Regulation of G protein function by an effector in GTP-dependent signal transduction: An inhibitory subunit of cGMP phosphodiesterase inhibits GTP hydrolysis by transducin in vertebrate rod photoreceptors

The regulation of cGMP phosphodiesterase in vertebrate rod photoreceptors is a typical G protein-dependent signal transduction mechanism. The interaction of Pγ, an inhibitory subunit of cGMP phosphodiesterase, with transducin α subunit (Tα) is essential for the activation of cGMP phosphodiesterase. It has been shown that, in a homogenized preparation of frog (Rana catesbiana) rods, Pγ interacts with GTP·Tα and remains tightly bound to GDP·Tα after GTP hydrolysis on Tα. Association of this complex with βγ subunits of transducin (Tβγ) triggers the release of Pγ from the complex and the subsequent inactivation of cGMP phosphodiesterase. In a system reconstituted with purified components, both GTP- and GDP-bound forms of Tα were found to interact with Pγ. Under these conditions, Pγ inhibited GTP hydrolysis by transducin in a noncompetitive manner with a K_i of 92 nM. Binding of an hydrolysis-resistant GTP analog to Tα was also inhibited by Pγ. These inhibitions of transducin function were resulted from the inhibition of both hydrolysis of GTP bound to Tα and interaction of GDP ·Tα with membrane-bound Tβγ. However, after GDP·Tα reassociated with membrane-bound Tβγ, the inhibitory effect of Pγ on the binding of an hydrolysis-resistant GTP analog to Tα was greatly diminished, suggesting that the GTP/GDP exchange on Tα was not inhibited by Pγ. These data indicate that the Tα function is altered during complexing with Pγ. G protein functions may be modified by interacting with an effector in the G protein-dependent signal transduction.