Peptide display on bacterial flagella: principles and applications.

Expression of foreign peptides as fusions to bacterial cell surface proteins has gained increasing attention in basic, as well as applied research during the last decade. A wide range of heterologous peptides have been expressed, and the spectrum of available carrier proteins is also wide. The choice of carrier protein is frequently ruled by the application of the fusion protein constructed. This review is focused on flagella display, which is based on genetic fusion of foreign peptides into a surface-exposed, dispensable region of flagellin, the flagellar major subunit present in thousands of copies per filament. Expression of these constructs in flagellin-deficient host strains results in hybrid flagella carrying the heterologous peptides in thousands of intimately-associated copies. The first and still most frequent application of flagella display is the construction of novel recombinant vaccines. Flagella display has also been used in peptide display as an alternative to the phage-display technique. One application involves fusion into a disulfide loop of Escherichia coli thioredoxin that has been inserted into flagellin, this system facilitates expression of random peptides in a conformationally constrained manner readily accessible on the flagellar surface. The random peptide library has been applied in antibody epitope mapping and is suitable for biopanning procedures in the study of ligand-receptor interactions. Many bacterial adhesins are of complex nature and thereby difficult to analyse by conventional methods. Direct flagella display has proven to be applicable also in bacterial adhesion technology since large fragments, up to 302 amino acid residues in length, of bacterial adhesins can be functionally expressed as fusions to flagellin. Hybrid flagella are easily purified and can easily be analysed for binding to various targets, such as immobilized proteins, tissue sections, as well as cell cultures.