HIGH EXPRESSION LEVELS IN CONES OF RGS9, THE PREDOMINANT GTPASE ACCELERATING PROTEIN OF RODS

RGS9 is a member of the RGS family of GTPase accelerating proteins (GA Ps) for heterotrimeric G proteins. We have explored its contribution t o GTPase acceleration in mammalian rod and cone photoreceptors. When R GS9 was specifically removed from detergent extracts of rod outer segm ents by immunodepletion, the extracts lost nearly all of their GAP act ivity stimulatable by the inhibitory subunit of cGMP phosphodiesterase . Immunolocalization using monoclonal antibodies and confocal microsco py revealed that RGS9 is present in cones at significantly higher leve ls than in rods. Thus, RGS9 is the predominant source of GAP activity in rod outer segments, and RGS9 concentration emerges as a potentially important determinant of the faster response kinetics and lower sensi tivity of mammalian cones, as compared with rods.