G-protein-coupled receptor kinase 7 (EC 2.7.11.14, GRK7, cone opsin kinase, iodopsin kinase) is a serine/threonine-specific protein kinase involved in phototransduction.^[1][2][3] This enzyme catalyses the phosphorylation of cone (color) photopsins in retinal cones during high acuity color vision primarily in the fovea.

GRK7 is a member of the family of G protein-coupled receptor kinases, and is officially named G protein-coupled receptor kinase 7. GRK7 is found primarily in mammalian retinal cone cells, where it phosphorylates light-activated photopsins, members of the family of G protein-coupled receptors that recognize light of various wavelengths (red, green, blue).^[3] Phosphorylated, light-activated photopsin binds to the cone arrestin protein arrestin-4 to terminate the light-activated signaling cascade.^[3] The related GRK1, also known as rhodopsin kinase, serves a similar function in retinal rod cells subserving dim light black-and-white peripheral vision outside the fovea.^[4][5] The post-translational modification of GRK7 by geranylgeranylation and α-carboxyl methylation is important for regulating the ability of the enzyme to recognize color opsins in cone outer segment disk membranes.^[2]

Arrestin-1 bound to rhodopsin in retinal rods prevents rhodopsin activation of the transducin protein to turn off photo-transduction completely.^[6] While cone visual transduction is much less well characterized, it is expected that arrestin-4 bound to GRK7-phosphorylated color photopsin prevents opsin activation of the transducin protein to turn off photo-transduction completely.