CRYSTAL-STRUCTURE OF A PEPTIDE COMPLEX OF ANTIINFLUENZA PEPTIDE ANTIBODY-FAB-26 9 - COMPARISON OF 2 DIFFERENT ANTIBODIES BOUND TO THE SAME PEPTIDE ANTIGEN/

The three-dimensional structure of the complex of a second anti-peptid e antibody (Fab 26/9) that recognizes the same six-residue epitope of an immunogenic peptide from influenza virus hemagglutinin (HA1; 75-110 ) as Fab 17/9 with the peptide has been determined at 2.8 Angstrom res olution. The amino acid sequence of the variable region of the 26/9 an tibody differs in 24 positions from that of 17/9, the first antibody i n this series for which several ligand-bound and free structures have been determined and refined. Comparison of the 26/9 peptide with the 1 7/9-peptide complex structures shows that the two Fabs are very simila r (r.m.s.d. 0.5 to 0.8 Angstrom) and that the peptide antigen (101-107 ) has virtually the same conformation (r.m.s.d. 0.3 to 0.8 Angstrom) w hen bound to both antibodies. A sequence difference in the 26/9 bindin g pocket (L94; His in 26/9, Asn in 17/9) results in an interaction wit h a bound water molecule that is not seen in the 17/9 structures. Epit ope mapping shows that the relative specificity of 26/9 and 17/9 antib odies for individual positions of the peptide antigen are slightly dif ferent. Amino acid substitutions in the peptide, particularly at posit ion SerP107, are tolerated to different extents by 17/9 and 26/9. Stru ctural and sequence analysis suggests that amino acid differences near the peptide-binding site are responsible for altering slightly the sp ecificity of 26/9 for three peptide residues and illustrates hew amino acid substitutions can modify antibody-antigen interactions and there by modulate antibody specificity.