PHOTORECEPTOR LIGHT-ADAPTATION MEDIATED BY S-MODULIN, A MEMBER OF A POSSIBLE REGULATORY PROTEIN FAMILY OF PROTEIN-PHOSPHORYLATION IN SIGNAL-TRANSDUCTION

In vertebrate retinal photoreceptors, cytoplasmic [Ca2+] decreases upon exposure to light. A Ca2+-binding protein, S-modulin, detects this [Ca2+] decrease and reduces the light-sensitivity of the cell to induce light-adaptation. The reduction of the sensitivity is attained by disinhibition or facilitation of rhodopsin phosphorylation, a quenching mechanism of light-activated rhodopsin. S-modulin-like proteins are found in the brain as well. Several of these proteins show similar S-modulin effects, suggesting that these proteins also participate in the regulation of protein phosphorylation in the signal transduction in their host cells.