In vitro binding properties of the hepatitis delta antigens to the hepatitis B virus envelope proteins: potential significance for the formation of delta particles

Abstract To investigate the possible existence of (a) reactive binding site(s) on the hepatitis B surface antigen (HBsAg) for the hepatitis delta antigen (δAg) in the hepatitis delta virus (HDV), we performed binding studies using recombinant (rec)Small, recMiddle, recLarge HBsAg and recombinant small (S) and large (L) hepatitis delta antigen (recSδAg, recLδAg). RecδAg was immobilized onto microtiter plates and incubated with recSmall, recMiddle and recLarge HBsAg. Of the three HBsAg proteins only the recMiddle HBsAg was found to bind to recSδAg. This binding was inhibited by the addition of synthetic PreS2 peptide but not by small HBsAg, indicating that the SδAg exhibits a PreS2 binding site. RecLδAg bound to all three forms of HBsAg. The binding of the HBsAg to recLδAg was saturable and could be blocked with an excess of HBsAg, but not with BSA. The region of the additional 19 amino acids of the LδAg is therefore responsible for the creation of the small HBsAg binding site on the LδAg. We therefore suggest that all HBsAg proteins but particularly the small HBsAg in the HDV coat seem to be involved in the interaction with the HDV core particle and that the PreS2 region of the middle HBsAg plays a crucial role in binding to small δAg during HDV particle formation, probably to increase the stability of the HDV particle.