A Molecular and Structural Perspective of Bluetongue Virus Entry and Assembly

Bluetongue virus (BTV), the prototype orbivirus, infects livestock, causing high morbidity and mortality and impacting global trade. BTV is a non-enveloped, double-capsid virus, composed of seven structural proteins and a genome of ten double-stranded RNA segments. This manuscript highlights our recent findings on the molecular and structural mechanisms underlying BTV entry and assembly during replication. Viral entry is a stepwise, pH-dependent process. The virus-neutralisation, outermost protein VP2 attaches to sialic acids and senses the acidic pH of early endosomes, triggering its dissociation. Subsequently, the second outer capsid protein, VP5, undergoes major changes in late endosomes, forming a membrane-penetrating pore that releases the transcriptionally-active inner core into the host cytoplasm. Core assembly also proceeds stepwise and requires accurate packaging of ten positive-sense RNA segments. These segments form an RNA–RNA interaction network independent of viral proteins, beginning with the smaller segments and guiding complete genome assortment. The small capsid protein VP6, interacts with VP3 to facilitate RNA encapsidation. While infectious cores assemble in-vitro without non-structural proteins, NS2 is essential for the in-vivo formation of viral inclusion bodies via liquid–liquid phase separation, concentrating viral components and promoting genome assembly. These comprehensive characterisations of BTV provide future control strategies for related reoviruses.