Hepatitis B viral polymerase fusion proteins are biologically active and can interact with the hepatitis C virus core protein in vivo

Hepadnaviruses and retroviruses are evolutionarily related families because they both require a process of reverse transcription for genome replicatio n. However, hepadnaviruses produce polymerase (pol) and core proteins separ ately, while retroviruses synthesize a gag-pol fusion protein that is subse quently cleaved by a virally encoded protease to release a functional polym erase. To test whether an additional sequence at the N-terminus of pol in h epatitis B virus (HBV) interferes with its function, we created two plasmid s expressing core-pol fusion proteins, Core 144-pol and core31-pol. Secrete d particles obtained from HuH-7 cells, which were cotransfected with a core -pol fusion protein-expressing plasmid and a core-expressing plasmid, showe d a positive signal of HBV DNA by the endogenous polymerase assay, indicati ng that the core-pol fusion proteins retain DNA priming, polymerization and RNase H activities. The fusion protein was detected in the cytoplasm of tr ansfected cells and in secreted virions by immunoprecipitation. Furthermore , we found by immunofluorescence staining that the HBV core-pol fusion prot ein colocalized with the hepatitis C virus (HCV) core protein in cytoplasm and in lipid droplets. Immunoprecipitation studies showed that the anti-HCV core complex contained the HCV core-pol fusion protein while the anti-HBV pol complex contained the HCV core protein, which supports the hypothesis t hat the HCV core protein can form a complex with the HBV core-pol fusion pr otein. Copyright (C) 2001 National Science Council, ROC and S. Karger AG, B asel.