GPCR-mediated ß-arrestin activation deconvoluted with single-molecule precision.

ß-arrestins bind G protein-coupled receptors to terminate G protein signaling and to facilitate other downstream signaling pathways. Using single-molecule fluorescence resonance energy transfer imaging, we show that ß-arrestin is strongly autoinhibited in its basal state. Its engagement with a phosphopeptide mimicking phosphorylated receptor tail efficiently releases the ß-arrestin tail from its N domain to assume distinct conformations. Unexpectedly, we find that ß-arrestin binding to phosphorylated receptor, with a phosphorylation barcode identical to the isolated phosphopeptide, is highly inefficient and that agonist-promoted receptor activation is required for ß-arrestin activation, consistent with the release of a sequestered receptor C tail. These findings, together with focused cellular investigations, reveal that agonism and receptor C-tail release are specific determinants of the rate and efficiency of ß-arrestin activation by phosphorylated receptor. We infer that receptor phosphorylation patterns, in combination with receptor agonism, synergistically establish the strength and specificity with which diverse, downstream ß-arrestin-mediated events are directed.

Signal transduction at GPCRs: Allosteric activation of the ERK MAPK by ß-arrestin.

ß-arrestins are multivalent adaptor proteins that bind active phosphorylated G protein-coupled receptors (GPCRs) to inhibit G protein signaling, mediate receptor internalization, and initiate alternative signaling events. ß-arrestins link agonist-stimulated GPCRs to downstream signaling partners, such as the c-Raf-MEK1-ERK1/2 cascade leading to ERK1/2 activation. ß-arrestins have been thought to transduce signals solely via passive scaffolding by facilitating the assembly of multiprotein signaling complexes. Recently, however, ß-arrestin 1 and 2 were shown to activate two downstream signaling effectors, c-Src and c-Raf, allosterically. Over the last two decades, ERK1/2 have been the most intensely studied signaling proteins scaffolded by ß-arrestins. Here, we demonstrate that ß-arrestins play an active role in allosterically modulating ERK kinase activity in vitro and within intact cells. Specifically, we show that ß-arrestins and their GPCR-mediated active states allosterically enhance ERK2 autophosphorylation and phosphorylation of a downstream ERK2 substrate, and we elucidate the mechanism by which ß-arrestins do so. Furthermore, we find that allosteric stimulation of dually phosphorylated ERK2 by active-state ß-arrestin 2 is more robust than by active-state ß-arrestin 1, highlighting differential capacities of ß-arrestin isoforms to regulate effector signaling pathways downstream of GPCRs. In summary, our study provides strong evidence for a new paradigm in which ß-arrestins function as active "catalytic" scaffolds to allosterically unlock the enzymatic activity of signaling components downstream of GPCR activation.

The GPCR-ß-arrestin complex allosterically activates C-Raf by binding its amino terminus.

G protein-coupled receptors (GPCRs) convert external stimuli into cellular signals through heterotrimeric guanine nucleotide-binding proteins (G-proteins) and ß-arrestins (ßarrs). In a ßarr-dependent signaling pathway, ßarrs link GPCRs to various downstream signaling partners, such as the Raf-mitogen-activated protein kinase extracellular signal-regulated kinase-extracellular signal-regulated kinase cascade. Agonist-stimulated GPCR-ßarr complexes have been shown to interact with C-Raf and are thought to initiate the mitogen-activated protein kinase pathway through simple tethering of these signaling partners. However, recent evidence shows that in addition to canonical scaffolding functions, ßarrs can allosterically activate downstream targets, such as the nonreceptor tyrosine kinase Src. Here, we demonstrate the direct allosteric activation of C-Raf by GPCR-ßarr1 complexes in vitro. Furthermore, we show that ßarr1 in complex with a synthetic phosphopeptide mimicking the human V2 vasopressin receptor tail that binds and functionally activates ßarrs also allosterically activates C-Raf. We reveal that the interaction between the phosphorylated GPCR C terminus and ßarr1 is necessary and sufficient for C-Raf activation. Interestingly, the interaction between ßarr1 and C-Raf was considerably reduced in the presence of excess activated H-Ras, a small GTPase known to activate C-Raf, suggesting that H-Ras and ßarr1 bind to the same region on C-Raf. Furthermore, we found that ßarr1 interacts with the Ras-binding domain of C-Raf. Taken together, these data suggest that in addition to canonical scaffolding functions, GPCR-ßarr complexes directly allosterically activate C-Raf by binding to its amino terminus. This work provides novel insights into how ßarrs regulate effector molecules to activate downstream signaling pathways.

