G protein-effector coupling: binding of rod phosphodiesterase inhibitory subunit to transducin

The cyclic GMP phosphodiesterase of retinal rods is composed of three distinct polypeptides: alpha (90 kDa), beta (86 kDa), and gamma (10 kDa). In this multimeric form, the enzyme is inhibited. Its activity is stimulated by the interaction with the GTP-bound form of the T alpha subunit of transducin and reversed upon the recombination of the inhibitory gamma subunit with the catalytic alpha beta subunit. We show here by a novel coimmunoprecipitation technique that the gamma subunit, but not the alpha beta subunit, forms a 1:1 complex with T alpha. The binding of gamma to T alpha is nucleotide-dependent and is facilitated by GTP gamma S or Gpp(NH)p. This study provides convincing evidence that the T alpha-GTP subunit of transducin stimulates phosphodiesterase activity by binding to gamma and physically carrying it away from alpha beta.