A single serine mutation on the nuclear localization signal of hepatitis B virus core protein abolishes the inhibition of nuclear transport by surface proteins.

The nuclear localization signal of hepatitis B virus core protein contained three SPRRR motifs. Substitution of a single serine residue on the third SPRRR motif enhanced significantly the nuclear localization of core protein. By performing immunofluorescence staining assays and subcellular fractionation experiments, it was demonstrated that although nuclear transport of the wild type core protein was markedly suppressed by co-expression of hepatitis B virus surface proteins, nuclear transport of this serine mutant protein was not affected. These results indicated that the serine residue in the SPRRR motifs played an important role in regulating the nuclear transport of core protein and the interaction between core and surface proteins.