Fig. 1

HBV virion secretion is blocked by mutations within a cytosolic small loop of the envelope protein (HBsAg). A The predicted topology of the folded HBV envelope protein on the ER membrane contains a cytosolic large loop CYL-I and a small loop CYL-II. A conceptual model for HBV virion assembly and secretion postulates a matrix role for these two cytosolic loops in their interactions with nucleocapsids. B A motif YMDD in the wild type reverse transcriptase (RT) domain has been engineered into a YMHA motif. Plasmid DNAs of wild type and mutant HBV DNAs were transfected into a human hepatoma cell line HuH-7. This YMHA mutant secreted no virions in the medium as detected by native agarose gel electrophoresis and Western blot analysis using an anti-HBc antibody. The polymerase gene overlaps with the envelope gene. C The YMHA polymerase mutation caused simultaneous changes of amino acid sequences from WMM to CTP at position 196–198 within the small loop of the envelope protein. The CTP triple mutations were dissociated by analyzing individual mutations 196C, 197T, and 198P, respectively. Right panel: Lack of virion secretion was detected only in the triple mutant CTP and a single proline substitution mutant 198P. Red asterisk * highlights the strongly reduced levels of HBc core protein signal of total virions in mutants CTP and M198P