Phosphorylation and nuclear localization of the hepatitis B virus core protein: significance of serine in the three repeated SPRRR motifs.

Hepatitis B virus core protein (antigen) is an important serologic marker of hepatitis B virus infection. This protein is found in the cytoplasm or the nuclei, or both, of infected hepatocytes. A nuclear localization signal has previously been identified in the core protein sequence. This signal overlaps three repeated SPRRR motifs. In this report, we demonstrate that substitution of all of the serine residues in these three SPRRR motifs with alanine can prevent almost entirely the phosphorylation of the core protein in Huh-7 hepatoma cells, enhance nuclear localization of the core protein in both Huh-7 and nonhepatic cells, and abolish cell cycle regulation of nuclear localization of the core protein. Since the three core protein mutants which retained only one serine residue of each of the three SPRRR motifs could be phosphorylated to similar degrees, these three serine residues likely could serve as the acceptor sites for phosphorylation with equal efficiency. These results, together with the observation that the three SPRRR motifs overlap the nuclear localization signal of the core protein, raise the possibility that nuclear localization of the core protein is negatively regulated by phosphorylation of the serine residues in the SPRRR motifs.