G-protein-coupled receptor kinase 7 (, ''GRK7'', ''cone opsin kinase'', ''iodopsin kinase'') is a

serine/threonine-specific protein kinase A serine/threonine protein kinase () is a kinase enzyme, in particular a protein kinase, that phosphorylates the OH group of the amino-acid residues serine or threonine, which have similar side chains. At least 350 of the 500+ human prot ...

involved in phototransduction. This enzyme catalyses the phosphorylation of cone (color) photopsins in

retinal cones Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...

during high acuity color vision primarily in the

fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...

More on GRK7

GRK7 is a member of the family of

G protein-coupled receptor kinases G protein-coupled receptor kinases (GPCRKs, GRKs) are a family of protein kinases within the AGC (protein kinase A, protein kinase G, protein kinase C) group of kinases. Like all AGC kinases, GRKs use ATP to add phosphate to Serine and Threonine ...

, 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 G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...

that recognize light of various wavelengths (red, green, blue). Phosphorylated, light-activated photopsin binds to the cone

arrestin Arrestins (abbreviated Arr) are a small family of proteins important for regulating signal transduction at G protein-coupled receptors. Arrestins were first discovered as a part of a conserved two-step mechanism for regulating the activity of ...

protein arrestin-4 to terminate the light-activated signaling cascade. 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. The

post-translational modification Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribos ...

of GRK7 by

geranylgeranylation Geranylgeranylation is a form of prenylation, which is a post-translational modification of proteins that involves the attachment of one or two 20-carbon lipophilic geranylgeranyl isoprene units from geranylgeranyl diphosphate to one or two cysteine ...

and α-carboxyl methylation is important for regulating the ability of the enzyme to recognize color opsins in cone outer segment disk membranes. Arrestin-1 bound to rhodopsin in retinal rods prevents rhodopsin activation of the

transducin Transducin (Gt) is a protein naturally expressed in vertebrate retina rods and cones and it is very important in vertebrate phototransduction. It is a type of heterotrimeric G-protein with different α subunits in rod and cone photoreceptors. L ...

protein to turn off photo-transduction completely. 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

protein to turn off photo-transduction completely.

References

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