Structure and assembly of Borna disease virus 1 nucleoprotein–RNA complexes

Structures of nucleoprotein (N)–RNA complexes of the Bornaviridae , a virus family in the order Mononegavirales , have remained unknown. Using cryo-electron microscopy (cryo-EM), we characterized N structures of Borna disease virus 1 (BoDV-1), the type species of the Bornaviridae, which causes fatal human encephalitis. N forms complexes in both RNA-free and RNA-bound states, revealing conserved features throughout the order, as well as BoDV-1-specific stoichiometry, thereby offering insights into viral evolution. We redefine assembly principles governing N–RNA complexes with the discovery of a previously unrecognized, RNA-independent mechanism involving domain-swapping and truncated subunits. Mutational analyses identified residues essential for nucleocapsid formation and RNA synthesis. Cryo-EM of mutant complexes captured RNA-free assemblies, suggesting that initial N oligomerization precedes RNA binding. These findings explain several unknowns in N–RNA complex structure and suggest an alternative to canonical RNA-driven assembly models, offering a new conceptual framework for nucleocapsid formation.