Crystal Structure of an Aquabirnavirus Particle: Insights into Antigenic Diversity and Virulence Determinism

Abstract

Infectious pancreatic necrosis virus (IPNV), a pathogen of salmon and trout, imposes a severe toll on the aquaculture and sea farming industries. IPNV belongs to the Aquabirnavirus genus in the Birnaviridae family of bisegmented double-stranded RNA viruses. The virions are nonenveloped with a T‫31؍‬l icosahedral capsid made by the coat protein VP2, the three-dimensional (3D) organization of which is known in detail for the family prototype, the infectious bursal disease virus (IBDV) of poultry. A salient feature of the birnavirus architecture is the presence of 260 trimeric spikes formed by VP2, projecting radially from the capsid. The spikes carry the principal antigenic sites as well as virulence and cell adaptation determinants. We report here the 3.4-Å resolution crystal structure of a subviral particle (SVP) of IPNV, containing 20 VP2 trimers organized with icosahedral symmetry. We show that, as expected, the SVPs have a very similar organization to the IBDV counterparts, with VP2 exhibiting the same overall 3D fold. However, the spikes are significantly different, displaying a more compact organization with tighter packing about the molecular 3-fold axis. Amino acids controlling virulence and cell culture adaptation cluster differently at the top of the spike, i.e., in a central bowl in IBDV and at the periphery in IPNV. In contrast, the spike base features an exposed groove, conserved across birnavirus genera, which contains an integrin-binding motif. Thus, in addition to revealing the viral antigenic determinants, the structure suggests that birnaviruses interact with different receptors for attachment and for cell internalization during entry.Birnaviruses form a distinct family of double-stranded RNA (dsRNA) viruses infecting vertebrates and invertebrates (18). Aquatic birnaviruses are the most abundant and diverse and are grouped in two separate genera: the Aquabirnavirus genus and the Blosnavirus genus, with infectious pancreatic necrosis virus (IPNV) and blotched snakehead virus (BSNV) (16) as respective type species. A third aquatic birnavirus of unassigned genus, Tellina virus 1, was recently described and found to be phylogenetically distant from the two established genera (40). However, the vast majority of aquatic birnaviruses are antigenically related to IPNV (i.e., belong to the Aquabirnavirus genus), regardless of host species or geographical origin (4,11,26,39). They are implicated as etiological agents of disease in a variety of mollusks and fish species important in aquaculture, causing pathologies such as infectious pancreatic necrosis in salmonids, nephroblastoma and branchionephritis in eels, and gill necrosis in clams (21,34,50). Viruses in the Aquabirnavirus genus display considerable antigenic diversity and have substantial differences in biological properties such as host range and optimal replication temperature. These features contrast with the properties of other birnaviruses, in particular those infecting terrestrial species (avibirnaviruses and entomobirnaviruses). Based on rec...

Aquabirnavirus polyploidy: a new strategy to modulate virulence?

“…In the present study, the visualization of higher numbers of bands in CsCl gradients was clearly related to the original quantity of virus: the patterns of bands 5 and 6 were only obtained when high-titre virus stocks were used for purification. Among the six fractions, only F1 and F2 showed a protein composition clearly consistent with that reported for SVPs (Coulibaly et al, 2010), which were most represented in both fractions. The expected protein pattern of IPNV virions was obtained in the remaining four fractions, corresponding to complete T513 icosahedral particles.…”

Espirito Santo Virus: a New Birnavirus That Replicates in Insect Cells

“…Unfortunately, comparisons cannot be inferred from the structure of ESV because of the large difference in the resolution of its cryo-EM reconstruction compared to the crystal structures of VP2 proteins from IBDV and IPNV. This variance in capsid proteins, even though slight among some members, is probably responsible for host ranges, tissue tropisms, and differences in virulence among the birnaviruses (14,33). In our experiments with flaviviruses, preliminary observations demonstrated that ESV grows to a higher number of particles only during coinfection with DENV-2 44/2.…”

“…The structural variations observed among these viruses suggest that entomobirnaviruses such as ESV in C6/36 cells have a capsid structure with a similar overall architecture but with underlying differences in the trimer contacts (14). By using the structure of VP2 from IPNV, previous reports described a model of the Tϭ13 capsid of the IPNV virion (14,33) by superimposing the VP2 trimer onto the previously described structure of IBDV (15). This forms a model that assumes that the intertrimer interactions that form the IPNV are similar to those observed for the IBDV particle.…”

“…They include viruses with a bisegmented dsRNA genome encapsidated within single-shelled, unenveloped, icosahedral particles (46). These viruses are approximately 70 nm in diameter and exhibit 260 spikes in a proposed Tϭ13 organization (5,14). Members of the genera Aquabirnavirus and Avibirnavirus include infectious pancreatic necrosis virus (IPNV) and infectious bursal disease virus (IBDV), respectively, which are two wellcharacterized viruses of economic importance to the aquaculture and poultry industries (1,10).…”

Binding chicken Anx2 is beneficial for infection with infectious bursal disease virus

“…The authors of those studies also found that those proteins did not strongly influence IBDV infection Delgui et al, 2009). Recently, researchers suspected that birnaviruses, including IBDV, might need to interact with several cell receptors during entry; some for attachment located at the top of the spike of VP2 and others for internalization located at the conserved groove of VP2 (Coulibaly et al, 2010;Lai et al, 2014). We speculate that Anx2 may serves as one component of the binding proteins for initial attachment, followed by interacting with cHsp90, alpha 4 beta 1 integrin, and probably, other cellular polypeptides required for IBDV binding and successful entry into the cell.…”