title: Heterodimers as the Structural Unit of the T=1 Capsid of the Fungal Double-Stranded RNA Rosellinia necatrix Quadrivirus 1.
abstract: Most double-stranded RNA (dsRNA) viruses are transcribed and replicated in a specialized icosahedral capsid with a T=1 lattice consisting of 60 asymmetric capsid protein (CP) dimers. These capsids help to organize the viral genome and replicative complex(es). They also act as molecular sieves that isolate the virus genome from host defense mechanisms and allow the passage of nucleotides and viral transcripts. Rosellinia necatrix quadrivirus 1 (RnQV1), the type species of the family Quadriviridae, is a dsRNA fungal virus with a multipartite genome consisting of four monocistronic segments (segments 1 to 4). dsRNA-2 and dsRNA-4 encode two CPs (P2 and P4, respectively), which coassemble into ∼450-Å-diameter capsids. We used three-dimensional cryo-electron microscopy combined with complementary biophysical techniques to determine the structures of RnQV1 virion strains W1075 and W1118. RnQV1 has a quadripartite genome, and the capsid is based on a single-shelled T=1 lattice built of P2-P4 dimers. Whereas the RnQV1-W1118 capsid is built of full-length CP, P2 and P4 of RnQV1-W1075 are cleaved into several polypeptides, maintaining the capsid structural organization. RnQV1 heterodimers have a quaternary organization similar to that of homodimers of reoviruses and other dsRNA mycoviruses. The RnQV1 capsid is the first T=1 capsid with a heterodimer as an asymmetric unit reported to date and follows the architectural principle for dsRNA viruses that a 120-subunit capsid is a conserved assembly that supports dsRNA replication and organization.
  
  
  OBJECTIVE
  Given their importance to health, members of the family Reoviridae are the basis of most structural and functional studies and provide much of our knowledge of dsRNA viruses. Analysis of bacterial, protozoal, and fungal dsRNA viruses has improved our understanding of their structure, function, and evolution, as well. Here, we studied a dsRNA virus that infects the fungus Rosellinia necatrix, an ascomycete that is pathogenic to a wide range of plants. Using three-dimensional cryo-electron microscopy and analytical ultracentrifugation analysis, we determined the structure and stoichiometry of Rosellinia necatrix quadrivirus 1 (RnQV1). The RnQV1 capsid is a T=1 capsid with 60 heterodimers as the asymmetric units. The large amount of genetic information used by RnQV1 to construct a simple T=1 capsid is probably related to the numerous virus-host and virus-virus interactions that it must face in its life cycle, which lacks an extracellular phase.
authors.affiliation: Department of Structure of Macromolecules, Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain.|Centro Nacional de Microbiología/ISCIII, Carretera de Majadahonda-Pozuelo, Majadahonda, Madrid, Spain.
authors.forename: Daniel
authors.initials: D
authors.lastname: Luque
authors.affiliation: Department of Structure of Macromolecules, Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain.
authors.forename: Carlos P
authors.initials: CP
authors.lastname: Mata
authors.affiliation: Centro Nacional de Microbiología/ISCIII, Carretera de Majadahonda-Pozuelo, Majadahonda, Madrid, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: González-Camacho
authors.affiliation: Department of Structure of Macromolecules, Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain.
authors.forename: José M
authors.initials: JM
authors.lastname: González
authors.affiliation: Department of Structure of Macromolecules, Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain.
authors.forename: Josué
authors.initials: J
authors.lastname: Gómez-Blanco
authors.affiliation: Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Alfonso
authors.affiliation: Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: Germán
authors.initials: G
authors.lastname: Rivas
authors.affiliation: Department of Plant Pathology, University of Kentucky, Lexington, Kentucky, USA.
authors.forename: Wendy M
authors.initials: WM
authors.lastname: Havens
authors.affiliation: Institute of Fruit Tree Science, National Agriculture and Food Research Organization (NARO), Morioka, Iwate, Japan.
authors.forename: Satoko
authors.initials: S
authors.lastname: Kanematsu
authors.affiliation: Institute of Plant Science and Resources, Okayama, Japan.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Suzuki
authors.affiliation: Department of Plant Pathology, University of Kentucky, Lexington, Kentucky, USA.
authors.forename: Said A
authors.initials: SA
authors.lastname: Ghabrial
authors.affiliation: Imaging Sciences Laboratory, CIT, NIH, Bethesda, Maryland, USA.
authors.forename: Benes L
authors.initials: BL
authors.lastname: Trus
authors.affiliation: Department of Structure of Macromolecules, Centro Nacional de Biotecnología (CNB-CSIC), Cantoblanco, Madrid, Spain jrcaston@cnb.csic.es.
authors.forename: José R
authors.initials: JR
authors.lastname: Castón
mesh_terms: D000595:Amino Acid Sequence; D002213:Capsid / Q000737:chemistry / Q000648:ultrastructure*; D036022:Capsid Proteins / Q000737:chemistry* / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D016679:Genome, Viral; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D012328:RNA Viruses / Q000737:chemistry / Q000648:ultrastructure*; D012367:RNA, Viral / Q000378:metabolism / Q000648:ultrastructure*; D014771:Virion / Q000737:chemistry / Q000648:ultrastructure*; D014779:Virus Replication
chemical_list: D036022:Capsid Proteins; D012367:RNA, Viral