HBV Polymerase Residues $Asp^{429}$ and $Asp^{551}$, Invariant at Motifs A and C are Essential to DNA Binding

HBV polymerase shares several regions of amino acid homology with other DNA-directed and RNA-directed polymerases. The amino acid residues $Asp^{429}$, $Gly^{518}$, $Asp^{551}$, $Lys^{585}$, and $Gly^{641}$ in the conserved motifs A, B', C, D, and E in the polymerase domain of HBV polymerase were mutated to alanine or histidine by in vitro site-directed mutagenesis. Those mutants were overexpressed, purified, and analyzed against DNA-dependent DNA polymerase activity and affinity for DNA binding. All those mutants did not show DNA-dependent DNA polymerase activities indicating that those five amino acid residues are all critical in DNA polymerase activity. South-Western analysis shows that amino acid residues $ASp^{429}$ and $ASp^{551}$ are essential to DNA binding, and $Gly^{318}$ and $Gly^{585}$ also affect DNA binding to a certain extent.