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.

This entry represents the Lipocalin conserved site. The sequences of most members of the family, the core or kernal lipocalins, are characterised by three short conserved stretches of residues [6]. Others, the outlier lipocalin group, share only one or two of these. This signature pattern was built around the first, common to all outlier and kernal lipocalins, which occurs near the start of the first beta-strand.

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

The lipocalins are a diverse, interesting, yet poorly understood family of proteins composed, in
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