Reciprocal control of retinal rod cyclic GMP phosphodiesterase by its γ subunit and transducin

Abstract

The switching on of the cGMP phosphodiesterase (PDE) in retinal rod outer segments by activated transducin (Tα-GTP) is a key step invisual excitation. The finding that trypsin activates PDE (αβγ) by degrading its γ subunit and the reversal of this activation by γ led to the proposal that Tα-GTP activates PDE by relieving an inhibitory constraint imposed by γ (Hurley and Stryer: J. Biol. Chem. 257:11094–11099, 1982). We report here studies showing that the addition of γ subunit also reverses the activation of PDE by Tα-GTP-γS. A procedure for preparing γ in high yield (50–80%) is presented. Analyses of SDS polyacrylamide gel slices confirmed that inhibitorya activity resides in the γ subunit. Nanomolar γ blocks the activation of PDE by micromolar Tα-GTPγS. The degree of activation of PDE depends reciprocally on the concentrations of γ and Tα-GTPγS. γ remains bound to the disk membrane during the activation of PDE by transducin. The binding of γ to the αβ subunits of native PDE is very tight; the dissociation constant is less than 10 pM, indicating that fewer than 1 in 1,700 PDE molecules in rod outer segments are activated in the absence of Tα-GTP.