Investigation of the 'switch-epitope' concept with random peptide libraries displayed as thioredoxin loop fusions

The 'FLITRX' random peptide library, consisting of dodecamer loop peptides displayed on a thioredoxin-flagellin scaffold on Escherichia coli, was used to select peptide sequences with affinity for a monoclonal antibody. These peptides were further screened for pH- and metal-sensitive antibody bindin g. Several zinc-sensitive peptides were identified, termed 'switch epitopes ', A soluble, monomeric thioredoxin loop ('Trxloop') insertion analog of a FLITRX switch epitope was constructed and its antibody binding properties w ere characterized by Western blots. Zinc-dependent antibody recognition was maintained in the Trxloop protein although the apparent antibody affinity was lower. This Trxloop protein bound to an immobilized metal affinity chro matography matrix, similar to a 'histidine-patch' thioredoxin variant, and was reversibly precipitated by 1 mM Zn2+ or Cu2+ ions. Residues important f or zinc and antibody binding were determined by site-directed mutagenesis. The Trxloop antibody affinity was increased by saturation mutagenesis. Biot inylated Trxloop ('Biotrxloop') variants of the original and improved affin ity Trxloop proteins were constructed and characterized by surface plasmon resonance measurements. Increased antibody affinity was partially due to a slower antibody desorption rate, although the relative adsorption rates wer e dependent on the amount of immobilized Biotrxloop protein, indicating an influence of avidity on the apparent affinity.