G␤5 exists as two splice variants, G␤5-S and G␤5-L, which interact with and stabilize the R7 members of the regulators of G-protein signaling (RGSs): RGS6, RGS7, RGS9, and RGS11. Although the role of G␤5-L and RGS9-1 is established in photoreceptors, the physiological functions of G␤5-S and other R7 RGS proteins remain unclear. We found that the electroretinogram of G␤5 Ϫ/Ϫ mice lacks the b-wave component and that G␤5-S and RGS11 colocalize with Go␣ at the tips of the ON-bipolar cell dendrites. Unexpectedly, we found a significant reduction in the number of synaptic triads in the outer plexiform layer (OPL) of the G␤5 Ϫ/Ϫ mice, which is evident at postnatal day 14. Transgenic expression of G␤5-L in rods failed to rescue the b-wave or the OPL defects. These results indicate that G␤5-S is indispensable for OPL integrity and normal light responses of the retina.

The data demonstrate the presence of a delayed ERG b-wave in SG711 mice and a functionally redundant role for RGS11 and -7 at the tips of ON-bipolar cell dendrites. These results suggest that RGS11 or -7 works as the major physiological GAP (GTPase acceleration protein) for Galphao1 in ON-bipolar cells.

We have generated a mouse with rod photoreceptors overexpressing the ␥ inhibitory subunit (PDE6␥) of the photoreceptor G-protein effector cGMP phosphodiesterase (PDE6). PDE6␥ 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␥ binding to activated transducin ␣ GTP (T ␣ -GTP) before the T ␣ -GTP binds to endogenous PDE6␥. Excess PDE6␥ 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␥ but lack RGS9. Our results show that the PDE6␥ binding sites of PDE6 ␣ and ␤ are accessible to excess (presumably cytoplasmic) PDE6␥ in the light, once endogenous PDE6␥ has been displaced from its binding site by T ␣ -GTP. They also suggest that in the presence of T ␣ -GTP, the PDE6␥ remains attached to the rest of the PDE6 molecule, but after conversion of T ␣ -GTP to T ␣ -GDP, the PDE6␥ 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. Key words: rod; phototransduction; retina; phosphodiesterase; G-protein; RGS protein IntroductionPhotoexcited rhodopsin in a vertebrate rod binds to and activates the G-protein transducin, facilitating the exchange of GTP for GDP on the transducin ␣ subunit (T ␣ or GNAT1). The T ␣ -GTP then binds to the inhibitory ␥ subunit (PDE6␥) of the phosphodiesterase effector enzyme (PDE6), relieving the inhibition of the PDE6 ␣ and ␤ catalytic subunits. Activated PDE6 hydrolyzes cGMP, leading to the closing of the cGMP-gated channels in the outer segment. This produces the hyperpolarizing light response that signals the detection of the light to the rest of the nervous system .The turnoff of the photoreceptor response and reopening of the channels requires the inactivation of rhodopsin by phosphorylation and subsequent binding of arrestin, as well as the return of the PDE6 to its dark resting level by hydrolysis of T ␣ -GTP back to T ␣ -GDP. The intrinsic rate of transducin GTP hydrolysis is slow but is facilitated by interaction of transducin with other proteins . The first of these to be identified was PDE6␥, which was initially thought to act by itself to accelerate GTP hydrolysis (Arshavsky and Bownds, 1992) but was later shown to have no effect on the rate of hydrolysis in isolation (Angleson and Wensel, 1993;Antonny et al., 1993) and to require additional components, subsequently identified as RGS9 -1 (He et al., 1998), G␤5 (Makino et al., 1999, and a membrane anchor protein, R9AP (Hu and Wensel, 2002). These together form a GTPase activating protein (GAP) complex that is essential for the rapid conve...

No abstract

No abstract