A 3RD FORM OF THE G-PROTEIN BETA-SUBUNIT .1. IMMUNOCHEMICAL IDENTIFICATION AND LOCALIZATION TO CONE PHOTORECEPTORS

Vertebrate retinal cones play a major role in both photopic vision and color perception. Although the molecular mechanism of visual excitation in the cone is not as well understood as in the rod, it is generally thought to involve a cone-specific G protein (cone transducin) that couples the cone visual pigment to a cGMP phosphodiesterase. Like all other G proteins, cone transducin is most likely a heterotrimer consisting of Galpha, Gbeta, and Ggamma subunits. A Galpha subunit of cone transducin has been localized to the outer segment of bovine cones, but its associated Gbeta and Ggamma subunits are unknown. To identify the Gbeta subunit involved in the phototransduction process of cones, we have developed a panel of antipeptide antisera against the most diverse region of the amino acid sequences encoded by Gbeta1, Gbeta2, and Gbeta3 cDNAs and used them to determine the distribution of the Gbeta isoforms in different retinal preparations. We found that the Gbeta3 subunit is present in bovine retinal transducin and phosducin-Tbetagamma complex preparations which were previously thought to contain only Gbeta1. Analysis of its subcellular distribution indicated that Gbeta3 is predominantly cytoplasmic. Immunocytochemical staining of bovine retinal sections with the anti-Gbeta3 antiserum further revealed a specific localization of Gbeta3 in cones but not in rods. In contrast, anti-Gbeta1 antiserum stained only the rods. These results suggest that Gbeta3 is the Gbeta subunit of cone transducin and confirms the proposition that rods and cones utilize distinct signaling proteins for phototransduction.