title: Low activation and fast inactivation of transducin in carp cones.
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.
authors.affiliation: Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan.
authors.forename: Shuji
authors.initials: S
authors.lastname: Tachibanaki
authors.forename: Shin-Ichi
authors.initials: S
authors.lastname: Yonetsu
authors.forename: Satoshi
authors.initials: S
authors.lastname: Fukaya
authors.forename: Yuki
authors.initials: Y
authors.lastname: Koshitani
authors.forename: Satoru
authors.initials: S
authors.lastname: Kawamura
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D002347:Carps; D019204:GTP-Binding Proteins / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000737:chemistry / Q000378:metabolism; D006868:Hydrolysis; D008027:Light; D008956:Models, Chemical; D008962:Models, Theoretical; D010858:Pigmentation; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism*; D012168:Retinal Pigments / Q000378:metabolism; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism; D015398:Signal Transduction; D013997:Time Factors; D015293:Transducin / Q000378:metabolism*
chemical_list: D012168:Retinal Pigments; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D019204:GTP-Binding Proteins; D015293:Transducin