The cryo-electron microscopy structure of Broad Bean Stain Virus suggests a common capsid assembly mechanism among comoviruses.

The Broad bean stain virus (BBSV) is a member of the genus Comovirus infecting Fabaceae. The virus is transmitted through seed and by plant weevils causing severe and widespread disease worldwide. BBSV has a bipartite, positive-sense, single-stranded RNA genome encapsidated in icosahedral particles. We present here the cryo-electron microscopy reconstruction of the BBSV and an atomic model of the capsid proteins refined at 3.22 Šresolution. We identified residues involved in RNA/capsid interactions revealing a unique RNA genome organization. Inspection of the small coat protein C-terminal domain highlights a maturation cleavage between Leu567 and Leu568 and interactions of the C-terminal stretch with neighbouring small coat proteins within the capsid pentameric turrets. These interactions previously proposed to play a key role in the assembly of the Cowpea mosaic virus suggest a common capsid assembly mechanism throughout all comovirus species.