Activation of retinal rod cyclic GMP‐phosphodiesterase by transducin: Characterization of the complex formed by phosphodiesterase inhibitor and transducin α‐subunit

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Abstract
The GTP‐binding subunit of transducin (Tα) activates the cGMP phosphodiesterase (PDE) of bovine retinal rods by relieving the constraint imposed by the inhibitory subunit PDEγ. We have isolated and characterized the complex Tα.GTPγS‐PDEγ formed when Tα is activated by the nonhydrolyzable analog GTPγS. Sedimentation and light‐scattering techniques demonstrate that, in contrast to free Tγ.GTPγS, which is soluble, the Tα.GTPγS‐PDEγ complex, as well as Tα.GTP‐PDEγ, is membrane bound at cytosolic ionic strength. It is eluted from the membrane at low ionic strength as a monomeric and 1:1 stoichiometric complex. The relative affinities of PDEγ for PDEαβ and for Tα.GTP are discussed.