Calcium-dependent regulation of rhodopsin phosphorylation.
Depending on ambient light conditions, a rod photoreceptor cell adapts to a light stimulus. For example, when it is kept in the light, its light sensitivity decreases because of light adaptation. The adaptational state is regulated by the Ca2+ concentration in the cytoplasm ([Ca2+]i). The [Ca2+]i is high in the dark and becomes low when the cell is light-adapted. The change in [Ca2+]i is detected by several Ca(2+)-binding proteins that change their conformations by binding Ca2+. S-modulin, found in frog rods, or its bovine homologue recoverin, is a 23 kDa Ca(2+)-binding protein that inhibits rhodopsin phosphorylation at high Ca2+ concentrations by inhibiting rhodopsin kinase. Since rhodopsin phosphorylation is an inactivating mechanism for light-activated rhodopsin (R*), the inhibition of this reaction will prolong the lifetime of R*. In this way, S-modulin is expected to increase the efficiency of phototransduction and therefore the light-sensitivity of rods in the dark. When rods are light-adapted, [Ca2+]i decreases so that the lifetime of the R* is expected to reduce, resulting in a decrease in light sensitivity. Even though it is generally agreed that S-modulin inhibits rhodopsin phosphorylation, its physiological function is not yet well understood.
Kawamura
S
S
Department of Biology, Graduate School of Science, Osaka University, Japan.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D002417:Cattle; D005136:Eye Proteins; D051597:Hippocalcin; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D010766:Phosphorylation; D051603:Recoverin; D012243:Rhodopsin / Q000378:metabolism
D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D051603:Recoverin; D051597:Hippocalcin; D000255:Adenosine Triphosphate; D012243:Rhodopsin; D002118:Calcium