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 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 catesbeiana) rods, P gamma interacts with GTP.T alpha and remains tightly bound to GDP.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 Ki 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.T alpha with membrane-bound T beta gamma. However, after GDP.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.
Kresge Eye Institute, Detroit, Michigan.
A
A
Yamazaki
M
M
Yamazaki
S
S
Tsuboi
A
A
Kishigami
K O
KO
Umbarger
L D
LD
Hutson
W T
WT
Madland
F
F
Hayashi
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006868:Hydrolysis; D007700:Kinetics; D010786:Photoreceptor Cells / Q000378:metabolism*; D011892:Rana catesbeiana; D015398:Signal Transduction; D015293:Transducin / Q000378:metabolism*
D006160:Guanosine Triphosphate; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D019204:GTP-Binding Proteins; D015293:Transducin