156:100-7. [PMID: 21893114 DOI: 10.1016/j.jbiotec.2011.08.023] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2011] [Revised: 07/26/2011] [Accepted: 08/14/2011] [Indexed: 10/17/2022]
Abstract
Group A rotavirus is a major leading cause of diarrhea in mammalian species worldwide. In Argentina, bovine rotavirus (BRV) is the main cause of neonatal diarrhea in calves. VP4, one of the outermost capsid proteins, is involved in various virus functions. Rotavirus infectivity requires proteolytic cleavage of VP4, giving an N-terminal non-glycosilated sialic acid-recognizing domain (VP8*), and a C-terminal fragment (VP5*) that remains associated with the virion. VP8* subunit is the major determinant of the viral infectivity and one of the neutralizing antigens. In this work, the C486 BRV VP8* protein was produced in tobacco chloroplasts. Transplastomic plants were obtained and characterized by Southern blot, northern blot and western blot. VP8* was highly stable in the transplastomic leaves, and formed insoluble aggregates that were partially solubilized by sonication. The recombinant protein yield was 600 μg/g of fresh tissue (FT). Both the soluble and insoluble fractions of the VP8* plant extracts were able to induce a strong immune response in female mice as measured by ELISA and virus neutralization test. Most important, suckling mice born to immunized dams were protected against oral challenge with virulent rotavirus. Results presented here contribute to demonstrate the feasibility of using antigens expressed in transplastomic plants for the development of subunit vaccines.
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Affiliation(s)
- E M Lentz
- Laboratorio de Virología y Biotecnología Vegetal, INGEBI-CONICET, Ciudad Autónoma de Buenos Aires, Argentina
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17
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Cross-linking of rotavirus outer capsid protein VP7 by antibodies or disulfides inhibits viral entry. J Virol 2011; 85:10509-17. [PMID: 21849465 DOI: 10.1128/jvi.00234-11] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Antibodies that neutralize rotavirus infection target outer coat proteins VP4 and VP7 and inhibit viral entry. The structure of a VP7-Fab complex (S. T. Aoki, et al., Science 324:1444-1447, 2009) led us to reclassify epitopes into two binding regions at inter- and intrasubunit boundaries of the calcium-dependent trimer. It further led us to show that antibodies binding at the intersubunit boundary inhibit uncoating of the virion outer layer. We have now tested representative antibodies for each of the defined structural epitope regions and find that antibodies recognizing epitopes in either binding region neutralize by cross-linking VP7 trimers. Antibodies that bind at the intersubunit junction neutralize as monovalent Fabs, while those that bind at the intrasubunit region require divalency. The VP7 structure has also allowed us to design a disulfide cross-linked VP7 mutant which recoats double-layered particles (DLPs) as efficiently as does wild-type VP7 but which yields particles defective in cell entry as determined both by lack of infectivity and by loss of α-sarcin toxicity in the presence of recoated particles. We conclude that dissociation of the VP7 trimer is an essential step in viral penetration into cells.
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18
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Rhesus rotavirus entry into a polarized epithelium is endocytosis dependent and involves sequential VP4 conformational changes. J Virol 2010; 85:2492-503. [PMID: 21191022 DOI: 10.1128/jvi.02082-10] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Rotavirus (RV) cell entry is an incompletely understood process, involving VP4 and VP7, the viral proteins composing the outermost layer of the nonenveloped RV triple-layered icosahedral particle (TLP), encasing VP6. VP4 can exist in three conformational states: soluble, cleaved spike, and folded back. In order to better understand the events leading to RV entry, we established a detection system to image input virus by monitoring the rhesus RV (RRV) antigens VP4, VP6, and VP7 at very early times postinfection. We provide evidence that decapsidation occurs directly after cell membrane penetration. We also demonstrate that several VP4 and VP7 conformational changes take place during entry. In particular, we detected, for the first time, the generation of folded-back VP5 in the context of the initiation of infection. Folded-back VP5 appears to be limited to the entry step. We furthermore demonstrate that RRV enters the cell cytoplasm through an endocytosis pathway. The endocytosis hypothesis is supported by the colocalization of RRV antigens with the early endosome markers Rab4 and Rab5. Finally, we provide evidence that the entry process is likely dependent on the endocytic Ca(2+) concentration, as bafilomycin A1 treatment as well as an augmentation of the extracellular calcium reservoir using CaEGTA, which both lead to an elevated intraendosomal calcium concentration, resulted in the accumulation of intact virions in the actin network. Together, these findings suggest that internalization, decapsidation, and cell membrane penetration involve endocytosis, calcium-dependent uncoating, and VP4 conformational changes, including a fold-back.
