title: cGMP influences guanine nucleotide binding to frog photoreceptor G-protein.
abstract: A rapid light-induced decrease in cGMP is thought to play a role in regulating the permeability or light sensitivity of photoreceptor membranes. Photo-excited rhodopsin activates a guanine nucleotide-binding protein (G-protein) by catalyzing the exchange of bound GDP for GTP. This G-protein X GTP complex activates the phosphodiesterase resulting in a decrease in cGMP concentration. We have observed two processes in vitro which may be relevant for the regulation of G-protein activation. First, we have found that free GDP binds to G-protein with an affinity similar to that of GTP. These two nucleotides appear to compete for a common site. Since G-protein X GDP does not activate phosphodiesterase, light-induced changes in the GTP/GDP ratio known to occur on illumination may serve to reduce G-protein activation and hence reduce phosphodiesterase activation. Second, addition of cGMP in the presence of equimolar GTP and GDP causes GTP binding to G-protein to be enhanced compared to GDP binding. This effect increases as the cGMP concentration is increased from 0.05 to 2 mM. Thus, light-induced decreases in cGMP concentration may also act as a feedback control in reducing G-protein activation. One or both of these processes may be involved in the desensitization (light adaptation) of rod photoreceptors.
mesh_terms: D000818:Animals; D001665:Binding Sites; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008027:Light; D010786:Photoreceptor Cells / Q000378:metabolism*; D011892:Rana catesbeiana; D011898:Ranidae; D012374:Rod Cell Outer Segment / Q000378:metabolism
chemical_list: D006150:Guanine Nucleotides; D006153:Guanosine Diphosphate; C016276:8-bromocyclic GMP; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D006152:Cyclic GMP