Visualization of arrestin recruitment by a G-protein-coupled receptor.

G-protein-coupled receptors (GPCRs) are critically regulated by ß-arrestins, which not only desensitize G-protein signalling but also initiate a G-protein-independent wave of signalling. A recent surge of structural data on a number of GPCRs, including the ß2 adrenergic receptor (ß2AR)-G-protein complex, has provided novel insights into the structural basis of receptor activation. However, complementary information has been lacking on the recruitment of ß-arrestins to activated GPCRs, primarily owing to challenges in obtaining stable receptor-ß-arrestin complexes for structural studies. Here we devised a strategy for forming and purifying a functional human ß2AR-ß-arrestin-1 complex that allowed us to visualize its architecture by single-particle negative-stain electron microscopy and to characterize the interactions between ß2AR and ß-arrestin 1 using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and chemical crosslinking. Electron microscopy two-dimensional averages and three-dimensional reconstructions reveal bimodal binding of ß-arrestin 1 to the ß2AR, involving two separate sets of interactions, one with the phosphorylated carboxy terminus of the receptor and the other with its seven-transmembrane core. Areas of reduced HDX together with identification of crosslinked residues suggest engagement of the finger loop of ß-arrestin 1 with the seven-transmembrane core of the receptor. In contrast, focal areas of raised HDX levels indicate regions of increased dynamics in both the N and C domains of ß-arrestin 1 when coupled to the ß2AR. A molecular model of the ß2AR-ß-arrestin signalling complex was made by docking activated ß-arrestin 1 and ß2AR crystal structures into the electron microscopy map densities with constraints provided by HDX-MS and crosslinking, allowing us to obtain valuable insights into the overall architecture of a receptor-arrestin complex. The dynamic and structural information presented here provides a framework for better understanding the basis of GPCR regulation by arrestins.

Distinct conformations of GPCR-ß-arrestin complexes mediate desensitization, signaling, and endocytosis.

ß-Arrestins (ßarrs) interact with G protein-coupled receptors (GPCRs) to desensitize G protein signaling, to initiate signaling on their own, and to mediate receptor endocytosis. Prior structural studies have revealed two unique conformations of GPCR-ßarr complexes: the "tail" conformation, with ßarr primarily coupled to the phosphorylated GPCR C-terminal tail, and the "core" conformation, where, in addition to the phosphorylated C-terminal tail, ßarr is further engaged with the receptor transmembrane core. However, the relationship of these distinct conformations to the various functions of ßarrs is unknown. Here, we created a mutant form of ßarr lacking the "finger-loop" region, which is unable to form the core conformation but retains the ability to form the tail conformation. We find that the tail conformation preserves the ability to mediate receptor internalization and ßarr signaling but not desensitization of G protein signaling. Thus, the two GPCR-ßarr conformations can carry out distinct functions.

Structure of active ß-arrestin-1 bound to a G-protein-coupled receptor phosphopeptide.

