Formation of transmembraneous hepatitis B e-antigen by cotranslational in vitro processing of the viral precore protein.

The gene encoding the major core protein P22c of hepatitis B virus is preceded by a precore sequence. Expression of the core gene with the precore in Escherichia coli results in a membrane protein of HBe antigenicity. Expression in mammalian cells generates secreted HBeAg. To study the biosynthetic pathway of HBeAg and the function of precore in this process, we translated mRNAs for core proteins with and without precore using reticulocyte lysates and microsomal vesicles. The precore sequence was cleaved cotranslationally as a signal peptide, probably at alanine 19. The processed product P23e was partially translocated to the lumen of the microsomes. The arginine-rich carboxy-terminal domain of P23e was however not translocated and susceptible to trypsin. Clusters of positive-charged amino acids seem to act as a novel type of translocation stop signal. Trypsin generated a P16e which no longer had a transmembraneous configuration. The findings may explain the biosynthesis and potential function of HBeAg in hepatitis B virus-infected hepatocytes.