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19
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Atomic model of an infectious rotavirus particle. EMBO J 2010; 30:408-16. [PMID: 21157433 DOI: 10.1038/emboj.2010.322] [Citation(s) in RCA: 215] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2010] [Accepted: 11/16/2010] [Indexed: 01/17/2023] Open
Abstract
Non-enveloped viruses of different types have evolved distinct mechanisms for penetrating a cellular membrane during infection. Rotavirus penetration appears to occur by a process resembling enveloped-virus fusion: membrane distortion linked to conformational changes in a viral protein. Evidence for such a mechanism comes from crystallographic analyses of fragments of VP4, the rotavirus-penetration protein, and infectivity analyses of structure-based VP4 mutants. We describe here the structure of an infectious rotavirus particle determined by electron cryomicroscopy (cryoEM) and single-particle analysis at about 4.3 Å resolution. The cryoEM image reconstruction permits a nearly complete trace of the VP4 polypeptide chain, including the positions of most side chains. It shows how the two subfragments of VP4 (VP8(*) and VP5(*)) retain their association after proteolytic cleavage, reveals multiple structural roles for the β-barrel domain of VP5(*), and specifies interactions of VP4 with other capsid proteins. The virion model allows us to integrate structural and functional information into a coherent mechanism for rotavirus entry.
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20
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Abstract
Experiments in cell-free systems have demonstrated that the VP5 cleavage fragment of the rotavirus spike protein, VP4, undergoes a foldback rearrangement that translocates three clustered hydrophobic loops from one end of the molecule to the other. This conformational change resembles the foldback rearrangements of enveloped virus fusion proteins. By recoating rotavirus subviral particles with recombinant VP4 and VP7, we tested the effects on cell entry of substituting hydrophilic for hydrophobic residues in the clustered VP5 loops. Several of these mutations decreased the infectivity of recoated particles without preventing either recoating or folding back. In particular, the V391D mutant had a diminished capacity to interact with liposomes when triggered to fold back by serial protease digestion in solution, and particles recoated with this mutant VP4 were 10,000-fold less infectious than particles recoated with wild-type VP4. Particles with V391D mutant VP4 attached normally to cells and internalized efficiently, but they failed in the permeabilization step that allows coentry of the toxin alpha-sarcin. These findings indicate that the hydrophobicity of the VP5 apex is required for membrane disruption during rotavirus cell entry.
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21
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Abstract
During rotavirus entry, a virion penetrates a host cell membrane, sheds its outer capsid proteins, and releases a transcriptionally active subviral particle into the cytoplasm. VP5, the rotavirus protein believed to interact with the membrane bilayer, is a tryptic cleavage product of the outer capsid spike protein, VP4. When a rotavirus particle uncoats, VP5 folds back, in a rearrangement that resembles the fusogenic conformational changes in enveloped-virus fusion proteins. We present direct experimental evidence that this rearrangement leads to membrane binding. VP5 does not associate with liposomes when mounted as part of the trypsin-primed spikes on intact virions, nor does it do so after it has folded back into a stably trimeric, low-energy state. But it does bind liposomes when they are added to virions before uncoating, and VP5 rearrangement is then triggered by addition of EDTA. The presence of liposomes during the rearrangement enhances the otherwise inefficient VP5 conformational change. A VP5 fragment, VP5CT, produced from monomeric recombinant VP4 by successive treatments with chymotrypsin and trypsin, also binds liposomes only when the proteolysis proceeds in their presence. A monoclonal antibody that neutralizes infectivity by blocking a postattachment entry event also blocks VP5 liposome association. We propose that VP5 binds lipid bilayers in an intermediate conformational state, analogous to the extended intermediate conformation of enveloped-virus fusion proteins.
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22
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Abstract
Trypsin primes rotavirus for efficient infectivity by cleaving the spike protein, VP4, into VP8* and VP5*. A recombinant VP5* fragment has a trimeric, folded-back structure. Comparison of this structure with virion spikes suggests that a rearrangement, analogous to those of enveloped virus fusion proteins, may mediate membrane penetration by rotavirus during entry. To detect this inferred rearrangement of virion-associated authentic VP5*, we raised conformation-specific monoclonal antibodies against the recombinant VP5* fragment in its putative post-membrane penetration conformation. Using one of these antibodies, we demonstrate that rotavirus uncoating triggers a conformational change in the cleaved VP4 spike to yield rearranged VP5*.
