Viral membrane fusion Harrison, Stephen C
Virology (New York, N.Y.),
05/2015, Letnik:
479
Journal Article
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Abstract Membrane fusion is an essential step when enveloped viruses enter cells. Lipid bilayer fusion requires catalysis to overcome a high kinetic barrier; viral fusion proteins are the agents that ...fulfill this catalytic function. Despite a variety of molecular architectures, these proteins facilitate fusion by essentially the same generic mechanism. Stimulated by a signal associated with arrival at the cell to be infected (e.g., receptor or co-receptor binding, proton binding in an endosome), they undergo a series of conformational changes. A hydrophobic segment (a “fusion loop” or “fusion peptide”) engages the target-cell membrane and collapse of the bridging intermediate thus formed draws the two membranes (virus and cell) together. We know of three structural classes for viral fusion proteins. Structures for both pre- and postfusion conformations of illustrate the beginning and end points of a process that can be probed by single-virion measurements of fusion kinetics.
Viral membrane fusion Harrison, Stephen C
Nature structural & molecular biology,
07/2008, Letnik:
15, Številka:
7
Journal Article
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Infection by viruses having lipid-bilayer envelopes proceeds through fusion of the viral membrane with a membrane of the target cell. Viral 'fusion proteins' facilitate this process. They vary ...greatly in structure, but all seem to have a common mechanism of action, in which a ligand-triggered, large-scale conformational change in the fusion protein is coupled to apposition and merger of the two bilayers. We describe three examples--the influenza virus hemagglutinin, the flavivirus E protein and the vesicular stomatitis virus G protein--in some detail, to illustrate the ways in which different structures have evolved to implement this common mechanism. Fusion inhibitors can be effective antiviral agents.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Kinetochores connect centromeric nucleosomes with mitotic-spindle microtubules through conserved, cross-interacting protein subassemblies. In budding yeast, the heterotetrameric MIND complex (Mtw1, ...Nnf1, Nsl1, Dsn1), ortholog of the metazoan Mis12 complex, joins the centromere-proximal components, Mif2 and COMA, with the principal microtubule-binding component, the Ndc80 complex (Ndc80C). We report the crystal structure of Kluyveromyces lactis MIND and examine its partner interactions, to understand the connection from a centromeric nucleosome to a much larger microtubule. MIND resembles an elongated, asymmetric Y; two globular heads project from a coiled-coil shaft. An N-terminal extension of Dsn1 from one head regulates interactions of the other head, blocking binding of Mif2 and COMA. Dsn1 phosphorylation by Ipl1/Aurora B relieves this autoinhibition, enabling MIND to join an assembling kinetochore. A C-terminal extension of Dsn1 recruits Ndc80C to the opposite end of the shaft. The structure and properties of MIND show how it integrates phospho-regulatory inputs for kinetochore assembly and disassembly.
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•MIND-complex crystal structure defines pivotal interactions in kinetochore assembly•One end of 200-Å-long MIND rod binds Mif2 and COMA; the other, Ndc80 complex•Phosphorylation of Dsn1 subunit relieves autoinhibition of Mif2 and COMA binding
The yeast MIND complex acts as a Y-shaped adaptor linking the centromeric nucleosome to the much larger microtubule.
