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A monoclonal antibody induces opening of a coiled coil. Global protection of amide protons from H/D exchange decreases by up to 1000-fold in antibody-bound triple-stranded coiled coil.

Antibody specificity is limited, and different antigens may cross-react to varying degrees with the same antibody. An example is monoclonal antibody 42PF elicited against a 29-residue random-coil peptide. 42PF cross-reacts with a triple-stranded coiled coil of related amino acid sequence, and there was circumstantial evidence for an antibody-induced opening of the coiled coil [Leder, L., Berger, C., Bornhauser, S., Wendt, H., Ackermann, F., Jelesarov, I. & Bosshard, H. R. (1995) Biochemistry 34, 16,509-16,518]. To reveal in a direct way that antibody 42PF induces the opening of the coiled coil, we have compared the rates of H/D exchange of amide protons in the free and in the antibody-bound coiled-coil peptide using electrospray-ionization mass spectrometry for the analysis of deuterium incorporation. Complete H/D exchange lasted several days in the tightly folded coiled-coil conformation while in the presence of antibody 42PF the overall exchange took only minutes, corresponding to a thousand-fold decrease of protection of the most slowly exchanging amide hydrogens of the folded coiled coil. This demonstrates that the antibody induced a considerable opening of the coiled coil.