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23
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Bellido D, Craig PO, Mozgovoj MV, Gonzalez DD, Wigdorovitz A, Goldbaum FA, Dus Santos MJ. Brucella spp. lumazine synthase as a bovine rotavirus antigen delivery system. Vaccine 2009; 27:136-45. [DOI: 10.1016/j.vaccine.2008.10.018] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2008] [Revised: 09/30/2008] [Accepted: 10/02/2008] [Indexed: 11/29/2022]
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24
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Kraschnefski MJ, Bugarcic A, Fleming FE, Yu X, von Itzstein M, Coulson BS, Blanchard H. Effects on sialic acid recognition of amino acid mutations in the carbohydrate-binding cleft of the rotavirus spike protein. Glycobiology 2008; 19:194-200. [PMID: 18974199 DOI: 10.1093/glycob/cwn119] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
The rotavirus spike protein VP4 mediates attachment to host cells and subsequent membrane penetration. The VP8(*) domain of VP4 forms the spike tips and is proposed to recognize host-cell surface glycans. For sialidase-sensitive rotaviruses such as rhesus (RRV), this recognition involves terminal sialic acids. We show here that the RRV VP8(*)(64-224) protein competes with RRV infection of host cells, demonstrating its relevance to infection. In addition, we observe that the amino acids revealed by X-ray crystallography to be in direct contact with the bound sialic acid derivative methyl alpha-D-N-acetylneuraminide, and that are highly conserved amongst sialidase-sensitive rotaviruses, are residues that are also important in interactions with host-cell carbohydrates. Residues Arg101 and Ser190 of the RRV VP8(*) carbohydrate-binding site were mutated to assess their importance for binding to the sialic acid derivative and their competition with RRV infection of host cells. The crystallographic structure of the Arg(101)Ala mutant crystallized in the presence of the sialic acid derivative was determined at 295 K to a resolution of 1.9 A. Our multidisciplinary study using X-ray crystallography, saturation transfer difference nuclear magnetic resonance spectroscopy, isothermal titration calorimetry, and competitive virus infectivity assays to investigate RRV wild-type and mutant VP8(*) proteins has provided the first evidence that the carbohydrate-binding cavity in RRV VP8(*) is used for host-cell recognition, and this interaction is not only with the sialic acid portion but also with other parts of the glycan structure.
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Affiliation(s)
- Mark J Kraschnefski
- Institute for Glycomics, Gold Coast Campus, Griffith University, Queensland 4222, Australia
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25
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Haselhorst T, Blanchard H, Frank M, Kraschnefski MJ, Kiefel MJ, Szyczew AJ, Dyason JC, Fleming F, Holloway G, Coulson BS, von Itzstein M. STD NMR spectroscopy and molecular modeling investigation of the binding of N-acetylneuraminic acid derivatives to rhesus rotavirus VP8* core. Glycobiology 2006; 17:68-81. [PMID: 16973731 DOI: 10.1093/glycob/cwl051] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
The VP8* subunit of rotavirus spike protein VP4 contains a sialic acid (Sia)-binding domain important for host cell attachment and infection. In this study, the binding epitope of the N-acetylneuraminic acid (Neu5Ac) derivatives has been characterized by saturation transfer difference (STD) nuclear magnetic resonance (NMR) spectroscopy. From this STD NMR data, it is proposed that the VP8* core recognizes an identical binding epitope in both methyl alpha-D-N-acetylneuraminide (Neu5Acalpha2Me) and the disaccharide methyl S-(alpha-D-N-acetylneuraminosyl)-(2-->6)-6-thio-beta-D-galactopyranoside (Neu5Ac-alpha(2,6)-S-Galbeta1Me). In the VP8*-disaccharide complex, the Neu5Ac moiety contributes to the majority of interaction with the protein, whereas the galactose moiety is solvent-exposed. Molecular dynamics calculations of the VP8*-disaccharide complex indicated that the galactose moiety is unable to adopt a conformation that is in close proximity to the protein surface. STD NMR experiments with methyl 9-O-acetyl-alpha-D-N-acetylneuraminide (Neu5,9Ac(2)alpha2Me) in complex with rhesus rotavirus (RRV) VP8* revealed that both the N-acetamide and 9-O-acetate moieties are in close proximity to the Sia-binding domain, with the N-acetamide's methyl group being saturated to a larger extent, indicating a closer association with the protein. RRV VP8* does not appear to significantly recognize the unsaturated Neu5Ac derivative [2-deoxy-2,3-didehydro-D-N-acetylneuraminic acid (Neu5Ac2en)]. Molecular modeling of the protein-Neu5Ac2en complex indicates that key interactions between the protein and the unsaturated Neu5Ac derivative when compared with Neu5Acalpha2Me would not be sustained. Neu5Acalpha2Me, Neu5Ac-alpha(2,6)-S-Galbeta1Me, Neu5,9Ac(2)alpha2Me, and Neu5Ac2en inhibited rotavirus infection of MA104 cells by 61%, 35%, 30%, and 0%, respectively, at 10 mM concentration. NMR spectroscopic, molecular modeling, and infectivity inhibition results are in excellent agreement and provide valuable information for the design of inhibitors of rotavirus infection.
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Affiliation(s)
- Thomas Haselhorst
- Institute for Glycomics, Griffith University, Gold Coast Campus, PMB 50 Gold Coast Mail Centre, Queensland 9726, Australia
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26
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Trask SD, Dormitzer PR. Assembly of highly infectious rotavirus particles recoated with recombinant outer capsid proteins. J Virol 2006; 80:11293-304. [PMID: 16971442 PMCID: PMC1642144 DOI: 10.1128/jvi.01346-06] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Assembly of the rotavirus outer capsid is the final step of a complex pathway. In vivo, the later steps include a maturational membrane penetration that is dependent on the scaffolding activity of a viral nonstructural protein. In vitro, simply adding the recombinant outer capsid proteins VP4 and VP7 to authentic double-layered rotavirus subviral particles (DLPs) in the presence of calcium and acidic pH increases infectivity by a factor of up to 10(7), yielding particles as infectious as authentic purified virions. VP4 must be added before VP7 for high-level infectivity. Steep dependence of infectious recoating on VP4 concentration suggests that VP4-VP4 interactions, probably oligomerization, precede VP4 binding to particles. Trypsin sensitivity analysis identifies two populations of VP4 associated with recoated particles: properly mounted VP4 that can be specifically primed by trypsin, and nonspecifically associated VP4 that is degraded by trypsin. A full complement of properly assembled VP4 is not required for efficient infectivity. Minimal dependence of recoating on VP7 concentration suggests that VP7 binds DLPs with high affinity. The parameters for efficient recoating and the characterization of recoated particles suggest a model in which, after a relatively weak interaction between oligomeric VP4 and DLPs, VP7 binds the particles and locks VP4 in place. Recoating will allow the use of infectious modified rotavirus particles to explore rotavirus assembly and cell entry and could lead to practical applications in novel immunization strategies.
