Figure 2. Multiple opsin-driven G protein-coupled signaling cascades.

(A) G_t-mediated transduction (e.g. mammalian rods and cones) couples an activated opsin to a G protein called transducin (G_t). G_t stimulates a phosphodiesterase (PDE), which causes a decline in cyclic GMP (cGMP) levels and subsequent closure of the cyclic nucleotide-gated (CNG) cation channels resulting in hyperpolarization.

(B) G_q-mediated transduction (e.g. Drosophila photoreceptor cells) couples an activated opsin to G_q, which stimulates phospholipase C (PLC), resulting in hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂). The subsequent openings of TRP channels result in depolarization. IP₃, inositol triphosphate; DAG, diacylglycerol.

(C) G_o-mediated transduction (e.g. scallop visual cells) links an activated opsin to G_o, which activates guanylyl cyclase (GC). This leads to opening of CNG K⁺ channels and hyperpolarization. G_o-mediated signaling cascades have also been observed in sea slug simple photoreceptors and in the lizard parietal eye. Several encephalopsins (pufferfish TMT and mosquito Opn3) are suggested to couple to G_o and G_i, but the downstream components are less clear. Moreover, vertebrate neuropsins (Opn5) couple to G_i and likely inhibits adenylyl cyclase activity.

(D) G_s-mediated transduction (e.g. tilapia erythrophores and box jellyfish visual cells) links an activated opsin to G_s, which activates adenylyl cyclase (AC). The consequent increase in cAMP opens CNG cation channels resulting in depolarization.