Isolation of isoelectric species of phosphorylated rhodopsin

Molecular mechanism of GPCR-mediated arrestin activation

Despite intense interest in discovering drugs that cause G-protein-coupled receptors (GPCRs) to selectively stimulate or block arrestin signalling, the structural mechanism of receptor-mediated arrestin activation remains unclear1,2. Here we reveal this mechanism through extensive atomic-level simulations of arrestin. We find that the receptor's transmembrane core and cytoplasmic tail-which bind distinct surfaces on arrestin-can each independently stimulate arrestin activation. We confirm this unanticipated role of the receptor core, and the allosteric coupling between these distant surfaces of arrestin, using site-directed fluorescence spectroscopy. The effect of the receptor core on arrestin conformation is mediated primarily by interactions of the intracellular loops of the receptor with the arrestin body, rather than the marked finger-loop rearrangement that is observed upon receptor binding. In the absence of a receptor, arrestin frequently adopts active conformations when its own C-terminal tail is disengaged, which may explain why certain arrestins remain active long after receptor dissociation. Our results, which suggest that diverse receptor binding modes can activate arrestin, provide a structural foundation for the design of functionally selective ('biased') GPCR-targeted ligands with desired effects on arrestin signalling.

Functional map of arrestin-1 at single amino acid resolution

Arrestins function as adapter proteins that mediate G protein-coupled receptor (GPCR) desensitization, internalization, and additional rounds of signaling. Here we have compared binding of the GPCR rhodopsin to 403 mutants of arrestin-1 covering its complete sequence. This comprehensive and unbiased mutagenesis approach provides a functional dimension to the crystal structures of inactive, preactivated p44 and phosphopeptide-bound arrestins and will guide our understanding of arrestin-GPCR complexes. The presented functional map quantitatively connects critical interactions in the polar core and along the C tail of arrestin. A series of amino acids (Phe375, Phe377, Phe380, and Arg382) anchor the C tail in a position that blocks binding of the receptor. Interaction of phosphates in the rhodopsin C terminus with Arg29 controls a C-tail exchange mechanism in which the C tail of arrestin is released and exposes several charged amino acids (Lys14, Lys15, Arg18, Lys20, Lys110, and Lys300) for binding of the phosphorylated receptor C terminus. In addition to this arrestin phosphosensor, our data reveal several patches of amino acids in the finger (Gln69 and Asp73-Met75) and the lariat loops (L249-S252 and Y254) that can act as direct binding interfaces. A stretch of amino acids at the edge of the C domain (Trp194-Ser199, Gly337-Gly340, Thr343, and Thr345) could act as membrane anchor, binding interface for a second rhodopsin, or rearrange closer to the central loops upon complex formation. We discuss these interfaces in the context of experimentally guided docking between the crystal structures of arrestin and light-activated rhodopsin.

Regulation of arrestin binding by rhodopsin phosphorylation level

Arrestins ensure the timely termination of receptor signaling. The role of rhodopsin phosphorylation in visual arrestin binding was established more than 20 years ago, but the effects of the number of receptor-attached phosphates on this interaction remain controversial. Here we use purified rhodopsin fractions with carefully quantified content of individual phosphorylated rhodopsin species to elucidate the impact of phosphorylation level on arrestin interaction with three biologically relevant functional forms of rhodopsin: light-activated and dark phosphorhodopsin and phospho-opsin. We found that a single receptor-attached phosphate does not facilitate arrestin binding, two are necessary to induce high affinity interaction, and three phosphates fully activate arrestin. Higher phosphorylation levels do not increase the stability of arrestin complex with light-activated rhodopsin but enhance its binding to the dark phosphorhodopsin and phospho-opsin. The complex of arrestin with hyperphosphorylated light-activated rhodopsin is less sensitive to high salt and appears to release retinal faster. These data suggest that arrestin likely quenches rhodopsin signaling after the third phosphate is added by rhodopsin kinase. The complex of arrestin with heavily phosphorylated rhodopsin, which appears to form in certain disease states, has distinct characteristics that may contribute to the phenotype of these visual disorders.

Identifying interactions between transmembrane helices from the adenosine A2A receptor

The human adenosine A(2A) receptor (A(2A)R) is an integral membrane protein and a member of the G-protein-coupled receptor (GPCR) superfamily, characterized by seven transmembrane (TM) helices. Although helix-helix association in the lipid bilayer is known to be an essential step in the folding of GPCRs, the determinants of their structures, folding, and assembly in the cell membrane are poorly understood. Previous studies in our group showed that while peptides corresponding to all seven TM domains of A(2A)R form stable helical structures in detergent micelles and lipid vesicles, they display significant variability in their helical propensity. This finding suggested to us that some TM domains might need to interact with other domains to properly insert and fold in hydrophobic environments. In this study, we assessed the ability of TM peptides to interact in pairwise combinations. We analyzed peptide interactions in hydrophobic milieus using circular dichroism spectroscopy and Förster resonance energy transfer. We find that specific interactions between TM helices occur, leading to additional helical content, especially in weakly helical TM domains, suggesting that some TM domains need a partner for proper folding in the membrane. The approach developed in this study will enable complete analysis of the TM domain interactions and the modeling of a folding pathway for A(2A)R.

Crystals of Native and Modified Bovine Rhodopsins and Their Heavy Atom Derivatives

Rhodopsin, the pigment protein responsible for dim-light vision, is a G protein-coupled receptor that converts light absorption into the activation of a G protein, transducin, to initiate the visual response. We have crystallised detergent-solubilised bovine rhodopsin in the native form and after chemical modifications as needles 10-40 microm in cross-section. The crystals belong to the trigonal space group P3(1), with two molecules of rhodopsin per asymmetric unit, related by a non-crystallographic 2-fold axis parallel with the crystallographic screw axis along c (needle axis). The unit cell dimensions are a=103.8 A, c=76.6 A for native rhodopsin, but vary over a wide range after heavy atom derivatisation, with a between 101.5 A and 113.9 A, and c between 76.6 A and 79.2 A. Rhodopsin molecules are packed with the bundle of transmembrane helices tilted from the c-axis by about 100 degrees . The two molecules in the asymmetric unit form contacts along the entire length of their transmembrane helices 5 in an antiparallel orientation, and they are stacked along the needle axis according to the 3-fold screw symmetry. Hence hydrophobic contacts are prominent at protein interfaces both along and normal to the needle axis. The best crystals of native rhodopsin in this crystal form diffracted X-rays from a microfocused synchrotron source to 2.55 A maximum resolution. We describe steps taken to extend the diffraction limit from about 10 A to 2.6 A.