CDNA isolation and characterization of a novel leucine zipper protein rkp1 expressed in photoreceptor cells

From the retina-enriched cDNA library which we constructed last year, thirty six clones were isolated as candidates for the retina-specific cDNA. These clones were sequenced, and used for Northern analysis to determine tissue expression. We focused on one clone BBS which was expressed only in retina and skeletal muscle among tissues we tested. We isolated a full length cDNA of 1938bp by using clone BB8(0.8 kb) as a probe from human retina cDNA library. It contained an open reading frame of 490 amino acid residues having leucine zipper motif and peroxisomal targeting sequence. The deduced amino acid sequence showed 32.3% identity in 402 amino acids with neuron specific olfactomedin-related protein and 31.3% identity in 179 amino acids with olfactomedin. This protein was designated RKP1. In situ RNA hybridization showed specific signal in the outer nuclear layer of retina where photoreceptor cells are located. Polyclonal rabbit anti-serum was raised against the recombinant RKP1 protein. Immunohistochemistry revealed that RKP1 protein is expressed prominently in the inner segment of photoreceptor cell and sarcoplasmic reticulum of skeletal muscle. These observations indicate that RKP1 may play a role in anchoring the intermediate filaments onto membrane to maintain its shape in a manner similar to peripherin.