Genetic and Biochemical Analyses of the Flaviviridae Capsid Proteins
The small, enveloped viruses of the family Flaviviridae are etiological agents of numerous important human and agricultural diseases including hepatitis C, yellow fever, and bovine viral diarrhea. Efficient dissemination of these viruses is dependent on the production of infectious particles, thought to arise by budding of the capsid protein and associated genomic RNA through a host cell-derived lipid membrane outfitted with envelope glycoproteins. The process of virion morphogenesis is not well understood, but the presumed involvement of numerous viral and cellular components makes it an attractive target for novel therapeutic drug design. To investigate the early events of Flaviviridae particle assembly, we examined the properties of a major virion structural component, the viral capsid proteins. Biochemical analysis of the bovine viral diarrhea virus (BVDV) capsid protein revealed a remarkably flexible molecule, capable of binding RNA with low affinity and low specificity in vitro. The ability of BVDV capsid to functionally replace a nonspecific RNA condensing sequence in vivo suggests a mechanism for its role in virion assembly. Hepatitis C virus (HCV) propagation in tissue culture has very recently become possible, allowing the role of the capsid (core) protein in the authentic viral life cycle to be studied for the first time. We performed a comprehensive deletion and mutational analysis of the HCV core protein, confirming its importance for infectious virus production and identifying numerous residues essential for this activity. Interestingly, investigation of the virion building blocks converged on a group of nonstructural proteins that may engineer the assembly process. The infectivity of several defective HCV core mutants could be rescued by compensatory mutations in p7, NS2, and NS3, adding to accumulating evidence that these nonstructural proteins are important for virion morphogenesis. The functional determinants of an analogous BVDV protein, uncleaved NS2-3, in infectious virus production were examined. These studies of the Flaviviridae capsid proteins provide insights into the mechanisms of viral genome packaging and highlight the importance of nonstructural accessory factors in the initial steps of infectious particle assembly.
flaviviridae, viral capsid proteins, hepatitis C virus (HCV), infectious particle assembly, Life Sciences
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