Light inhibition of bovine retinal rod guanylyl cyclase mediated by beta gamma-transducin.
Photoreceptor guanylyl cyclase (ROS-GC), converting GTP into cGMP and pyrophosphate, is a key enzyme in the regulation of the visual transduction cascade. ROS-GC requires GC-activating proteins (GCAPs) and low free [Ca] for full activity. We found that when choline or potassium were the major cations present, light caused a 70% inhibition of stimulated ROS-GC in native unstripped membranes. In the presence of sodium ions, however, no inhibition was observed. ROS-GC activity of ROS membranes, stripped of transducin and other components, was not affected by light when reconstituted with GCAP1 only. However, when stripped ROS membranes were reconstituted with both GCAP1 and either transducin (T alpha beta gamma) or the T beta gamma-subunits, the inhibition of ROS-GC by light was restored. The T alpha-subunit alone was ineffective. These results suggest that under saturating light conditions, ROS-GC may be regulated by T beta gamma and cations, providing a possible mechanism of desensitization and light adaptation.
Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, Alberta, Canada. gwolbrin@ucalgary.ca
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G
Wolbring
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Baehr
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K
Palczewski
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PP
Schnetkamp
D000818:Animals; D002414:Cations, Monovalent / Q000494:pharmacology; D002417:Cattle; D003124:Colorimetry; D004789:Enzyme Activation / Q000187:drug effects; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D019811:Hydroxylamine / Q000494:pharmacology; D008027:Light; D008954:Models, Biological; D010777:Photochemistry; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000201:enzymology*; D012964:Sodium / Q000494:pharmacology; D015293:Transducin / Q000037:antagonists & inhibitors / Q000502:physiology*
D002414:Cations, Monovalent; D019811:Hydroxylamine; D012243:Rhodopsin; D012964:Sodium; D015293:Transducin; D006162:Guanylate Cyclase