FLAG- and HA-tagged M2 muscarinic receptors from coinfected Sf9 cells have been purified in digitonin-cholate and reconstituted into phospholipid vesicles. The purified receptor was predominantly monomeric: it showed no detectable coimmunoprecipitation; it migrated as a monomer during electrophoresis before or after cross-linking with bis(sulfosuccinimidyl)suberate; and it bound agonists and antagonists in a manner indicative of identical and mutually independent sites. Receptor cross-linked after reconstitution or after reconstitution and subsequent solubilization in digitonin-cholate migrated almost exclusively as a tetramer. The binding properties of the reconstituted receptor mimicked those reported previously for cardiac muscarinic receptors. The apparent capacity for N-[3H]methylscopolamine (NMS) was only 60% of that for [3H]quinuclidinylbenzilate (QNB), yet binding at saturating concentrations of [3H]QNB was inhibited fully and in a noncompetitive manner at comparatively low concentrations of unlabeled NMS. Reconstitution of the receptor with a saturating quantity of functional G proteins led to the appearance of three classes of sites for the agonist oxotremorine-M in assays with [3H]QNB; GMP-PNP caused an apparent interconversion from highest to lowest affinity and the concomitant emergence of a fourth class of intermediate affinity. All of the data can be described quantitatively in terms of cooperativity among four interacting sites, presumably within a tetramer; the effect of GMP-PNP can be accommodated as a shift in the distribution of tetramers between two states that differ in their cooperative properties. Monomers of the M2 receptor therefore can be assembled into tetramers with binding properties that closely resemble those of the muscarinic receptor in myocardial preparations.

Although anti-inflammatory macrophages perform macropinocytosis constitutively, proinflammatory macrophages do not, due to impaired signaling of the calcium-sensing receptor. However, macropinocytosis can be elicited in proinflammatory macrophages by agonists that induce the recruitment of phosphatidylinositol 3-kinase to the plasma membrane.

The M 2 muscarinic receptor has two topographically distinct sites: the orthosteric site and an allosteric site recognized by compounds such as gallamine. It also can exhibit cooperative effects in the binding of orthosteric ligands, presumably to the orthosteric sites within an oligomer. Such effects would be difficult to interpret, however, if those ligands also bound to the allosteric site. Monomers of the hemagglutinin (HA)-and FLAGtagged human M 2 receptor therefore have been purified from coinfected Sf9 cells and examined for any effect of the antagonist N-methyl scopolamine or the agonist oxotremorine-M on the rate at which N-[ 3 H]methyl scopolamine dissociates from the orthosteric site (k obsd ). The predominantly monomeric status was confirmed by coimmunoprecipitation and by crosslinking with bis(sulfosuccinimidyl)suberate. Both N-methyl scopolamine and oxotremorine-M acted in a cooperative manner to decrease k obsd by 4.5-and 9.1-fold, respectively; the corresponding estimates of affinity (log K L ) are Ϫ2.55 Ϯ 0.13 and Ϫ2.29 Ϯ 0.14. Gallamine and the allosteric ligand obidoxime decreased k obsd by more than 100-fold (log K L ϭ Ϫ4.12 Ϯ 0.04) and by only 1.1-fold (log K L ϭ Ϫ1.73 Ϯ 0.91), respectively. Obidoxime reversed the effect of N-methyl scopolamine, oxotremorine-M, and gallamine in a manner that could be described by a model in which all four ligands compete for a common allosteric site. Ligands generally assumed to be exclusively orthosteric therefore can act at the allosteric site of the M 2 receptor, albeit at comparatively high concentrations.

Muscarinic and other G protein-coupled receptors exhibit an agonist-specific heterogeneity that tracks efficacy and commonly is attributed to an effect of the G protein on an otherwise homogeneous population of sites. To examine this notion, M2 muscarinic receptors were purified from Sf9 cells as monomers devoid of G protein and reconstituted as tetramers in phospholipid vesicles. In assays with N-[(3)H]methylscopolamine, seven agonists revealed a dispersion of affinities indicative of two or more classes of sites. Unlabeled N-methylscopolamine and the antagonist quinuclidinylbenzilate recognized one class of sites; atropine recognized two classes with a preference that was the opposite of that of agonists, as indicated by the effects of N-ethylmaleimide. The data were inconsistent with an explicit model of constitutive asymmetry within a tetramer, and the fit improved markedly upon the introduction of cooperative interactions (P < 0.00001). Purified monomers appeared to be homogeneous or nearly so to all ligands except the partial agonists pilocarpine and McN-A-343, where heterogeneity emerged from intramolecular cooperativity between the orthosteric site and an allosteric site. The breadth of each dispersion was quantified empirically as the area between the fitted curve for two classes of sites and the theoretical curve for a single class of lower affinity, which approximates the expected effect of GTP if a G protein were present. The areas measured for 10 ligands at reconstituted tetramers correlated with similar measures of heterogeneity and with intrinsic activities reported previously for binding and response in natural membranes (P ≤ 0.00002). The data suggest that the GTP-sensitive heterogeneity typically revealed by agonists at M2 receptors is intrinsic to the receptor in its tetrameric state. It exists independently of the G protein, and it appears to arise at least in part from cooperativity between linked orthosteric sites.

Background:The allosteric interaction between agonists and guanylyl nucleotides reports on the interaction between G protein-coupled receptors and G proteins. Results: Such allostery differs in kind between reconstituted monomers and tetramers of the M 2 muscarinic receptor. Conclusion: Monomers and tetramers mediate allostery via different mechanisms. Significance: Only tetramers resemble muscarinic receptors in myocardial membranes in the nature of their sensitivity to guanylyl nucleotides.

The M2 muscarinic receptor is the prototypic model of allostery in GPCRs, yet the molecular and the supramolecular determinants of such effects are unknown. Monomers and oligomers of the M2 muscarinic receptor therefore have been compared to identify those allosteric properties that are gained in oligomers. Allosteric interactions were monitored by means of a FRET-based sensor of conformation at the allosteric site and in pharmacological assays involving mutants engineered to preclude intramolecular effects. Electrostatic, steric, and conformational determinants of allostery at the atomic level were examined in molecular dynamics simulations. Allosteric effects in monomers were exclusively negative and derived primarily from intramolecular electrostatic repulsion between the allosteric and orthosteric ligands. Allosteric effects in oligomers could be positive or negative, depending upon the allosteric-orthosteric pair, and they arose from interactions within and between the constituent protomers. The complex behavior of oligomers is characteristic of muscarinic receptors in myocardial preparations.DOI: http://dx.doi.org/10.7554/eLife.11685.001

The COVID-19 pandemic has highlighted the urgent need for the identification of new antiviral drug therapies for a variety of diseases. COVID-19 is caused by infection with the human coronavirus SARS-CoV-2, while other related human coronaviruses cause diseases ranging from severe respiratory infections to the common cold. We developed a computational approach to identify new antiviral drug targets and repurpose clinically-relevant drug compounds for the treatment of a range of human coronavirus diseases. Our approach is based on graph convolutional networks (GCN) and involves multiscale host-virus interactome analysis coupled to off-target drug predictions. Cell-based experimental assessment reveals several clinically-relevant drug repurposing candidates predicted by the in silico analyses to have antiviral activity against human coronavirus infection. In particular, we identify the MET inhibitor capmatinib as having potent and broad antiviral activity against several coronaviruses in a MET-independent manner, as well as novel roles for host cell proteins such as IRAK1/4 in supporting human coronavirus infection, which can inform further drug discovery studies.

No abstract