Fig. 6

Schematic of the 2c-Cyclop working model. a Scheme of the Cr2c-Cyclop1 structure with important amino acids. Rhodopsin domain is embedded in the membrane with both termini in the cytosolic side. The key K298 residue, located in the last transmembrane helix, binds retinal covalently. Histidine kinase domain is depicted with DHp (dimerization and histidine phosphotransferase domain) and CA (catalytic and ATPase domain) in the red modules, including key residues H352, T356, and G533. Response regulator is drawn in a green module with key D1092 residue to accept phosphoryl group. Guanylyl cyclase (GC) domain is illustrated in the purple module, producing cGMP from substrate GTP. b A model for the cascade reaction within 2c-Cyclops. Green light is detected by the 8 TM rhodopsin, which in turn inhibits the histidine kinase. Without inhibition, the histidine kinase performs autophosphorylation using a phosphoryl group from ATP and then it transfers the phosphoryl group to the response regulator. The phosphorylated response regulator in turn activates the guanylyl cyclase to produce cGMP from GTP. The cGMP then acts as an effector molecule to trigger cellular processes