Abstract
Many lines of evidence show that membranes contain microdomains, "lipid rafts", that are different from the rest of the membrane in specific lipid and protein composition. In several biological systems, they were shown to be necessary for trafficking and signal transduction. Here, we investigate if lipid rafts have a role in the regulation of the G protein-mediated pathway underlying vertebrate phototransduction. Photoreceptor membranes contain detergent-resistant membrane (DRM) rafts. Rhodopsin and cGMP phosphodiesterase are found in raft and nonraft portions of the membrane; guanylate cyclase is found exclusively in the raft. Distribution of these proteins does not change in the light or dark. In contrast, the G protein transducin, the RGS9-1 -G beta 5L complex, and the p44 isoform of arrestin undergo dramatic translocation to the raft upon illumination. Phosphorylation of RGS9-1 occurs exclusively in the raft. GTP gamma S or pertussis toxin prevent the light-mediated translocation of transducin and RGS9-1, whereas AIF sub(4) super(-) causes both proteins to move to the raft in the dark. This shows that the G alpha t-RGS9-1-G beta 5L complex has the highest affinity to rafts in the transition state of the GTPase. GTP gamma S binds to transducin at a significantly slower rate in the raft, indicating that this translocation results in a reduced rhodopsintransducin coupling. Thus, an external signal can rearrange components of a G protein pathway in specific domains of the cell membrane, changing its signaling properties. These findings could reveal a novel mechanism utilized by the cells for regulation of G protein-mediated signal transduction.
