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 photorecept ors is a typical G protein-dependent signal transduction mechanism. Th e interaction of Pgamma, an inhibitory subunit of cGMP phosphodiestera se, with transducin alpha subunit (Talpha) is essential for the activa tion of cGMP phosphodiesterase. It has been shown that, in a homogeniz ed preparation of frog (Rana catesbiana) rods, Pgamma interacts with G TP.Talpha and remains tightly bound to GDP.Talpha after GTP hydrolysis on Talpha. Association of this complex with betagamma subunits of tra nsducin (Tbetagamma) triggers the release of Pgamma from the complex a nd the subsequent inactivation of cGMP phosphodiesterase. In a system reconstituted with purified components, both GTP- and GDP-bound forms of Talpha were found to interact with Pgamma. Under these conditions, Pgamma inhibited GTP hydrolysis by transducin in a noncompetitive mann er with a K(i) of 92 nm. Binding of an hydrolysis-resistant GTP analog to Talpha was also inhibited by Pgamma. These inhibitions of transduc in function were resulted from the inhibition of both hydrolysis of GT P bound to Talpha and interaction of GDP.Talpha with membrane-bound Tb etagamma. However, after GDP.Talpha reassociated with membrane-bound T betagamma, the inhibitory effect of Pgamma on the binding of an hydrol ysis-resistant GTP analog to Talpha was greatly diminished, suggesting that the GTP/GDP exchange on Talpha was not inhibited by Pgamma. Thes e data indicate that the Talpha function is altered during complexing with Pgamma. G protein functions may be modified by interacting with a n effector in the G protein-dependent signal transduction.