CRYSTAL-STRUCTURE OF A DOMINANT B-CELL EPITOPE FROM THE PRES2 REGION OF HEPATITIS-B VIRUS IN THE FORM OF AN INSERTED PEPTIDE SEGMENT IN MALTODEXTRIN-BINDING PROTEIN

We report here the crystal structure of MalE-B363, a recombinant const ruction of maltodextrin-binding protein bearing a dominant B-cell epit ope sequence from the preS2 region of the hepatitis B surface antigen. The inserted peptide sequence, which replaces the seven carboxy-termi nal residues of maltodextrin-binding: protein, carries the 14 amino ac id residue epitope contained between residues 132 and 145 from the pre S2 region. The epitope sequence is flanked on either side by additiona l residues that result from the genetically engineered insertion, brin ging the total length of the foreign peptide to 26 amino acid residues . The hybrid protein has been previously shown to be recognised by mon oclonal antibodies elicited by the native viral antigen. Three indepen dent molecules of MalE-B363 are present in the asymmetric unit of the crystal. All 14 epitope residues could be traced for one molecule, ten epitope residues had significant electron density for the second, but no density was visible for the epitope of the third. The conformation of the amino-terminal segment of the epitope from Gln132(e) to Gly138 (e) is similar in the two molecules of MalE-B363 for which the foreign peptide could be traced. Moreover, the conformation of a smaller segm ent, comprising residues Asp133(e) to Arg137(e), is similar to that pr esent in the previously determined crystal structure of MalE-B133, ano ther insertion/deletion mutant of maltodextrin-binding protein bearing the preS2 epitope. The presence of a common structural motif for the same sequence in disparate molecular environments suggests that this c onformation might be present also in the native viral antigen. This co uld provide a structural basis to explain the cross-reactivity of anti -preS2 monoclonal antibodies with these hybrid proteins. (C) 1998 Acad emic Press.