Lipocalin, OBP-like (OBP-like)
Type: Family

The crystal structures of several lipocalins have been solved and show a novel 8-stranded anti-parallel β-barrel fold well conserved within the family. Sequence similarity within the family is at a much lower level and would seem to be restricted to conserved disulphides and 3 motifs, which form a juxtaposed cluster that may act as a common cell surface receptor site . By contrast, at the more variable end of the fold are found an internal ligand binding site and a putative surface for the formation of macromolecular complexes . The anti-parallel β-barrel fold is also exploited by the fatty acid-binding proteins (which function similarly by binding small hydrophobic molecules), by avidin and the closely related metalloprotease inhibitors, and by triabin. Similarity at the sequence level, however, is less obvious, being confined to a single short N-terminal motif. The lipocalin family can be subdivided into kernal and outlier sets. The kernal lipocalins form the largest self consistent group, comprising the subfamily of odour-binding proteins. The outlier lipocalins form several smaller distinct subgroups: the OBPs, the von Ebner's gland proteins, alpha-1-acid glycoproteins, tick histamine binding proteins and the nitrophorins.

Odour Binding Proteins (OBPs) are associated with olfactory tissue, and seem able to bind odorant molecules with high specificity. Rattus norvegicus (Rat) OBP is localised to the lateral nasal, or Sterno's, gland, the largest of the 20 discrete nasal glands of the rat. A similar protein from the olfactory tissue of Rana pipiens (Northern leopard frog), which was named protein BG (Bowman's gland), has been identified, cloned and sequenced. It is thought that the OBPs may function by concentrating and delivering odorant molecules to their receptors.

Aphrodisin is the major macromolecular component of hamster vaginal discharge, and is secreted by vaginal tissue and the Bartholin's gland. These secretions, acting via the vomeronasal organ, are known to elicit a copulatory response in male hamsters. Aphrodisin is a mammalian proteinaceous pheromone.

Probasin is a lipocalin originally isolated from the nuclei of rat dorsolateral prostate epithelial cells. Probasin mRNA expression, which is regulated by androgens, gives rise to both a secreted and a nuclear form of probasin, the relative abundance of the two forms being correlated with cell type. Probasin concentration also seems to be closely linked with cell age and state of differentiation.

Bos taurus (Bovine) lipocalin allergen Bos d 2 is found in the secretory cells of skin apocrine sweat glands and the basement membranes of the epithelium and hair follicles. Immunohistochemistry with a monoclonal anti-Bos d 2 antibody has confirmed that skin is the only tissue where mRNA encoding Bos d 2 is detected. This suggest that Bos d 2 is produced in sweat glands and transported to the skin surface as a carrier of a pheromone. Because dander allergens of several mammalian species are lipocalins, the biological function of pheromone transport appears to be a common feature of an important group of aeroallergens .

These lipocalins belong to the OBP group and may also act as odorant/pheromone carriers .