Functional comparison of RGS9 splice isoforms in a living cell

Two isoforms of the GTPase-activating protein, regulator of G protein signaling 9 (RGS9), control such fundamental functions as vision and behavior. RGS9–1 regulates phototransduction in rods and cones, and RGS9–2 regulates dopamine and opioid signaling in the basal ganglia. To determine their functional differences in the same intact cell, we replaced RGS9–1 with RGS9–2 in mouse rods. Surprisingly, RGS9–2 not only supported normal photoresponse recovery under moderate light conditions but also outperformed RGS9–1 in bright light. This versatility of RGS9–2 results from its ability to inactivate the G protein, transducin, regardless of its effector interactions, whereas RGS9–1 prefers the G protein-effector complex. Such versatility makes RGS9–2 an isoform advantageous for timely signal inactivation across a wide range of stimulus strengths and may explain its predominant representation throughout the nervous system

Replacing the rod with the cone transducin α subunit decreases sensitivity and accelerates response decay

Cone vision is less sensitive than rod vision. Much of this difference can be attributed to the photoreceptors themselves, but the reason why the cones are less sensitive is still unknown. Recent recordings indicate that one important factor may be a difference in the rate of activation of cone transduction; that is, the rising phase of the cone response per bleached rhodopsin molecule (Rh*) has a smaller slope than the rising phase of the rod response per Rh*, perhaps because some step between Rh* and activation of the phosphodiesterase 6 (PDE6) effector molecule occurs with less gain. Since rods and cones have different G-protein α subunits, and since this subunit (Tα) plays a key role both in the interaction of G-protein with Rh* and the activation of PDE6, we investigated the mechanism of the amplification difference by expressing cone Tα in rod Tα-knockout rods to produce so-called GNAT2C mice. We show that rods in GNAT2C mice have decreased sensitivity and a rate of activation half that of wild-type (WT) mouse rods. Furthermore, GNAT2C responses recover more rapidly than WT responses with kinetic parameters resembling those of native mouse cones. Our results show for the first time that part of the difference in sensitivity and response kinetics between rods and cones may be the result of a difference in the G-protein α subunit. They also indicate more generally that the molecular nature of G-protein α may play an important role in the kinetics of G-protein cascades for metabotropic receptors throughout the body