Molecular Cloning and Characterization of the G Protein Subunit of Cone Photoreceptors

The phototransduction process in cones has been proposed to involve a G protein that couples the signal from light-activated visual pigment to the effector cyclic GMP phosphodiesterase. Previously, we have identified and purified a Gβ complex composed of a Gβ 3 isoform and an immunochemically distinct G subunit (G 8 ) from bovine retinal cones (Fung, B. K.-K., Lieberman, B. S., and Lee, R. H. (1992) J. Biol. Chem. 267, 24782-24788; Lee, R. H., Lieberman, B. S., Yamane, H. K., Bok, D., and Fung, B. K.-K. (1992a) J. Biol. Chem. 267, 24776-24781). Based on the partial amino acid sequence of this cone G 8 , we screened a bovine retinal cDNA library and isolated a cDNA clone encoding G 8 . The cDNA insert of this clone includes an open reading frame of 207 bases encoding a 69-amino acid protein. The predicted protein sequence of G 8 shares a high degree of sequence identity (68%) with the G (G 1 ) subunit of rod transducin. Similar to rod G 1 , it terminates in a CIIS motif that is the site for post-translational modification by farnesylation. Messenger RNA for G 8 is present at a high level in the retina and at a very low level in the lung, but is undetectable in other tissues. Immunostaining of bovine retinal sections with an antipeptide antibody against the N-terminal region of G 8 further shows a differential localization of G 8 to cones with a pattern indistinguishable from that of Gβ 3 . This finding suggests that Gβ 3 8 is a component of cone transducin involved in cone phototransduction and color vision.