The entry mechanism of hepatitis B virus (HBV) has not been defined, and this impedes development of
antiviral therapies aimed at an early step in the viral life cycle. HBV
infection has both host and tissue specificities. For the related duck hepatitis B virus (DHBV), duck
carboxypeptidase D (DCPD) has been proposed as the species-specific docking receptor, while
glycine decarboxylase (DGD) may serve as a tissue-specific cofactor or secondary receptor. DGD binds to several truncated versions of the viral large envelope
protein but not to the full-length
protein, suggesting a need for proteolytic cleavage of the envelope
protein by a
furin-like
proprotein convertase. In the present study, we found that transfected DCPD could confer DHBV binding to non-duck cell lines but that this was followed by rapid virus release from cells. Coexpression of
furin led to DCPD cleavage and increased virus retention. Treatment of DHBV particles with endosome prepared from duck liver led to cleavage of the large envelope
protein, and such viral preparation could generate a small amount of covalently closed
circular DNA in LMH cells, a chicken
hepatoma cell line resistant to DHBV
infection. A
furin inhibitor composed of decanoyl-RVKR-chloromethylketone blocked endosomal cleavage of the large envelope
protein in vitro and suppressed DHBV
infection of primary duck hepatocytes in vivo. These findings suggest that
furin or a
furin-like
proprotein convertase facilitates DHBV
infection by cleaving both the docking receptor and the viral large envelope
protein.