Photoreceptor specific guanylate cyclases in vertebrate phototransduction

Two membrane bound guanylate cyclases are expressed in vertebrate photoreceptor cells. They serve a key function in photoreceptor physiology as they synthesize the intracellular transmitter of photoexcitation guanosine 3',5'-cyclic monophosphate (cGMP). Both cyclases named ROS-GC1 and ROS-GC2 form a subclass of membrane bound cyclases and differ in many aspects from hormone peptide receptor guanylate cyclases. One unique feature is their regulation by three small Ca2+-binding proteins called GCAPs. These regulatory proteins sense changes in the cytoplasmic Ca2+-concentration [Ca2+] during illumination and activate ROS-GCs when the [Ca2+] decreases below the value in a dark adapted cell of 500-600 nM. Recent work has identified the target regions of GCAP-1 in ROS-GC1. In addition to GCAPs several other proteins including aktin, tubulin, a glutamic-acid-rich protein and a GTPase accelerating protein (RGS9) were found to interact with ROS-GC1 and probably form a multiprotein complex.