Megabirnavirus structure reveals a putative 120-subunit capsid formed by asymmetrical dimers with distinctive large protrusions

Abstract

Rosellinia necatrix megabirnavirus 1 (RnMBV1) W779 is a bi-segmented dsRNA virus and a strain of the type species Rosellinia necatrix megabirnavirus 1 of the family Megabirnaviridae. RnMBV1 causes severe reduction of both mycelial growth of Rosellinia necatrix in synthetic medium and fungal virulence to plant hosts, and thus has strong potential for virocontrol (biological control using viruses) of white rot. The structure of RnMBV1 was examined by cryo-electron microscopy and three-dimensional reconstruction at 15.7 Å resolution. The diameter of the RnMBV1 capsid was 520 Å , and the capsid was composed of 60 asymmetrical dimers in the T51 (so-called T52) lattice that is well conserved among dsRNA viruses. However, RnMBV1 has putatively 120 large protrusions with a width of ,45 Å and a height of ,50 Å on the virus surface, making it distinguishable from the other dsRNA viruses.

Novel, diverse RNA viruses from Mediterranean isolates of the phytopathogenic fungus, Rosellinia necatrix: insights into evolutionary biology of fungal viruses

“…5A). However, unlike megabirnaviruses, which are able to form icosahedral particles (Miyazaki et al, 2015), rigid virions were not reported for any of the fusagraviruses, suggesting their capsidless nature.…”

The infectious particle of insect-borne totivirus-like Omono River virus has raised ridges and lacks fibre complexes

“…Arthropod hosts such as insects are known to be important transmitters of dsRNA viruses 3 7 8 . Ten distinct families of the dsRNA viruses have been established so far: Birnaviridae , Botybirnaviridae , Chrysoviridae , Cystoviridae , Megabirnaviridae , Partitiviridae , Picobirnaviridae , Quadriviridae , Reoviridae , and Totiviridae 9 . Most dsRNA viruses share a similar capsid architecture with a T = 1 capsid lattice that comprises 120 chemically identical capsid proteins (CPs) 9 , except for Birnaviridae 10 and Chrysoviridae 11 .…”

“…Totiviruses, partitiviruses and chrysoviruses primarily infect prokaryotes and primitive eukaryotes, such as protozoa and fungi, while partiviruses and reoviruses infect a broad host range from prokaryotes to higher eukaryotes, including plants and vertebrates 9 . The fungal and protozoan dsRNA viruses propagate only intracellularly: vertically due to the frequent cell division of the primitive hosts 15 16 17 or upon cell fusion, e.g.…”

Capsid Structure of dsRNA Fungal Viruses

“…They have also been described for members of the family Cystoviridae —bacteriophages that infect the prokaryote Pseudomonas syringae [ 25 , 26 ]. Members of the Toti- [ 27 , 28 , 29 ], Partiti- [ 30 , 31 ], Megabirna- [ 32 ], Chryso- [ 33 , 34 , 35 ], and Quadriviridae [ 36 , 37 ] families, which infect unicellular and simple eukaryotes, such as fungi, protozoa, but also some plants, also have these capsids. Members of the family Birnaviridae, which infect vertebrates, mollusks, insects, and rotifers, are exceptions, since they lack the T = 1 core of 60 CP dimers [ 38 , 39 ].…”