Functional role of the three conserved cysteines in the N domain of visual arrestin-1

Functional Role of Arrestin-1 Residues Interacting with Unphosphorylated Rhodopsin Elements

Arrestin-1, or visual arrestin, exhibits an exquisite selectivity for light-activated phosphorylated rhodopsin (P-Rh*) over its other functional forms. That selectivity is believed to be mediated by two well-established structural elements in the arrestin-1 molecule, the activation sensor detecting the active conformation of rhodopsin and the phosphorylation sensor responsive to the rhodopsin phosphorylation, which only active phosphorylated rhodopsin can engage simultaneously. However, in the crystal structure of the arrestin-1-rhodopsin complex there are arrestin-1 residues located close to rhodopsin, which do not belong to either sensor. Here we tested by site-directed mutagenesis the functional role of these residues in wild type arrestin-1 using a direct binding assay to P-Rh* and light-activated unphosphorylated rhodopsin (Rh*). We found that many mutations either enhanced the binding only to Rh* or increased the binding to Rh* much more than to P-Rh*. The data suggest that the native residues in these positions act as binding suppressors, specifically inhibiting the arrestin-1 binding to Rh* and thereby increasing arrestin-1 selectivity for P-Rh*. This calls for the modification of a widely accepted model of the arrestin-receptor interactions.

The Role of Arrestin-1 Middle Loop in Rhodopsin Binding

Arrestins preferentially bind active phosphorylated G protein-coupled receptors (GPCRs). The middle loop, highly conserved in all arrestin subtypes, is localized in the central crest on the GPCR-binding side. Upon receptor binding, it directly interacts with bound GPCR and demonstrates the largest movement of any arrestin element in the structures of the complexes. Comprehensive mutagenesis of the middle loop of rhodopsin-specific arrestin-1 suggests that it primarily serves as a suppressor of binding to non-preferred forms of the receptor. Several mutations in the middle loop increase the binding to unphosphorylated light-activated rhodopsin severalfold, which makes them candidates for improving enhanced phosphorylation-independent arrestins. The data also suggest that enhanced forms of arrestin do not bind GPCRs exactly like the wild-type protein. Thus, the structures of the arrestin-receptor complexes, in all of which different enhanced arrestin mutants and reengineered receptors were used, must be interpreted with caution.

The finger loop as an activation sensor in arrestin

The finger loop in the central crest of the receptor-binding site of arrestins engages the cavity between the transmembrane helices of activated G-protein-coupled receptors. Therefore, it was hypothesized to serve as the sensor that detects the activation state of the receptor. We performed comprehensive mutagenesis of the finger loop in bovine visual arrestin-1, generated mutant radiolabeled proteins by cell-free translation, and determined the effects of mutations on the in vitro binding of arrestin-1 to purified phosphorylated light-activated rhodopsin. This interaction is driven by two factors, rhodopsin activation and rhodopsin-attached phosphates. Therefore, the binding of arrestin-1 to light-activated unphosphorylated rhodopsin is low. To evaluate the role of the finger loop specifically in the recognition of the active receptor conformation, we tested the effects of these mutations in the context of truncated arrestin-1 that demonstrates much higher binding to unphosphorylated activated and phosphorylated inactive rhodopsin. The majority of finger loop residues proved important for arrestin-1 binding to light-activated rhodopsin, with six mutations affecting the binding exclusively to this form. Thus, the finger loop is the key element of arrestin-1 activation sensor. The data also suggest that arrestin-1 and its enhanced mutant bind various functional forms of rhodopsin differently.

