The delta subunit of type 6 phosphodiesterase reduces light-induced cGMP hydrolysis in rod outer segments

The delta subunit of the rod photoreceptor PDE has previously been shown to copurify with the soluble form of the enzyme and to solubilize the membran e-bound form (1), To determine the physiological effect of the delta subuni t on the light response of bovine rod outer segments, we measured the real time accumulation of the products of cGMP hydrolysis in a preparation of pe rmeablized rod outer segments. The addition of delta subunit GST fusion pro tein (delta -GST) to this preparation caused a reduction in the maximal rat e of cGMP hydrolysis in response to light. The maximal reduction of the lig ht response was about 80%, and the half-maximal effect occurred at 385 nM d elta subunit, Several experiments suggest that this effect was not due to t he effects of delta -GST on transducin or rhodopsin kinase. Immunoblots dem onstrated that exogenous delta -GST solubilized the majority of the PDE in ROS but did not affect the solubility of transducin. Therefore, changes in the solubility of transducin cannot account for the effects of delta -GST i n the pH assay. The reduction in cGMP hydrolysis was independent of ATP, wh ich indicates that it was not due to effects of delta -GST on rhodopsin kin ase. In addition to the effect on cGMP hydrolysis, the delta -GST fusion pr otein slowed the turn-off of the system. This is probably due, at least in part, to an observed reduction in the GTPase rate of transducin in the pres ence of delta -GST. These results demonstrate that delta -GST can modify th e activity of the phototransduction cascade in preparations of broken rod o uter segments, probably due to a functional uncoupling of the transducin to PDE step of the signal transduction cascade and suggest that the delta sub unit may play a similar role in the intact outer segment.