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.