In search of novel human lipocalin receptors

Lipocalins are a broad family of proteins, which transport small hydrophobic molecules such as steroids, bilins, retinoids, and lipids. They share limited regions of sequence homology but a common tertiary structure architecture. These proteins are found in gram-negative bacteria, vertebrates, invertebrates, and in plants. Lipocalins have been associated with many biological processes, among them immune response, inflammation, detoxification, pheromone transport, biological prostaglandin synthesis, retinoid binding, and cancer cell interactions. Because most lipocalins are extracellular proteins their intracellular effects depend on interaction with specific cellular receptors. Although progress in this field has accelerated in recent years the number of lipocalin receptors identified is still limited. Therefore, the major goal of this project is to perform a search for novel lipocalin receptors with a focus on human proteins. For this purpose we will use different experimental approaches. We will use a set of biochemical methods, including in vivo crosslinking, affinity purification and mass spectrometry analysis for identification of specific receptors for the lipocalins ApoD and the major dog and cat allergens Can f 1 and Fel d 4. These target proteins were chosen/selected because ApoD plays an important role in aging and neuronal differentiation, which is clearly receptor-dependent. Allergy against pet allergens Can f 1 and Fel d 4 is widely distributed in the Western world but the mechanism is largely unclear. In our preparatory work we found a receptor-mediated uptake of these allergens by antigen presenting cells. Isolation of receptors for human odorant-binding proteins (hOBP) by using phage-display technology is another approach to be applied within this project. The role of hOBPs is fully unclear and isolation of a specific receptor might add significant knowledge to the biological function of these lipocalins. Finally, the receptors identified will be further characterized. For this purpose the genes encoding the putative receptors will be expressed in CHO/insect cells and the recombinant proteins will be used for biochemical assays using surface plasmon resonance. Alternatively, cells expressing the recombinant receptors will be incubated with labelled lipocalins and tested for binding/uptake. In sum, identification of novel lipocalin receptors should help to understand the mechanism by which specific lipocalins exert their biological effects, since for many lipocalins receptor-binding is supposed to be directly related to their function. It will also help to clarify to what extent these receptor are structurally and functionally related, and it will enforce studies concerning the structural features of receptor-lipocalin interactions. In addition, knowledge about some of these receptors might be of significant relevance in medicine (e.g. receptor/s for lipocalin allergens or receptors for ApoD).

Lipocalins are a large family of proteins, which transport small hydrophobic molecules such as steroids, bilins, retinoids, and lipids. These proteins are found in gram-negative bacteria, vertebrates, invertebrates, and in plants. They share limited regions of sequence homology but a common tertiary architecture which resembles a cup like structure. Lipocalins have been associated with many biological processes, among them immune response, inflammation, detoxification, pheromone transport, biological prostaglandin synthesis, retinoid binding, and cancer cell interactions. Because most lipocalins are extracellular proteins their intracellular effects depend on interaction with specific cellular receptors. Although progress in this field has accelerated in recent years the number of lipocalin receptors identified is still limited. Therefore, the major goal of this project was to perform a search for novel lipocalin receptors with a focus on human proteins. In fact, we were able to identify a Heparan Sulfate Proteoglycan (HSPG), which is present on almost all eukaryotic cells, as novel group of receptors for lipocalins. Lipocalin uptake by HSPGs is also highly relevant for uptake of external lipocalins, like ß-lactoglobulin or lipocalin allergens, in human cells.