UCLA

We have generated a mouse with rod photoreceptors overexpressing the gamma inhibitory subunit ( PDE6 gamma) of the photoreceptor G-protein effector cGMP phosphodiesterase ( PDE6). PDE6 gamma overexpression decreases the rate of rise of the rod response at dim intensities, indicating a reduction in the gain of transduction that may be the result of cytoplasmic PDE6 gamma binding to activated transducin alpha GTP (T-alpha-GTP) before the T-alpha-GTP binds to endogenous PDE6 gamma. Excess PDE6 gamma also produces a marked acceleration in the falling phase of the light response and more rapid recovery of sensitivity and circulating current after prolonged light exposure. These effects are not mediated by accelerating GTP hydrolysis through the GAP ( GTPase activating protein) complex, because the decay of the light response is also accelerated in rods that overexpress PDE6 gamma but lack RGS9. Our results show that the PDE6 gamma binding sites of PDE6 alpha and beta are accessible to excess (presumably cytoplasmic) PDE6 gamma in the light, once endogenous PDE6 has been displaced from its binding site by T-alpha-GTP. They also suggest that in the presence of T alpha-GTP, the PDE6 gamma remains attached to the rest of the PDE6 molecule, but after conversion of T-alpha-GTP to T-alpha-GDP, the PDE6 gamma may dissociate from the PDE6 and exchange with a cytoplasmic pool. This pool may exist even in wild- type rods and may explain the decay of rod photoresponses in the presence of nonhydrolyzable analogs of GTP.