Structural characterizations of an epitope domain and its mutant escape variant of hepatitis B virus surface antigen
1Institute of Biomedical Sciences, 2Institute of Biological Chemistry, Academia Sinica, Taipei 115, Taiwan
The small hepatitis B virus surface antigen (HBsAg) is the major constituent of the envelope of hepatitis B virus and is composed of 226 amino acids. The proposed topology of HBsAg shows that it consists of four membrane-spanning helices and two hydrophilic exposed segments. The second hydrophilic domains residing in residues 124-137 and 139-147 has been identified to contain the main conformational epitopes and is defined the “a” determinant of the S antigen. Furthermore, vaccinated children who cannot gain protection against chronic infection were found to harbor several HBV variants possessing the altered antigenic epitopes. Among these HBV variants, the most frequently found variant is Arg145 instead of the native Gly145 of S antigen. In the present study, we carried out the structural studies of the small circular epitope linked by a disulfide bridge, HBsAg139-147 and its G145R mutant in the aqueous solution as well as in 30% TFE using NMR and CD experiments. Multiple conformers were identified for both HBsAg139-147 and G145R mutant due to the cis/trans isomerization of Pro142, and the trans/cis ratio was increased for both peptides when the sample temperature was increased. Based on the 3D solution structures generated using X-PLOR program, the conformational differences between HBsAg139-147 and G145R mutant are discussed.