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Affiliation(s)
- Shane D Trask
- Children's Hospital, Enders 673, 320 Longwood Ave., Boston, MA 02115, USA
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27
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Yoder JD, Dormitzer PR. Alternative intermolecular contacts underlie the rotavirus VP5* two- to three-fold rearrangement. EMBO J 2006; 25:1559-68. [PMID: 16511559 PMCID: PMC1440311 DOI: 10.1038/sj.emboj.7601034] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2005] [Accepted: 02/14/2006] [Indexed: 11/08/2022] Open
Abstract
The spike protein VP4 is a key component of the membrane penetration apparatus of rotavirus, a nonenveloped virus that causes childhood gastroenteritis. Trypsin cleavage of VP4 produces a fragment, VP5*, with a potential membrane interaction region, and primes rotavirus for cell entry. During entry, the part of VP5* that protrudes from the virus folds back on itself and reorganizes from a local dimer to a trimer. Here, we report that a globular domain of VP5*, the VP5* antigen domain, is an autonomously folding unit that alternatively forms well-ordered dimers and trimers. Because the domain contains heterotypic neutralizing epitopes and is soluble when expressed directly, it is a promising potential subunit vaccine component. X-ray crystal structures show that the dimer resembles the spike body on trypsin-primed virions, and the trimer resembles the folded-back form of the spike. The same structural elements pack differently to form key intermolecular contacts in both oligomers. The intrinsic molecular property of alternatively forming dimers and trimers facilitates the VP5* reorganization, which is thought to mediate membrane penetration during cell entry.
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Affiliation(s)
- Joshua D Yoder
- Program in Virology, Laboratory of Molecular Medicine, Harvard Medical School, Children's Hospital, Boston, MA, USA
| | - Philip R Dormitzer
- Program in Virology, Laboratory of Molecular Medicine, Harvard Medical School, Children's Hospital, Boston, MA, USA
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28
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Monnier N, Higo-Moriguchi K, Sun ZYJ, Prasad BVV, Taniguchi K, Dormitzer PR. High-resolution molecular and antigen structure of the VP8* core of a sialic acid-independent human rotavirus strain. J Virol 2006; 80:1513-23. [PMID: 16415027 PMCID: PMC1346936 DOI: 10.1128/jvi.80.3.1513-1523.2006] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
The most intensively studied rotavirus strains initially attach to cells when the "heads" of their protruding spikes bind cell surface sialic acid. Rotavirus strains that cause disease in humans do not bind this ligand. The structure of the sialic acid binding head (the VP8* core) from the simian rotavirus strain RRV has been reported, and neutralization epitopes have been mapped onto its surface. We report here a 1.6-A resolution crystal structure of the equivalent domain from the sialic acid-independent rotavirus strain DS-1, which causes gastroenteritis in humans. Although the RRV and DS-1 VP8* cores differ functionally, they share the same galectin-like fold. Differences between the RRV and DS-1 VP8* cores in the region that corresponds to the RRV sialic acid binding site make it unlikely that DS-1 VP8* binds an alternative carbohydrate ligand in this location. In the crystals, a surface cleft on each DS-1 VP8* core binds N-terminal residues from a neighboring molecule. This cleft may function as a ligand binding site during rotavirus replication. We also report an escape mutant analysis, which allows the mapping of heterotypic neutralizing epitopes recognized by human monoclonal antibodies onto the surface of the VP8* core. The distribution of escape mutations on the DS-1 VP8* core indicates that neutralizing antibodies that recognize VP8* of human rotavirus strains may bind a conformation of the spike that differs from those observed to date.
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29
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Liakatos A, Kiefel MJ, Fleming F, Coulson B, von Itzstein M. The synthesis and biological evaluation of lactose-based sialylmimetics as inhibitors of rotaviral infection. Bioorg Med Chem 2006; 14:739-57. [PMID: 16214356 DOI: 10.1016/j.bmc.2005.08.057] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2005] [Revised: 08/29/2005] [Accepted: 08/29/2005] [Indexed: 01/09/2023]
Abstract
Rotaviruses are the most significant cause of gastroenteritis in young children and are responsible for over 600,000 infant deaths annually. The rotaviral haemagglutinin protein (VP8*) of some strains has been implicated in early recognition and binding events of host cell-surface sialoglycoconjugates, and is therefore an attractive target for potential therapeutic intervention. Since N-acetylneuraminic acid alpha(2,3)-linked to galactose is believed to be the minimum binding epitope of rotavirus to host cells, we report here our development of an efficient and flexible synthetic route to a range of lactose-based sialylmimetics of alpha(2,3)-linked thiosialosides. These compounds were biologically evaluated as inhibitors of rotaviral infection using an in vitro neutralisation assay. The results suggest that these lactose-based sialylmimetics are not inhibitors of the rhesus rotavirus strain; however, they do exhibit modest inhibition of the human (Wa) strain, presumably through inhibition of the rotaviral adhesion process.
