Host cell proteins binding to the encapsidation signal epsilon in hepatitis B virus RNA

The highly conserved encapsidation signal (epsilon) of hepatitis B viral (HBV) pregenomic RNA has been reported as an essential component for encapsidation and protein priming of HBV polymerase. Here, we report that two HBV epsilon RNA-binding host proteins (80 and 43 kDa) and a copurifying protein (100 kDa) were purified and characterized by the combined methods of UV cross-linking analysis with the epsilon RNA and column chromatography. Amino-terminal microsequencing showed that 80- and 43-kDa proteins were identified as the heterodimeric nuclear factor of activated T cells (NF90/NF45) and 100 kDa as a molecular chaperone, the GRP94. The heterodimeric factor interacted preferentially with the upper-bulge region of HBV epsilon RNA helping the HBV polymerase bind the lower-bulge region. Using in vitro protein priming analysis, the initial oligonucleotide of the protein-priming product was deduced as 5'-GAAC-3', which is the complementary sequence of both regions of DR1 and epsilon in the pregenomic RNA. Previously, we also proposed that the GRP94 was associated with HBV polymerase in the human liver cell HepG2. These results suggest that the heterodimeric factor plays an important role in the priming activity of HBV polymerase.