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Meiotic recombination is initiated by the formation of DNA double-strand breaks (DSBs), essential for fertility and genetic diversity. In the mouse, DSBs are formed by the catalytic TOPOVIL complex consisting of SPO11 and TOPOVIBL. To preserve genome integrity, the activity of the TOPOVIL complex is finely controlled by several meiotic factors including REC114, MEI4 and IHO1, but the underlying mechanism is poorly understood. Here, we report that mouse REC114 forms homodimers, that it associates with MEI4 as a 2:1 heterotrimer that further dimerizes, and that IHO1 forms coiled-coil based tetramers. Using AlphaFold2 modelling combined with biochemical characterization we uncovered the molecular details of these assemblies. Finally, we show that IHO1 directly interacts with the PH domain of REC114 by recognizing the same surface as TOPOVIBL and another meiotic factor ANKRD31. These results provide strong evidence for the existence of a ternary IHO1-REC114-MEI4 complex and show that REC114 is a potential regulatory platform mediating mutually exclusive interactions with several partners.

Background Galloway-Mowat syndrome (GAMOS) is characterized by neurodevelopmental defects and a progressive nephropathy, which typically manifests as steroid-resistant nephrotic syndrome. The prognosis of GAMOS is poor, and the majority of children progress to renal failure. The discovery of monogenic causes of GAMOS has uncovered molecular pathways involved in the pathogenesis of disease. Methods Homozygosity mapping, whole-exome sequencing, and linkage analysis were used to identify mutations in four families with a GAMOS-like phenotype, and high-throughput PCR technology was applied to 91 individuals with GAMOS and 816 individuals with isolated nephrotic syndrome. In vitro and in vivo studies determined the functional significance of the mutations identified. Results Three biallelic variants of the transcriptional regulator PRDM15 were detected in six families with proteinuric kidney disease. Four families with a variant in the protein's zinc-finger (ZNF) domain have additional GAMOS-like features, including brain anomalies, cardiac defects, and skeletal defects. All variants destabilize the PRDM15 protein, and the ZNF variant additionally interferes with transcriptional activation. Morpholino oligonucleotide-mediated knockdown of Prdm15 in Xenopus embryos disrupted pronephric development. Human wild-type PRDM15 RNA rescued the disruption, but the three PRDM15 variants did not. Finally, CRISPR-mediated knockout of PRDM15 in human podocytes led to dysregulation of several renal developmental genes. Conclusions Variants in PRDM15 can cause either isolated nephrotic syndrome or a GAMOS-type syndrome on an allelic basis. PRDM15 regulates multiple developmental kidney genes, and is likely to play an essential role in renal development in humans.

Soft law in international trade, and to an extent in similar international relations, is subject to certain paradoxes. The lex mercatoria doctrine divides the academic community, not only as to the conceptual question but also the implications in terms of the relevant applicable law to the potential dispute. However, the case is different for respective individual non-state rules. Their existence is accepted and highly valued both in terms of their use in contracting and formulation of the contract’s content, as well as in court or arbitral tribunal decision-making. The impending fundamental discrepancy is the degree of conceptualism in the evaluation of each of the non-state rules and their subsequent transposition into a coherent doctrine that seeks to compete with the existing private international law concepts. Lex mercatoria, or the transnational law of international trade (as a more appropriate name would perhaps be for a concept detached from national laws), has emerged in strong interaction with international practice. The emergence was, and still is, both spontaneous and unwritten, as well as formulated in various legal rules. Such rules’ essential characteristics are their familiarity, applicability, and actual usage in international trade practice. However, in all forms, they originate outside the regular legislative process. It is precisely this deficit of proper law-making that, among other things, can be used against the very existence of the lex mercatoria doctrine. The question is no longer “whether” lex mercatoria exists. The rules, at least in their soft law form, do exist, are identifiable, and are commonly applied. They are observably used in the contracting practice and decision-making. The question is instead “when” and “under what conditions” is the lex mercatoria applied. This book presents two fundamental concepts of understanding the non-state rules, as they appear in the field of international trade –integrative and transnational.

