Differential distribution of signal-transducing G-proteins in retina

We used specific antibodies in immunoblot studies of membrane fractions derived from bovine retina, and in immunohistochemical studies of sections of rat retina to determine the distribution of two guanine nucleotide binding proteins G_o (a G-protein of unknown function discovered in the brain) and transducin, in retina. Both G_o and transducin were readily detected in membranes derived from whole retina, and in crude rod outer segment membranes. Purification of rod outer segment membranes by sucrose density gradient centrifugation resulted in enrichment in transducin and depletion of G_o immunoreactivity. Transducin-α immunoreactivity was localized to photoreceptor inner and outer segments and the outer nuclear layer. In contrast, G_o-α immunoreactivity was localized in the inner and outer plexiform layers and ganglion cell layer. The results indicate that G_o unlike transducin, is not associated with rod outer segment membranes and is therefore unlikely to function in phototransduction. G_o is, however, relatively abundant in neural layers of retina where it may be involved in signal transduction.