Neurocalcin. Role in neuronal signaling.

Abstract

The interaction of Ca2+with calcium-binding proteins is one mechanism by which Ca2+is thought to regulate cellular processes (). In photoreceptors, Ca2+ is thought to act as the key modulator of light adaptation, a process whereby absorption of each additional photon is less effective in activating the phototransduction pathway, thus producing smaller alterations in the conductance of the cell. Both biochemical and electrophysiological experiments implicate the enzyme-retinal guanylyl cyclase or rod outer-segment guanylyl cyclase (ROS-GC) as one of the sites of Ca2+action (). Following illumination, this key enzyme catalyzes the formation of cyclic GMP from GTP, which, in turn, opens cation channels in the outer-segment plasma membrane and reestablishes the dark potential of the cell. The decrease in intracellular Ca2+ that occurs upon illumination stimulates cyclase activity (). It has only recently been established that a calcium-binding protein called Guanylate cyclase activating protein (GCAP) is responsible for this regulation ().