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

Organon Teknika, Boxtel, The Netherlands 1 Departments of Gastroenterology and Hepatology, Academic Medical Center, University of Amsterdam, The Netherlands 2 Erasmus Medical Centre Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands 3 Author for correspondence: R. A. Heijtink.Fax +31 10 4089485. e-mail heijtink{at}viro.fgg.eur.nl In a search for monoclonal antibodies (MAbs) that can bind hepatitis B virus surface antigen (HBsAg) with amino acid substitutions in the immune dominant `a' region (escape mutants) we investigated the epitope recognition site of the human MAb 4-7B. Pepscan analysis and experiments with alanine substitution as well as substitutions known from nature pointed to residues 178–186 in the small S protein with the amino acid sequence PFVQWFVGL (key amino acids in bold) as the minimal epitope. Single amino acid substitutions at 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 4-7B. Synthetic peptides (residues 175–189) including the 4-7B epitope sequence were able to evoke an anti-HBs response in rabbits. According to established polypeptide models, the 4-7B epitope region is located in the lipid layer of 20 nm HBsAg particles. The present results, however, suggest that residues 178–186 are exposed on the surface of the 20 nm particle. This may change our view of the structure of HBsAg.