Nucleocapsid-like cryo-EM structure of influenza B virus

Influenza viruses belong to the Orthomyxoviridae family, they are categorized into four types: A, B, C, and D. Influenza B viruses co-circulate annually with influenza A strains during seasonal flu epidemics in humans, causing severe disease. The segmented RNA encapsidated by multiple copies of the nucleoprotein (NP) and attached to the heterotrimeric polymerase forms the central replicative unit called the viral ribonucleoprotein (vRNP). All influenza NP proteins share the same core domain folding, only NP of influenza B virus (B/NP) has an extended unfolded N-terminal tail of 70 amino-acids. This disordered N-terminal tail is required for nuclear localization of the protein, and may be involved in viral RNA transcription and replication regulation. In this study, we report that in absence of the extended N-terminal tail, the truncated NP maintains RNA binding ability and NP oligomeric state in vitro. We then reconstituted RNP-like particles by incubating truncated B/NP with synthetic RNA and we solved the cryo-EM structure at 4.1 Å resolution. Their morphology appears identical to native vRNPs extracted from viruses when observed by negative-stain electron microscopy. Overall, our results suggest that vRNPs from influenza virus type A, B and D share the same right-handed antiparallel helical conformation and that the B/NP N-terminal tail does not participate to the helical architecture stabilization once the vRNPs are assembled.