Nucleolin is a multifunctional RNA Binding Protein (RBP) with diverse subcellular localizations, including the nucleolus in all eukaryotic cells, the plasma membrane in tumor cells, and the axon in neurons. Here we show that the glycine arginine rich (GAR) domain of nucleolin drives subcellular localization via protein‐protein interactions with a kinesin light chain. In addition, GAR sequences mediate plasma membrane interactions of nucleolin. Both these modalities are in addition to the already reported involvement of the GAR domain in liquid‐liquid phase separation in the nucleolus. Nucleolin transport to axons requires the GAR domain, and heterozygous GAR deletion mice reveal reduced axonal localization of nucleolin cargo mRNAs and enhanced sensory neuron growth. Thus, the GAR domain governs axonal transport of a growth controlling RNA‐RBP complex in neurons, and is a versatile localization determinant for different subcellular compartments. Localization determination by GAR domains may explain why GAR mutants in diverse RBPs are associated with neurodegenerative disease., The glycine arginine rich (GAR) domain of nucleolin controls subcellular localization in axons by directly binding the kinesin motor.

BACKGROUND: Galloway-Mowat syndrome (GAMOS) is characterized by neurodevelopmental defects and a progressive nephropathy, which typically manifests as steroid-resistant nephrotic syndrome. The prognosis of GAMOS is poor, and the majority of children progress to renal failure. The discovery of monogenic causes of GAMOS has uncovered molecular pathways involved in the pathogenesis of disease. METHODS: Homozygosity mapping, whole-exome sequencing, and linkage analysis were used to identify mutations in four families with a GAMOS-like phenotype, and high-throughput PCR technology was applied to 91 individuals with GAMOS and 816 individuals with isolated nephrotic syndrome. In vitro and in vivo studies determined the functional significance of the mutations identified. RESULTS: Three biallelic variants of the transcriptional regulator PRDM15 were detected in six families with proteinuric kidney disease. Four families with a variant in the protein’s zinc-finger (ZNF) domain have additional GAMOS-like features, including brain anomalies, cardiac defects, and skeletal defects. All variants destabilize the PRDM15 protein, and the ZNF variant additionally interferes with transcriptional activation. Morpholino oligonucleotide-mediated knockdown of Prdm15 in Xenopus embryos disrupted pronephric development. Human wild-type PRDM15 RNA rescued the disruption, but the three PRDM15 variants did not. Finally, CRISPR-mediated knockout of PRDM15 in human podocytes led to dysregulation of several renal developmental genes. CONCLUSIONS: Variants in PRDM15 can cause either isolated nephrotic syndrome or a GAMOS-type syndrome on an allelic basis. PRDM15 regulates multiple developmental kidney genes, and is likely to play an essential role in renal development in humans.

1. Different groups of specialised herbivores often exhibit highly variable responses to host plant traits and phylogeny. Gall-forming insects and mites on willows are highly adapted to their hosts and represent one of the richest communities of gallers associated with a single genus of host plants. 2. The present study evaluated the effects of host plant secondary metabolites (salicylates, flavonoids, condensed tannins), physical traits (trichome density), nutrient content (N:C) and phylogeny on the abundance and richness of gall-forming arthropods associated with eight willow species and Populus tremula. 3. Galler abundance was affected by N:C rather than by willow defensive traits or phylogeny, suggesting that gallers respond differently to host plant traits than to less specialised guilds, such as leaf-chewing insects. None of the studied defensive traits had a significant effect on gall abundance. Gall morphospecies richness was correlated with the host phylogeny, mainly with the nodes representing the inner division of the willow subgenus Vetrix. This suggests that the radiation of some willow taxa could have been important for the speciation of gallers associated with willows. 4. In conclusion, it is shown that whereas willow traits, such as nutrient content, appeared to affect abundances of gallers, it is probably willow radiation that drives galler speciation.

