CLONING OF A CYCLIC-GMP PHOSPHODIESTERASE GAMMA-SUBUNIT FROM THE GROUND-SQUIRREL RETINA

Recent evidence suggests that the two main classes of cones are not on ly equipped with different photopigments, but also exhibit differences in their downstream phototransduction cascade. An antibody against th e gamma subunit of retinal cGMP phosphodiesterase (PDE) had previously been found to label all photoreceptors in the ground squirrel (Spermo philus tridecemlineatus). This property was utilized for the cloning o f a cDNA fragment encoding the corresponding polypeptide. A lambda gt1 1 cDNA library was constructed and screened with the antibody. Positiv e clones were isolated, subcloned, and sequenced. Clones were used as probes in RNA blot hybridization, and the obtained sequence informatio n was compared to other available gamma-PDE sequences. Four virtually identical cDNA clones were isolated. Both nucleic acid and amino acid sequence alignment placed this gene in the same group as bovine and hu man cone gamma-phosphodiesterase subunits. The predicted length of the translated protein was 84 amino acids, and its molecular weight 11 kD . The cDNA hybridized with an 0.6-kb transcript in retinal RNA from gr ound squirrel, Syrian golden hamster, and mouse, and also with a 1.4-k b transcript in the ground squirrel. We conclude that the isolated cDN A fragment encodes a gamma subunit of cone cGMP-PDE. This subunit is e xpressed in middle-wavelength sensitive cones (the predominant photore ceptor type in this species). It remains to be determined if this subu nit is expressed in short-wavelength sensitive cones as well, or if th ese photoreceptors are equipped with another, unique gamma-PDE subunit . (C) 1998 Elsevier Science B.V.