RBP3

Retinol-binding protein 3, interstitial (RBP3), also known as IRBP is a is_associated_with::protein that in humans is encoded by the RBP3 is_associated_with::gene. RBP3 is_associated_with::orthologs have been identified in most is_associated_with::eutherians except is_associated_with::tenrecs and is_associated_with::armadillos.

Function
The inter-photoreceptor is_associated_with::retinoid-binding protein is a large is_associated_with::glycoprotein known to bind retinoids and found primarily in the is_associated_with::interphotoreceptor matrix of the retina between the retinal pigment epithelium and the photoreceptor cells. It is thought to transport retinoids between the retinal pigment epithelium and the photoreceptors, a critical role in the visual process.

Gene
The human IRBP gene is approximately 9.5 kbp in length and consists of four is_associated_with::exons separated by three is_associated_with::introns. The introns are 1.6-1.9 kbp long. The gene is transcribed by photoreceptor and retinoblastoma cells into an approximately 4.3-kilobase mRNA that is translated and processed into a glycosylated protein of 135,000 Da.

Structure
The amino acid sequence of human IRBP can be divided into four contiguous homology domains with 33-38% identity, suggesting a series of gene duplication events. In the gene, the boundaries of these domains are not defined by exon-intron junctions, as might have been expected. The first three homology domains and part of the fourth are all encoded by the first large exon, which is 3,180 base pairs long. The remainder of the fourth domain is encoded in the last three exons, which are 191, 143, and approximately 740 base pairs long, respectively.

Application
The rbp3 gene is commonly used in animals as a is_associated_with::nuclear DNA phylogenetic marker. The exon 1 has first been used in a pioneer study to provide evidence for is_associated_with::monophyly of is_associated_with::Chiroptera. Then, it has been used to infer the is_associated_with::phylogeny of is_associated_with::placental is_associated_with::mammal orders,  and of the major is_associated_with::clades of is_associated_with::Rodentia, is_associated_with::Macroscelidea, and is_associated_with::Primates. RBP3 is also useful at lower is_associated_with::taxonomic levels, e.g., in muroid rodents and Malagasy primates, at the is_associated_with::phylogeography level in Geomys and Apodemus rodents, and even for is_associated_with::carnivora species identification purposes.

Note that the RBP3 intron 1 has also been used to investigate the is_associated_with::platyrrhine primates phylogenetics.