Removal of phosphorylation sites of gamma subunit of phosphodiesterase 6 alters rod light response.

The phosphodiesterase 6 gamma (PDE6 gamma) inhibitory subunit of the rod PDE6 effector enzyme plays a central role in the turning on and off of the visual transduction cascade, since binding of PDE6 gamma to the transducin alpha subunit (T alpha) initiates the hydrolysis of the second messenger cGMP, and PDE6 gamma in association with RGS9-1 and the other GAP complex proteins (G beta 5, R9AP) accelerates the conversion of T alpha GTP to T alpha GDP, the rate-limiting step in the decay of the rod light response. Several studies have shown that PDE6 gamma can be phosphorylated at two threonines, T22 and T35, and have proposed that phosphorylation plays some role in the physiology of the rod. We have examined this possibility by constructing mice in which T22 and/or T35 were replaced with alanines. Our results show that T35A rod responses rise and decay more slowly and are less sensitive to light than wild-type (WT). T22A responses show no significant difference in initial time course with WT but decay more rapidly, especially at dimmer intensities. When the T22A mutation is added to T35A, double mutant rods no longer showed the prolonged decay of T35A rods but remained slower than WT in initial time course. Our experiments suggest that the polycationic domain of PDE6 gamma containing these two phosphorylation sites can influence the rate of PDE6 activation and deactivation and raise the possibility that phosphorylation or dephosphorylation of PDE6 gamma could modify the time course of transduction, thereby influencing the wave form of the light response.