Light inhibition of bovine retinal rod guanylyl cyclase mediated by beta gamma-transducin

Photoreceptor guanylyl cyclase (ROS-GC), converting GTP into cGMP and pyrop hosphate, is a key enzyme in the regulation of the visual transduction casc ade. ROS-GC requires GC-activating proteins (GCAPs) and low free [Ca] for f ull-activity. We found that when choline or potassium were the major cation s present, light caused a 70% inhibition of stimulated ROS-GC in native uns tripped membranes. In the presence of sodium ions, however, no inhibition w as observed, ROS-GC activity of ROS membranes, stripped of transducin and o ther components, was not affected by light when reconstituted with GCAP1 on ly. However, when stripped ROS membranes were reconstituted with both GCAP1 and either transducin (T-alpha beta gamma) Or the T-beta gamma-subunits, t he inhibition of ROS-GC by light was restored. The T-alpha-subunit alone wa s 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.