Classical cadherins mediate specific adhesion at intercellular adherens junctions. Interactions between cadherin ectodomains from apposed cells mediate cell-cell contact, whereas the intracellular ...region functionally links cadherins to the underlying cytoskeleton. Structural, biophysical, and biochemical studies have provided important insights into the mechanism and specificity of cell-cell adhesion by classical cadherins and their interplay with the cytoskeleton. Adhesive binding arises through exchange of beta strands between the first extracellular cadherin domains (EC1) of partner cadherins from adjacent cells. This "strand-swap" binding mode is common to classical and desmosomal cadherins, but sequence alignments suggest that other cadherins will bind differently. The intracellular region of classical cadherins binds to p120 and beta-catenin, and beta-catenin binds to the F-actin binding protein alpha-catenin. Rather than stably bridging beta-catenin to actin, it appears that alpha-catenin actively regulates the actin cytoskeleton at cadherin-based cell-cell contacts.
Dendritic cell specific intracellular adhesion molecule-3 (ICAM-3) grabbing nonintegrin (DC-SIGN), a C-type lectin present on the surface of dendritic cells, mediates the initial interaction of ...dendritic cells with T cells by binding to ICAM-3. DC-SIGN and DC-SIGNR, a related receptor found on the endothelium of liver sinusoids, placental capillaries, and lymph nodes, bind to oligosaccharides that are present on the envelope of human immunodeficiency virus (HIV), an interaction that strongly promotes viral infection of T cells. Crystal structures of carbohydrate-recognition domains of DC-SIGN and of DC-SIGNR bound to oligosaccharide, in combination with binding studies, reveal that these receptors selectively recognize endogenous high-mannose oligosaccharides and may represent a new avenue for developing HIV prophylactics.
Syntaxin 1a and neuronal Sec1 (nSec1) form an evolutionarily conserved heterodimer that is essential for vesicle trafficking and membrane fusion. The crystal structure of the nSec1-syntaxin 1a ...complex, determined at 2.6 A resolution, reveals that major conformational rearrangements occur in syntaxin relative to both the core SNARE complex and isolated syntaxin. We identify regions of the two proteins that seem to determine the binding specificity of particular Sec1 proteins for syntaxin isoforms, which is likely to be important for the fidelity of membrane trafficking. The structure also indicates mechanisms that might couple the action of upstream effector proteins to conformational changes in syntaxin 1a and nSec1 that lead to core complex formation and membrane fusion.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Wnt signaling activates target genes by promoting association of the co‐activator β‐catenin with TCF/LEF transcription factors. In the absence of β‐catenin, target genes are silenced by TCF‐mediated ...recruitment of TLE/Groucho proteins, but the molecular basis for TLE/TCF‐dependent repression is unclear. We describe the unusual three‐dimensional structure of the N‐terminal Q domain of TLE1 that mediates tetramerization and binds to TCFs. We find that differences in repression potential of TCF/LEFs correlates with their affinities for TLE‐Q, rather than direct competition between β‐catenin and TLE for TCFs as part of an activation–repression switch. Structure‐based mutation of the TLE tetramer interface shows that dimers cannot mediate repression, even though they bind to TCFs with the same affinity as tetramers. Furthermore, the TLE Q tetramer, not the dimer, binds to chromatin, specifically to K20 methylated histone H4 tails, suggesting that the TCF/TLE tetramer complex promotes structural transitions of chromatin to mediate repression.
Synopsis
Bill Weis and colleagues present an unusual tetramer fold for the N‐terminal domain of the Groucho/TLE1 repressor. Their functional results propose chromatin binding, rather than competition for TCF as mechanism for Wnt‐target gene repression.
The N‐terminal tetramerization domain of the transcriptional co‐repressor TLE1 forms an extended, interdigitated dimer of dimers
TLE1 binds the repressive TCF3 and TCF4 proteins more strongly than the activating TCF1 and LEF1 proteins
There is no direct competition between TLE and ß‐catenin for TCF/LEF binding, suggesting that other factors mediate the switch between repression and activation of Wnt target genes
The TLE N‐terminal domain can bind chromatin through its interaction with K20 methylated H4 tails.
Bill Weis and colleagues present an unusual tetramer fold for the N‐terminal domain of the Groucho/TLE1 repressor. Their functional results propose chromatin binding, rather than competition for TCF as mechanism for Wnt‐target gene repression.
