The co-receptor Tspan12 directly captures Norrin to promote ligand-specific ß-catenin signaling.

Wnt/ß-catenin signaling directs animal development and tissue renewal in a tightly controlled, cell- and tissue-specific manner. In the central nervous system, the atypical ligand Norrin controls angiogenesis and maintenance of the blood-brain barrier and blood-retina barrier through the Wnt/ß-catenin pathway. Like Wnt, Norrin activates signaling by binding and heterodimerizing the receptors Frizzled (Fzd) and Low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6), leading to membrane recruitment of the intracellular transducer Dishevelled (Dvl); this ultimately results in the stabilization of the transcriptional coactivator ß-catenin. Unlike Wnt, the cysteine-knot ligand Norrin only signals through Fzd4 and additionally requires the co-receptor Tspan12; however, the mechanism underlying Tspan12-mediated signal enhancement is unclear. It has been proposed that Tspan12 integrates into the Norrin-Fzd4 complex to enhance Norrin-Fzd4 affinity or otherwise allosterically modulate Fzd4 signaling. Here, we measure direct, high-affinity binding between purified Norrin and Tspan12 in a lipid environment and use AlphaFold models to interrogate this interaction interface. We find that Tspan12 and Fzd4 can simultaneously bind Norrin and that a pre-formed Tspan12/Fzd4 heterodimer, as well as cells co-expressing Tspan12 and Fzd4, more efficiently capture low concentrations of Norrin than Fzd4 alone. We also show that Tspan12 competes with both heparan sulfate proteoglycans and LRP6 for Norrin binding and that Tspan12 does not impact Fzd4-Dvl affinity in the presence or absence of Norrin. Our findings suggest that Tspan12 does not allosterically enhance Fzd4 binding to Norrin or Dvl, but instead functions to directly capture Norrin upstream of signaling.

Multi-level Force-dependent Allosteric Enhancement of αE-catenin Binding to F-actin by Vinculin.

Classical cadherins are transmembrane proteins whose extracellular domains link neighboring cells, and whose intracellular domains connect to the actin cytoskeleton via ß-catenin and α-catenin. The cadherin-catenin complex transmits forces that drive tissue morphogenesis and wound healing. In addition, tension-dependent changes in αE-catenin conformation enables it to recruit the actin-binding protein vinculin to cell-cell junctions, which contributes to junctional strengthening. How and whether multiple cadherin-complexes cooperate to reinforce cell-cell junctions in response to load remains poorly understood. Here, we used single-molecule optical trap measurements to examine how multiple cadherin-catenin complexes interact with F-actin under load, and how this interaction is influenced by the presence of vinculin. We show that force oriented toward the (-) end of the actin filament results in mean lifetimes 3-fold longer than when force was applied towards the barbed (+) end. We also measured force-dependent actin binding by a quaternary complex comprising the cadherin-catenin complex and the vinculin head region, which cannot itself bind actin. Binding lifetimes of this quaternary complex increased as additional complexes bound F-actin, but only when load was oriented toward the (-) end. In contrast, the cadherin-catenin complex alone did not show this form of cooperativity. These findings reveal multi-level, force-dependent regulation that enhances the strength of the association of multiple cadherin/catenin complexes with F-actin, conferring positive feedback that may strengthen the junction and polarize F-actin to facilitate the emergence of higher-order cytoskeletal organization.

Mechanism of the cadherin-catenin F-actin catch bond interaction.

