XXXX cones secrete a two-repeat interphotoreceptor retinoid-binding protein.

@entity1 @entity1522 @entity844 @entity535 @entity904 @entity1274 @entity10613 @entity1718 @entity21232 @entity2083
@entity904 and @entity1718 are critical to photoreceptor structure, function, and development. The transport of these nutrients between the pigment epithelium and @entity1274 is mediated by interphotoreceptor retinoid-binding protein ( @entity21232 ). @entity21232 , a 133-kDa ( @entity1 ) glycolipoprotein, is the major protein component of the extracellular matrix separating these two cell layers. In amphibians and mammals, @entity21232 consists of four homologous repeats of about 300 @entity1522 which form two @entity904 and four @entity844 -binding sites. Here we show that @entity21232 in teleosts is a simpler protein composed of only two repeats. Western blot analysis shows that @entity2083 @entity21232 is half the size (70 kDa) of @entity21232 in higher vertebrates. Metabolic labeling studies employing @entity10613 taken together with in situ hybridization studies and the presence of a signal peptide show that @entity2083 @entity21232 is secreted by the cone photoreceptors. The translated @entity535 sequence has a calculated molecular weight of 66.7 kDa. The primary structure consists of only two homologous repeats with a similarity score of 52.5%. The last repeats of @entity1 and @entity2083 IRBPs are 69.1% similar with hydrophobic regions being the most similar. These data suggest that two repeats were lost during the evolution of the ray-finned fish (Actinopterygii), or that the @entity21232 gene duplicated between the emergence of bony fish (Osteichthyes) and amphibians. Acquisition of a multirepeat structure may reflect evolutionary pressure to efficiently transport higher levels of hydrophobic molecules within a finite space. Quadruplication of an ancestral @entity21232 gene may have been an important event in the evolution of photo-receptors in higher vertebrates.