Lysine in the lariat loop of arrestins does not serve as phosphate sensor

Arrestins demonstrate strong preference for phosphorylated over unphosphorylated receptors, but how arrestins "sense" receptor phosphorylation is unclear. A conserved lysine in the lariat loop of arrestins directly binds the phosphate in crystal structures of activated arrestin-1, -2, and -3. The lariat loop supplies two negative charges to the central polar core, which must be disrupted for arrestin activation and high-affinity receptor binding. Therefore, we hypothesized that receptor-attached phosphates pull the lariat loop via this lysine, thus removing the negative charges and destabilizing the polar core. We tested the role of this lysine by introducing charge elimination (Lys->Ala) and reversal (Lys->Glu) mutations in arrestin-1, -2, and -3. These mutations in arrestin-1 only moderately reduced phospho-rhodopsin binding and had no detectable effect on arrestin-2 and -3 binding to cognate non-visual receptors in cells. The mutations of Lys300 in bovine and homologous Lys301 in mouse arrestin-1 on the background of pre-activated mutants had variable effects on the binding to light-activated phosphorylated rhodopsin, while affecting the binding to unphosphorylated rhodopsin to a greater extent. Thus, conserved lysine in the lariat loop participates in receptor binding, but does not play a critical role in phosphate-induced arrestin activation.

Structural biology of G protein-coupled receptor signaling complexes

G protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors that mediate numerous cell signaling pathways, and are targets of more than one-third of clinical drugs. Thanks to the advancement of novel structural biology technologies, high-resolution structures of GPCRs in complex with their signaling transducers, including G-protein and arrestin, have been determined. These 3D complex structures have significantly improved our understanding of the molecular mechanism of GPCR signaling and provided a structural basis for signaling-biased drug discovery targeting GPCRs. Here we summarize structural studies of GPCR signaling complexes with G protein and arrestin using rhodopsin as a model system, and highlight the key features of GPCR conformational states in biased signaling including the sequence motifs of receptor TM6 that determine selective coupling of G proteins, and the phosphorylation codes of GPCRs for arrestin recruitment. We envision the future of GPCR structural biology not only to solve more high-resolution complex structures but also to show stepwise GPCR signaling complex assembly and disassembly and dynamic process of GPCR signal transduction.

Arrestin mutations: Some cause diseases, others promise cure

Arrestins play a key role in homologous desensitization of G protein-coupled receptors (GPCRs) and regulate several other vital signaling pathways in cells. Considering the critical roles of these proteins in cellular signaling, surprisingly few disease-causing mutations in human arrestins were described. Most of these are loss-of-function mutations of visual arrestin-1 that cause excessive rhodopsin signaling and hence night blindness. Only one dominant arrestin-1 mutation was discovered so far. It reduces the thermal stability of the protein, which likely results in photoreceptor death via unfolded protein response. In case of the two nonvisual arrestins, only polymorphisms were described, some of which appear to be associated with neurological disorders and altered response to certain treatments. Structure-function studies revealed several ways of enhancing arrestins' ability to quench GPCR signaling. These enhanced arrestins have potential as tools for gene therapy of disorders associated with excessive signaling of mutant GPCRs.

Molecular Defects of the Disease-Causing Human Arrestin-1 C147F Mutant

Purpose

The purpose of this study was to identify the molecular defect in the disease-causing human arrestin-1 C147F mutant.

Methods

The binding of wild-type (WT) human arrestin-1 and several mutants with substitutions in position 147 (including C147F, which causes dominant retinitis pigmentosa in humans) to phosphorylated and unphosphorylated light-activated rhodopsin was determined. Thermal stability of WT and mutant human arrestin-1, as well as unfolded protein response in 661W cells, were also evaluated.

Results

WT human arrestin-1 was selective for phosphorylated light-activated rhodopsin. Substitutions of Cys-147 with smaller side chain residues, Ala or Val, did not substantially affect binding selectivity, whereas residues with bulky side chains in the position 147 (Ile, Leu, and disease-causing Phe) greatly increased the binding to unphosphorylated rhodopsin. Functional survival of mutant proteins with bulky substitutions at physiological and elevated temperature was also compromised. C147F mutant induced unfolded protein response in cultured cells.

Conclusions

Bulky Phe substitution of Cys-147 in human arrestin-1 likely causes rod degeneration due to reduced stability of the protein, which induces unfolded protein response in expressing cells.