Structure-function relationships of human lipocalin 1

Lipocalin 1 (Lcn-1) is a protein which is produced by a number of human secretory glands and tissues. Based on amino acid sequence similarities and due to its biochemical properties it was found to be a member of the Lipocalin protein superfamily. The Lipocalins are known to act as physiological binding and transport proteins of a large number of hydrophobic or lipophilic ligands. The most well known example is the plasma retinol-binding protein of mammalia, which is involved in transport of the water insoluble retinol. The common and most important structural feature of all Lipocalins investigated so far, is their cup shaped hydrophobic pocket based on eight antiparallel ß-sheets. Whithin this pocket the hydrophobic ligands are bound. Lcn-1 is a very unusual member of the Lipocalins, since in addition to its binding of a large number of hydrophobic compounds it is able to act as an inhibitor of cysteine proteinases. Within the finished FWF project we could clearly demonstrate that the main biological function of Lcn-1 is to act as a clearance factor of potentially harmful lipophilic compounds, which were released or produced by oxidative stress, infection and inflammation. Most noteworthy, we found that Lcn-1 is highly overproduced in bronchotracheal sectretions of patients suffering from Cystic Fibrosis. Overproduction strictly correlates with the degree of disease and with an increase of bacterial infections, which are commonly found in these patients. Therefore, Lcn-1 might be a novel and useful marker, especially in monitoring the success of antibiotic treatment of the disease. By molecularbiological studies we were able to define important structural motifs involved in ligand binding of Lcn-1. In addition, we could demonstrate that the binding of lipophilic ligands could be modulated by interacting proteins, such as thioredoxin. This mechanism might be important especially under conditions of inflammation. Part of our research within the finished FWF project dealt with the structural basis of the second activity of Lcn-1, namely inhibition of cysteine proteinases. Since there is increasing evidence that several proteins may have dual functions, our investigations could therefore be of general biological interest. We found that within the N-terminal part of Lcn-1 there are three amino acid motifs, which resemble the motifs mediating cysteine proteinase inhibition in cystatins, well known physiological protease inhibitors. Experiments using recombinant Lcn-1 and investigations on engineered protein variants demonstrated that these motifs indeed confer proteinase inhibition in Lcn-1.