Insertions within the hepatitis B virus capsid protein influence capsid formation and RNA encapsidation

Hepatitis B virus (HBV) capsid proteins, termed core proteins, with two- to four-amino-acid insertions were assessed for capsid formation, RNA encapsidation, and the ability to support reverse transcription of the pregenome by the polymerase molecule. Velocity sedimentation analysis of insect cell-expressed recombinant core proteins revealed that only two of the nine insertion mutant proteins formed capsids with the tight banding patterns of wild-type capsids. The remaining mutant core proteins were spread over the gradients, suggesting aggregate formation, or at the top of the gradients, suggesting lack of stable capsid formation. The mutant capsid proteins were coexpressed in Huh7 cells with an HBV genome lacking a functional core gene to test for trans complementation of HBV replication. Three of the mutant core proteins formed capsids containing HBV RNA, but only two of these contained reverse-transcribed HBV DNA. While the core protein has shown resiliency in capsid formation following insertion of foreign residues into the major B-cell epitope, several of the small insertions severely reduced the efficiency of capsid formation and inhibited capsid function.