Localization of a unique hepatitis B virus epitope sheds new light on the structure of hepatitis B virus surface antigen

In a search for monoclonal antibodies (MAbs) that can bind hepatitis B viru s surface antigen (HBsAg) with amino acid substitutions in the immune domin ant 'a' region (escape mutants) we investigated the epitope recognition sit e of the human MAb 4-7B, Pepscan analysis and experiments with alanine subs titution as well as substitutions known from nature pointed to residues 178 -186 in the small S protein with the amino acid sequence PFVQWFVGL (key ami no acids in bold) as the minimal epitope. Single amino acid substitutions a t positions 122(R/K)(d/y), 134(Y/F), 145(G/R), 148(T/A) and 160(K/R)(w/r), representing 'a' region variants in recombinant HBsAg COS-I cells, did not influence binding of MAb 4-7 B. Synthetic peptides (residues 175-189) inclu ding the 4-7B epitope sequence were able to evoke an anti-HBs response in r abbits. According to established polypeptide models, the 4-7 B epitope regi on is located in the lipid layer of 20 nm HBsAg particles. The present resu lts, however, suggest that residues 178-186 are exposed on the surface of t he 20 nm particle. This may change our view of the structure of HBsAg.