As a component of adherens junctions and the Wnt signaling pathway, β-catenin binds cadherins, Tcf family transcription factors, and the tumor suppressor APC. We have determined the crystal ...structures of both unphosphorylated and phosphorylated E-cadherin cytoplasmic domain complexed with the arm repeat region of β-catenin. The interaction spans all 12 arm repeats, and features quasi-independent binding regions that include helices which interact with both ends of the arm repeat domain and an extended stretch of 14 residues which closely resembles a portion of XTcf-3. Phosphorylation of E-cadherin results in interactions with a hydrophobic patch of β-catenin that mimics the binding of an amphipathic XTcf-3 helix. APC contains sequences homologous to the phosphorylated region of cadherin, and is likely to bind similarly.
The transcriptional coactivator β-catenin mediates Wnt growth factor signaling. In the absence of a Wnt signal, casein kinase 1 (CK1) and glycogen synthase kinase-3β (GSK-3β) phosphorylate cytosolic ...β-catenin, thereby flagging it for recognition and destruction by the ubiquitin/proteosome machinery. Phosphorylation occurs in a multiprotein complex that includes the kinases, β-catenin, axin, and the Adenomatous Polyposis Coli (APC) protein. The role of APC in this process is poorly understood. CK1ϵ and GSK-3β phosphorylate APC, which increases its affinity for β-catenin. Crystal structures of phosphorylated and nonphosphoryated APC bound to β-catenin reveal a phosphorylation-dependent binding motif generated by mutual priming of CK1 and GSK-3β substrate sequences. Axin is shown to act as a scaffold for substrate phosphorylation by these kinases. Phosphorylated APC and axin bind to the same surface of, and compete directly for, β-catenin. The structural and biochemical data suggest a novel model for how APC functions in β-catenin degradation.
Lectins are responsible for cell surface sugar recognition in bacteria, animals, and plants. Examples include bacterial toxins; animal receptors that mediate cell-cell interactions, uptake of ...glycoconjugates, and pathogen neutralization; and plant toxins and mitogens. The structural basis for selective sugar recognition by members of all of these groups has been investigated by x-ray crystallography. Mechanisms for sugar recognition have evolved independently in diverse protein structural frameworks, but share some key features. Relatively low affinity binding sites for monosaccharides are formed at shallow indentations on protein surfaces. Selectivity is achieved through a combination of hydrogen bonding to the sugar hydroxyl groups with van der Waals packing, often including packing of a hydrophobic sugar face against aromatic amino acid side chains. Higher selectivity of binding is achieved by extending binding sites through additional direct and water-mediated contacts between oligosaccharides and the protein surface. Dramatically increased affinity for oligosaccharides results from clustering of simple binding sites in oligomers of the lectin polypeptides. The geometry of such oligomers helps to establish the ability of the lectins to distinguish surface arrays of polysaccharides in some instances and to crosslink glycoconjugates in others.
Protein‐carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar‐binding proteins) mediate both pathogen recognition and cell‐cell interactions using ...structurally related Ca2+‐dependent carbohydrate‐recognition domains (C‐type CRDs). Pathogen recognition by soluble collections such as serum mannose‐binding protein and pulmonary surfactant proteins, and also the macrophage cell‐surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide‐binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non‐self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C‐type lectin‐like domains (CTLDs) that are evolutionarily divergent from the C‐type lectins and which would be predicted to function through different mechanisms.
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.
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•3 Å cryo-EM structure of the CB1-Gi complex bound to potent agonist MDMB-Fubinaca•MDMB-Fubinaca locks “toggle switch” residues F2003.36/W3566.48 in active conformation•Quantum mechanics calculations reveal the mechanism for the high affinity of Fubinaca•Molecular dynamic simulations reveal a path for ligand entry between TM1 and TM7
Looking at how a toxic, synthetic ligand locks cannabinoid receptor 1 into a signaling conformation points to ways to understand and modulate receptor activity.
Valosin-containing protein (VCP)/p97 is an AAA family ATPase that has been implicated in the removal of misfolded proteins from the endoplasmic reticulum and in membrane fusion. p97 forms a ...homohexamer whose protomers consist of an N-terminal (N) domain responsible for binding to effector proteins, followed by two AAA ATPase domains, D
1 and D
2. Small-angle X-ray scattering (SAXS) measurements of p97 in the presence of AMP-PNP (ATP state), ADP-AlF
x (hydrolysis transition state), ADP, or no nucleotide reveal major changes in the positions of the N domains with respect to the hexameric ring during the ATP hydrolysis cycle. Nucleotide binding and hydrolysis experiments indicate that D
2 inhibits nucleotide exchange by D
1. The data suggest that the conversion of the chemical energy of ATP hydrolysis into mechanical work on substrates involves transmission of conformational changes generated by D
2 through D
1 to move N.
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