Interphotoreceptor retinoid-binding protein (IRBP) is the first example of an extracellular matrix protein that plays a role in transporting, buffering, or mediating the actions of retinoids and fatty acids in the interphotoreceptor space. This laboratory has isolated and characterized recombinant DNA molecules necessary for the study of the structure and expression of IRBP. We determined the primary structure of the IRBP gene and its protein, prerequisites to thorough study of IRBP gene expression. The DNA clones are important substrates that provide the tools for studies of how IRBP is synthesized and functions. IRBP is a single polypeptide that contains four 300 amino acid long repeats, with 30%-40% identity among the repeats. These sequences have been helpful in the analysis of the uveitogenic peptides in IRBP. The cell in the retina that contains IRBP mRNA is the photoreceptor. IRBP mRNA is large and usually gives only one band on a Northern blot. But two sizes of IRBP mRNA are present in rat and mouse. Having analyzed the IRBP gene in many species, especially the human, we have determined that there is only one IRBP gene per haploid genome. The chromosomal location of the IRBP gene is 10q21.1 for human and 14 for mouse, chromosomal localizations that rule out IRBP as the defective gene in many inherited eye disorders. Moreover, we have demonstrated that IRBP cannot cause fundus albipunctatus, a rare autosomal recessive form of stationary night blindness with a defect in the vitamin A cycle. The IRBP gene structure, which is compact for the size of the protein, has only three introns. The remarkable quadruplication within the gene suggests an interesting evolution, possibly involving a processed gene intermediate and two unequal crossovers. We have begun to characterize the elements regulating IRBP gene expression, including both cis-elements (the DNA sequences) and transacting factors (DNA binding proteins). We find two homologous areas of sequence in the 5' flanking regions of the bovine and mouse IRBP genes, one from -1 to -350 and another at -1200 to -1410. At least two blocks of sequence (one in each homologous area) and at least one protein of 120,000 MW form DNA-protein complexes in this promoter by gel-shift assays, DNAse footprinting, and Southwestern blotting. A region including sequences 1.7 kb upstream from the start of transcription is important for maximum expression in transient assays. Shorter segments have lesser promoter activity.
