Regulatory protein that inhibits guanylyl cyclase when free calcium ions concentration is elevated. This Ca(2+)-sensitive regulation of retinal guanylyl cyclase is a key event in recovery of the dark state of rod photoreceptors following light exposure. Retina. Binds three calcium ions (via EF-hands 2, 3 and 4) when calcium levels are high. Binds Mg(2+) when calcium levels are low.