Non-identity of cGMP as the guanine nucleotide stimulated to bind to ROS by light and ATP.

Light, in the presence of ATP, has been reported to stimulate cGMP binding to a 58 kDa protein in ROS (rod outer segments, Fesenko and Krapivinsky, 1986b, Photobiochem. Photobiophys. 13 345-58). This apparent light-related redistribution of ROS cGMP has been suggested to eliminate any requirement for phosphodiesterase-promoted hydrolysis of cGMP in the mechanism subserving phototransduction. Using conditions identical to those previously reported, this effect of light and ATP was examined further by characterizing the metabolic products that arise and the nucleotides that become liganded. The increased binding of radiolabeled guanine nucleotide upon illumination of ROS in the presence of ATP was confirmed, but the species of guanine nucleotide that were stimulated to bind under these conditions were identified as [32P]GDP and [32P]GTP rather than [32P]cGMP. The precautions to prevent enzymic hydrolysis of cGMP, which included conducting the reactions at 0 degrees and the addition of 3-isobutyl-l-methylxanthine (250 microM) to the reaction mixture did not prevent about a 20-fold increase in the rate of phosphodiesterase-catalyzed hydrolysis of radiolabeled cGMP by light when ATP was also present. This stimulation of phosphodiesterase activity is undoubtedly related to transphosphorylation by exogenous ATP of endogenous GMP and GDP involving catalytic actions of guanylate kinase and nucleoside diphosphate kinase in isolated ROS. These enzymes can also serve to generate [32P]GDP and [32P]GTP, which subsequently bind to ROS components. Such a mechanism involving ATP as phosphoryl donor was supported by observing that an analog of ATP (beta,gamma-methyleneadenosine 5'-triphosphate), which cannot serve as a phosphoryl donor, did not increase radiolabeled guanine nucleotide binding. Although several ROS proteins can form filter-retainable complexes with GDP and GTP, the properties of the 58 kDa protein found to be photoaffinity labeled with radioactive guanine nucleotide are most characteristic of those attributable to tubulin. The previous report that illumination in the presence of ATP stimulates the binding of cGMP to ROS components finds no support from the data obtained in the present studies.