Conserved cysteines of the hepatitis B virus core protein are not required for assembly of replication-competent core particles nor for their envelopment.

Replication of hepatitis B virus (HBV) proceeds by reverse transcription of an RNA intermediate inside the viral nucleocapsid formed by the core protein. This protein contains four Cys residues which occur at equivalent positions in the core proteins of all known mammalian hepadnaviruses, suggesting that they might be of structural and/or functional importance. The four His residues of the core protein are located strikingly close to the three N-proximal cysteines. This arrangement is likewise conserved and might indicate the presence of an unconventional Cys-His box element similar to that required for nucleic acid binding in all retroviral NC proteins. In order to test the potential involvement of the core protein cysteines in virus assembly, we transiently expressed in HuH7 cells a mutant HBV genome encoding a core protein in which all cysteines are replaced by serine residues and analyzed the formation of replication-competent cores using the endogenous polymerase reaction. The mutant genome yielded products that were nearly indistinguishable from those produced by a corresponding wild-type genome, virtually ruling out the presence of a functional Cys-His box element in the hepadnaviral core protein. Density gradient analysis showed that the mutant cores were enveloped, though the efficiency of envelopment and/or the stability of the mutant enveloped particles was lowered compared to the wild-type. These data indicate that none of the steps in the viral life cycle from reverse transcription to envelopment was principally impaired. The conservedness of the cysteines might then be related to virus infectivity rather than replication; alternatively, the Cys residues might not be important for the core protein itself, but for the alternative C gene product HBeAg.