FIGURE 15-21 Inhibition of rhodopsin signaling by rhodopsin kinase. Light-activated rhodopsin (R*), but not dark-adapted rhodopsin, is a substrate for rhodopsin kinase. The extent of rhodopsin phosphorylation is proportional to the amount of time each rhodopsin molecule spends in the light-activated form; the greater the extent of the phosphorylation, the more the ability of R* to activate transducin is reduced. Arrestin binds to the completely phosphorylated opsin, forming a complex that cannot activate transducin at all. See A. Mendez et al., 2000, Neuron 28:153, and V. Arshavsky, 2002, Trends Neurosci. 25:124.