Low Activation and Fast Inactivation of Transducin in Carp Cones |
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Authors: | Shuji Tachibanaki Shin-Ichi Yonetsu Satoshi Fukaya Yuki Koshitani Satoru Kawamura |
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Affiliation: | From the ‡Graduate School of Frontier Biosciences, and ;§Department of Biological Science, Graduate School of Science, Osaka University, Suita, Osaka 565-0871, Japan |
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Abstract: | Cone photoreceptors show lower light sensitivity and briefer light responses than rod photoreceptors. The light detection signal in these cells is amplified through a phototransduction cascade. The first step of amplification in the cascade is the activation of a GTP-binding protein, transducin (Tr), by light-activated visual pigment (R*). We quantified transducin activation by measuring the binding of GTPγS in purified carp rod and cone membrane preparations with the use of a rapid quench apparatus and found that transducin activation by an R* molecule is ∼5 times less efficient in cones than in rods. Transducin activation terminated in less than 1 s in cones, more quickly than in rods. The rate of GTP hydrolysis in Tr*, and thus the rate of Tr* inactivation, was ∼25 times higher in cones than in rods. This faster inactivation of Tr* ensures briefer light responses in cones. The expression level of RGS9 was found to be ∼20 times higher in cones than in rods, which explains higher GTP hydrolytic activity and, thus, faster Tr* inactivation in cones than in rods. Although carp rods and cones express rod- or cone-versions of visual pigment and transducin, these molecules themselves do not seem to induce the differences significantly in the transducin activation and Tr* inactivation in rods and cones. Instead, the differences seem to be brought about in a rod or cone cell-type specific manner. |
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Keywords: | G proteins Photoreceptors Phototransduction Physiology Signal Transduction Cones Rods Signal Amplification |
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