Parietal-eye phototransduction components and their potential evolutionary implications

The parietal-eye photoreceptor is unique because it has two antagonistic light signaling pathways in the same cell - a hyperpolarizing pathway maximally sensitive to blue light and a depolarizing pathway maximally sensitive to green light. Here, we report the molecular components of these two pathways. We found two opsins in the same cell: the blue-sensitive pinopsin and a previously unidentified green-sensitive opsin, which we name parietopsin. Signaling components included gustducin-alpha and G alpha(o), but not rod or cone transducin-alpha. Single-cell recordings demonstrated that G(o) mediates the depolarizing response. Gustducin-alpha resembles transducin-alpha functionally and likely mediates the hyperpolarizing response. The parietopsin-G(o) signaling pair provides clues about how rod and cone phototransduction might have evolved.