Phage display reveals a novel interaction of human tear lipocalin and thioredoxin which is relevant for ligand binding

Human tear lipocalin (TL) is an unusual member of the lipocalin protein family, since it is known to bind a large variety of lipophilic ligands in vivo and acts as a cysteine proteinase inhibitor in vitro. It is suggested to function as a physiological protection factor by scavenging lipophilic potentially harmful compounds. Since protein-protein interaction or macromolecular complexation is a common feature of many lipocalins, we applied phage display technology to identify TL interacting proteins. By panning of a human prostate cDNA phagemid library against purified TL we isolated a thioredoxin (Trx) encoding phage clone. Biochemical analysis revealed that TL indeed interacts with Trx and is reduced by this redox protein. Reduction of the TL-specific disulfide bond is of functional relevance, since the reduced protein shows a nine-fold increase in ligand affinity when tested with retinoic acid as ligand.