Mechanotransduction at cell-cell adhesions is crucial for the structural integrity, organization, and morphogenesis of epithelia. At cell-cell junctions, ternary E-cadherin/ß-catenin/αE-catenin complexes sense and transmit mechanical load by binding to F-actin. The interaction with F-actin, described as a two-state catch bond, is weak in solution but is strengthened by applied force due to force-dependent transitions between weak and strong actin-binding states. Here, we provide direct evidence from optical trapping experiments that the catch bond property principally resides in the αE-catenin actin-binding domain (ABD). Consistent with our previously proposed model, the deletion of the first helix of the five-helix ABD bundle enables stable interactions with F-actin under minimal load that are well described by a single-state slip bond, even when αE-catenin is complexed with ß-catenin and E-cadherin. Our data argue for a conserved catch bond mechanism for adhesion proteins with structurally similar ABDs. We also demonstrate that a stably bound ABD strengthens load-dependent binding interactions between a neighboring complex and F-actin, but the presence of the other αE-catenin domains weakens this effect. These results provide mechanistic insight to the cooperative binding of the cadherin-catenin complex to F-actin, which regulate dynamic cytoskeletal linkages in epithelial tissues.

PI(4,5)P2-stimulated positive feedback drives the recruitment of Dishevelled to Frizzled in Wnt-ß-catenin signaling.

In the Wnt-ß-catenin pathway, Wnt binding to Frizzled (Fzd) and LRP5 or LRP6 (LRP5/6) co-receptors inhibits the degradation of the transcriptional coactivator ß-catenin by recruiting the cytosolic effector Dishevelled (Dvl). Polymerization of Dvl at the plasma membrane recruits the ß-catenin destruction complex, enabling the phosphorylation of LRP5/6, a key step in inhibiting ß-catenin degradation. Using purified Fzd proteins reconstituted in lipid nanodiscs, we investigated the factors that promote the recruitment of Dvl to the plasma membrane. We found that the affinity of Fzd for Dvl was not affected by Wnt ligands, in contrast to other members of the GPCR superfamily for which the binding of extracellular ligands affects the affinity for downstream transducers. Instead, Fzd-Dvl binding was enhanced by increased concentration of the lipid PI(4,5)P2, which is generated by Dvl-associated lipid kinases in response to Wnt and which is required for LRP5/6 phosphorylation. Moreover, binding to Fzd did not promote Dvl DEP domain dimerization, which has been proposed to be required for signaling downstream of Fzd. Our findings suggest a positive feedback loop in which Wnt-stimulated local PI(4,5)P2 production enhances Dvl recruitment and further PI(4,5)P2 production to support Dvl polymerization, LRP5/6 phosphorylation, and ß-catenin stabilization.

The C-terminal actin-binding domain of talin forms an asymmetric catch bond with F-actin.

SignificanceTalin is a mechanosensitive adaptor protein that links integrins to the actin cytoskeleton at cell-extracellular matrix adhesions. Although the C-terminal actin-binding domain ABS3 of talin is required for function, it binds weakly to actin in solution. We show that ABS3 binds actin strongly only when subjected to mechanical forces comparable to those generated by the cytoskeleton. Moreover, the interaction between ABS3 and actin depends strongly on the direction of force in a manner predicted to organize actin to facilitate adhesion growth and efficient cytoskeletal force generation. These characteristics can explain how force sensing by talin helps to nucleate adhesions precisely when and where they are required to transmit force between the cytoskeleton and the extracellular matrix.

Reconstitution of purified membrane protein dimers in lipid nanodiscs with defined stoichiometry and orientation using a split GFP tether.

Many membrane proteins function as dimers or larger oligomers, including transporters, channels, certain signaling receptors, and adhesion molecules. In some cases, the interactions between individual proteins may be weak and/or dependent on specific lipids, such that detergent solubilization used for biochemical and structural studies disrupts functional oligomerization. Solubilized membrane protein oligomers can be captured in lipid nanodiscs, but this is an inefficient process that can produce stoichiometrically and topologically heterogeneous preparations. Here, we describe a technique to obtain purified homogeneous membrane protein dimers in nanodiscs using a split GFP (sGFP) tether. Complementary sGFP tags associate to tether the coexpressed dimers and control both stoichiometry and orientation within the nanodiscs, as assessed by quantitative Western blotting and negative-stain EM. The sGFP tether confers several advantages over other methods: it is highly stable in solution and in SDS-PAGE, which facilitates screening of dimer expression and purification by fluorescence, and also provides a dimer-specific purification handle for use with GFP nanobody-conjugated resin. We used this method to purify a Frizzled-4 homodimer and a Frizzled-4/low-density lipoprotein receptor-related protein 6 heterodimer in nanodiscs. These examples demonstrate the utility and flexibility of this method, which enables subsequent mechanistic molecular and structural studies of membrane protein pairs.