Multiconfiguration pair-density functional (MC-PDFT) theory provides an economical way to calculate the ground-state and excited-state energetics of strongly correlated systems. The energy is calculated from the kinetic energy, density, and on-top pair-density of a multiconfiguration wave function as the sum of kinetic energy, classical Coulomb energy, and on-top density functional energy. We have usually found good results with the translated Perdew–Burke–Ernzerhof (tPBE) on-top density functional, and in this article, we examine whether the results can be systematically improved by introducing scaling constants into the exchange and correlation terms. We find that only a small improvement is possible for electronic excitation energies and that no improvement is possible for bond energies.

The Mason-Pfizer monkey virus is a type D retrovirus, which assembles its immature particles in the cytoplasm prior to their transport to the host cell membrane. The association with the membrane is mediated by the N-terminally myristoylated matrix protein. To reveal the role of particular residues which are involved in the capsid-membrane interaction, covalent labelling of arginine, lysine and tyrosine residues of the Mason-Pfizer monkey virus matrix protein bound to artificial liposomes containing 95% of phosphatidylcholine and 5% phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P2 ) was performed. The experimental results were interpreted by multiscale molecular dynamics simulations. The application of these two complementary approaches helped us to reveal that matrix protein specifically recognizes the PI(4,5)P2 molecule by the residues K20, K25, K27, K74, and Y28, while the residues K92 and K93 stabilizes the matrix protein orientation on the membrane by the interaction with another PI(4,5)P2 molecule. Residues K33, K39, K54, Y66, Y67, and K87 appear to be involved in the matrix protein oligomerization. All arginine residues remained accessible during the interaction with liposomes which indicates that they neither contribute to the interaction with membrane nor are involved in protein oligomerization. Proteins 2016; 84:1717-1727. © 2016 Wiley Periodicals, Inc.

Mouse REC114 is essential for meiotic DNA double-strand break formation and forms a complex with IHO1. Its N-terminal region forms a Pleckstrin homology domain, while its C-terminal region is interacting with MEI4., Programmed formation of DNA double-strand breaks (DSBs) initiates the meiotic homologous recombination pathway. This pathway is essential for proper chromosome segregation at the first meiotic division and fertility. Meiotic DSBs are catalyzed by Spo11. Several other proteins are essential for meiotic DSB formation, including three evolutionarily conserved proteins first identified in Saccharomyces cerevisiae (Mer2, Mei4, and Rec114). These three S. cerevisiae proteins and their mouse orthologs (IHO1, MEI4, and REC114) co-localize on the axes of meiotic chromosomes, and mouse IHO1 and MEI4 are essential for meiotic DSB formation. Here, we show that mouse Rec114 is required for meiotic DSB formation. Moreover, MEI4 forms a complex with REC114 and IHO1 in mouse spermatocytes, consistent with cytological observations. We then demonstrated in vitro the formation of a stable complex between REC114 C-terminal domain and MEI4 N-terminal domain. We further determine the structure of the REC114 N-terminal domain that revealed similarity with Pleckstrin homology domains. These analyses provide direct insights into the architecture of these essential components of the meiotic DSB machinery.

The detailed functional mechanism of recoverin, which acts as a myristoyl switch at the rod outer-segment disk membrane, is elucidated by direct and replica-exchange molecular dynamics. In accord with NMR structural evidence and calcium binding assays, simulations point to the key role of enhanced calcium binding to the EF3 loop of the semiopen state of recoverin as compared to the closed state. This 2-4-order decrease in calcium dissociation constant stabilizes the semiopen state in response to the increase of cytosolic calcium concentration in the vicinity of recoverin. A second calcium ion then binds to the EF2 loop and, consequently, the structure of the protein changes from the semiopen to the open state. The latter has the myristoyl chain extruded to the cytosol, ready to act as a membrane anchor of recoverin.

