FIGURE 7.

Schematic overview of the nucleotide cycle in photoreceptor cells. Cyclic GMP is the second messenger of photoreceptor excitation. After light-triggered degradation to 5′-GMP by the PDE6, 5′-GMP diffuses to the inner segment of the photoreceptor. Here it can be further degraded to guanosine or is recycled by the guanylate kinase (GMP → GDP) and the nucleoside diphosphate kinase (GDP → GTP). RD3 interacts with GUK and upregulates its activity. GTP then diffuses back to the outer segments, where the enzymes of the phototransduction cascade use it, specifically the GCs synthesizing cGMP from GTP.