Characterization of a 120-kilodalton pre-S-binding protein as a candidate duck hepatitis B virus receptor

Infection by human and animal hepadnaviruses displays remarkable host and tissue tropism, The infection cycle probably initiates with binding of the pre-S domain of viral envelope protein to surface receptors present on the hepatocyte, Three types of neutralizing monoclonal antibodies against duck hepatitis B virus (DHBV) have their binding sites clustered within residues 83 to 107 of the pre-S protein, suggesting that this region may constitute a major receptor binding site, A 170- or 180-kDa duck protein (p170 or gp180) which binds DHBV particles through this part of the pre-S sequence has been identified recently, Although the p170 binding protein is host (duck) specific, its distribution is not restricted to DHBV-infectible tissues, Using the pre-S protein fused to glutathione S-transferase and immobilized on Sepharose beads, we have now identified an additional binding protein with a size of 120 kDa (p120), p120 expression is restricted to the liver, kidney, and pancreas, the three major organs of DHBV replication, While optimal p170 binding requires an intact pre-S protein, binding to p120 occurs much more efficiently with a few N- or C-terminally truncated forms, The p120 binding site was mapped to residues 98 to 102 of the pre-S region, which overlaps with a cluster of known virus-neutralizing epitopes, Site-directed mutagenesis revealed residues 100 to 102 (Phe-Arg-Arg) as the critical p120 contact site; nonconservative substitution in any of the three positions abolished p120 binding, Double mutations at positions 100 to 102 markedly reduced DHBV infectivity in cell culture, Short pre-S peptides covering the clustered neutralizing epitopes (also p170 and p120 binding sites) reduced DHBV infectivity in primary duck hepatocyte cultures, Thus, p120 represents a candidate component of the DHBV receptor complex.