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Each receptor molecule binds its own arrestin

Arrestins are multi‐functional regulators of the most numerous family of signaling proteins, G protein‐coupled receptors. Two models of arrestin‐receptor interaction have been proposed: the binding of one arrestin to an individual receptor or to two receptors in a dimer. To determine the binding stoichiometry in vivo, we used rod photoreceptors where rhodopsin and arrestin are expressed at comparably high levels and where arrestin localization in the light is determined by its binding to the activated rhodopsin. Genetic manipulation of the expression of both proteins shows that the maximum amount of arrestin which moves to the rhodopsin‐containing compartment exceeds 80%, but not 100%, of the molar amount of rhodopsin present. In vitro experiments with purified proteins confirm that arrestin “saturates” rhodopsin at a one‐to‐one ratio. We also show that arrestin dimers and tetramers completely dissociate in the process of its binding to rhodopsin, so the observed stoichiometry cannot be explained by arrestin dimer binding a rhodopsin dimer. Thus, a single rhodopsin molecule is necessary and sufficient to bind arrestin. Remarkable structural conservation among receptors and arrestins strongly suggests that all arrestin subtypes bind individual molecules of their cognate receptors. These results suggest that receptor dimerization in not necessary for arrestin binding. Conceivably, dimers or larger arrays of receptor‐arrestin complexes facilitate arrestin‐mediated GPCR signaling and scaffolding of MAP kinase cascades.