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Interaction of retinal transducin with gtp analogues specificity of the gamma phosphate binding region


, : Interaction of retinal transducin with gtp analogues specificity of the gamma phosphate binding region. Biochemistry 25(20): 6149-6153

The interaction of six hydrolysis-resistant analogues of GTP with transducin, the signal-coupling protein in vertebrate photoreceptors, was investigated. GppNHp and GppCH2p differ from GTP at the bridging position between the .beta.- and .gamma.-phosphate groups. The other analogues studied (GTP.gamma.F, GTP.gamma.OMe, GTP.gamma.OPh, and GTP.gamma.S) differ from GTP in containing a substituent on the .gamma.-phosphorus atom or at a nonbridging .gamma.-oxygen atom. Competition binding experiments were carried out by adding an analogue, [.alpha.-32P]GTP, and a catalytic amount of photoexcited rhodopsin (R*) to transducin and measuring the amount of bound [.gamma.-32P]GTP. The order of effectiveness of these analogues in binding to transducin was GTP.gamma.S > GTP .mchgt. GppNHp > GTP.gamma.OPh > GTP.gamma.OMe > GppCH2p > GTP.gamma.F A second assay measured the effectiveness of GTP.gamma.S, GppNHp, and GppCH2p in eluting transducin from disc membranes containing R*. The basis of this assay is that transducin is released from disc membranes when it is activated to the GTP form. The relative potency of these three analogues in converting transducin from a membrane-bound to a soluble form was 1000, 75, and 1, respectively. Stimulation of cGMP phosphodiesterase activity served as a third criterion of the interaction of these analogues with transducin. The order of effectiveness of these analogues in promoting the transducin-mediated activation of the phosphodiesterase was GTP.gamma.S > GTP .mchgt. GppNHp > GTP.gamma.OPh .mchgt. GppCH2p > GTP.gamma.OMe > GTP.gamma.F GTP.gamma.S was more than a 1000 times as potent as GTP.gamma.F in activating the phosphodiesterase. For GTP.gamma.OPh, GTP.gamma.OMe, and GTP.gamma.F, the order of effectiveness in stimulating the phosphodiesterase was the same as previously reported for the activation of adenylate cyclase in pigeon erythrocyte membranes [Pfeuffer, T., and Eckstein, F. (1976) FEBS Lett. 67, 354-358] but the opposite of that found with bacterial elongation factor G [Eckstein, F., Bruns, W., and Parmeggiani, A. (1975) Biochemistry 14, 5225-5232]. Thus, transducin and the stimulatory G protein have similar binding sites for the .gamma.-phosphoryl group of GTP, whereas that of elongation factor G is significantly different.

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