The mechanism of an immature secretion phenotype of a highly frequent naturally occurring missense mutation at codon 97 of human hepatitis B virus core antigen

A very frequent missense mutation at codon 97 of human hepatitis B virus (H BV) core antigen (HBcAg) has been found in chronic carriers worldwide. Func tional characterization of this mutant revealed one intracellular and two e xtracellular phenotypes in contrast to wild-type HBV: (i) a 6- to 12-fold d ecrease in the level of the full-length relaxed circular DNA, a 4- to 5-fol d decrease ire the plus-strand DNA, and an approximately 1.8-fold decrease in the minus-strand and overall DNA levels in the intracellular viral core particles; (ii) a 5.7-fold increase in the immature secretion of Bane parti cles, containing minus-strand, single-stranded virion DNA; and (iii) a sign ificant reduction of nonenveloped core particles in the medium. The steady- state levels of mutant and wild-type core proteins expressed from the same vector appeared to be similar. Using a complementation assay and gradient c entrifugation analysis, we demonstrated that this mutant core protein alone is necessary and sufficient for immature secretion. The decreased level of intracellular HBV DNA is caused by both the cis defect of the mutant genom e and the trans defect of the mutant core protein. We have dissected furthe r the relationship between the intracellular and extracellular phenotypes o f mutant F97L. The pleiotropic effects of the HBcAg codon 97 mutation were observed consistently in several different experimental settings. The mecha nism and biological significance of these findings are discussed.