ZEBRAFISH INTERPHOTORECEPTOR RETINOID-BINDING PROTEIN - DIFFERENTIAL CIRCADIAN EXPRESSION AMONG CONE SUBTYPES

Retinoid trafficking between the photoreceptors and pigmented epitheli um is probably mediated by interphotoreceptor retinoid-binding protein (IRBP), a 124-145 kDa glycolipoprotein in mammals and amphibians. In these animals, IRBP is composed of four homologous regions (modules) 3 00 amino acids in length. We have determined the primary structure of zebrafish IRBP and its expression pattern by northern analysis, revers e transcriptase-polymerase chain reaction and in situ hybridization un der a variety of lighting conditions. Zebrafish IRBP is half the size (66.3 kDa) of mammalian IRBP because it is composed of only two module s, similar to goldfish IRBP. The first half of the zebrafish protein i s most similar to the first module of mammalian IRBP and the second ha lf to the fourth module of mammalian IRBP. This suggests that during t he evolution of the ray-finned fish (Actinopterygii), the middle two m odules were lost. Each of the modules contains conserved hydrophobic d omains which may form the ligand-binding pocket. The expression of zeb rafish IRBP mRNA is sevenfold higher in the middle of the light period (at mid-light) than in the middle of the dark period (at mid-dark). T his rhythm persists for 2 days under conditions of constant light or c onstant darkness, then dampens to an intermediate level by 8 days of c onstant conditions. At mid-light, IRBP mRNA is expressed by all cone t ypes and to a lesser extent by the rods. At mid-dark, the mRNA is rest ricted to the ultraviolet-sensitive short single cones. These data sug gest that IRBP expression is regulated by circadian and light-driven m echanisms that act differentially on the various photoreceptor subtype s in the zebrafish retina.