Invertebrate colouration protein (IPR003057)

Short name: Invtbrt_color

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. 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.

A number of lipocalins act in invertebrate colouration and are represented in this entry. These include: bilin binding protein from the cabbage white butterfly (Pieris brassicae), the closely related protein insecticyanin from Manduca sexta (Tobacco hawkmoth) and the lobster protein crustacyanin. Like other members of the family, they bind small molecules, and gain their colourant properties from interaction with their ligands.

Crustacyanin (meaning `shell blue') is the general name given to the carotenoprotein complex found in the epicuticle, or calcified outer layer, of the lobster carapace. It acts as the dominant pigment of the lobster shell, giving rise to its characteristic blue colour. In solution, crustacyanin exists as an equilibrium mixture between several distinct forms, differing in their physical and spectral properties. The native, blue form (alpha-crustacyanin), which predominates in vivo, will, at low ionic strength, form alpha'-crustacyanin; this in turn changes to purple beta-crustacyanin on standing. The alpha to alpha' transition is favoured by low ionic strength and is reversible, while conversion into beta- crustacyanin is irreversible. Native alpha-crustacyanin is an octamer of heterodimers, totalling 16 separate polypeptide chains, each dimer binding two molecules of astaxanthin, beta-crustacyanin corresponding to the free heterodimer.

GO terms

Biological Process

No terms assigned in this category.

Molecular Function

GO:0031409 pigment binding

Cellular Component

No terms assigned in this category.

Contributing signatures

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