Guanylate Cyclase-Activating Protein (GCAP)

Vertebrate photoreceptors respond to light by a transient hyperpolarization of the plasma membrane. Hyperpolarization is achieved when cGMP-gated cation channels, located in the plasma membrane, are closed after hydrolysis of the gating ligand, cGMP ( 1,28). To re-open the channels, a prerequisite to return photoreceptors to the dark-adapted state, cGMP has to be produced at an accelerated rate by a guanylate cyclase (GC). As shown by numerous electrophysiological and biochemical experiments, activation of GC is regulated bv Ca2+ ions.