The ectodomain of Toll-like receptor 9 is cleaved to generate a functional receptor

Mammalian Toll-like receptors (TLRs) 3, 7, 8 and 9 initiate immune responses to infection by recognizing microbial nucleic acids; however, these responses come at the cost of potential autoimmunity owing to inappropriate recognition of self nucleic acids. The localization of TLR9 and TLR7 to intracellular compartments seems to have a role in facilitating responses to viral nucleic acids while maintaining tolerance to self nucleic acids, yet the cell biology regulating the transport and localization of these receptors remains poorly understood. Here we define the route by which TLR9 and TLR7 exit the endoplasmic reticulum and travel to endolysosomes in mouse macrophages and dendritic cells. The ectodomains of TLR9 and TLR7 are cleaved in the endolysosome, such that no full-length protein is detectable in the compartment where ligand is recognized. Notably, although both the full-length and cleaved forms of TLR9 are capable of binding ligand, only the processed form recruits MyD88 on activation, indicating that this truncated receptor, rather than the full-length form, is functional. Furthermore, conditions that prevent receptor proteolysis, including forced TLR9 surface localization, render the receptor non-functional. We propose that ectodomain cleavage represents a strategy to restrict receptor activation to endolysosomal compartments and prevent TLRs from responding to self nucleic acids.     

Structure of the insulin receptor substrate IRS-1 defines a unique signal transduction protein.

Since the discovery of insulin nearly 70 years ago, there has been no problem more fundamental to diabetes research than understanding how insulin works at the cellular level. Insulin binds to the alpha subunit of the insulin receptor which activates the tyrosine kinase in the beta subunit, but the molecular events linking the receptor kinase to insulin-sensitive enzymes and transport processes are unknown. Our discovery that insulin stimulates tyrosine phosphorylation of a protein of relative molecular mass between 165,000 and 185,000, collectively called pp185, showed that the insulin receptor kinase has specific cellular substrates. The pp185 is a minor cytoplasmic phosphoprotein found in most cells and tissues; its phosphorylation is decreased in cells expressing mutant receptors defective in signalling. We have now cloned IRS-1, which encodes a component of the pp185 band. IRS-1 contains over ten potential tyrosine phosphorylation sites, six of which are in Tyr-Met-X-Met motifs. During insulin stimulation, the IRS-1 protein undergoes tyrosine phosphorylation and binds phosphatidylinositol 3-kinase, suggesting that IRS-1 acts as a multisite 'docking' protein to bind signal-transducing molecules containing Src-homology 2 and Src-homology-3 domains. Thus IRS-1 may link the insulin receptor kinase and enzymes regulating cellular growth and metabolism.     

Crystal structure of fibroblast growth factor receptor ectodomain bound to ligand and heparin

Fibroblast growth factors (FGFs) are a large family of structurally related proteins with a wide range of physiological and pathological activities. Signal transduction requires association of FGF with its receptor tyrosine kinase (FGFR) and heparan sulphate proteoglycan in a specific complex on the cell surface. Direct involvement of the heparan sulphate glycosaminoglycan polysaccharide in the molecular association between FGF and its receptor is essential for biological activity. Although crystal structures of binary complexes of FGF-heparin and FGF-FGFR have been described, the molecular architecture of the FGF signalling complex has not been elucidated. Here we report the crystal structure of the FGFR2 ectodomain in a dimeric form that is induced by simultaneous binding to FGF1 and a heparin decasaccharide. The complex is assembled around a central heparin molecule linking two FGF1 ligands into a dimer that bridges between two receptor chains. The asymmetric heparin binding involves contacts with both FGF1 molecules but only one receptor chain. The structure of the FGF1-FGFR2-heparin ternary complex provides a structural basis for the essential role of heparan sulphate in FGF signalling.     

Structure of the yeast polarity protein Sro7 reveals a SNARE regulatory mechanism

Polarized exocytosis requires coordination between the actin cytoskeleton and the exocytic machinery responsible for fusion of secretory vesicles at specific sites on the plasma membrane. Fusion requires formation of a complex between a vesicle-bound R-SNARE and plasma membrane Qa, Qb and Qc SNARE proteins. Proteins in the lethal giant larvae protein family, including lethal giant larvae and tomosyn in metazoans and Sro7 in yeast, interact with Q-SNAREs and are emerging as key regulators of polarized exocytosis. The crystal structure of Sro7 reveals two seven-bladed WD40 beta-propellers followed by a 60-residue-long 'tail', which is bound to the surface of the amino-terminal propeller. Deletion of the Sro7 tail enables binding to the Qbc SNARE region of Sec9 and this interaction inhibits SNARE complex assembly. The N-terminal domain of Sec9 provides a second, high-affinity Sro7 interaction that is unaffected by the tail. The results suggest that Sro7 acts as an allosteric regulator of exocytosis through interactions with factors that control the tail. Sequence alignments indicate that lethal giant larvae and tomosyn have a two-beta-propeller fold similar to that of Sro7, but only tomosyn appears to retain the regulatory tail.     

