InterPro domain: IPR002345

The lipocalins are a diverse, interesting, yet poorly understood family of proteins composed, in the main, of extracellular ligand-binding proteins displaying high specificity for small hydrophobic molecules [ 1 ]. Functions of these proteins include transport of nutrients, control of cell regulation, pheromone transport, cryptic colouration, and the enzymatic synthesis of prostaglandins. For example, retinol-binding protein 4 transfers retinol from the stores in the liver to peripheral tissues [ 2 ].

The crystal structures of several lipocalins have been solved and show a novel 8-stranded anti-parallel beta-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 [ 3 , 4 ]. 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 [ 5 ]. The anti-parallel beta-barrel fold is also exploited by the fatty acid-binding proteins, which function similarly by binding small hydrophobic molecules. Similarity at the sequence level, however, is less obvious, being confined to a single short N-terminal motif.

1. Homology of beta-lactoglobulin, serum retinol-binding protein, and protein HC. Science 228, 335-7
2. Studies on the transport and cellular distribution of vitamin A in normal and vitamin A-deficient rats with special reference to the vitamin A-binding plasma protein. J. Biol. Chem. 248, 4009-22
3. Mouse oncogene protein 24p3 is a member of the lipocalin protein family. Biochem. Biophys. Res. Commun. 180, 69-74
4. Structure and sequence relationships in the lipocalins and related proteins. Protein Sci. 2, 753-61
5. Multiple molecular recognition properties of the lipocalin protein family. J. Mol. Recognit. 8, 185-95