Insights into the structure of RNPs from segmented negative-sense RNA viruses.

The genome of segmented negative-sense single-stranded RNA viruses, such as influenza virus and bunyaviruses, is coated by viral nucleoproteins (NPs), forming a ribonucleoprotein (RNP). In this issue of Structure, Dick et al.1 expand our knowledge on the RNPs of these viruses by solving the structures of Thogoto virus NP and RNP.

Structural characterization of Thogoto Virus nucleoprotein provides insights into viral RNA encapsidation and RNP assembly.

Orthomyxoviruses, such as influenza and thogotoviruses, are important human and animal pathogens. Their segmented viral RNA genomes are wrapped by viral nucleoproteins (NPs) into helical ribonucleoprotein complexes (RNPs). NP structures of several influenza viruses have been reported. However, there are still contradictory models of how orthomyxovirus RNPs are assembled. Here, we characterize the crystal structure of Thogoto virus (THOV) NP and found striking similarities to structures of influenza viral NPs, including a two-lobed domain architecture, a positively charged RNA-binding cleft, and a tail loop important for trimerization and viral transcription. A low-resolution cryo-electron tomography reconstruction of THOV RNPs elucidates a left-handed double helical assembly. By providing a model for RNP assembly of THOV, our study suggests conserved NP assembly and RNA encapsidation modes for thogoto- and influenza viruses.

Tissue Microarrays to Visualize Influenza D Attachment to Host Receptors in the Respiratory Tract of Farm Animals.

The trimeric hemagglutinin-esterase fusion protein (HEF) of influenza D virus (IDV) binds 9-O-acetylated sialic acid receptors, which are expressed in various host species. While cattle are the main reservoir for IDV, the viral genome has also been detected in domestic pigs. In addition, antibodies against IDV have been detected in other farm animals such as sheep, goats, and horses, and even in farmers working with IDV positive animals. Viruses belonging to various IDV clades circulate, but little is known about their differences in host and tissue tropism. Here we used recombinantly produced HEF proteins (HEF S57A) from the major clades D/Oklahoma (D/OK) and D/Oklahoma/660 (D/660) to study their host and tissue tropism and receptor interactions. To this end, we developed tissue microarrays (TMA) composed of respiratory tissues from various farm animals including cattle, domestic pigs, sheep, goats, and horses. Protein histochemical staining of farm animal respiratory tissue-microarrays with HEF proteins showed that cattle have receptors present over the entire respiratory tract while receptors are only present in the nasal and pharyngeal epithelium of pigs, sheep, goats, and horses. No differences in tropism for tissues and animals were observed between clades, while hemagglutination assays showed that D/OK has a 2-fold higher binding affinity than D/660 for receptors on red blood cells. The removal of O-acetylation from receptors via saponification treatment confirmed that receptor-binding of both clades was dependent on O-acetylated sialic acids.

Sinu virus, a novel and divergent orthomyxovirus related to members of the genus Thogotovirus isolated from mosquitoes in Colombia.

The genome and structural organization of a novel insect-specific orthomyxovirus, designated Sinu virus, is described. Sinu virus (SINUV) was isolated in cultures of C6/36 cells from a pool of mosquitoes collected in northwestern Colombia. The virus has six negative-sense ssRNA segments. Genetic analysis of each segment demonstrated the presence of six distinct ORFs encoding the following genes: PB2 (Segment 1), PB1, (Segment 2), PA protein (Segment 3), envelope GP gene (Segment 4), the NP (Segment 5), and M-like gene (Segment 6). Phylogenetically, SINUV appears to be most closed related to viruses in the genus Thogotovirus.

An Open Receptor-Binding Cavity of Hemagglutinin-Esterase-Fusion Glycoprotein from Newly-Identified Influenza D Virus: Basis for Its Broad Cell Tropism.

Influenza viruses cause seasonal flu each year and pandemics or epidemic sporadically, posing a major threat to public health. Recently, a new influenza D virus (IDV) was isolated from pigs and cattle. Here, we reveal that the IDV utilizes 9-O-acetylated sialic acids as its receptor for virus entry. Then, we determined the crystal structures of hemagglutinin-esterase-fusion glycoprotein (HEF) of IDV both in its free form and in complex with the receptor and enzymatic substrate analogs. The IDV HEF shows an extremely similar structural fold as the human-infecting influenza C virus (ICV) HEF. However, IDV HEF has an open receptor-binding cavity to accommodate diverse extended glycan moieties. This structural difference provides an explanation for the phenomenon that the IDV has a broad cell tropism. As IDV HEF is structurally and functionally similar to ICV HEF, our findings highlight the potential threat of the virus to public health.

Proteomics computational analyses suggest that baculovirus GP64 superfamily proteins are class III penetrenes.

BACKGROUND: Members of the Baculoviridae encode two types of proteins that mediate virus:cell membrane fusion and penetration into the host cell. Alignments of primary amino acid sequences indicate that baculovirus fusion proteins of group I nucleopolyhedroviruses (NPV) form the GP64 superfamily. The structure of these viral penetrenes has not been determined. The GP64 superfamily includes the glycoprotein (GP) encoded by members of the Thogotovirus genus of the Orthomyxoviridae. The entry proteins of other baculoviruses, group II NPV and granuloviruses, are class I penetrenes. RESULTS: Class III penetrenes encoded by members of the Rhabdoviridae and Herpesviridae have an internal fusion domain comprised of beta sheets, other beta sheet domains, an extended alpha helical domain, a membrane proximal stem domain and a carboxyl terminal anchor. Similar sequences and structural/functional motifs that characterize class III penetrenes are located collinearly in GP64 of group I baculoviruses and related glycoproteins encoded by thogotoviruses. Structural models based on a prototypic class III penetrene, vesicular stomatitis virus glycoprotein (VSV G), were established for Thogoto virus (THOV) GP and Autographa california multiple NPV (AcMNPV) GP64 demonstrating feasible cysteine linkages. Glycosylation sites in THOV GP and AcMNPV GP64 appear in similar model locations to the two glycosylation sites of VSV G. CONCLUSION: These results suggest that proteins in the GP64 superfamily are class III penetrenes.