International audience; The programmed formation of hundreds of DNA double-strand breaks (DSBs) is essential for proper meiosis and fertility. In mice and humans, the location of these breaks is determined by the meiosis-specific protein PRDM9, through the DNA-binding specificity of its zinc-finger domain. PRDM9 also has methyltransferase activity. Here, we show that this activity is required for H3K4me3 and H3K36me3 deposition and for DSB formation at PRDM9-binding sites. By analyzing mice that express two PRDM9 variants with distinct DNA-binding specificities, we show that each variant generates its own set of H3K4me3 marks independently from the other variant. Altogether, we reveal several basic principles of PRDM9-dependent DSB site determination, in which an excess of sites are designated through PRDM9 binding and subsequent histone methylation, from which a subset is selected for DSB formation.

International audience; Long non‐coding RNAs (lncRNAs) regulating gene expression at the chromatin level are widespread among eukaryotes. However, their functions and the mechanisms by which they act are not fully understood. Here, we identify new fission yeast regulatory lncRNAs that are targeted, at their site of transcription, by the YTH domain of the RNA‐binding protein Mmi1 and degraded by the nuclear exosome. We uncover that one of them, nam1, regulates entry into sexual differentiation. Importantly, we demonstrate that Mmi1 binding to this lncRNA not only triggers its degradation but also mediates its transcription termination, thus preventing lncRNA transcription from invading and repressing the downstream gene encoding a mitogen‐activated protein kinase kinase kinase (MAPKKK) essential to sexual differentiation. In addition, we show that Mmi1‐mediated termination of lncRNA transcription also takes place at pericentromeric regions where it contributes to heterochromatin gene silencing together with RNA interference (RNAi). These findings reveal an important role for selective termination of lncRNA transcription in both euchromatic and heterochromatic lncRNA‐based gene silencing processes.

Recoverin is a neuronal calcium sensor involved in vision adaptation that reversibly associates with cellular membranes via its calcium-activated myristoyl switch. While experimental evidence shows that the myristoyl group significantly enhances membrane affinity of this protein, molecular details of the binding process are still under debate. Here, we present results of extensive molecular dynamics simulations of recoverin in the proximity of a phospholipid bilayer. We capture multiple events of spontaneous membrane insertion of the myristoyl moiety and confirm its critical role in the membrane binding. Moreover, we observe that the binding strongly depends on the conformation of the N-terminal domain. We propose that a suitable conformation of the N-terminal domain can be stabilized by the disordered C-terminal segment or by binding of the target enzyme, i.e., rhodopsin kinase. Finally, we find that the presence of negatively charged lipids in the bilayer stabilizes a physiologically functional orientation of the membrane-bound recoverin., Molecular dynamics simulations provide atomistic insights into the reversible membrane binding of recoverin, demonstrating the key role of the myristoyl moiety for the membrane association.

Chemical diversity: Two SAM-dependent N-methyltransferases-LmbJ from the biosynthesis of the antibiotic lincomycin and CcbJ from celesticetin biosynthesis-have been characterized and compared. Both tested enzymes form multimers and are able to utilize N-demethyllincomycin, the natural substrate of LmbJ, with comparable efficiency.

Bovine viral diarrhoea virus (BVDV) is an economically important animal pathogen which is closely related to Hepatitis C virus. Of the structural proteins, the envelope glycoprotein E2 of BVDV is the major antigen which induces neutralizing antibodies; thus, BVDV E2 is considered as an ideal target for use in subunit vaccines. Here, the expression, purification of wild-type and mutant forms of the ectodomain of BVDV E2 and subsequent crystallization and data collection of two crystal forms grown at low and neutral pH are reported. Native and multiple-wavelength anomalous dispersion (MAD) data sets have been collected and structure determination is in progress.

