Iodopsin (a red-sensitive cone visual pigment) and rhodopsin (a rod pigment) were isolated from chicken retina. They were separately reconstituted into phosphatidylcholine liposomes and then mixed with rod transducin (Tα and Tβγ) purified from bovine retina. Iodopsin enhanced, only when irradiated, the binding of GppNHp to Tα to a similar extent to irradiated rhodopsin. Furthermore, the binding of GppNHp to Tα in the presence of a photobleaching intermediate of iodopsin preferably required Tβγ-2 rather than Tβγ-1, which is very similar in profile to that in the presence of the intermediate of rhodopsin (J. Biol. Chem., in press). These results indicate that the binding domain for transducin in iodopsin should closely resemble that in rhodopsin.
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