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∗.
