GOLDFISH CONES SECRETE A 2-REPEAT INTERPHOTORECEPTOR RETINOID-BINDINGPROTEIN

Vitamin A and fatty acids are critical to photoreceptor structure, fun ction, and development. The transport of these nutrients between the p igment epithelium and neural retina is mediated by interphotoreceptor retinoid-binding protein (IRBP). IRBP, a 133-kDa (human) glycolipoprot ein, is the major protein component of the extracellular matrix separa ting these two cell layers, In amphibians and mammals, IRBP consists o f four homologous repeats of about 300 amino acids which form two reti nol and four fatty acid-binding sites, Here we show that IRBP in teleo sts is a simpler protein composed of only two repeats. Western blot an alysis shows that goldfish IRBP is half the size (70 kDa) of IRBP in h igher vertebrates. Metabolic labeling studies employing Brefeldin A ta ken together with in situ hybridization studies and the presence of a signal peptide show that goldfish IRBP is secreted by the cone photore ceptors. The translated amino acid sequence has a calculated molecular weight of 66.7 kDa. The primary structure consists of only two homolo gous repeats with a similarity score of 52.5%, The last repeats of hum an and goldfish IRBPs are 69.1% similar with hydrophobic regions being the most similar, These data suggest that two repeats were lost durin g the evolution of the ray-finned fish (Actinopterygii), or that the I RBP gene duplicated between the emergence of bony fish (Osteichthyes) and amphibians. Acquisition of a multirepeat structure may reflect evo lutionary pressure to efficiently transport higher levels of hydrophob ic molecules within a finite space, Quadruplication of an ancestral IR BP gene may have been an important event in the evolution of photorece ptors in higher vertebrates.