Rhodopsin kinase is a member of the family of
G protein-coupled receptor kinases, and is officially named G protein-coupled receptor kinase 1, or GRK1. Rhodopsin kinase is found primarily in mammalian retinal rod cells, where it phosphorylates light-activated rhodopsin, a member of the family of
G protein-coupled receptors that recognizes light. Phosphorylated, light-activated rhodopsin binds to the protein
arrestin to terminate the light-activated signaling cascade. The related
GRK7, also known as cone opsin kinase, serves a similar function in retinal cone cells subserving high-acuity color vision in the
fovea. The
post-translational modification of GRK1 by
farnesylation and α-carboxyl methylation is important for regulating the ability of the enzyme to recognize rhodopsin in rod outer segment disk membranes. Arrestin-1 bound to rhodopsin prevents rhodopsin activation of the
transducin protein to turn off
photo-transduction completely. Rhodopsin kinase is inhibited by the calcium-binding protein
recoverin in a graded manner that maintains rhodopsin sensitivity to light despite large changes in ambient light conditions. That is, in retinas exposed to only dim light, calcium levels are high in retinal rod cells and recoverin is bound to and inhibits rhodopsin kinase, leaving rhodopsin exquisitely sensitive to photons to mediate low-light, low-acuity vision; in bright light, rod cell calcium levels are low so recoverin cannot bind or inhibit rhodopsin kinase, resulting in greater rhodopsin kinase/arrestin inhibition of rhodopsin signaling at baseline to preserve visual sensitivity. According to a proposed model, the
N-terminus of rhodopsin kinase is involved in its own activation. It's suggested that an activated rhodopsin binds to the N-terminus, which is also involved in the stabilization of the kinase domain to induce an active conformation. == Eye disease ==