Cellular phosphoinositides and the maturation of bluetongue virus, a non-enveloped capsid virus

BACKGROUND: Bluetongue virus (BTV), a member of Orbivirus genus in the Reoviridae family is a double capsid virus enclosing a genome of 10 double-stranded RNA segments. A non-structural protein of BTV, NS3, which is associated with cellular membranes and interacts with outer capsid proteins, has been shown to be involved in virus morphogenesis in infected cells. In addition, studies have also shown that during the later stages of virus infection NS3 behaves similarly to HIV protein Gag, an enveloped viral protein. Since Gag protein is known to interact with membrane lipid phosphatidylinositol (4,5) bisphosphate [PI(4,5)P(2)] and one of the known binding partners of NS3, cellular protein p11 also interacts with annexin a PI(4,5)P(2) interacting protein, this study was designed to understand the role of this negatively charged membrane lipid in BTV assembly and maturation. METHODS: Over expression of cellular enzymes that either depleted cells of PI(4,5)P(2) or altered the distribution of PI(4,5)P(2), were used to analyze the effect of the lipid on BTV maturation at different times post-infection. The production of mature virus particles was monitored by plaque assay. Microscopic techniques such as confocal microscopy and electron microscopy (EM) were also undertaken to study localization of virus proteins and virus particles in cells, respectively. RESULTS: Initially, confocal microscopic analysis demonstrated that PI(4,5)P(2) not only co-localized with NS3, but it also co-localized with VP5, one of the outer capsid proteins of BTV. Subsequently, experiments involving depletion of cellular PI(4,5)P(2) or its relocation demonstrated an inhibitory effect on normal BTV maturation and it also led to a redistribution of BTV proteins within the cell. The data was supported further by EM visualization showing that modulation of PI(4,5)P(2) in cells indeed resulted in less particle production. CONCLUSION: This study to our knowledge, is the first report demonstrating involvement of PI(4,5)P(2) in a non-enveloped virus assembly and release. As BTV does not have lipid envelope, this finding is unique for this group of viruses and it suggests that the maturation of capsid and enveloped viruses may be more closely related than previously thought.