Lattice micropatterning for cryo-electron tomography studies of cell-cell contacts.

Cryo-electron tomography is the highest resolution tool available for structural analysis of macromolecular complexes within their native cellular environments. At present, data acquisition suffers from low throughput, in part due to the low probability of positioning a cell such that the subcellular structure of interest is on a region of the electron microscopy (EM) grid that is suitable for imaging. Here, we photo-micropatterned EM grids to optimally position endothelial cells so as to enable high-throughput imaging of cell-cell contacts. Lattice micropatterned grids increased the average distance between intercellular contacts and thicker cell nuclei such that the regions of interest were sufficiently thin for direct imaging. We observed a diverse array of membranous and cytoskeletal structures at intercellular contacts, demonstrating the utility of this technique in enhancing the rate of data acquisition for cellular cryo-electron tomography studies.

Distinct intramolecular interactions regulate autoinhibition of vinculin binding in αT-catenin and αE-catenin.

α-Catenin binds directly to ß-catenin and connects the cadherin-catenin complex to the actin cytoskeleton. Tension regulates α-catenin conformation. Actomyosin-generated force stretches the middle (M)-region to relieve autoinhibition and reveal a binding site for the actin-binding protein vinculin. It is not known whether the intramolecular interactions that regulate epithelial (αE)-catenin binding are conserved across the α-catenin family. Here, we describe the biochemical properties of testes (αT)-catenin, an α-catenin isoform critical for cardiac function and how intramolecular interactions regulate vinculin-binding autoinhibition. Isothermal titration calorimetry showed that αT-catenin binds the ß-catenin-N-cadherin complex with a similar low nanomolar affinity to that of αE-catenin. Limited proteolysis revealed that the αT-catenin M-region adopts a more open conformation than αE-catenin. The αT-catenin M-region binds the vinculin N-terminus with low nanomolar affinity, indicating that the isolated αT-catenin M-region is not autoinhibited and thereby distinct from αE-catenin. However, the αT-catenin head (N- and M-regions) binds vinculin 1000-fold more weakly (low micromolar affinity), indicating that the N-terminus regulates the M-region binding to vinculin. In cells, αT-catenin recruitment of vinculin to cell-cell contacts requires the actin-binding domain and actomyosin-generated tension, indicating that force regulates vinculin binding. Together, our results show that the αT-catenin N-terminus is required to maintain M-region autoinhibition and modulate vinculin binding. We postulate that the unique molecular properties of αT-catenin allow it to function as a scaffold for building specific adhesion complexes.

Structural analysis of carbohydrate binding by the macrophage mannose receptor CD206.

The human mannose receptor expressed on macrophages and hepatic endothelial cells scavenges released lysosomal enzymes, glycopeptide fragments of collagen, and pathogenic microorganisms and thus reduces damage following tissue injury. The receptor binds mannose, fucose, or N-acetylglucosamine (GlcNAc) residues on these targets. C-type carbohydrate-recognition domain 4 (CRD4) of the receptor contains the site for Ca2+-dependent interaction with sugars. To investigate the details of CRD4 binding, glycan array screening was used to identify oligosaccharide ligands. The strongest signals were for glycans that contain either Manα1-2Man constituents or fucose in various linkages. The mechanisms of binding to monosaccharides and oligosaccharide substructures present in many of these ligands were examined in multiple crystal structures of CRD4. Binding of mannose residues to CRD4 results primarily from interaction of the equatorial 3- and 4-OH groups with a conserved principal Ca2+ common to almost all sugar-binding C-type CRDs. In the Manα1-2Man complex, supplementary interactions with the reducing mannose residue explain the enhanced affinity for this disaccharide. Bound GlcNAc also interacts with the principal Ca2+ through equatorial 3- and 4-OH groups, whereas fucose residues can bind in several orientations, through either the 2- and 3-OH groups or the 3- and 4-OH groups. Secondary contacts with additional sugars in fucose-containing oligosaccharides, such as the Lewis-a trisaccharide, provide enhanced affinity for these glycans. These results explain many of the biologically important interactions of the mannose receptor with both mammalian glycoproteins and microbes such as yeast and suggest additional classes of ligands that have not been previously identified.

