Rhodopsin phosphorylation as a mechanism of cyclic GMP phosphodiesterase regulation by S-modulin.
During light-adaptation by the vertebrate eye, the rods are desensitized and the light response is accelerated. When light is absorbed by the rods, a phosphodiesterase is activated that hydrolyses cyclic GMP. A light-induced decrease in cytoplasmic Ca2+ concentration is part of this light-adaptation process. The protein S-modulin (M(r) 26,000) is known to increase the fraction of light-activated cyclic GMP-phosphodiesterase (PDE) at high Ca2+ concentrations in frog rod photoreceptors. Here I present evidence that S-modulin lengthens the lifetime of active PDE (PDE*) at high Ca2+ concentrations. These S-modulin effects are observed in the physiological range of Ca2+ concentration (30 nM to 1 microM; half-maximum effects at 200-400 nM). At the high Ca2+ concentrations at which S-modulin prolongs the lifetime of PDE*, S-modulin inhibits rhodopsin phosphorylation (half-maximum effect at approximately 100 nM Ca2+). ATP is necessary for the S-modulin effects on PDE activation. I therefore conclude that the Ca(2+)-dependent regulation of PDE by S-modulin is mediated by rhodopsin phosphorylation. This regulation seems to be the principal mechanism of light adaptation in vertebrate photoreceptors.
Department of Physiology, Keio University School of Medicine, Tokyo, Japan.
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Kawamura
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000951:Antigens, Neoplasm / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D005136:Eye Proteins; D051597:Hippocalcin; D066298:In Vitro Techniques; D008027:Light; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D010766:Phosphorylation; D011892:Rana catesbeiana; D051603:Recoverin; D012243:Rhodopsin / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism
D000951:Antigens, Neoplasm; D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D051603:Recoverin; D051597:Hippocalcin; D000255:Adenosine Triphosphate; D012243:Rhodopsin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D002118:Calcium