title: Enhancement of rod outer segment GTPase accelerating protein activity by the inhibitory subunit of cGMP phosphodiesterase.
abstract: The cGMP phosphodiesterase (PDE) of retinal rod outer segments (ROS) is activated by the GTP-bound form of the G protein, transducin (Gt alpha). This activation can be reversed by the inhibitory gamma subunit of PDE through two distinct mechanisms: acceleration of GTP hydrolysis and direct inactivation independent of GTP hydrolysis. We have found that acceleration of Gt alpha GTPase by PDE gamma does not occur upon formation of a Gt alpha PDE gamma complex but rather reflects enhanced activity toward this complex of a membrane-bound GTPase accelerating protein. GTPase rate constants for Gt alpha in the presence of 3.3 microM PDE gamma were as high as 0.7 s-1 with hypotonically washed ROS membranes at 40 microM rhodopsin but were more than 10-fold lower when protein-free vesicles containing ROS lipids were substituted for ROS membranes. Acceleration of Gt alpha GTPase by PDE gamma was also barely detectable at low ROS concentrations (e.g. 4 microM rhodopsin) or if ROS treated with trypsin or urea were used. GTPase-independent inactivation by PDE gamma occurred efficiently at much lower membrane concentrations. Inhibition of Gt alpha-activated PDE was much slower than inhibition of PDE alpha beta by PDE gamma. Effects of PDE gamma upon successive additions of GTP suggested formation of a complex of PDE gamma and Gt alpha-GDP that is refractory to reactivation.
authors.affiliation: Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030.
authors.forename: J K
authors.initials: JK
authors.lastname: Angleson
authors.forename: T G
authors.initials: TG
authors.lastname: Wensel
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000201:enzymology / Q000378:metabolism*; D015293:Transducin / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D006160:Guanosine Triphosphate; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D020558:GTP Phosphohydrolases; D015293:Transducin