A substantial fraction of rhodopsin kinase (Grk1) is bound to recoverin (Rcvrn) in darkness, when internal Ca2+ levels are high. Rcvrn is an EF-hand protein that functions as a myristoyl switch. With Ca2+ bound, the myristoyl group is exposed to attach Rcvrn to the membrane. When Ca2+ levels drop with light exposure, Ca2+ dissociates from Rcvrn and Grk1 is released. Higher levels of free Grk1 accelerate the phosphorylation and shutoff of photoexcited rhodopsin (MII) (Chen et al. 1995, Dizhoor et al. 1991, Gorodovikova et al. 1994).