Structural Evolution of Reoviridae Revealed by Oryzavirus in Acquiring the Second Capsid Shell

Abstract

The conservation of the core structure and diversification of the external features among the turreted reoviruses appear to be relevant to structural evolution in facilitating the infection of diverse host species. The structure of Rice ragged stunt virus (RRSV), in the genus Oryzavirus of the family Reoviridae, is determined to show a core composed of capsid shell, clamps, and long turrets. The RRSV core structure is equivalent to the core structure of Orthoreovirus and the virion structure of Cytoplasmic polyhedrosis virus (CPV). In RRSV, five peripheral trimers surround each long turret and sit at the Q trimer position in the T‫31؍‬l icosahedral symmetry, a structural feature unique to turreted reoviruses. That is, the core of RRSV is partially covered by 60 copies of the peripheral trimer. In contrast, the core of Orthoreovirus is covered by 200 copies of the trimer that sit at the Q, R, S, and T trimer positions. Our results suggest that among the three viruses, RRSV has a structure intermediate between that of Orthoreovirus and the CPV virion. This conclusion coincides with the results of the phylogenetic analysis of amino acid sequences of RNA-dependent RNA polymerases.Reoviridae is the largest and most diverse family of doublestranded RNA (dsRNA) viruses. It includes 12 established genera, namely Aquareovirus, Coltivirus, Cypovirus, Fijivirus, Idnoreovirus, Mycoreovirus, Orbivirus, Orthoreovirus, Oryzavirus, Phytoreovirus, Rotavirus, and Seadornavirus. The hosts of these viruses include plants, vertebrates, insects, and fungi (22). All known viruses in this family are 600 to 800 Å in diameter and consist of an inner core that is surrounded by a few layers of protein, with the exception of the single-layered Cytoplasmic polyhedrosis virus (CPV), which encapsidates 9 to 12 segments of dsRNA and the enzymes involved in transcription. Whereas the precise morphology varies among genera, the morphologies of the innermost capsid shells are similar in spite of the absence of significant sequence homology among component proteins. The conserved innermost capsid of reoviruses is composed of 120 copies of thin crescent-shaped proteins, and the respective subunits exhibit similar overall folding, with substantial modifications that appear to have developed during viral evolution (11,24,28,37,42). Except in CPV, the innermost capsid shell is covered by additional outer layers, the organization and structure of which vary among the genera in the family. The outer capsid shell appears to play important roles in maintaining the stability of the thin innermost capsid shell and sequestering the dsRNA genome, as well as in conferring host specificity and mediating entry into host cells.Reoviruses can be divided structurally into two subgroups, the turreted and nonturreted reoviruses, on the basis of a critical structural feature. Members of the seven genera Aquareovirus, Cypovirus, Fijivirus, Idnoreovirus, Mycoreovirus, Orthoreovirus, and Oryzavirus are classified as turreted reoviruses (22), with distinctive pentameric tu...

Development of an Insect Vector Cell Culture and RNA Interference System To Investigate the Functional Role of Fijivirus Replication Protein

“…SRBSDV was crudely purified from infected rice plants as previously described by Miyazaki et al (22). The proteins and solution of purified viruses were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), transferred onto a polyvinylidene difluoride (PVDF) membrane, and detected on immunoblots using prepared antibodies as described previously (27).…”

Atomic model of a cypovirus built from cryo-EM structure provides insight into the mechanism of mRNA capping

“…For this subgroup of reoviruses, a pentameric turret formed by five copies of turret proteins on fivefold vertex of the innermost shell functions in the catalysis of mRNA 5′ cap synthesis (2)(3)(4)(5). Cryo-EM or crystal structures are available for four turreted virus genera of the family Reoviridae: the Cypovirus (6), Orthoreovirus (2), Oryzavirus (7), and Aquareovirus (8,9) genera. These viruses have a segmented genome consisting of 10-12 linear segments of dsRNA and their hosts include vertebrates (orthoreoviruses and aquareovriuses), invertebrates (cypoviruses), and plants (oryzaviruses).…”

A Newly Isolated Reovirus Has the Simplest Genomic and Structural Organization of Any Reovirus

“…Orthoreoviruses contain a third clamp binding site near the 2-fold symmetry axis. These interactions are similar but not identical, as the clamp can also interact with the turret or with dissimilarly oriented domains (3,4,11,14). No protein density was observed that would sterically hinder the clamp from binding at its usual site.…”

“…FAKV was determined to be a single-shelled reovirus with gross morphology similar to that of Bombyx mori CPV or to rice ragged stunt virus (RRSV) (see Fig. S3 in the supplemental material) (3,34,35). Each of the 60 asymmetric units in FAKV can be segmented into 4 density regions using Chimera and Gorgon, which are annotated in different colors (Fig.…”

“…Spinareovirinae viruses, such as the recently emerged human pathogen Melaka virus and the mammalian orthoreovirus type 3, are called "turreted" reoviruses, because they encode spikes (protrusions) on the surface of the virion. The subfamily Sedoreovirinae contains the "nonturreted" viruses with multiple smooth shells, which include human and other animal pathogens, such as rotaviruses and orbiviruses (3,4). In addition to this structural classification, amino acid identities of Ͼ30% in the polymerase sequence and the pres-ence and sequence of conserved terminal nucleotide (nt) motifs have been used as criteria for the classification of genera within this family (2,(5)(6)(7)(8)(9).…”