INTERPHOTORECEPTOR RETINOID-BINDING PROTEIN (IRBP), A MAJOR 124 KDA GLYCOPROTEIN IN THE INTERPHOTORECEPTOR MATRIX OF XENOPUS-LAEVIS - CHARACTERIZATION, MOLECULAR-CLONING AND BIOSYNTHESIS

We have demonstrated that the neural retina of Xenopus laevis secretes into the extracellular matrix surrounding the inner and outer segment s of its photoreceptors a glycoprotein containing hydrophobic domains conserved in mammalian interphotoreceptor retinoid-binding proteins (I RBPs). The soluble extract of the interphotoreceptor matrix contains a 124 kDa protein that cross-reacts with anti-bovine IRBP immunoglobuli ns. In vitro [H-3]fucose incorporation studies combined with in vivo l ight and electron microscopic autoradiographic analysis, showed that t he IRBP-like glycoprotein is synthesized by the neural retina and secr eted into the interphotoreceptor matrix. A 1.2 kb Xenopus IRBP cDNA wa s isolated by screening a stage 42 (swimming tadpole) lambdaZap II lib rary with a human IRBP cDNA under low-stringency conditions. The cDNA hybridizes with a 4.2 kb mRNA in adult Xenopus neural retina, tadpole heads as well as a less-abundant mRNA of the same size in brain. Durin g development, IRBP and opsin mRNA expression correlates with photorec eptor differentiation. The translated amino acid sequence of the Xenop us IRBP clone has an overall 70% identity with the fourth repeat of th e human protein. Sequence alignment with the four repeats of human IRB P showed three highly conserved regions, rich in hydrophobic residues. This focal conservation predicts domains important to the protein's f unction, which presumably is to facilitate the exchange of 11-cis reti nal and all-trans retinol between the pigment epithelium and photorece ptors, and to the transport of fatty acids through the hydrophilic int erphotoreceptor matrix.