The guanylate cyclase signaling system in zebrafish photoreceptors

Zebrafish express in the retina a large variety of three different membrane-bound guanylate cyclases and six different guanylate cyclase-activating proteins (zGCAPs) belonging to the family of neuronal calcium sensor proteins. Although these proteins are predominantly localized in rod and cone photoreceptor cells of the retina, they differ in their spatial-temporal expression profiles. Further, each zGCAP has a different affinity for Ca2+ and displays different Ca2+-sensitivities of guanylate cyclase activation. Thus, zGCAPs operate as cytoplasmic Ca2+-sensors that sense incremental changes of cytoplasmic Ca2+-concentration in rod and cone cells and control the activity of their target guanylate cyclases in a Ca2+-relay mode fashion.