Lipocalin, ApoD type (IPR022271)

Short name: Lipocalin_ApoD

Overlapping homologous superfamilies

Family relationships


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 [PMID: 2580349]. 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 [PMID: 4708098].

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 [PMID: 1834059, PMID: 7684291]. 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 [PMID: 8573354]. 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 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 [PMID: 11058760]. Insect Lazarillo is an homologue of ApoD [PMID: 23777559].

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.