CHARACTERIZATION OF NUCLEAR-LOCALIZATION OF A HEPATITIS-B VIRUS PRECORE PROTEIN DERIVATIVE P22

Both of hepatitis B virus core protein and a precore protein derivativ e, named P22, have been shown to localize in the nucleus. Although P22 has ten additional amino acid residues at its amino-terminus, both pr oteins contain the same nuclear localization signal. In order to under stand the mechanism that regulates the activity of this signal, we hav e studied the nuclear localization of P22 and compared it with that of core protein. It was found that both cytosolic and nuclear fractions of P22 were phosphorylated but to a lesser extent when compared with c ytosolic core protein. This distinction was likely attributed to diffe rent conformations between these two proteins since the density gradie nt analysis revealed a different particle formation for P22 in the cyt osol. When expressed in Vero cells synchronized by serum deprivation, P22 remained in the cytosol during GO and G1 phases, accumulated gradu ally in the nucleus during S phase, and largely localized in the nucle us when cells were confluent. On the other hand, the core protein was transported into the nucleus during mid-G1 phase, shuttled back to the cytosol in S phase and again accumulated in the nucleus when cells we re confluent. Interestingly, when aphidicolin was used to arrest the c ells in late G1 phase, both proteins were found to accumulate in the n uclei. These results indicated that although both P22 and core protein s possessed the same nuclear localization signal, the cellular regulat ion of their nuclear transport was not identical and might involve dif ferent molecular mechanisms.