Alternative binding proteins: affibody binding proteins developed from a small three-helix bundle scaffold.

In recent years, classical antibody-based affinity reagents have been challenged by novel types of binding proteins developed by combinatorial protein engineering principles. One of these classes of binding proteins of non-Ig origin are the so-called affibody binding proteins, functionally selected from libraries of a small (6 kDa), non-cysteine three-helix bundle domain used as a scaffold. During the first 10 years since they were first described, high-affinity affibody binding proteins have been selected towards a large number of targets for use in a variety of applications, such as bioseparation, diagnostics, functional inhibition, viral targeting and in vivo tumor imaging/therapy. The small size offers the possibility to produce functional affibody binding proteins also by chemical synthesis production routes, which has been found to be advantageous for the site-specific introduction of various labels and radionuclide chelators.