Structural basis of αE-catenin-F-actin catch bond behavior.

Cell-cell and cell-matrix junctions transmit mechanical forces during tissue morphogenesis and homeostasis. α-Catenin links cell-cell adhesion complexes to the actin cytoskeleton, and mechanical load strengthens its binding to F-actin in a direction-sensitive manner. Specifically, optical trap experiments revealed that force promotes a transition between weak and strong actin-bound states. Here, we describe the cryo-electron microscopy structure of the F-actin-bound αE-catenin actin-binding domain, which in solution forms a five-helix bundle. In the actin-bound structure, the first helix of the bundle dissociates and the remaining four helices and connecting loops rearrange to form the interface with actin. Deletion of the first helix produces strong actin binding in the absence of force, suggesting that the actin-bound structure corresponds to the strong state. Our analysis explains how mechanical force applied to αE-catenin or its homolog vinculin favors the strongly bound state, and the dependence of catch bond strength on the direction of applied force.

Author Correction: Structural insights into µ-opioid receptor activation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Limited dishevelled/Axin oligomerization determines efficiency of Wnt/ß-catenin signal transduction.

In Wnt/ß-catenin signaling, the transcriptional coactivator ß-catenin is regulated by its phosphorylation in a complex that includes the scaffold protein Axin and associated kinases. Wnt binding to its coreceptors activates the cytosolic effector Dishevelled (Dvl), leading to the recruitment of Axin and the inhibition of ß-catenin phosphorylation. This process requires interaction of homologous DIX domains present in Dvl and Axin, but is mechanistically undefined. We show that Dvl DIX forms antiparallel, double-stranded oligomers in vitro, and that Dvl in cells forms oligomers typically <10 molecules at endogenous expression levels. Axin DIX (DAX) forms small single-stranded oligomers, but its self-association is stronger than that of DIX. DAX caps the ends of DIX oligomers, such that a DIX oligomer has at most four DAX binding sites. The relative affinities and stoichiometry of the DIX-DAX interaction provide a mechanism for efficient inhibition of ß-catenin phosphorylation upon Axin recruitment to the Wnt receptor complex.

Stem cells can give rise to many types of specialized cells through a process called differentiation, which is partly regulated by changes in the levels of a protein known as ß-catenin. On one hand, a 'destruction complex' can keep ß-catenin levels low; this complex includes a protein called Axin and an enzyme known as GSK-3, which can tag ß-catenin for degradation. On the other hand, when ß-catenin levels need to increase, another protein called Dishevelled is activated. By binding to Axin, Dishevelled can bring the destruction complex in contact with other proteins, which leads to the deactivation of GSK-3. Dishevelled and Axin interact via a region that is similar in the two proteins, called DIX in Dishevelled and DAX in Axin. Studies of DIX and DAX have shown that both regions can form polymers ­ that is, a high number of similar units can bind together to form larger structures. However, these experiments were at higher concentrations than would be found in the cell. It was thought that, when combined, DIX and DAX might form these long chains together, preventing Axin from carrying out its role in destroying ß-catenin. Kan et al. set out to better understand this process by studying how DIX and DAX behave separately, and how they interact. The proteins were examined using a technique called cryo-electron microscopy, which allows scientists to dissect the structure of large proteins. When there was a high concentration of DIX in the sample, the molecules attached to one another to form long double-stranded helices. Similarly, DAX also formed helices, but these were shorter and only single-stranded. When the two proteins were combined, DAX bound only to the ends of short DIX chains, so that there are not more than four DAX chains attached to each DIX double helix. To see if this behaviour happens naturally, Kan et al. attached fluorescent tags to Dishevelled proteins and followed them in living cells: this showed that Dishevelled forms smaller chains with fewer than ten molecules. Together these results highlight how Dishevelled binds to Axin to deactivate GSK-3, to prevent the enzyme from promoting ß-catenin destruction. Mutations in the genes that encode ß-catenin or its regulators are associated with cancer. Ultimately, a better understanding of how ß-catenin is regulated could help to identify new opportunities for drug development.