Binding of vanadium compounds perturbs conformation and aggregation state of insulin

The interactions between zinc-free insulin and vanadium compounds, NaVO3, VO(acac)2 and VO(ma)2, have been investigated by fluorescence spectroscopy, circular dichroism (CD) and Fourier-transformed infrared (FT-IR) spectroscopy. The results showed that binding of vanadium compounds produced a static quenching of the intrinsic fluorescence of insulin. The apparent association constants were determined to be (0.17±0.01)×104 L*mol-1 for NaVO3, (2.8±0.2)×104 L*mol-1 for VO(acac)2, and (4.0±0.1)×104 L*mol-1 for VO(ma)2, respectively. The light scattering intensity of insulin decreased upon incubation with the vanadium compounds, suggesting the disaggregation of insulin. The attenuation of the band at 273 nm of insulin CD spectra also supported the disaggregation of insulin observed above. A new band at 1650～1653 cm-1 appeared in the FT-IR spectra of insulin upon incubation with the vanadium compounds, indicating the formation of an α-helix structure at B (9-19) motif. This α-helix structure suggests a structural change of insulin from an extended conformation (T state) to a helical conformation (R state), which is essential for binding of insulin to its receptor. In conclusion, binding of vanadium compounds results in conformational changes and disaggregation of insulin. These changes might account for the enhancement of binding affinity for insulin to its receptor in the presence of vanadium compounds.     

Structure of the dimerized hormone-binding domain of a guanylyl-cyclase-coupled receptor

The atrial natriuretic peptide (ANP) hormone is secreted by the heart in response to an increase in blood pressure. ANP exhibits several potent anti-hypertensive actions in the kidney, adrenal gland and vascular system. These actions are induced by hormone binding extracellularly to the ANP receptor, thereby activating its intracellular guanylyl cyclase domain for the production of cyclic GMP. Here we present the crystal structure of the glycosylated dimerized hormone-binding domain of the ANP receptor at 2.0-A resolution. The monomer comprises two interconnected subdomains, each encompassing a central beta-sheet flanked by alpha-helices, and exhibits the type I periplasmic binding protein fold. Dimerization is mediated by the juxtaposition of four parallel helices, arranged two by two, which brings the two protruding carboxy termini into close relative proximity. From affinity labelling and mutagenesis studies, the ANP-binding site maps to the side of the dimer crevice and extends to near the dimer interface. A conserved chloride-binding site is located in the membrane distal domain, and we found that hormone binding is chloride dependent. These studies suggest mechanisms for hormone activation and the allostery of the ANP receptor.     

The structure of HLA-B27 reveals nonamer self-peptides bound in an extended conformation

X-ray crystallography reveals electron density in the antigen-binding site of HLA-B27 that is an interpretable image of nonameric peptides in a largely extended conformation. Clear density exists for the main chain and several side chains and is consistent with the sequence of 11 nonameric self-peptides eluted from HLA-B27. Pockets in the antigen-binding cleft bind four side chains and the amino and carboxyl termini of the peptide.     

Structure of the human CLC-7/Ostm1 complex reveals a novel state

CLC-7 functions as a Cl?/H+ exchanger in lysosomes. Defects in CLC-7 and its β-subunit, Ostm1, result in osteopetrosis and neurodegeneration. Here, we present the cryogenic electron microscopy (cryo-EM) structure of the human CLC-7/Ostm1 complex (HsCLC-7/Ostm1) at a resolution of 3.6 ?. Our structure reveals a new state of the CLC-7/Ostm1 heterotetramer, in which the cytoplasmic domain of CLC-7 is absent, likely due to high flexibility. The disordered cytoplasmic domain is probably not able to restrain CLC-7 subunits and thus allow their relative movements. The movements result in an approximately half smaller interface between the CLC-7 transmembrane domains than that in a previously reported CLC-7/Ostm1 structure with a well-folded cytoplasmic domain. Key interactions involving multiple osteopetrosis-related residues are affected by the interface change.     

