Structure and Function of G-protein--Signal Coupling Protein

视紫红质对光子的吸收触发了一个转导级联，导致视网膜感光器中的电信号。脊椎动物光感受器的光传导现已确定，g蛋白（Gt: transducin）将光激发的视紫红质与cGMP水解偶联，cGMP控制质膜中的阳离子通透性。对于信号偶联蛋白或无脊椎动物光感受器中阳离子通透性的控制，我们所知甚少。我们发现章鱼光感受器膜上的Gtpase （g蛋白）与脊椎动物的光感受器一样被光激活。在本工作中，g蛋白的特征是百日咳毒素和霍乱毒素催化ADP核糖基化。Gip α，章鱼视紫红质Gip的α亚基，被百日咳毒素核糖化，而不是被霍乱毒素核糖化，就像激素系统中的Gi一样。Gip α的分子量为41 kDa，与Gip相似，比Gt α大约2 kDa。然而，光和核苷酸对Gip α的标记作用与脊椎动物光感受器的gt α一样；在黑暗条件下，章鱼ADP的Gip α在GDP β S存在下核糖化最多。光线抑制贴标。GTP hamma S和GppnHp在黑暗中也有抑制作用。转导蛋白（Gt）被百日咳毒素和霍乱毒素标记。在章鱼微绒毛中，霍乱毒素底物为46 kDa蛋白（Gsp α），而不是Gip α (41 kDa)。46kda蛋白的合理标记程度取决于核苷酸的存在，GppNHp和GTP γ S显著增强了标记，但GTP和GppNHp具有抑制作用。GSP的标记不受光的影响，这表明GSP可能不参与光导。

Photon absorption by rhodopsin triggers a transduction cascade, leading to an electrical signal in the photoreceptor of retina. Phototransduction of vertebrate photoreceptors are now established that G-protein (Gt: transducin) couples the photoexcited rhodopsin to cGMP hydrolysis and cGMP controls the cation permeability in the plasma membranes. Much less in known about either signal coupling proteins or the control of cation permeability in invertebrate photoreceptors. We found Gtpase (G-protein) in octopus photoreceptor membranes was activated by light just like vertebrate photoreceptors. In present work, the G-proteins were characterized by pertussis toxin and cholera toxin catalyzed ADP ribosylation. Gip alpha, the alpha-subunit of Gip of octopus rhodopsin, is ADP ribosylated by pertussis toxin, but not by cholera toxin, in the same way as Gi in the hormon system. The molecular weight of Gip alpha is 41 kDa like gip which is about 2 kDa more than Gt alpha. However the effect of light and nucleotides on the labeling of Gip alpha is just like gt alpha of vertebrate photoreceptors; Gip alpha of octopus ADP ribosylated most in the presence of GDP beta S in the dark. Light inhibits labeling. Both GTP hamma S and GppnHp were inhibitory even in the dark. Transducin (Gt) is labeled by both pertussis toxin and cholera toxin. In octopus microvilli, cholera toxin substrate is 46 kDa protein (Gsp alpha), but not Gip alpha(41 kDa). The extent of ratiolabeling of the 46 kDa protein depend on nucleotide present, GppNHp and GTP gamma S, greatly enhanced albeling, but GDP beta S and GTP are inhibitory. Labeling of GSP was not influenced by light, which suggests GSP might not be involved in phototransduction.