Summary The Male-Specific Lethal (MSL) complex regulates dosage compensation of the male X chromosome in Drosophila . Here, we report the crystal structure of its MSL1/MSL2 core, where two MSL2 subunits bind to a dimer formed by two molecules of MSL1. Analysis of structure-based mutants revealed that MSL2 can only interact with the MSL1 dimer, but MSL1 dimerization is MSL2 independent. We show that Msl1 is a substrate for Msl2 E3 ubiquitin ligase activity. ChIP experiments revealed that Msl1 dimerization is essential for targeting and spreading of the MSL complex on X-linked genes; however, Msl1 binding to promoters of male and female cells is independent of the dimer status and other MSL proteins. Finally, we show that loss of Msl1 dimerization leads to male-specific lethality. We propose that Msl1-mediated dimerization of the entire MSL complex is required for Msl2 binding, X chromosome recognition, and spreading along the X chromosome.

We present a probabilistic model to minimize the fingerprinting effort associated with the implementation of the “breeding without breeding” scheme under partial pedigree reconstruction. Our approach is directed at achieving a declared target population’s minimum effective population size (Ne), following the pedigree reconstruction and genotypic selection and is based on the graph theory algorithm. The primary advantage of the proposed method is to reduce the cost associated with fingerprinting before the implementation of the pedigree reconstruction for seed parent–offspring derived from breeding arboreta and production or natural populations. Stochastic simulation was conducted to test the method’s efficiency assuming a simple polygenic model and a single trait. Hypothetical population consisted of 30 parental trees that were paired at random (selfing excluded), resulting in 600 individuals (potential candidates for forwards selection). The male parentage was assumed initially unknown. The model was used to estimate the minimum genotyping sample size needed to reaching the prescribed Ne. Results were compared with the known pedigree data. The model was successful in revealing the true relationship pattern over the whole range of Ne. Two to three offspring entered genotyping to meet the Ne = 2 while 41 to 43 were required to satisfy the Ne = 14. Importantly, genetic gain was affected at the lower limits of the genotyping effort. Doubling the number of parents resulted in considerable reduction of the genotyping effort at higher Ne values.

Here, we report the biochemical characterization of the nonspecific lethal (NSL) complex (NSL1, NSL2, NSL3, MCRS2, MBD-R2, and WDS) that associates with the histone acetyltransferase MOF in both Drosophila and mammals. Chromatin immunoprecipitation-Seq analysis revealed association of NSL1 and MCRS2 with the promoter regions of more than 4000 target genes, 70% of these being actively transcribed. This binding is functional, as depletion of MCRS2, MBD-R2, and NSL3 severely affects gene expression genome wide. The NSL complex members bind to their target promoters independently of MOF. However, depletion of MCRS2 affects MOF recruitment to promoters. NSL complex stability is interdependent and relies mainly on the presence of NSL1 and MCRS2. Tethering of NSL3 to a heterologous promoter leads to robust transcription activation and is sensitive to the levels of NSL1, MCRS2, and MOF. Taken together, we conclude that the NSL complex acts as a major transcriptional regulator in Drosophila.

Dehydration of protein crystals is rarely used, despite being a post-crystallization method that is useful for the improvement of crystal diffraction properties, as it is difficult to reproduce and monitor. A novel device for hydration control of macromolecular crystals in a standard data-collection environment has been developed. The device delivers an air stream of precise relative humidity that can be used to alter the amount of water in macromolecular crystals. The device can be rapidly installed and is fully compatible with most standard synchrotron X-ray beamlines. Samples are mounted in cryoloops and the progress of dehydration can be monitored both optically and by the acquisition of diffraction images. Once the optimal hydration level has been obtained, cryocooling is easy to achieve by hand or by using a sample changer. The device has been thoroughly tested on several ESRF beamlines and is available to users.

