Rhodopsin, the rod cell photoreceptor, undergoes rapid desensitization upon exposure to light, resulting in uncoupling of the receptor from its G protein, transducin (G t ). Phosphorylation of serine and threonine residues located in the COOH terminus of rhodopsin is the first step in this process, followed by the binding of arrestin. In this study, a series of mutants was generated in which these COOH-terminal phosphorylation substrate sites were substituted with alanines. These mutants were expressed in HEK-293 cells and analyzed for their ability to be phosphorylated by rhodopsin kinase and to bind arrestin. The results demonstrate that rhodopsin kinase can efficiently phosphorylate other serine and threonine residues in the absence of the sites reported to be the preferred substrates for rhodopsin kinase. A correlation was observed between the level of rhodopsin phosphorylation and the amount of arrestin binding to these mutants. However, mutants T340A and S343A demonstrated a significant reduction in arrestin binding even though the level of phosphorylation was similar to that of wild-type rhodopsin. Substitution of Thr-340 and Ser-343 with glutamic acid residues (T340E and S343E, respectively) was not sufficient to promote the binding of arrestin in the absence of phosphorylation by rhodopsin kinase. When S343E was phosphorylated, its ability to bind arrestin was similar to that of wild-type rhodopsin. Surprisingly, arrestin binding to phosphorylated T340E did not increase to the level observed for wild-type rhodopsin. These results suggest that 2 amino acids, Thr-340 and Ser-343, play important but distinct roles in promoting the binding of arrestin to rhodopsin.Receptor desensitization is a critical process in the regulation of G protein-coupled receptor signaling pathways. Serine and threonine residues located either in the COOH terminus or in the third cytoplasmic loop of these receptors serve as substrates for phosphorylation by members of the G protein-coupled receptor kinase family. The G protein-coupled receptor kinases are unique serine/threonine kinases that phosphorylate only the ligand-activated form of G protein-coupled receptors (1, 2). Phosphorylation is followed by the binding of arrestin to the receptor, resulting in rapid termination of G protein activation (3-5). The physiological importance of rapid receptor desensitization has been demonstrated directly in studies of the visual signal transduction system, in which expression of a truncated form of rhodopsin missing the phosphorylation sites and a selective reduction in the levels of arrestin both lead to extended rhodopsin activity (6, 7).Rhodopsin, the photoreceptor of the rod cell, has been used extensively as a model for investigating the regulation of G protein-coupled receptor desensitization (4). As many as 7 serines and threonines located in the COOH terminus of rhodopsin are substrates in vitro for rhodopsin kinase, the rod cell G protein-coupled receptor kinase, when rhodopsin is activated by light (8). However, several studie...

Arginine-135, located at the border between the third transmembrane domain and the second cytoplasmic loop of rhodopsin, is one of the most highly conserved amino acids in the family of G protein-coupled receptors. The effect of mutation at Arg-135 on the ability of rhodopsin to undergo desensitization was investigated. Four mutants, R135K, R135Q, R135A, and R135L, were examined for their ability to be phosphorylated by rhodopsin kinase, to bind arrestin, and to activate the rod cell G protein, transducin (Gt). All of the mutants were phosphorylated, bound arrestin, and were able to activate Gt when reconstituted with 11-cis-retinal. Surprisingly, several of the mutants could be phosphorylated by rhodopsin kinase and could bind arrestin in the absence of 11-cis-retinal but were not able to activate Gt. These observations represent the first demonstration of a mutant G protein-coupled receptor that assumes a conformation able to interact with its G protein-coupled receptor kinase and arrestin, but not with its G protein, in the absence of ligand.

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