MOLECULAR-CLONING AND CHARACTERIZATION OF THE G-PROTEIN GAMMA-SUBUNIT OF CONE PHOTORECEPTORS

The phototransduction process in cones has been pro posed 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 G beta gamma complex composed of a G beta(3) isoform and an immunochemically distinct G gamma subunit (G gamma(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 gamma(8), we screened a bovine retinal cDNA library and isolated a cDNA clone encoding G gamma(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 gamma(8) shares a high degree of sequence identity (68%) with the G gamma (G gamma(1)) subunit of rod transducin. Similar to rod G gamma(1), it terminates in a CIIS motif that is the site for post-translational modification by farnesylation. Messenger RNA for G gamma(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 gamma(8) further shows a differential localization of G gamma(8) to cones with a pattern indistinguishable from that of G beta(3). This finding suggests that G beta(3) gamma(8) is a component of cone transducin involved in cone phototransduction and color vision.