MOLECULAR BIOLOGY OF OUTER RETINA-SPECIFIC PROTEINS

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The retinal pigment epithelial (RPE) cells and the photoreceptor (PR) cells are functionally and developmentally closely integrated. We are characterizing a 65 kD RPE-specific protein, which we have found is membrane-associated, hydrophobic, and dependent upon detergent solubilization for its extraction. In cell fractionation experiments, it does not copurify with cytoskeletal proteins. We have obtained its amino acid analysis and the sequences of several tryptic peptides. Its amino terminus is blocked. Despite certain suggestive similarities with other proteins in the database, we conclude that the 65 kD protein is likely a novel entity. We are screening a bovine RPE cDNA library for a cDNA for this protein. We have subcloned DNA fragments corresponding to several fragments of bovine IRBP into a bacterial expression vector. IRBP is involved in the transport of retinoids, a functional relationship between the RPE and the PR. The resultant expressed protein fragments were tested for their retinoid-binding functions. One of the fragments, which comprises the first two and one-third repeats, binds retinol, indicating that all of the four repeats of IRBP are not required for retinoid binding activity.