Nonsense-mediated decay (NMD) is a eukaryotic quality control mechanism that degrades mRNAs carrying premature stop codons. In mammalian cells, NMD is triggered when UPF2 bound to UPF3 on a downstream exon junction complex interacts with UPF1 bound to a stalled ribosome. We report structural studies on the interaction between the C-terminal region of UPF2 and intact UPF1. Crystal structures, confirmed by EM and SAXS, show that the UPF1 CH-domain is docked onto its helicase domain in a fixed configuration. The C-terminal region of UPF2 is natively unfolded but binds through separated alpha-helical and beta-hairpin elements to the UPF1 CH-domain. The alpha-helical region binds sixfold more weakly than the beta-hairpin, whereas the combined elements bind 80-fold more tightly. Cellular assays show that NMD is severely affected by mutations disrupting the beta-hairpin binding, but not by those only affecting alpha-helix binding. We propose that the bipartite mode of UPF2 binding to UPF1 brings the ribosome and the EJC in close proximity by forming a tight complex after an initial weak encounter with either element.

Viral fusion proteins mediate the merger of host and viral membranes during cell entry for all enveloped viruses. Baculovirus glycoprotein gp64 (gp64) is unusual in promoting entry into both insect and mammalian cells and is distinct from established class I and class II fusion proteins. We report the crystal structure of its postfusion form, which explains a number of gp64's biological properties including its cellular promiscuity, identifies the fusion peptides and shows it to be the third representative of a new class (III) of fusion proteins with unexpected structural homology with vesicular stomatitis virus G and herpes simplex virus type 1 gB proteins. We show that domains of class III proteins have counterparts in both class I and II proteins, suggesting that all these viral fusion machines are structurally more related than previously thought.

Germline-specific Piwi-interacting RNAs (piRNAs) protect animal genomes against transposons and are essential for fertility. piRNAs targeting active transposons are amplified by the ping-pong cycle, which couples Piwi endonucleolytic slicing of target RNAs to biogenesis of new piRNAs. Here, we describe the identification of a transient Amplifier complex that mediates biogenesis of secondary piRNAs in insect cells. Amplifier is nucleated by the DEAD box RNA helicase Vasa and contains the two Piwi proteins participating in the ping-pong loop, the Tudor protein Qin/Kumo and antisense piRNA guides. These components assemble on the surface of Vasa's helicase domain, which functions as an RNA clamp to anchor Amplifier onto transposon transcripts. We show that ATP-dependent RNP remodeling by Vasa facilitates transfer of 5′ sliced piRNA precursors between ping-pong partners, and loss of this activity causes sterility in Drosophila. Our results reveal the molecular basis for the small RNA amplification that confers adaptive immunity against transposons. Refereed/Peer-reviewed

The subunits of the nonspecific lethal (NSL) complex, which include the histone acetyltransferase MOF (males absent on the first), play important roles in various cellular functions, including transcription regulation and stem cell identity maintenance and reprogramming, and are frequently misregulated in disease. Here, we provide the first biochemical and structural insights into the molecular architecture of this large multiprotein assembly. We identified several direct interactions within the complex and show that KANSL1 acts as a scaffold protein interacting with four other subunits, including WDR5, which in turn binds KANSL2. Structural analysis of the KANSL1/WDR5/KANSL2 subcomplex reveals how WDR5 is recruited into the NSL complex via conserved linear motifs of KANSL1 and KANSL2. Using structure-based KANSL1 mutants in transgenic flies, we show that the KANSL1–WDR5 interaction is required for proper assembly, efficient recruitment of the NSL complex to target promoters, and fly viability. Our data clearly show that the interactions of WDR5 with the MOF-containing NSL complex and MLL/COMPASS histone methyltransferase complexes are mutually exclusive. We propose that rather than being a shared subunit, WDR5 plays an important role in assembling distinct histone-modifying complexes with different epigenetic regulatory roles.

