Proteaselike sequence in hepatitis B virus core antigen is not required for e antigen generation and may not be part of an aspartic acid-type protease

Abstract

The hepatitis B virus (HBV) C gene directs the synthesis of two major gene products: HBV core antigen (HBcAg[p21c]), which forms the nucleocapsid, and HBV e antigen (HBeAg [p17e]), a secreted antigen that is produced by several processing events during its maturation. These proteins contain an amino acid sequence similar to the active-site residues of aspartic acid and retroviral proteases. On the basis of this sequence similarity, which is highly conserved among mammalian hepadnaviruses, a model has been put forward according to which processing to HBeAg is due to self-cleavage of p21c involving the proteaselike sequence. Using site-directed mutagenesis in conjunction with transient expression of HBV proteins in the human hepatoma cell line HepG2, we tested this hypothesis. Our results with HBV mutants in which one or two of the conserved amino acids have been replaced by others suggest strongly that processing to HBeAg does not depend on the presence of an intact proteaselike sequence in the core protein. Attempts to detect an influence of this sequence on the processing of HBV P gene products into enzymatically active viral polymerase also gave no conclusive evidence for the existence of an HBV protease. Mutations replacing the putatively essential aspartic acid showed little effect on polymerase activity. Additional substitution of the likewise conserved threonine residue by alanine, in contrast, almost abolished the activity of the polymerase. We conclude that an HBV protease, if it exists, is functionally different from aspartic acid and retroviral proteases.