Anti-metatype peptides, a molecular tool with high sensitivity and specificity to monitor small ligands

To develop a new immunological detection system of gibberellins (GAs), a class of phytohormones, peptides that interact with an antibody against GA4 in a GA4-dependent manner, were screened from phage display random peptide libraries. The biopanning procedure yielded peptides designated as anti-metatype peptides (AM-peps), which showed specific binding to the complex of the antibody and its ligand GA4; that is, the antibody could not be replaced with the other anti-GA4 antibody, and GA4 could not be replaced with GA1, another ligand of the antibody. Together with computational analyses such as analysis of structural propensity of the AM-peps and docking simulation of the AM-peps and the 8/E9-GA4 complex, it was suggested that AM-peps formed a helix in their central region and interacted with a part of the 8/E9-GA4 complex located in close proximity to the GA4 molecule. Based on the property of AM-peps to make a ternary complex with antibody and its ligand, a noncompetitive enzyme-linked immunosorbent assay (ELISA) system corresponding to sandwich ELISA was developed to detect GA4. GA4 as low as 30 pg, which could not be achieved by conventional competitive ELISA, could be detected by the new system, demonstrating the feasibility of this system.