Retinol binding protein/Purpurin (IPR002449)

Short name: Retinol-bd/Purpurin

Overlapping homologous superfamilies

Family relationships


Proteins in this family include plasma retinol binding protein (pRBP) and purpurin. They mediate retinol transport in blood plasma [PMID: 5541771]. Binding to RBP allows the hydrophobic vitamin to circulate in blood, but retinol dissociates from the protein prior to entering target cells [PMID: 22665496].

Retinol circulates in the plasma without significant loss because it is bound to pRBP. This protein is synthesized primarily in the liver, where it requires the binding of retinol to trigger its secretion [PMID: 6682115]. In mammals, pRBP binds to transthyretin in the plasma, preventing the loss of pRBP through glomerular filtration [PMID: 5675424]. It has been suggested that a surface cell receptor would recognise the retinol-pRBP-transthyretin complex and, in that way, retinol would be delivered into the cell. However, such a protein remains to be identified. In fish, no transthyretin homologue has been found, though fish pRBP is capable of binding mammalian transthyretin [PMID: 1740159]. In Cyprinus carpio, pRBP is N-glycosylated and it has been suggested that pRBP filtration through kidney glomeruli may be reduced by a glycosylation-dependent increase in the molecular size and negative charge of the protein, since kidney filtration of anionic proteins is less than half that of neutral protein of the same size [PMID: 11278316].

The role of pRBP in retinol transport enables it to fulfill a number of physiological functions:1) it facilitates the transfer of insoluble retinol between tissues, principally transport from storage sites in the liver to peripheral tissues, 2) pRBP protects bound retinol from oxidation and tissues from the indiscriminate distribution of such a biologically active molecule, 3) the synthesis of pRBP regulates retinol release from the liver and mediates the specificity of its uptake by target cells, and 4) pRBP is believed to have an important role in the transfer of retinol from maternal circulation to the developing fetus in mammals [PMID: 6369133].

Purpurin is a constituent of adherons, high molecular weight glycoprotein complexes that are released into the growth medium of cultured cells. Adherons mediate the adhesive interactions of many cell types, including those of embryonic chick neural retina. Purpurin promotes cell-adheron adhesion by interacting with a cell surface heparan sulphate proteoglycan. It also prolongs the survival of cultured neural retina cells [PMID: 2993313]. It is located almost exclusively in the neural cells of the retina [PMID: 2993313, PMID: 3754874], and it is synthesised in photoreceptor cells before incorporation into the extracellular matrix [PMID: 3652208]. The role of purpurin as a trophic factor, mediating both cell adhesion and survival, seems clear but it may also have a subsidiary role as a minor retinol transporter in the retina based on its retinol binding capacity [PMID: 3652208]. pRBP is also able to stimulate the adhesion of neural retina cells, although the serum protein is less active than purpurin [PMID: 3754874].

Structural notes: This subgroup shares a common beta-barrel fold with the parent group. See PIRSF036893 for a detailed description.

GO terms

Biological Process

No terms assigned in this category.

Molecular Function

GO:0005501 retinoid binding
GO:0034632 retinol transmembrane transporter activity

Cellular Component

No terms assigned in this category.

Contributing signatures

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