Abstract
Picornaviruses are responsible for a range of human and animal diseases, but how their RNA genome is packaged remains poorly understood. A particularly poorly studied group within this family are those that lack the internal coat protein, VP4. Here we report the atomic structure of one such virus, Ljungan virus, the type member of the genus Parechovirus B, which has been linked to diabetes and myocarditis in humans. The 3.78-Å resolution cryo-electron microscopy structure shows remarkable features, including an extended VP1 C terminus, forming a major protuberance on the outer surface of the virus, and a basic motif at the N terminus of VP3, binding to which orders some 12% of the viral genome. This apparently charge-driven RNA attachment suggests that this branch of the picornaviruses uses a different mechanism of genome encapsidation, perhaps explored early in the evolution of picornaviruses.
Highlights
Picornaviruses are responsible for a range of human and animal diseases, but how their RNA genome is packaged remains poorly understood
Numerous attempts to identify an RNA encapsidation signal in enterovirus genomes have failed[6], a recent study reported that PV (Enterovirus genus) encapsidation is facilitated by the interactions of capsid proteins and the RNA replication complex[7] and Aichi virus (Kobuvirus genus) was reported to contain a 50-terminal RNA stem–loop critical for viral RNA encapsidation[8]
Ljungan virus (LV), a picornavirus originally isolated from voles, has been proposed as a zoonotic virus, potentially associated with type-1 diabetes mellitus, myocarditis and Guillain–Barresyndrome in humans[9]
Summary
Picornaviruses are responsible for a range of human and animal diseases, but how their RNA genome is packaged remains poorly understood. The 3.78-Å resolution cryo-electron microscopy structure shows remarkable features, including an extended VP1 C terminus, forming a major protuberance on the outer surface of the virus, and a basic motif at the N terminus of VP3, binding to which orders some 12% of the viral genome. This apparently charge-driven RNA attachment suggests that this branch of the picornaviruses uses a different mechanism of genome encapsidation, perhaps explored early in the evolution of picornaviruses. Structural comparisons place the virus close to HAV within the evolutionary hierarchy of picornaviruses, while substantial differences on both the inner and outer capsid surfaces, including the visualization of 12% of the RNA genome forming icosahedrally ordered interactions with the capsid, suggest the use of a charge-driven mechanism for genome encapsidation
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