Chromium(III) .beta.,.gamma.-bidentate guanine nucleotide complexes as probes of the GTP-activated cGMP cascade of retinal rod outer segments

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Abstract
The exchange-inert Cr(III), .beta.-.gamma.-bidentate guanine nucleotide complexes Cr(III)GTP and Cr-(III)Gpp(NH)p were used to probe the role of transducin in activating the retinal cGMP cascade. The Cr(III) nucleotide complexes was found to have lower binding affinity for transducin as compared to the Mg2+ complexes. However, the rate of hydrolysis of the transducin-bound Cr(III)GTP was similar to that of Mg(II)GTP. Cr(III)Gpp(NH)p activated the cGMP phosphodiesterase of photolyzed rod outer segment membranes up to 75% of the Mg(II)Gpp(NH)p level but lacked the ability to dissociate the transducin subunits from the rod outer segment membrane. This result implies that the activation of the phosphodiesterase by transducin-GTP complex is a membrane-associated event and the formation of a soluble complex of transducin-GTP with the inhibitory peptide of the phosphodiesterase may not be an obligatory step. Both the .DELTA. and .LAMBDA. screw sense stereoisomers of Cr(III)Gpp(NH)p were capable of activating the cGMP cascade with no apparent stereoselectivity. The nature of the interaction of the metal ion and GTP at the nucleotide-binding site of transducin is discussed together with the results from previous studies using the phosphorothioate GTP analogues [Yamanaka, G., Eckstein, F., and Stryer, L. (1985) Biochemistry 24, 8094-8101] and is compared to the site found in homologous GTP-binding proteins such as elongation factor Tu [Jurnak, F. (1985) Science (Washington, D.C.) 230, 32-36; la Cour, T. F. M., Nyborg, J., Thirup, S., and Clark, B. F. C. (1985) EMBO J. 4, 2385-2388]. The implications of the observed results on the molecular mechanism of visual signal transduction are discussed.