Alternative binding proteins: anticalins - harnessing the structural plasticity of the lipocalin ligand pocket to engineer novel binding activities.

Antibodies are the paradigm for binding proteins, with their hypervariable loop region supported by a structurally rigid framework, thus providing the vast repertoire of antigen-binding sites in the immune system. Lipocalins are another family of proteins that exhibit a binding site with high structural plasticity, which is composed of four peptide loops mounted on a stable beta-barrel scaffold. Using site-directed random mutagenesis and selection via phage display against prescribed molecular targets, it is possible to generate artificial lipocalins with novel ligand specificities, so-called anticalins. Anticalins have been successfully selected both against small hapten-like compounds and against large protein antigens and they usually possess high target affinity and specificity. Their structural analysis has yielded interesting insights into the phenomenon of molecular recognition. Compared with antibodies, they are much smaller, have a simpler molecular architecture (comprising just one polypeptide chain) and they do not require post-translational modification. In addition, anticalins exhibit robust biophysical properties and can easily be produced in microbial expression systems. As their structure-function relationships are well understood, rational engineering of additional features such as site-directed pegylation or fusion with functional effector domains, dimerization modules or even with another anticalin, can be readily achieved. Thus, anticalins offer many applications, not only as reagents for biochemical research but also as a new class of potential drugs for medical therapy.