Structure of the Tilapia lake virus nucleoprotein bound to RNA

Tilapia Lake virus (TiLV) belongs to the Amnoonviridae family within the Articulavirales order of segmented negative-strand RNA viruses and is highly diverged from more familiar orthomyxoviruses, such as influenza. The viral nucleoprotein (NP), a key component of the replication machinery, packages the viral genome into protective ribonucleoprotein particles. Here we describe the electron cryo-microscopy (cryo-EM) structure of TiLV-NP bound to RNA within in vitro reconstituted, small ring-like, pseudo-symmetrical oligomers. Although TiLV-NP is considerably smaller than its influenza counterpart and unrelated in sequence, it maintains the same topology and domain organisation. This comprises a head and body domain between which is a negatively-charged groove where single-stranded RNA binds. In addition, an oligomerisation loop inserts into a hydrophobic pocket in the neighbouring NP, the flexible hinges of which allow considerable variability orientation of neighbouring NPs. Focussed cryo-EM maps (up to 2.9 A resolution) unambiguously define the 5' to 3' direction of the bound RNA, confirmed by double stranded, A-form RNA regions that extrude out from some of the NP-NP interfaces. This is the first description of how RNA binds to an articulaviral NP and superposition with orthomyxoviral NPs suggest that the mode of RNA binding is likely conserved across the Articulavirales order.