Stimulates synthesis of cGMP in photoreceptors. Thought to mediate Ca(2+)-sensitive regulation of retinal guanylyl cyclase (GC), a key event in recovery of the dark state of rod photoreceptors following light exposure (By similarity). Undergoes dimerization at low calcium ions concentration, while the presence of calcium ions inhibits its dimerization. Dimerization correlates with its ability to activate GC. Retina and pineal gland. Binds three calcium ions.