THE PRECORE SEQUENCE OF HEPATITIS-B VIRUS IS REQUIRED FOR NUCLEAR-LOCALIZATION OF THE CORE PROTEIN

The cellular localization of the precore/core and core proteins was st udied by immunofluorescence following transfection of 143 thymidine ki nase-negative (TK-) and Hep-G2 cells with expression constructs contai ning wild-type (hepatitis B e antigen [HBeAg]-positive) and precore mu tant (HBeAg-negative) sequences, Precore/core constructs with the wild -type phenotype result in strong nuclear staining, while, in contrast, constructs expressing core antigen alone have strong cytoplasmic stai ning, These differences in the pattern of immunofluorescence staining may be caused by expression of the precore/core protein, some of which may be translocated into the nucleus, following removal of the signal peptide, In vitro translation experiments showed that the main protei n products obtained in the presence of microsomal membranes were the p recore/core protein and a truncated product representing the same prot ein without its signal peptide. Core protein expression from the preco re mutant constructs was very much reduced, indicating that translatio nal re-initiation was not very efficient. The significance of the prec ore/core protein being present in the nucleus is not clear, but sugges ts that it may be important in the replicative cycle of the virus, Fin ally, HBeAg produced by some of the constructs could not be detected b ecause amino acid substitutions affected antibody-binding epitopes.