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Affiliation(s)
- Angela Liakatos
- Institute for Glycomics, Griffith University (Gold Coast Campus), PMB 50 Gold Coast Mail Centre, Qld 9726, Australia
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30
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Molecular Virology of Enteric Viruses (with Emphasis on Caliciviruses). VIRUSES IN FOODS 2006:43-100. [PMCID: PMC7120911 DOI: 10.1007/0-387-29251-9_3] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
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31
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Patton JT, Vasquez-Del Carpio R, Tortorici MA, Taraporewala ZF. Coupling of Rotavirus Genome Replication and Capsid Assembly. Adv Virus Res 2006; 69:167-201. [PMID: 17222694 DOI: 10.1016/s0065-3527(06)69004-0] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
The Reoviridae family represents a diverse collection of viruses with segmented double-stranded (ds)RNA genomes, including some that are significant causes of disease in humans, livestock, and plants. The genome segments of these viruses are never detected free in the infected cell but are transcribed and replicated within viral cores by RNA-dependent RNA polymerase (RdRP). Insight into the replication mechanism has been provided from studies on Rotavirus, a member of the Reoviridae whose RdRP can specifically recognize viral plus (+) strand RNAs and catalyze their replication to dsRNAs in vitro. These analyses have revealed that although the rotavirus RdRP can interact with recognition signals in (+) strand RNAs in the absence of other proteins, the conversion of this complex to one that can support initiation of dsRNA synthesis requires the presence and partial assembly of the core capsid protein. By this mechanism, the viral polymerase can carry out dsRNA synthesis only when capsid protein is available to package its newly made product. By preventing the accumulation of naked dsRNA within the cell, the virus avoids triggering dsRNA-dependent interferon signaling pathways that can induce expression and activation of antiviral host proteins.
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Affiliation(s)
- John T Patton
- Laboratory of Infectious Diseases, National Institutes of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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32
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Scott SA, Holloway G, Coulson BS, Szyczew AJ, Kiefel MJ, von Itzstein M, Blanchard H. Crystallization and preliminary X-ray diffraction analysis of the sialic acid-binding domain (VP8*) of porcine rotavirus strain CRW-8. Acta Crystallogr Sect F Struct Biol Cryst Commun 2005; 61:617-20. [PMID: 16511112 PMCID: PMC1952324 DOI: 10.1107/s1744309105013849] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2005] [Accepted: 05/03/2005] [Indexed: 11/10/2022]
Abstract
Rotavirus recognition and attachment to host cells involves interaction with the spike protein VP4 that projects outwards from the surface of the virus particle. An integral component of these spikes is the VP8* domain, which is implicated in the direct recognition and binding of sialic acid-containing cell-surface carbohydrates and facilitates subsequent invasion by the virus. The expression, purification, crystallization and preliminary X-ray diffraction analysis of VP8* from porcine CRW-8 rotavirus is reported. Diffraction data have been collected to 2.3 A resolution, enabling the determination of the VP8* structure by molecular replacement.
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Affiliation(s)
- Stacy A. Scott
- Institute for Glycomics, Griffith University (Gold Coast Campus) PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
| | - Gavan Holloway
- Department of Microbiology and Immunology, The University of Melbourne, Victoria 3010, Australia
| | - Barbara S. Coulson
- Department of Microbiology and Immunology, The University of Melbourne, Victoria 3010, Australia
| | - Alex J. Szyczew
- Institute for Glycomics, Griffith University (Gold Coast Campus) PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
| | - Milton J. Kiefel
- Institute for Glycomics, Griffith University (Gold Coast Campus) PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
| | - Mark von Itzstein
- Institute for Glycomics, Griffith University (Gold Coast Campus) PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
| | - Helen Blanchard
- Institute for Glycomics, Griffith University (Gold Coast Campus) PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
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33
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Dormitzer PR, Nason EB, Prasad BVV, Harrison SC. Structural rearrangements in the membrane penetration protein of a non-enveloped virus. Nature 2004; 430:1053-8. [PMID: 15329727 PMCID: PMC1780043 DOI: 10.1038/nature02836] [Citation(s) in RCA: 166] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2004] [Accepted: 07/14/2004] [Indexed: 01/14/2023]
Abstract
Non-enveloped virus particles (those that lack a lipid-bilayer membrane) must breach the membrane of a target host cell to gain access to its cytoplasm. So far, the molecular mechanism of this membrane penetration step has resisted structural analysis. The spike protein VP4 is a principal component in the entry apparatus of rotavirus, a non-enveloped virus that causes gastroenteritis and kills 440,000 children each year. Trypsin cleavage of VP4 primes the virus for entry by triggering a rearrangement that rigidifies the VP4 spikes. We have determined the crystal structure, at 3.2 A resolution, of the main part of VP4 that projects from the virion. The crystal structure reveals a coiled-coil stabilized trimer. Comparison of this structure with the two-fold clustered VP4 spikes in a approximately 12 A resolution image reconstruction from electron cryomicroscopy of trypsin-primed virions shows that VP4 also undergoes a second rearrangement, in which the oligomer reorganizes and each subunit folds back on itself, translocating a potential membrane-interaction peptide from one end of the spike to the other. This rearrangement resembles the conformational transitions of membrane fusion proteins of enveloped viruses.
