Interphotoreceptor retinoid-binding protein: biochemistry and molecular biology

Interphotoreceptor retinoid-binding protein (IRBP) is a recent and still somewhat enigmatic newcomer in the family of photoreceptor-specific proteins involved in vision. It has been isolated, characterized, cloned and sequenced from several species in less than a decade. Its extracellular localization and the increased amount of bound all-trans retinol following illumination are consistent with the proposed function of IRBP in extracellular transport or buffering of retinol and retinal isomers in the IPM. Its capacity to bind a great many other hydrophobic ligands, however, may indicate a multi-purpose function. Many questions concerning biosynthesis and processing, secretion, tertiary structure and ligand binding remain to be answered. Perhaps most interesting, from a molecular biology point of view, will be the regulation of transcription and tissue-specific expression of this photoreceptor protein, and possible involvement of its gene in human retinal degenerations. Constructs consisting of IRBP regulatory sequences and foreign antigens incorporated into the germ line of transgenic mice will help in the characterization of promoters and enhancer elements, while suppression of IRBP gene expression and its consequences may give us a more precise answer about IRBP function.