The functions of G-protein-coupled receptors (GPCRs) are primarily mediated and modulated by three families of proteins: the heterotrimeric G proteins, the G-protein-coupled receptor kinases (GRKs) and the arrestins. G proteins mediate activation of second-messenger-generating enzymes and other effectors, GRKs phosphorylate activated receptors, and arrestins subsequently bind phosphorylated receptors and cause receptor desensitization. Arrestins activated by interaction with phosphorylated receptors can also mediate G-protein-independent signalling by serving as adaptors to link receptors to numerous signalling pathways. Despite their central role in regulation and signalling of GPCRs, a structural understanding of ß-arrestin activation and interaction with GPCRs is still lacking. Here we report the crystal structure of ß-arrestin-1 (also called arrestin-2) in complex with a fully phosphorylated 29-amino-acid carboxy-terminal peptide derived from the human V2 vasopressin receptor (V2Rpp). This peptide has previously been shown to functionally and conformationally activate ß-arrestin-1 (ref. 5). To capture this active conformation, we used a conformationally selective synthetic antibody fragment (Fab30) that recognizes the phosphopeptide-activated state of ß-arrestin-1. The structure of the ß-arrestin-1-V2Rpp-Fab30 complex shows marked conformational differences in ß-arrestin-1 compared to its inactive conformation. These include rotation of the amino- and carboxy-terminal domains relative to each other, and a major reorientation of the 'lariat loop' implicated in maintaining the inactive state of ß-arrestin-1. These results reveal, at high resolution, a receptor-interacting interface on ß-arrestin, and they indicate a potentially general molecular mechanism for activation of these multifunctional signalling and regulatory proteins.

Small-Molecule Positive Allosteric Modulators of the ß2-Adrenoceptor Isolated from DNA-Encoded Libraries.

Conventional drug discovery efforts at the ß2-adrenoceptor (ß2AR) have led to the development of ligands that bind almost exclusively to the receptor's hormone-binding orthosteric site. However, targeting the largely unexplored and evolutionarily unique allosteric sites has potential for developing more specific drugs with fewer side effects than orthosteric ligands. Using our recently developed approach for screening G protein-coupled receptors (GPCRs) with DNA-encoded small-molecule libraries, we have discovered and characterized the first ß2AR small-molecule positive allosteric modulators (PAMs)-compound (Cmpd)-6 [(R)-N-(4-amino-1-(4-(tert-butyl)phenyl)-4-oxobutan-2-yl)-5-(N-isopropyl-N-methylsulfamoyl)-2-((4-methoxyphenyl)thio)benzamide] and its analogs. We used purified human ß2ARs, occupied by a high-affinity agonist, for the affinity-based screening of over 500 million distinct library compounds, which yielded Cmpd-6. It exhibits a low micro-molar affinity for the agonist-occupied ß2AR and displays positive cooperativity with orthosteric agonists, thereby enhancing their binding to the receptor and ability to stabilize its active state. Cmpd-6 is cooperative with G protein and ß-arrestin1 (a.k.a. arrestin2) to stabilize high-affinity, agonist-bound active states of the ß2AR and potentiates downstream cAMP production and receptor recruitment of ß-arrestin2 (a.k.a. arrestin3). Cmpd-6 is specific for the ß2AR compared with the closely related ß1AR. Structure-activity studies of select Cmpd-6 analogs defined the chemical groups that are critical for its biologic activity. We thus introduce the first small-molecule PAMs for the ß2AR, which may serve as a lead molecule for the development of novel therapeutics. The approach described in this work establishes a broadly applicable proof-of-concept strategy for affinity-based discovery of small-molecule allosteric compounds targeting unique conformational states of GPCRs.

Role of the V2R-ßarrestin-Gßγ complex in promoting G protein translocation to endosomes.

Classically, G protein-coupled receptors (GPCRs) promote signaling at the plasma membrane through activation of heterotrimeric Gαßγ proteins, followed by the recruitment of GPCR kinases and ßarrestin (ßarr) to initiate receptor desensitization and internalization. However, studies demonstrated that some GPCRs continue to signal from internalized compartments, with distinct cellular responses. Both ßarr and Gßγ contribute to such non-canonical endosomal G protein signaling, but their specific roles and contributions remain poorly understood. Here, we demonstrate that the vasopressin V2 receptor (V2R)-ßarr complex scaffolds Gßγ at the plasma membrane through a direct interaction with ßarr, enabling its transport to endosomes. Gßγ subsequently potentiates Gαs endosomal translocation, presumably to regenerate an endosomal pool of heterotrimeric Gs. This work shines light on the mechanism underlying G protein subunits translocation from the plasma membrane to the endosomes and provides a basis for understanding the role of ßarr in mediating sustained G protein signaling.

Kloeckera taiwanica sp. nov., an ascomycetous apiculate yeast species isolated from mushroom fruiting bodies.

