Binding of GTP to transducin is not inhibited by arrestin and phosphorylated rhodopsin

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Abstract
In the presence of a photobleaching intermediate of unphosphorylated or phosphorylated rhodopsin (Rh∗), the binding of GppNHp to transducin was measured with or without arrestin for elucidation of the shut‐off mechanism of the visual transduction process in bovine rod outer segments. The ability of Rh∗to catalyze the formation of the transducin‐GppNHp complex in the absence of arrestin was independent of the degree of phosphorylation of Rh∗. Furthermore, the catalyzing ability of the phosphorylated Rh∗was not reduced by the addition of arrestin. These observations indicate that the interaction between phosphorylated Rh∗and transducin was not inhibited by arrestin. Thus, the hypothesis was not supported that the PDE shut‐off process is a simple competition between transducin and arrestin for binding to phosphorylated Rh∗.