Figure 1.

Main signaling steps in phototransduction. Photo-activation of rhodopsin (Rh to Rh*) leads to GDP/GTP exchange at the G protein transducin (T) which in turn activates its effector PDE. Hydrolysis of cGMP is catalyzed by activated PDE; resynthesis of cGMP by guanylate cyclase (GC) is under control of a negative Ca²⁺-feedback involving the GC-activating proteins (GCAP) Ca²⁺-sensor proteins. Ca²⁺ enters the cell via the cyclic nucleotide-gated (CNG)-channel and is extruded by the exchanger. Rh* is phosphorylated by GRK1, when inhibition by Ca²⁺-bound recoverin is relieved. Arrestin can bind to phosphorylated Rh* preventing further activation of transducin.