The insect baculovirus AcMNPV (Autographa californica multiple nuclear polyhedrosis virus) enters many mammalian cell lines, prompting its application as a general eukaryotic gene delivery agent, but the basis of entry is poorly understood. For adherent mammalian cells, we show that entry is favoured by low pH and by increasing the available cell-surface area through a transient release from the substratum. Low pH also stimulated baculovirus entry into mammalian cells grown in suspension which, optimally, could reach 90% of the transduced population. The basic loop, residues 268–281, of the viral surface glycoprotein gp64 was required for entry and a tetra mutant with increasing basicity increased entry into a range of mammalian cells. The same mutant failed to plaque in Sf9 cells, instead showing individual cell entry and minimal cell-to-cell spread, consistent with an altered fusion phenotype. Viruses grown in different insect cells showed different mammalian cell entry efficiencies, suggesting that additional factors also govern entry.

The male-specific lethal (MSL) complex is required for dosage compensation in Drosophila melanogaster, and analogous complexes exist in mammals. We report structures of binary complexes of mammalian MSL3 and the histone acetyltransferase (HAT) MOF with consecutive segments of MSL1. MSL1 interacts with MSL3 as an extended chain forming an extensive hydrophobic interface, whereas the MSL1-MOF interface involves electrostatic interactions between the HAT domain and a long helix of MSL1. This structure provides insights into the catalytic mechanism of MOF and enables us to show analogous interactions of MOF with NSL1. In Drosophila, selective disruption of Msl1 interactions with Msl3 or Mof severely affects Msl1 targeting to the body of dosage-compensated genes and several high-affinity sites, without affecting promoter binding. We propose that Msl1 acts as a scaffold for MSL complex assembly to achieve specific targeting to the X chromosome.

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism by which eukaryotic cells detect and degrade transcripts containing premature termination codons. Three 'up-frameshift' proteins, UPF1, UPF2 and UPF3, are essential for this process in organisms ranging from yeast to human. We present a crystal structure at a resolution of 1.95 A of the complex between the interacting domains of human UPF2 and UPF3b, which are, respectively, a MIF4G (middle portion of eIF4G) domain and an RNP domain (ribonucleoprotein-type RNA-binding domain). The protein-protein interface is mediated by highly conserved charged residues in UPF2 and UPF3b and involves the beta-sheet surface of the UPF3b RNP domain, which is generally used by these domains to bind nucleic acids. We show that the UPF3b RNP does not bind RNA, whereas the UPF2 construct and the complex do. Our results advance understanding of the molecular mechanisms underlying the NMD quality control process.

Nerve terminals release neurotransmitters from vesicles into the synaptic cleft upon transient increases in intracellular Ca(2+). This exocytotic process requires the formation of trans SNARE complexes and is regulated by accessory proteins including the complexins. Here we report the crystal structure of a squid core complexin-SNARE complex at 2.95-A resolution. A helical segment of complexin binds in anti-parallel fashion to the four-helix bundle of the core SNARE complex and interacts at its C terminus with syntaxin and synaptobrevin around the ionic zero layer of the SNARE complex. We propose that this structure is part of a multiprotein fusion machinery that regulates vesicle fusion at a late pre-fusion stage. Accordingly, Ca(2+) may initiate membrane fusion by acting directly or indirectly on complexin, thus allowing the conformational transitions of the trans SNARE complex that are thought to drive membrane fusion.

Single-wavelength anomalous dispersion of sulfur atoms (S-SAD) is an elegant phasing method to determine crystal structures that does not require heavy atom incorporation or selenomethionine derivatization. Nevertheless this technique has been limited by the paucity of the signal at usual X-ray wavelengths, requiring very accurate measurement of the anomalous differences. Here we report the data collection and structure solution of the N-terminal domain of the ectodomain of Hepatitis C virus (HCV) E1, from crystals that diffracted very weakly. By combining the data from 32 crystals it was possible to solve the sulfur substructure and calculate initial maps at 7Å resolution, and after density modification and phase extension, using a higher resolution native dataset, to 3.5Å resolution, model building was achievable. The crystal structure of the N-terminal domain of reveals a complex network of covalently linked intertwined homodimers that do not harbor the expected truncated class II fusion protein fold.