Influence of GDP on interaction of transducin with cyclic nucleotide phosphodiesterase and rhodopsin from bovine retinal rods

In the presence of guanine nucleotides and rhodopsin-containing membranes from bovine retinal rod outer segments transducin stimulates light-sensitive cyclic nucleotide phosphodiesterase 5.5- to 7-fold. The activation constant (K/sub act/) for GTP and Gpp(NH)p is equal to 0.25 ..mu..M, while that for GDP and GDP..beta..S is 14 and 110 ..mu..M, respectively. GDP free of admixtures of other nucleotides does not activate phosphodiesterase at concentrations up to 1 mM, but is bound to transducin and inhibits the Gpp(NH)p-dependent activation of phosphodiesterase. The nature of the interaction of transducin with depolarized rhodopsin also depends on the type of guanine nucleotide bound: in the presence of GDP rhodopsin-containing membranes bind 70-100% of the transducin, whereas in the presence of Gpp(NH)p only 13% of the protein is bound. The data obtained indicate that GDP and GTP convert transducin to two different functional states: the transducin-GTP complex is bound to phosphodiesterase and activates it, while the transducin-GDP complex is bound primarily to rhodopsin.