Molecular cloning of a rod opsin cDNA from the skate retina

Skates (Raja erinacea and R. ocellata) are among the few animals that have an exclusively rod retina. However, skate rods are unusual in that they are capable of adapting to extremely high levels of illumination that initially saturate the rod photocurrent. This adaptive process restores the ability of the visual cells to respond to incremental photic stimuli and enables them to function under ambient conditions that are subserved by the cone mechanism in mixed (rod/cone) retinae. As a first step towards exploring the molecular basis of visual adaptation in the skate retina, we have cloned and analyzed the opsin cDNA from a skate retina library. The cDNA codes for a protein 354 amino acids (aa) long and 39.7 kDa predicted molecular mass, and labels a single abundant transcript of 1.7 kb in retinal RNA. Amino acid alignments and a parsimony analysis of nucleotide alignments show the skate opsin to be homologous to other rod opsins. An analysis of the aa sequence reveals a high degree of conservation of those residues thought to be important for most aspects of rhodopsin function. However, a few critical aa replacements may indicate alterations in the interactions of skate rhodopsin with other proteins in the phototransduction cascade. In particular, replacements of Glu150 with serine and Cys323 with leucine are in cytoplasmic domains thought to interact with transducin and rhodopsin kinase. The latter change eliminates one of the conserved acylation sites in the carboxyl terminal tail. These substitutions increase the similarity of the cytoplasmic domains of skate opsin to those of blue-sensitive visual pigments.