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 []. 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 [].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. Similarity at the sequence level, however, is less obvious, being confined to a single short N-terminal motif.This entry represents ApoD-type lipocalins, including retinol-binding protein 4 as well as other retinol-binding proteins. Apolipoprotein D (ApoD) is mainly associated with high-density lipoproteins (HDL) and appears to be able to transport a variety of ligands in a number of different contexts []. Insect Lazarillo is an homologue of ApoD [].
