Upon absorption of light, rhodopsin activates transducin (Gt) through gdp-GTP exchange reaction on the alpha-subunit of transducin. This activation triggers the G-protein-mediated signal transduction cascade that even...
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Upon absorption of light, rhodopsin activates transducin (Gt) through gdp-GTP exchange reaction on the alpha-subunit of transducin. This activation triggers the G-protein-mediated signal transduction cascade that eventually generates electrical responses in the photoreceptor cells. The exchange reaction proceeds through at least three steps, which include the binding of photoactivated rhodopsin to the gdp-bound Gt, the dissociation of gdp from the complex, and the binding of GTP to the nucleotide-unbound Gt. These steps have been thought to occur after the formation of the rhodopsinintermediate, meta-Ⅱ. However, the extra formation of meta-Ⅱ, which reflects the formation of a complex with Gt, was inhibited in the presence of excess gdp, suggesting that Gt in the meta-Ⅱ-Gt complex is in a nucleotide-unbound form. Therefore, identification of the intermediate state that binds to the gdp-bound Gt is important for a better understanding of the molecular mechanism of G-protein activation by rhodopsin.
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