G protein-coupled receptor (GPCR) kinases (GRKs) play a key role in homologous desensitization of GPCRs. It is widely assumed that most GRKs selectively phosphorylate only active GPCRs. Here, we show that although this seems to be the case for the GRK2/3 subfamily, GRK5/6 effectively phosphorylate inactive forms of several GPCRs, including β2-adrenergic and M2 muscarinic receptors, which are commonly used as representative models for GPCRs. Agonist-independent GPCR phosphorylation cannot be explained by constitutive activity of the receptor or membrane association of the GRK, suggesting that it is an inherent ability of GRK5/6. Importantly, phosphorylation of the inactive β2-adrenergic receptor enhanced its interactions with arrestins. Arrestin-3 was able to discriminate between phosphorylation of the same receptor by GRK2 and GRK5, demonstrating preference for the latter. Arrestin recruitment to inactive phosphorylated GPCRs suggests that not only agonist activation but also the complement of GRKs in the cell regulate formation of the arrestin-receptor complex and thereby G protein-independent signaling.

Members of the arrestin superfamily have great propensity of self-association, but the physiological significance of this phenomenon is unclear. To determine the biological role of visual arrestin-1 oligomerization in rod photoreceptors, we expressed mutant arrestin-1 with severely impaired self-association in mouse rods and analyzed mice of both sexes. We show that the oligomerization-deficient mutant is capable of quenching rhodopsin signaling normally, as judged by electroretinography and single-cell recording. Like wild type, mutant arrestin-1 is largely excluded from the outer segments in the dark, proving that the normal intracellular localization is not due the size exclusion of arrestin-1 oligomers. In contrast to wild type, supraphysiological expression of the mutant causes shortening of the outer segments and photoreceptor death. Thus, oligomerization reduces the cytotoxicity of arrestin-1 monomer, ensuring long-term photoreceptor survival.SIGNIFICANCE STATEMENT Visual arrestin-1 forms dimers and tetramers. The biological role of its oligomerization is unclear. To test the role of arrestin-1 self-association, we expressed oligomerization-deficient mutant in arrestin-1 knock-out mice. The mutant quenches light-induced rhodopsin signaling like wild type, demonstrating that in vivo monomeric arrestin-1 is necessary and sufficient for this function. In rods, arrestin-1 moves from the inner segments and cell bodies in the dark to the outer segments in the light. Nonoligomerizing mutant undergoes the same translocation, demonstrating that the size of the oligomers is not the reason for arrestin-1 exclusion from the outer segments in the dark. High expression of oligomerization-deficient arrestin-1 resulted in rod death. Thus, oligomerization reduces the cytotoxicity of high levels of arrestin-1 monomer.

Arrestin-1 binds light-activated phosphorhodopsin and ensures timely signal shutoff. We show that high transgenic expression of an arrestin-1 mutant with enhanced rhodopsin binding and impaired oligomerization causes apoptotic rod death in mice. Dark rearing does not prevent mutant-induced cell death, ruling out the role of arrestin complexes with light-activated rhodopsin. Similar expression of WT arrestin-1 that robustly oligomerizes, which leads to only modest increase in the monomer concentration, does not affect rod survival. Moreover, WT arrestin-1 co-expressed with the mutant delays retinal degeneration. Thus, arrestin-1 mutant directly affects cell survival via binding partner(s) other than light-activated rhodopsin. Due to impaired self-association of the mutant its high expression dramatically increases the concentration of the monomer. The data suggest that monomeric arrestin-1 is cytotoxic and WT arrestin-1 protects rods by forming mixed oligomers with the mutant and/or competing with it for the binding to non-receptor partners. Thus, arrestin-1 self-association likely serves to keep low concentration of the toxic monomer. The reduction of the concentration of harmful monomer is an earlier unappreciated biological function of protein oligomerization.