title: Squid rhodopsin and GTP-binding protein crossreact with vertebrate photoreceptor enzymes.
abstract: The activation of photoreceptor GTP-binding protein by rhodopsin was studied in squid photoreceptors and in crossreactions between the squid and bovine proteins. Turbidity changes were observed in the far-red after photoexcitation of rhodopsin with brief flashes and were used to probe interactions between photoreceptor membrane suspensions and soluble protein extracts. Our findings are squid photoreceptors contain a GTP-binding protein detectable by light- and GTP-sensitive turbidity changes and by limited sequence homology of a 46-kilodalton polypeptide to the alpha-subunit of bovine GTP-binding protein; the squid membranes activate bovine GTP-binding protein qualitatively in the same way as bovine rhodopsin; the 46-kilodalton component is present in a membrane-bound fraction but is more abundant in a crude, soluble fraction of squid rhabdomes, and this soluble fraction can interact with either squid or bovine rhodopsin-containing membranes; light-activated GTPase activities in all of these preparations are consistent with the light-induced turbidity changes. These results show that rhodopsin activation of GTP-binding protein is highly conserved in vertebrate and cephalopod photoreceptors. Since squid rhodopsin is immobilized in precisely ordered microvilli, this suggests that activation of GTP-binding protein in cephalopod photoreceptors occurs in the absence of rhodopsin diffusion. The rhodopsin immobility may be compensated by higher mobility of the soluble GTP-binding protein.
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D049832:Decapodiformes; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D019204:GTP-Binding Proteins; D006160:Guanosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012168:Retinal Pigments / Q000378:metabolism*; D012243:Rhodopsin / Q000378:metabolism*; D012542:Scattering, Radiation
chemical_list: D011956:Receptors, Cell Surface; D012168:Retinal Pigments; D006160:Guanosine Triphosphate; D012243:Rhodopsin; D010744:Phosphoric Monoester Hydrolases; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins