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
Yamazaki, A.; Yamazaki, M.; Tsuboi, S.; Kishigami, A.; Umbarger, K.O.; Hutson, L.D.; Madland, W.T.; Hayashi, F.
Journal of Biological Chemistry 268(12): 8899-8907
The regulation of cGMP phosphodiesterase in vertebrate rod photoreceptors is a typical G protein-dependent signal transduction mechanism. The interaction of P-gamma, an inhibitory subunit of cGMP phosphodiesterase, with transducin alpha subunit (T-alpha) is essential for the activation of cGMP phosphodiesterase. It has been shown that, in a homogenized preparation of frog (Rana catesbiana) rods, P-gamma interacts with GTP cntdot T-alpha and remains tightly bound to GDP cntdot T-alpha after GTP hydrolysis on T-alpha. Association of this complex with beta-gamma subunits of transducin (T-beta-gamma) triggers the release of P-gamma from the complex and the subsequent inactivation of cGMP phosphodiesterase. In a system reconstituted with purified components, both GTP- and GDP-bound forms of T-alpha were found to interact with P-gamma. Under these conditions, P-gamma inhibited GTP hydrolysis by transducin in a noncompetitive manner with a K-i of 92 nm. Binding of an hydrolysis-resistant GTP analog to T-alpha was also inhibited by P-gamma. These inhibitions of transducin function were resulted from the inhibition of both hydrolysis of GTP bound to T-alpha and interaction of GDP cntdot T-alpha with membrane-bound T-beta-gamma. However, after GDP cntdot T-alpha reassociated with membrane-bound T-beta-gamma, the inhibitory effect of P-gamma on the binding of an hydrolysis-resistant GTP analog to T-alpha was greatly diminished, suggesting that the GTP/GDP exchange on T-alpha was not inhibited by P-gamma. These data indicate that the T-alpha function is altered during complexing with P-gamma. G protein functions may be modified by interacting with an effector in the G protein-dependent signal transduction.