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Affiliation(s)
- Philip R Dormitzer
- Department of Pediatrics, Harvard Medical School, and the Laboratory of Molecular Medicine, Children's Hospital, Boston, Massachusetts 02115, USA.
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34
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Jayaram H, Estes MK, Prasad BVV. Emerging themes in rotavirus cell entry, genome organization, transcription and replication. Virus Res 2004; 101:67-81. [PMID: 15010218 DOI: 10.1016/j.virusres.2003.12.007] [Citation(s) in RCA: 81] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Rotaviruses, causative agents of gastroenteritis in young animals and humans, are large icosahedral viruses with a complex architecture. The double-stranded RNA (dsRNA) genome composed of 11 segments, which codes for 6 structural and 6 non-structural proteins, is enclosed within three concentric capsid layers. In addition to facilitating host-specific interactions, the design of the capsid architecture in rotaviruses as in other dsRNA viruses should also be conducive to the requirement of transcribing the enclosed genome segments repeatedly and simultaneously within the capsid interior. Several non-structural proteins facilitate the subsequent processes of genome replication and packaging. Electron cryomicroscopy studies of intact virions, recombinant virus-like particles, functional complexes, together with recent X-ray crystallographic studies on rotavirus proteins have provided structural insights into the capsid architecture, genome organization, antibody interaction, cell entry, trypsin-enhanced infectivity, endogenous transcription and replication. These studies underscore contrasting features and unifying themes between rotavirus and other dsRNA viruses.
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Affiliation(s)
- Hariharan Jayaram
- Program in Structural and Computational Biology and Molecular Biophysics, Houston, TX 77030, USA
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35
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Golantsova NE, Gorbunova EE, Mackow ER. Discrete domains within the rotavirus VP5* direct peripheral membrane association and membrane permeability. J Virol 2004; 78:2037-44. [PMID: 14747568 PMCID: PMC369428 DOI: 10.1128/jvi.78.4.2037-2044.2004] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Cleavage of the rotavirus spike protein, VP4, is required for rotavirus-induced membrane permeability and viral entry into cells. The VP5* cleavage product selectively permeabilizes membranes and liposomes and contains an internal hydrophobic domain that is required for membrane permeability. Here we investigate VP5* domains (residues 248 to 474) that direct membrane binding. We determined that expressed VP5 fragments containing residues 248 to 474 or 265 to 474, including the internal hydrophobic domain, bind to cellular membranes but are not present in Triton X-100-resistant membrane rafts. Expressed VP5 partitions into aqueous but not detergent phases of Triton X-114, suggesting that VP5 is not integrally inserted into membranes. Since high-salt or alkaline conditions eluted VP5 from membranes, our findings demonstrate that VP5 is peripherally associated with membranes. Interestingly, mutagenesis of residue 394 (W-->R) within the VP5 hydrophobic domain, which abolishes VP5-directed permeability, had no effect on VP5's peripheral membrane association. In contrast, deletion of N-terminal VP5 residues (residues 265 to 279) abolished VP5 binding to membranes. Alanine mutagenesis of two positively charged residues within this domain (residues 274R and 276K) dramatically reduced (>95%) binding of VP5 to membranes and suggested their potential interaction with polar head groups of the lipid bilayer. Mutations in either the VP5 hydrophobic or basic domain blocked VP5-directed permeability of cells. These findings indicate that there are at least two discrete domains within VP5* required for pore formation: an N-terminal basic domain that permits VP5* to peripherally associate with membranes and an internal hydrophobic domain that is essential for altering membrane permeability. These results provide a fundamental understanding of interactions between VP5* and the membrane, which are required for rotavirus entry.
