Observation of arenavirus nucleoprotein heptamer assembly

Arenaviruses are enveloped viruses containing a segmented, negative, and ambisense single‐stranded RNA genome wrapped with a nucleoprotein (NP). The NP is the most abundant viral protein in infected cells and plays a critical role in both replication/transcription and virion assembly. The NP associates with RNA to form a ribonucleoprotein (RNP) complex, and this implies self‐assembly while the exact structure of this polymer is not yet known. Here, we report a measurement of the full‐length Mopeia virus NP by negative stain transmission electron microscopy. We observed RNP complex particles with diameter 15 ± 1 nm as well as symmetric circular heptamers of the same diameter, consistent with previous observations. The nucleoprotein associates with RNA to form a ribonucleoprotein complex. The nucleoprotein trends to self‐assemble into heptameric rings. Theses rings represent stable intermediate states representative of the average full turn of the Arenaviridae ribonucleoprotein filament. They are the first observation of the complexed nucleoprotein‐RNA as a polymer and hint that oligomerization occurs as it happens for other Bunyavirales nucleoproteins.