Identification of a cold receptor reveals a general role for TRP channels in thermosensation

The cellular and molecular mechanisms that enable us to sense cold are not well understood. Insights into this process have come from the use of pharmacological agents, such as menthol, that elicit a cooling sensation. Here we have characterized and cloned a menthol receptor from trigeminal sensory neurons that is also activated by thermal stimuli in the cool to cold range. This cold- and menthol-sensitive receptor, CMR1, is a member of the TRP family of excitatory ion channels, and we propose that it functions as a transducer of cold stimuli in the somatosensory system. These findings, together with our previous identification of the heat-sensitive channels VR1 and VRL-1, demonstrate that TRP channels detect temperatures over a wide range and are the principal sensors of thermal stimuli in the mammalian peripheral nervous system.     

Structure of a beta1-adrenergic G-protein-coupled receptor

G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.     

Structure of the sulphiredoxin-peroxiredoxin complex reveals an essential repair embrace

Typical 2-Cys peroxiredoxins (Prxs) have an important role in regulating hydrogen peroxide-mediated cell signalling. In this process, Prxs can become inactivated through the hyperoxidation of an active site Cys residue to Cys sulphinic acid. The unique repair of this moiety by sulphiredoxin (Srx) restores peroxidase activity and terminates the signal. The hyperoxidized form of Prx exists as a stable decameric structure with each active site buried. Therefore, it is unclear how Srx can access the sulphinic acid moiety. Here we present the 2.6 A crystal structure of the human Srx-PrxI complex. This complex reveals the complete unfolding of the carboxy terminus of Prx, and its unexpected packing onto the backside of Srx away from the Srx active site. Binding studies and activity analyses of site-directed mutants at this interface show that the interaction is required for repair to occur. Moreover, rearrangements in the Prx active site lead to a juxtaposition of the Prx Gly-Gly-Leu-Gly and Srx ATP-binding motifs, providing a structural basis for the first step of the catalytic mechanism. The results also suggest that the observed interactions may represent a common mode for other proteins to bind to Prxs.     

大鼠肝细胞膜胰岛素受体与胰岛素的相互作用

采用125I-胰岛素标记法研究大鼠肝细胞膜中胰岛素受体与胰岛素的结合性质,用31P-核磁共振法(31P-NMR)研究大鼠肝细胞膜中胰岛素刺激下的胰岛素受体体外自磷酸化,实验结果表明大鼠肝细胞膜胰岛素受体与胰岛素结合的最佳酸度大约为pH=7.5,并且表明大鼠肝细胞膜上的胰岛素受体为高亲和、低容量和低亲和、高容量两型受体.非放射性的31P-NMR波谱法检测到胰岛素刺激下的大鼠肝细胞膜胰岛素受体的体外自磷酸说明所提取纯化的胰岛素受体具有生物活性.     

Interaction with the NMDA receptor locks CaMKII in an active conformation.

Calcium- and calmodulin-dependent protein kinase II (CaMKII) and glutamate receptors are integrally involved in forms of synaptic plasticity that may underlie learning and memory. In the simplest model for long-term potentiation, CaMKII is activated by Ca2+ influx through NMDA (N-methyl-D-aspartate) receptors and then potentiates synaptic efficacy by inducing synaptic insertion and increased single-channel conductance of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. Here we show that regulated CaMKII interaction with two sites on the NMDA receptor subunit NR2B provides a mechanism for the glutamate-induced translocation of the kinase to the synapse in hippocampal neurons. This interaction can lead to additional forms of potentiation by: facilitated CaMKII response to synaptic Ca2+; suppression of inhibitory autophosphorylation of CaMKII; and, most notably, direct generation of sustained Ca2+/calmodulin (CaM)-independent (autonomous) kinase activity by a mechanism that is independent of the phosphorylation state. Furthermore, the interaction leads to trapping of CaM that may reduce down-regulation of NMDA receptor activity. CaMKII-NR2B interaction may be prototypical for direct activation of a kinase by its targeting protein.     

Structure of Sindbis virus core protein reveals a chymotrypsin-like serine proteinase and the organization of the virion

Sindbis virus consists of a nucleocapsid core surrounded by a lipid membrane through which penetrate 80 glycoprotein trimers. The structure of the core protein comprising the coat surrounding the genomic RNA has been determined. The polypeptide fold from residue 114 to residue 264 is homologous to that of chymotrypsin-like serine proteinases with catalytic residues His 141, Asp 163 and Ser 215 of the core protein positioned as in other serine proteinases. The C-terminal tryptophan remains in the P1 substrate site subsequent to the autocatalytic cis cleavage of the capsid protein, thus rendering the proteinase inactive. Model building of the Sindbis core protein dimer shows that the nucleocapsid is likely to have T = 4 quasisymmetry.