In a former life, I spent 30 years running a laboratory that studies how light is changed into a nerve signal in our eyes. Much of our work centered on the visual pigment rhodopsin, which starts an excitation cascade after its excitation by light by binding to the alpha subunit of a G-protein. I am in awe of new technologies that have, since my work, revealed many of the finer details of this process. Thus I can't resist showing this beautiful graphic from a recent review by Schwartz and Hubbell, describing work by Sheerer et al.
a, Rhodopsin, shown here in its inactivated conformation, is a light-sensing receptor found in cell membranes. It consists of a protein (opsin, green) and a ligand (retinal, pink, also shown in its inactivated conformation). When activated by light, rhodopsin binds to part of an adjacent G protein (binding region in red), triggering a cascade of biological responses. The protein plug (blue) is part of the extracellular domain of opsin, and immobilizes the extracellular transmembrane segments of the receptor. b, Scheerer et al. have determined the activated structure of opsin in complex with the receptor-binding peptide fragment of the G protein (the Galpha peptide). The most notable difference when compared with the inactivated receptor is that transmembrane helix 6 (TM-VI) has moved substantially outward (indicated by the red arrow), thereby creating the binding pocket for the G-protein peptide.