CA2+-DEPENDENT INTERACTION OF RECOVERIN WITH RHODOPSIN KINASE

Recoverin (Rv) is a myristoylated Ca2+-binding protein present primarily in bovine photoreceptors. It represents a newly identified family of neuronal specific Ca2+-binding proteins that includes neurocalcin, hippocalcin, and guanylyl cyclase-activating protein. To investigate the function of Rv in photoreceptors, we identified proteins that bind immobilized Rv in a Ca2+-dependent manner. Rhodopsin kinase (RK), interphotoreceptor retinoid-binding protein, and tubulin interact with Rv in the presence of Ca2+. The importance of the Rv/RK interaction was further characterized. RK, purified using immobilized Rv as an affinity matrix, catalyzed the light-dependent and Ca2+-independent incorporation of phosphates into rhodopsin when reconstituted with urea-stripped rod outer segment membranes. When only a small fraction (0.04%) of rhodopsin was photolyzed, as many as 700 phosphates were incorporated per photolyzed rhodopsin, a phenomenon known as ''high gain'' phosphorylation. When recoverin was added, the activity of RK became sensitive to free Ca2+, with EC(50) = 3 mu M. The N-terminal myristoyl residue of Rv enhances the inhibitory effect of Rv and introduces cooperativity to the Ca2+-dependent inhibition of rhodopsin phosphorylation. Rv neither interacts with other members of the G-protein-coupled receptor kinase family such as beta-adrenergic receptor kinase 1 nor inhibits beta-adrenergic receptor kinase 1 activity. The specific and Ca2+-dependent Rv/RK interaction is necessary for the inhibitory effect of Rv on rhodopsin phosphorylation and may play an important role in photoreceptor light adaptation.