Three apiculate yeast strains, EJ7M09(T), GJ5M15 and GJ15M04, isolated from mushrooms in Taiwan were found to represent a novel species of the genus Kloeckera. The phylogenetically closest relative of this novel species is Hanseniaspora occidentalis, but the type strain of H. occidentalis differed by 4.6 % divergence (25 substitutions; 5 gaps) in the sequence of the D1/D2 domain of the large subunit rRNA gene. This difference clearly suggests that the three strains represent a distinct species. As none of the strains that were examined in this study produced ascospores or exhibited conjugation on common sporulation medium either alone or in a pairwise mixture, this species could be considered as an anamorphic member of the genus Hanseniaspora, and a novel species, Kloeckera taiwanica sp. nov., is proposed, with EJ7M09(T) ( = BCRC 23182(T) = CBS 11434(T)) as the type strain.

Changes in conformation at the cytoplasmic ends of the fifth and sixth transmembrane helices of a yeast G protein-coupled receptor in response to ligand binding.

The third intracellular loop (IL3) of G protein-coupled receptors (GPCRs) is an important contact domain between GPCRs and their G proteins. Previously, the IL3 of Ste2p, a Saccharomyces cerevisiae GPCR, was suggested to undergo a conformational change upon activation as detected by differential protease susceptibility in the presence and absence of ligand. In this study using disulfide cross-linking experiments we show that the Ste2p cytoplasmic ends of helix 5 (TM5) and helix 6 (TM6) that flank the amino and carboxyl sides of IL3 undergo conformational changes upon ligand binding, whereas the center of the IL3 loop does not. Single Cys substitution of residues in the middle of IL3 led to receptors that formed high levels of cross-linked Ste2p, whereas Cys substitution at the interface of IL3 and the contiguous cytoplasmic ends of TM5 and TM6 resulted in minimal disulfide-mediated cross-linked receptor. The alternating pattern of residues involved in cross-linking suggested the presence of a 3(10) helix in the middle of IL3. Agonist (WHWLQLKPGQPNleY) induced Ste2p activation reduced cross-linking mediated by Cys substitutions at the cytoplasmic ends of TM5 and TM6 but not by residues in the middle of IL3. Thus, the cytoplasmic ends of TM5 and TM6 undergo conformational change upon ligand binding. An α-factor antagonist (des-Trp, des-His-α-factor) did not influence disulfide-mediated Ste2p cross-linking, suggesting that the interaction of the N-terminus of α-factor with Ste2p is critical for inducing conformational changes at TM5 and TM6. We propose that the changes in conformation revealed for residues at the ends of TM5 and TM6 are affected by the presence of G protein but not G protein activation. This study provides new information about role of specific residues of a GPCR in signal transduction and how peptide ligand binding activates the receptor.

Saturnispora serradocipensis sp. nov. and Saturnispora gosingensis sp. nov., two ascomycetous yeasts from ephemeral habitats.

Two novel ascomycetous yeast species, Saturnispora serradocipensis and Saturnispora gosingensis, were isolated from leaf detritus in a tropical stream of Southeastern Brazil and a mushroom collected in Taiwan, respectively. Analysis of the D1/D2 domains of the large-subunit of the rRNA gene of these strains showed that these species are related to Saturnispora hagleri, their closest relative. Saturnispora serradocipensis and S. gosingensis differed from S. hagleri, respectively, by seven nucleotide substitutions and two indels and three nucleotide substitutions and three indels in D1/D2 rRNA sequences. The two new species differ from each another by four nucleotide substitutions and one indel in D1/D2 rRNA sequences. However, the ITS sequences of S. serradocipensis, S. gosingensis and S. hagleri were quite divergent, showing that they are genetically separate species. The type strain of S. serradocipensis is UFMG-DC-198(T) (=CBS 11756(T) = NRRL Y-48717(T)), and of S. gosingensis GA4M05(T) is (CBS 11755(T) = NRRL Y-48718(T)).

Mechanism of ß2AR regulation by an intracellular positive allosteric modulator.