Binding partner- and force-promoted changes in αE-catenin conformation probed by native cysteine labeling.

Adherens Junctions (AJs) are cell-cell adhesion complexes that sense and propagate mechanical forces by coupling cadherins to the actin cytoskeleton via ß-catenin and the F-actin binding protein αE-catenin. When subjected to mechanical force, the cadherin•catenin complex can tightly link to F-actin through αE-catenin, and also recruits the F-actin-binding protein vinculin. In this study, labeling of native cysteines combined with mass spectrometry revealed conformational changes in αE-catenin upon binding to the E-cadherin•ß-catenin complex, vinculin and F-actin. A method to apply physiologically meaningful forces in solution revealed force-induced conformational changes in αE-catenin when bound to F-actin. Comparisons of wild-type αE-catenin and a mutant with enhanced vinculin affinity using cysteine labeling and isothermal titration calorimetry provide evidence for allosteric coupling of the N-terminal ß-catenin-binding and the middle (M) vinculin-binding domain of αE-catenin. Cysteine labeling also revealed possible crosstalk between the actin-binding domain and the rest of the protein. The data provide insight into how binding partners and mechanical stress can regulate the conformation of full-length αE-catenin, and identify the M domain as a key transmitter of conformational changes.

CD23 is a glycan-binding receptor in some mammalian species.

CD23, the low-affinity IgE receptor found on B lymphocytes and other cells, contains a C-terminal lectin-like domain that resembles C-type carbohydrate-recognition domains (CRDs) found in many glycan-binding receptors. In most mammalian species, the CD23 residues required to form a sugar-binding site are present, although binding of CD23 to IgE does not involve sugars. Solid-phase binding competition assays, glycoprotein blotting experiments, and glycan array analysis employing the lectin-like domains of cow and mouse CD23 demonstrate that they bind to mannose, GlcNAc, glucose, and fucose and to glycoproteins that bear these sugars in nonreducing terminal positions. Crystal structures of the cow CRD in the presence of α-methyl mannoside and GlcNAcß1-2Man reveal that a range of oligosaccharide ligands can be accommodated in an open binding site in which most interactions are with a single terminal sugar residue. Although mouse CD23 shows a pattern of monosaccharide and glycoprotein binding similar to cow CD23, the binding is weaker. In contrast, no sugar binding was observed in similar experiments with human CD23. The absence of sugar-binding activity correlates with accumulation of mutations in the gene for CD23 in the primate lineage leading to humans, resulting in loss of key sugar-binding residues. These results are consistent with a role for CD23 in many species as a receptor for potentially pathogenic microorganisms as well as IgE. However, the ability of CD23 to bind several different ligands varies between species, suggesting that it has distinct functions in different organisms.

Structural insights into the aPKC regulatory switch mechanism of the human cell polarity protein lethal giant larvae 2.

Metazoan cell polarity is controlled by a set of highly conserved proteins. Lethal giant larvae (Lgl) functions in apical-basal polarity through phosphorylation-dependent interactions with several other proteins as well as the plasma membrane. Phosphorylation of Lgl by atypical protein kinase C (aPKC), a component of the partitioning-defective (Par) complex in epithelial cells, excludes Lgl from the apical membrane, a crucial step in the establishment of epithelial cell polarity. We present the crystal structures of human Lgl2 in both its unphosphorylated and aPKC-phosphorylated states. Lgl2 adopts a double ß-propeller structure that is unchanged by aPKC phosphorylation of an unstructured loop in its second ß-propeller, ruling out models of phosphorylation-dependent conformational change. We demonstrate that phosphorylation controls the direct binding of purified Lgl2 to negative phospholipids in vitro. We also show that a coil-helix transition of this region that is promoted by phosphatidylinositol 4,5-bisphosphate (PIP2) is also phosphorylation-dependent, implying a highly effective phosphorylative switch for membrane association.

