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

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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.