Identification of the discontinuous epitope in human complement protein C9 recognized by anti-melittin antibodies

Polyclonal rabbit antibodies against melittin recognize human C protein C9 and retard C9-mediated hemolysis. Human C9 contains a tetrameric and a pentameric sequence (amino acids 293-296 and 528-532, respectively) that together match a continuous segment in the melittin sequence, i.e., residues 8-16. It has been suggested that the tetrameric and the pentameric regions on C9 form a discontinuous epitope on folded C9 that mimics the structure of melittin. To further test this hypothesis, antibodies to C9-sequence-specific peptides were prepared. Peptides containing either the homologous tetrameric or the homologous pentameric sequence together with short stretches of the respective amino- and carboxyl-terminal flanking regions were synthesized, as well as a composite peptide predicted to resemble the discontinuous epitope as a linear, nine-amino acid sequence. Direct and competitive binding assays demonstrated that the tetrameric and the pentameric sequences are part of the epitope on human C9 that is recognized by anti-melittin IgG. However, only antibodies directed against the complete epitope are capable of inhibiting hemolysis. Because neither anti-tetramer nor anti-pentamer antibodies affect hemolysis whereas anti-melittin and anti-composite antibodies do, we propose that human C9 changes conformation around a hinge located between residues 296 and 528 and that the latter two antibodies inhibit unfolding required for membrane insertion and subsequent hemolysis.