Combined proteomic-RNAi screen for host factors involved in human hepatitis delta virus replication

Human hepatitis delta virus (HDV) is the only animal virus known to replicate its RNA genome using a host polymerase because its only virally encoded proteins, the small and large hepatitis delta antigens (HDAg-S and HDAg-L), lack polymerase activity. Although this makes HDV an ideal model system to study RNA-directed transcription in mammalian cells, little is known about the host factors involved in its replication. To comprehensively identify such host factors, we created a stable cell line carrying a functional FLAG-HDAg-S. Anti-Flag immunopurification and mass spectrometry identified > 100 proteins associated with FLAG-HDAg-S, many of which had predicted roles in RNA metabolism. The biological relevance of this screen was strongly supported by the identification of nine out of the 12 subunits of the RNA polymerase II complex thought to mediate HDV replication. To further investigate the significance of these factors for HDV replication, we selected 65 proteins to look for factors that would also affect the accumulation of HDV RNA following siRNA knockdown. Fifteen and three factors were found to regulate HDV RNA accumulation negatively and positively, respectively, upon RNAi knockdown. Our results provide a valuable resource for future research to advance our mechanistic understanding of HDV replication and RNA-directed transcription in mammalian cells.