Arrestin with a Single Amino Acid Substitution Quenches Light-Activated Rhodopsin in a Phosphorylation-Independent Fashion

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Abstract
Arrestins are members of a superfamily of regulatory proteins that participate in the termination of G protein-mediated signal transduction. In the phototransduction cascade of vertebrate rods, which serves as a prototypical G protein-mediated signaling pathway, the binding of visual arrestin is stimulated by phosphorylation of the C-terminus of photoactivated rhodopsin (Rh*). Arrestin is very selective toward light-activated phosphorhodopsin (P-Rh*). Previously we reported that a single amino acid substitution in arrestin, Arg175Gln, results in a dramatic increase in arrestin binding to Rh* [Gurevich, V. V., & Benovic, J. L. (1995) J. Biol. Chem. 270, 6010-6016]. Here we demonstrate that a similar mutant, arrestin(R175E), binds to light-activated rhodopsin independent of phosphorylation. Arrestin(R175E) binds with high affinity not only to P-Rh* and Rh* but also to light-activated truncated rhodopsin in which the C-terminus phosphorylation sites have been proteolytically removed. In an in vitro assay that monitored rhodopsin-dependent activation of cGMP phosphodiesterase (PDE), wild type arrestin quenched PDE response only when ATP was present to support rhodopsin phosphorylation. In contrast, as little as 30 nM arrestin(R175E) effectively quenched PDE activation in the absence of ATP. Arrestin(R175E) had no effect when the lifetime of Rh* no longer contributed to the time course of PDE activity, suggesting that it disrupts signal transduction at the level of rhodopsin-transducin interaction.