Drugs targeting the orthosteric, primary binding site of G protein-coupled receptors are the most common therapeutics. Allosteric binding sites, elsewhere on the receptors, are less well-defined, and so less exploited clinically. We report the crystal structure of the prototypic ß2-adrenergic receptor in complex with an orthosteric agonist and compound-6FA, a positive allosteric modulator of this receptor. It binds on the receptor's inner surface in a pocket created by intracellular loop 2 and transmembrane segments 3 and 4, stabilizing the loop in an α-helical conformation required to engage the G protein. Structural comparison explains the selectivity of the compound for ß2- over the ß1-adrenergic receptor. Diversity in location, mechanism, and selectivity of allosteric ligands provides potential to expand the range of receptor drugs.

Structure of an endosomal signaling GPCR-G protein-ß-arrestin megacomplex.

Classically, G-protein-coupled receptors (GPCRs) are thought to activate G protein from the plasma membrane and are subsequently desensitized by ß-arrestin (ß-arr). However, some GPCRs continue to signal through G protein from internalized compartments, mediated by a GPCR-G protein-ß-arr 'megaplex'. Nevertheless, the molecular architecture of the megaplex remains unknown. Here, we present its cryo-electron microscopy structure, which shows simultaneous engagement of human G protein and bovine ß-arr to the core and phosphorylated tail, respectively, of a single active human chimeric ß2-adrenergic receptor with the C-terminal tail of the arginine vasopressin type 2 receptor (ß2V2R). All three components adopt their canonical active conformations, suggesting that a single megaplex GPCR is capable of simultaneously activating G protein and ß-arr. Our findings provide a structural basis for GPCR-mediated sustained internalized G protein signaling.

Unnatural amino acid replacement in a yeast G protein-coupled receptor in its native environment.

Ste2p is the G protein-coupled receptor (GPCR) for the tridecapeptide pheromone alpha factor of Saccharomyces cerevisiae. This receptor-pheromone pair has been used extensively as a paradigm for investigating GPCR structure and function. Expression in yeast harboring a cognate tRNA/aminoacyl-tRNA synthetase pair specifically evolved to incorporate p-benzoyl- l-phenylalanine (Bpa) in response to the amber codon allowed the biosynthesis of Bpa-substituted Ste2p in its native cell. We replaced natural amino acid residues in Ste2p with Bpa by engineering amber TAG stop codons into STE2 encoded on a plasmid. Several of the expressed Bpa-substituted Ste2p receptors exhibited high-affinity ligand binding, and incorporation of Bpa into Ste2p influenced biological activity as measured by growth arrest of whole cells in response to alpha factor. We found that, at concentrations of 0.1-0.5 mM, a dipeptide containing Bpa could be used to enhance delivery of Bpa into the cell, while at 2 mM, both dipeptide and Bpa were equally effective. The application of a peptide delivery system for unnatural amino acids will extend the use of the unnatural amino acid replacement methodology to amino acids that are impermeable to yeast. Incorporation of Bpa into Ste2p was verified by mass spectrometric analysis, and two Bpa-Ste2p mutants were able to selectively capture alpha factor into the ligand-binding site after photoactivation. To our knowledge, this is the first experimental evidence documenting an unnatural amino acid replacement in a GPCR expressed in its native environment and the use of a mutated receptor to photocapture a peptide ligand.

Mechanism-of-action determination of GMP synthase inhibitors and target validation in Candida albicans and Aspergillus fumigatus.

Mechanism-of-action (MOA) studies of bioactive compounds are fundamental to drug discovery. However, in vitro studies alone may not recapitulate a compound's MOA in whole cells. Here, we apply a chemogenomics approach in Candida albicans to evaluate compounds affecting purine metabolism. They include the IMP dehydrogenase inhibitors mycophenolic acid and mizoribine and the previously reported GMP synthase inhibitors acivicin and 6-diazo-5-oxo-L-norleucine (DON). We report important aspects of their whole-cell activity, including their primary target, off-target activity, and drug metabolism. Further, we describe ECC1385, an inhibitor of GMP synthase, and provide biochemical and genetic evidence supporting its MOA to be distinct from acivicin or DON. Importantly, GMP synthase activity is conditionally essential in C. albicans and Aspergillus fumigatus and is required for virulence of both pathogens, thus constituting an unexpected antifungal target.