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Affiliation(s)
- Nina E Golantsova
- Department of Medicine, Stony Brook University, Stony Brook, New York 11794, USA
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Kovacs-Nolan J, Yoo D, Mine Y. Fine mapping of sequential neutralization epitopes on the subunit protein VP8 of human rotavirus. Biochem J 2003; 376:269-75. [PMID: 12901721 PMCID: PMC1223744 DOI: 10.1042/bj20021969] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2002] [Revised: 08/05/2003] [Accepted: 08/05/2003] [Indexed: 11/17/2022]
Abstract
The epitopes of the HRV (human rotavirus), especially those involved in virus neutralization, have not been determined in their entirety, and would have significant implications for HRV vaccine development. In the present study, we report on the epitope mapping and identification of sequential neutralization epitopes, on the Wa strain HRV subunit protein VP8, using synthetic overlapping peptides. Polyclonal antibodies against recombinant Wa VP8 were produced previously in chicken, and purified from egg yolk, which showed neutralizing activity against HRV in vitro. Overlapping VP8 peptide fragments were synthesized and probed with the anti-VP8 antibodies, revealing five sequential epitopes on VP8. Further analysis suggested that three of the five epitopes detected, M1-L10, I55-D66 and L223-P234, were involved in virus neutralization, indicating that sequential epitopes may also be important for the HRV neutralization. The interactions of the antibodies with the five epitopes were characterized by an examination of the critical amino acids involved in antibody binding. Epitopes comprised primarily of hydrophobic amino acid residues, followed by polar and charged residues. The more critical amino acids appeared to be located near the centre of the epitopes, with proline, isoleucine, serine, glutamine and arginine playing an important role in the binding of antibody to the VP8 epitopes.
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Enouf V, Chwetzoff S, Trugnan G, Cohen J. Interactions of rotavirus VP4 spike protein with the endosomal protein Rab5 and the prenylated Rab acceptor PRA1. J Virol 2003; 77:7041-7. [PMID: 12768023 PMCID: PMC156175 DOI: 10.1128/jvi.77.12.7041-7047.2003] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Rotavirus spike protein VP4 is implicated in several important functions, such as cell attachment, penetration, hemagglutination, neutralization, virulence, and host range. It is present at the plasma membrane and colocalizes with the cytoskeleton in infected cells. We looked for cellular partners responsible for the localization of VP4 by two-hybrid screening of a monkey CV1 cell cDNA library. In the screen we isolated repeatedly three cDNAs encoding either two isoforms (a and c) of Rab5 protein or the prenylated Rab acceptor (PRA1). The small GTPase Rab5 is a molecule regulating the vesicular traffic and the motility of early endosomes along microtubules. Rab5 interacts with a large number of effectors, in particular with PRA1. Interactions of VP4 with both partners, Rab5 and PRA1, were confirmed by coimmunoprecipitation from infected- or transfected-cell lysates. Interaction of Rab5 and PRA1 was restricted to free VP4, since neither triple-layered particles nor NSP4-VP4-VP7 heterotrimeric complexes could be coprecipitated. Site-directed and deletion mutants of VP4 were used to map a VP4 domain(s) interacting with Rab5 or PRA1. Of the 10 mutants tested, 2 interacted exclusively with a single partner. In contrast, the domain extending from amino acids 560 to 722 of VP4 is essential for both interactions. These results suggest that Rab5 and PRA1 may be involved in the localization and trafficking of VP4 in infected cells.
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Affiliation(s)
- Vincent Enouf
- Virologie Moléculaire et Structurale, UMR CNRS-INRA 2472, F-91190 Gif-sur-Yvette, France
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Affiliation(s)
- Milton J Kiefel
- Centre for Biomolecular Science and Drug Discovery, Griffith University (Gold Coast Campus), PMB 50, Gold Coast Mail Centre, Queensland 9726, Australia
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Dormitzer PR, Sun ZYJ, Blixt O, Paulson JC, Wagner G, Harrison SC. Specificity and affinity of sialic acid binding by the rhesus rotavirus VP8* core. J Virol 2002; 76:10512-7. [PMID: 12239329 PMCID: PMC136543 DOI: 10.1128/jvi.76.20.10512-10517.2002] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2002] [Accepted: 07/02/2002] [Indexed: 11/20/2022] Open
Abstract
Nuclear magnetic resonance spectroscopy demonstrates that the rhesus rotavirus hemagglutinin specifically binds alpha-anomeric N-acetylneuraminic acid with a K(d) of 1.2 mM. The hemagglutinin requires no additional carbohydrate moieties for binding, does not distinguish 3' from 6' sialyllactose, and has approximately tenfold lower affinity for N-glycolylneuraminic than for N-acetylneuraminic acid. The broad specificity and low affinity of sialic acid binding by the rotavirus hemagglutinin are consistent with this interaction mediating initial cell attachment prior to the interactions that determine host range and cell type specificity.
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Affiliation(s)
- Philip R Dormitzer
- Laboratory of Molecular Medicine, Enders 673, Children's Hospital, Harvard Medical School, 320 Longwood Avenue, Boston, MA 02115, USA.
