Light-sensitivity modulating protein in frog rods.
Cyclic GMP is the second messenger in the phototransduction mechanism in rod photoreceptors. Light-induced activation of cGMP phosphodiesterase (PDE), the hydrolyzing enzyme of cGMP, reduces cytoplasmic cGMP concentration to close the cGMP-activated channel and thereby causes a hyperpolarizing light response. Ca2+ concentration decreases during light-adaptation and this decrease is thought to be at least one of the underlying mechanisms of light-adaptation. Our previous electrophysiological work suggested that PDE in frog rod photoreceptors is regulated by this Ca2+ concentration decrease. In the present work, we isolated a protein that binds to disk membranes at high Ca2+ concentrations. In the presence of this protein (a 26 kDa protein), PDE light sensitivity becomes high at high Ca2+ concentrations. The effect was observed at physiological ranges of Ca2+ concentrations. Thus we could explain high light-sensitivity of photoreceptors under the dark-adapted condition. According to its function, we termed the 26 kDa protein 'sensitivity-modulating protein' or 'S-modulin'. During the purification we noticed that there are additional mechanisms present that may contribute to light-adaptation in frog rod photoreceptors.
Kawamura
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Department of Physiology, Keio University School of Medicine, Tokyo, Japan.
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism; D000818:Animals; D000951:Antigens, Neoplasm / Q000302:isolation & purification; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000302:isolation & purification; D006152:Cyclic GMP / Q000378:metabolism; D005136:Eye Proteins; D051597:Hippocalcin; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D010777:Photochemistry; D010786:Photoreceptor Cells / Q000378:metabolism* / Q000528:radiation effects; D011506:Proteins / Q000302:isolation & purification / Q000378:metabolism* / Q000528:radiation effects; D011892:Rana catesbeiana; D051603:Recoverin
D000951:Antigens, Neoplasm; D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D011506:Proteins; D051603:Recoverin; D051597:Hippocalcin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D006152:Cyclic GMP; D002118:Calcium