Assembly of hepatitis delta virus particles: package of multimeric hepatitis delta virus genomic RNA and role of phosphorylation.

We previously demonstrated that both casein kinase II (CKII) and protein kinase C (PKC) positively modulate the hepatitis delta virus (HDV) RNA replication but not the assembly of the empty hepatitis delta antigen (HDAg) particle. In this study, we investigated whether phosphorylation of HDAg by these two kinases plays a role in assembly of the HDV virion. As demonstrated by in vivo labeling and kinase inhibitor experiments, the phosphorylation level of large HDAg but not small HDAg in HDAg-expressing HuH-7 cells was diminished by CKII inhibitor (DRB), whereas no effect was observed for the phosphorylation level of two HDAgs when treated with protein kinase A (PKA) inhibitor (HA1004) or PKC inhibitor (H7). Cotransfection experiment also demonstrated that packaging of HDV genomic RNA was not affected by the kinase inhibitor DRB or H7 and mutation at the putative CKII phosphorylation sites (serine-2, serine-123, or both), and the putative PKC site (serine-210) of HDAg did not elicit any significant effect on the HDV virion assembly. Therefore, based on the previous work and the present study, it seems that the status and biological significance of phosphorylation of HDAg vary depending on the HDV life cycle. Although in the HDV RNA replication cycle, phosphorylation of small HDAg by CKII or PKC plays important role in HDV replication, phosphorylation of the same HDAg by these two kinases does not occur during the HDV RNA virion assembly, and phosphorylation of the large HDAg by CKII does not confer any regulatory role in the assembly of HDV virion and empty viral particles. Our study also showed that the large HDAg without the small HDAg could efficiently assemble both monomeric and dimeric HDV genomic RNAs into secreted HBV-enveloped virus-like particles. Increasing the transfected small HDAg-expressing plasmid led to an enhancement of the packaging efficiency for the monomeric HDV genomic RNA with little effect on the packaging of dimeric HDV RNA. Similarly, HDAgs could package the trimeric HDV genomic RNA, albeit less efficiently. CsCl density gradient centrifugation confirmed that HDAgs and the monomeric and multimeric (dimer and trimer) HDV genomic RNAs formed an HBV-enveloped virus-like particle at a density of 1.23-1.25 g/ml. Thus, the assembly of the HDV virion seems to not impose much restriction on the size of HDV RNA for packaging.