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Sapin C, Colard O, Delmas O, Tessier C, Breton M, Enouf V, Chwetzoff S, Ouanich J, Cohen J, Wolf C, Trugnan G. Rafts promote assembly and atypical targeting of a nonenveloped virus, rotavirus, in Caco-2 cells. J Virol 2002; 76:4591-602. [PMID: 11932424 PMCID: PMC155075 DOI: 10.1128/jvi.76.9.4591-4602.2002] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Rotavirus follows an atypical pathway to the apical membrane of intestinal cells that bypasses the Golgi. The involvement of rafts in this process was explored here. VP4 is the most peripheral protein of the triple-layered structure of this nonenveloped virus. High proportions of VP4 associated with rafts within the cell as early as 3 h postinfection. In the meantime a significant part of VP4 was targeted to the Triton X-100-resistant microdomains of the apical membrane, suggesting that this protein possesses an autonomous signal for its targeting. At a later stage the other structural rotavirus proteins were also found in rafts within the cells together with NSP4, a nonstructural protein required for the final stage of virus assembly. Rafts purified from infected cells were shown to contain infectious particles. Finally purified VP4 and mature virus were shown to interact with cholesterol- and sphingolipid-enriched model lipid membranes that changed their phase preference from inverted hexagonal to lamellar structures. Together these results indicate that a direct interaction of VP4 with rafts promotes assembly and atypical targeting of rotavirus in intestinal cells.
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Affiliation(s)
- Catherine Sapin
- INSERM U 538, CHU Saint Antoine, Université Pierre et Marie Curie, 75012 Paris, France
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Ciarlet M, Ludert JE, Iturriza-Gómara M, Liprandi F, Gray JJ, Desselberger U, Estes MK. Initial interaction of rotavirus strains with N-acetylneuraminic (sialic) acid residues on the cell surface correlates with VP4 genotype, not species of origin. J Virol 2002; 76:4087-95. [PMID: 11907248 PMCID: PMC136071 DOI: 10.1128/jvi.76.8.4087-4095.2002] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
We examined 41 human and animal rotavirus strains representative of all known P genotypes for their dependency on cellular N-acetylneuraminic (sialic) acid (SA) residues for infectivity. Our results showed that all rotaviruses studied, whether of animal or human origin, belonging to P genotypes [1], [2], [3], and [7] depended on SA residues on the cell surface for efficient infectivity but that all human and animal rotavirus strains representative of the remaining known P genotypes were SA independent. The SA residue requirement for efficient infectivity did not change for reassortant rotavirus strains with altered VP4-VP7 combinations. The initial interaction of rotavirus strains with SA residues on the cell surface correlated with VP4 genotype specificity, not with species of origin or VP7 G serotype specificity (P = 0.001; r2 = 1.00, Pearson's correlation coefficient). In addition to being a requirement for infectivity, the presence of SA residues on the cell surface is a requirement for efficient growth in cell culture; recognition of the association of specific P genotypes with the binding of rotavirus to SA residues will facilitate our understanding of the molecular basis of the early events of rotavirus-cell interactions in cell culture models and of pathogenicity in vivo.
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Affiliation(s)
- Max Ciarlet
- Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, USA.
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Dormitzer PR, Sun ZYJ, Wagner G, Harrison SC. The rhesus rotavirus VP4 sialic acid binding domain has a galectin fold with a novel carbohydrate binding site. EMBO J 2002; 21:885-97. [PMID: 11867517 PMCID: PMC125907 DOI: 10.1093/emboj/21.5.885] [Citation(s) in RCA: 271] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Cell attachment and membrane penetration are functions of the rotavirus outer capsid spike protein, VP4. An activating tryptic cleavage of VP4 produces the N-terminal fragment, VP8*, which is the viral hemagglutinin and an important target of neutralizing antibodies. We have determined, by X-ray crystallography, the atomic structure of the VP8* core bound to sialic acid and, by NMR spectroscopy, the structure of the unliganded VP8* core. The domain has the beta-sandwich fold of the galectins, a family of sugar binding proteins. The surface corresponding to the galectin carbohydrate binding site is blocked, and rotavirus VP8* instead binds sialic acid in a shallow groove between its two beta-sheets. There appears to be a small induced fit on binding. The residues that contact sialic acid are conserved in sialic acid-dependent rotavirus strains. Neutralization escape mutations are widely distributed over the VP8* surface and cluster in four epitopes. From the fit of the VP8* core into the virion spikes, we propose that VP4 arose from the insertion of a host carbohydrate binding domain into a viral membrane interaction protein.
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Affiliation(s)
- Philip R. Dormitzer
- Laboratory of Molecular Medicine, Enders 673, Children’s Hospital, 320 Longwood Avenue, Boston, MA 02115, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115 and Howard Hughes Medical Institute and the Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138, USA Corresponding author e-mail:
| | - Zhen-Yu J. Sun
- Laboratory of Molecular Medicine, Enders 673, Children’s Hospital, 320 Longwood Avenue, Boston, MA 02115, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115 and Howard Hughes Medical Institute and the Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138, USA Corresponding author e-mail:
| | - Gerhard Wagner
- Laboratory of Molecular Medicine, Enders 673, Children’s Hospital, 320 Longwood Avenue, Boston, MA 02115, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115 and Howard Hughes Medical Institute and the Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138, USA Corresponding author e-mail:
| | - Stephen C. Harrison
- Laboratory of Molecular Medicine, Enders 673, Children’s Hospital, 320 Longwood Avenue, Boston, MA 02115, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115 and Howard Hughes Medical Institute and the Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138, USA Corresponding author e-mail:
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