A Molecular and Structural Perspective on Bluetongue Virus Entry and Assembly.

Bluetongue virus (BTV), the prototype of the genus 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 10 double-stranded RNA segments. This manuscript highlights our group's recent findings on the molecular and structural mechanisms underlying BTV entry and assembly during replication. Viral entry is a stepwise, pH-dependent process. The outermost protein, VP2, attaches to sialic acids and senses the acidic pH of early endosomes, triggering their 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 the accurate packaging of 10 positive-sense RNA segments. These segments form an RNA-RNA interaction network independent of viral proteins, beginning with the smaller segments and guiding the 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 characterizations of BTV provide a foundation for future control strategies against related reoviruses.