title: Kinetic mechanism of RGS9-1 potentiation by R9AP.
abstract: The duration of the photoreceptor's response to a light stimulus determines the speed at which an animal adjusts to ever-changing conditions of the visual environment. One critical component which regulates the photoresponse duration on the molecular level is the complex between the ninth member of the regulators of G protein signaling family (RGS9-1) and its partner, type 5 G protein beta-subunit (Gbeta5L). RGS9-1.Gbeta5L is responsible for the activation of the GTPase activity of the photoreceptor-specific G protein, transducin. Importantly, this function of RGS9-1.Gbeta5L is regulated by its membrane anchor, R9AP, which drastically potentiates the ability of RGS9-1.Gbeta5L to activate transducin GTPase. In this study, we address the kinetic mechanism of R9AP action and find that it consists primarily of a direct increase in the RGS9-1.Gbeta5L activity. We further showed that the binding site for RGS9-1.Gbeta5L is located within the N-terminal putative trihelical domain of R9AP, and even though this domain is sufficient for binding, it takes the entire R9AP molecule to potentiate the activity of RGS9-1.Gbeta5L. The mechanism revealed in this study is different from and complements another well-established mechanism of regulation of RGS9-1.Gbeta5L by the effector enzyme, cGMP phosphodiesterase, which is based entirely on the enhancement in the affinity between RGS9-1.Gbeta5L and transducin. Together, these mechanisms ensure timely transducin inactivation in the course of the photoresponse, a requisite for normal vision.
authors.affiliation: Department of Ophthalmology, Duke University, Durham, North Carolina 27710, USA.
authors.forename: Sheila A
authors.initials: SA
authors.lastname: Baker
authors.forename: Kirill A
authors.initials: KA
authors.lastname: Martemyanov
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Shavkunov
authors.forename: Vadim Y
authors.initials: VY
authors.lastname: Arshavsky
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D002417:Cattle; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000493:pharmacokinetics; D019204:GTP-Binding Proteins / Q000737:chemistry; D007700:Kinetics; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D011485:Protein Binding; D025941:Protein Interaction Mapping; D020710:RGS Proteins / Q000737:chemistry*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D012492:Salts / Q000737:chemistry; D015398:Signal Transduction; D012996:Solutions / Q000737:chemistry; D013329:Structure-Activity Relationship
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D008565:Membrane Proteins; D020710:RGS Proteins; C464657:RGS9BP protein, human; D012492:Salts; D012996:Solutions; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; C072504:transducin GTP phosphohydrolase