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.