Solving the structure of Lgl2, a difficult blind test of unsupervised structure determination.

In the companion paper by Ufimtsev and Levitt [Ufimtsev IS, Levitt M (2019) Proc Natl Acad Sci USA, 10.1073/pnas.1821512116], we presented a method for unsupervised solution of protein crystal structures and demonstrated its utility by solving several test cases of known structure in the 2.9- to 3.45-Å resolution range. Here we apply this method to solve the crystal structure of a 966-amino acid construct of human lethal giant larvae protein (Lgl2) that resisted years of structure determination efforts, at 3.2-Å resolution. The structure was determined starting with a molecular replacement (MR) model identified by unsupervised refinement of a pool of 50 candidate MR models. This initial model had 2.8-Å RMSD from the solution. The solved structure was validated by comparison with a model subsequently derived from an alternative crystal form diffracting to higher resolution. This model could phase an anomalous difference Fourier map from an Hg derivative, and a single-wavelength anomalous dispersion phased density map made from these sites aligned with the refined structure.

Author Correction: Structural insights into the activation of metabotropic glutamate receptors.

The surname of author Toon Laeremans was misspelled 'Laermans'. This error has been corrected online.

Resolving indexing ambiguities in X-ray free-electron laser diffraction patterns.

Processing X-ray free-electron laser (XFEL) diffraction images poses challenges, as an XFEL pulse is powerful enough to destroy or damage the diffracting volume and thereby yields only one diffraction image per volume. Moreover, the crystal is stationary during the femtosecond pulse, so reflections are generally only partially recorded. Therefore, each XFEL diffraction image must be scaled individually and, ideally, corrected for partiality prior to merging. An additional complication may arise owing to indexing ambiguities when the symmetry of the Bravais lattice is higher than that of the space group, or when the unit-cell dimensions are similar to each other. Here, an automated method is presented that diagnoses these indexing ambiguities based on the Brehm-Diederichs algorithm [Brehm & Diederichs (2014), Acta Cryst. D70, 101-109] and produces a consistent indexing choice for the large majority of diffraction images. This method was applied to an XFEL diffraction data set measured from crystals of the neuronal SNARE-complexin-1-synaptotagmin-1 complex. After correcting the indexing ambiguities, substantial improvements were observed in the merging statistics and the atomic model refinement R values. This method should be a useful addition to the arsenal of tools for the processing of XFEL diffraction data sets.

Structural insights into the activation of metabotropic glutamate receptors.

Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains-the 7-transmembrane domains-in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling.

Structure of a Signaling Cannabinoid Receptor 1-G Protein Complex.

Cannabis elicits its mood-enhancing and analgesic effects through the cannabinoid receptor 1 (CB1), a G protein-coupled receptor (GPCR) that signals primarily through the adenylyl cyclase-inhibiting heterotrimeric G protein Gi. Activation of CB1-Gi signaling pathways holds potential for treating a number of neurological disorders and is thus crucial to understand the mechanism of Gi activation by CB1. Here, we present the structure of the CB1-Gi signaling complex bound to the highly potent agonist MDMB-Fubinaca (FUB), a recently emerged illicit synthetic cannabinoid infused in street drugs that have been associated with numerous overdoses and fatalities. The structure illustrates how FUB stabilizes the receptor in an active state to facilitate nucleotide exchange in Gi. The results compose the structural framework to explain CB1 activation by different classes of ligands and provide insights into the G protein coupling and selectivity mechanisms adopted by the receptor.