CLONING OF THE CDNA FOR A NOVEL PHOTORECEPTOR PROTEIN

A subtractive cDNA cloning strategy was used to isolate a 1381-base pa ir human retina-specific cDNA, human retinal gene 4 (HRG4), which hybr idized to a 1.4-kilobase message in the retina and encoded a 240 amino acid acidic protein with a calculated molecular mass of 26,964 Da. Th e proximal 1/4 of the conceptual protein sequence was rich in glycine (18%) and proline (20%), had a predicted secondary structure of turns, and showed a loose similarity (19-24%) to various alpha-collagen sequ ences, while the distal 3/4 consisted of a mixture of alpha-helices, b eta-sheets, and turns. Genomic Southern analysis with HRG4 showed cros s-hybridizing sequences in six different species, and HRG4 was 92% hom ologous with a 1264-base pair rat cDNA (rat retinal gene 4; RRG4) at t he protein level. The region of 100% identity between the two sequence s corresponded to the distal 3/4 of the protein sequence consisting of mixed secondary structures, suggesting a functionally important domai n, In vitro transcription and translation corroborated the open readin g frames corresponding to HRG4 and RRG4 in the cDNAs, Expression of HR G4 in the retina was localized to the photoreceptors by in, situ hybri dization. Developmentally, RRG4 began to be highly expressed around po stnatal day 5 in the rat outer retina when the photoreceptors bean to differentiate and rapidly increased in expression to reach the mature adult level by postnatal day 23. No diurnal fluctuation in expression of RRG4 was seen.