Progression of nonalcoholic steatohepatitis (NASH) is incompletely characterized. We analyzed data on longitudinal changes in liver histology, hepatic venous pressure gradient (HVPG), and serum ...markers of fibrosis in 475 patients with NASH with bridging fibrosis (F3) or compensated cirrhosis (F4) enrolled in two phase 2b, placebo‐controlled trials of simtuzumab. The trials were terminated after 96 weeks because of lack of efficacy, so data from treatment groups were combined. Liver biopsies and HVPG measurements (only for patients with F4 fibrosis) were collected at screening and at weeks 48 and 96. Patients were assessed for Ishak fibrosis stage, hepatic collagen content and alpha‐smooth muscle actin (by morphometry), NAFLD Activity Score (NAS), and serum markers of fibrosis. Associations with progression to cirrhosis (in patients with F3 fibrosis) and liver‐related clinical events (in patients with F4 fibrosis) were determined. Progression to cirrhosis occurred in 22% (48/217) of F3 patients, and liver‐related clinical events occurred in 19% (50/258) of patients with cirrhosis. Factors significantly associated with progression to cirrhosis included higher baseline values of and greater increases in hepatic collagen content, level of alpha‐smooth muscle actin, and Enhanced Liver Fibrosis score. Similar factors, plus lack of fibrosis stage improvement (hazard ratio, 9.30; 95% confidence interval, 1.28‐67.37), higher HVPG at baseline, and greater increase in HVPG over time, were associated with an increased risk of liver‐related clinical events in patients with cirrhosis. Disease progression was not associated with the NAS at baseline or changes in NAS during treatment after adjustment for fibrosis stage. Conclusion: In patients with advanced fibrosis due to NASH, the primary determinant of clinical disease progression is fibrosis and its change over time.
Kinetochores connect mitotic-spindle microtubules with chromosomes, allowing microtubule depolymerization to pull chromosomes apart during anaphase while resisting detachment as the microtubule ...shortens. The heterodecameric DASH/Dam1 complex (DASH/Dam1c), an essential component of yeast kinetochores, assembles into a microtubule-encircling ring. The ring associates with rodlike Ndc80 complexes to organize the kinetochore-microtubule interface. We report the cryo-electron microscopy structure (at ~4.5-angstrom resolution) of a DASH/Dam1c ring and a molecular model of its ordered components, validated by evolutionary direct-coupling analysis. Integrating this structure with that of the Ndc80 complex and with published interaction data yields a molecular picture of kinetochore-microtubule attachment, including how flexible, C-terminal extensions of DASH/Dam1c subunits project and contact widely separated sites on the Ndc80 complex rod and how phosphorylation at previously identified sites might regulate kinetochore assembly.
Eukaryotic kinetochores connect spindlemicrotubules to chromosomal centromeres. A group of proteins called the Ctf19 complex (Ctf19c) in yeast and the constitutive centromere associated network ...(CCAN) in other organisms creates the foundation of a kinetochore. The Ctf19c/CCAN influences the timing of kinetochore assembly, sets its location by associating with a specialized nucleosome containing the histone H3 variant Cse4/CENP-A, and determines the organization of the microtubule attachment apparatus. We present here the structure of a reconstituted 13-subunit Ctf19c determined by cryo-electron microscopy at ~4 Å resolution. The structure accounts for known and inferred contacts with the Cse4 nucleosome and for an observed assembly hierarchy. We describe its implications for establishment of kinetochores and for their regulation by kinases throughout the cell cycle.
Clathrin is a molecular scaffold for vesicular uptake of cargo at the plasma membrane, where its assembly into cage-like lattices underlies the clathrin-coated pits of classical endocytosis. This ...review describes the structures of clathrin, major cargo adaptors, and other proteins that participate in forming a clathrin-coated pit, loading its contents, pinching off the membrane as a lattice-enclosed vesicle, and recycling the components. It integrates as much of the structural information as possible at the time of writing into a sketch of the principal steps in coated-pit and coated-vesicle formation.
Accurate noninvasive tests (NITs) are needed to replace liver biopsy for identifying advanced fibrosis caused by nonalcoholic steatohepatitis (NASH). We analyzed screening data from two phase 3 ...trials of selonsertib to assess the ability of NITs to discriminate advanced fibrosis. Centrally read biopsies from the STELLAR studies, which enrolled patients with bridging fibrosis and compensated cirrhosis, were staged according to the NASH Clinical Research Network classification. We explored associations between fibrosis stage and NITs, including the nonalcoholic fatty liver disease fibrosis score (NFS), fibrosis‐4 (FIB‐4) index, Enhanced Liver Fibrosis (ELF) test, and liver stiffness by vibration‐controlled transient elastography (LS by VCTE). The performance of these tests to discriminate advanced fibrosis, either alone or in combinations, was evaluated using areas under the receiver operating characteristic curve (AUROCs) with 5‐fold cross‐validation repeated 100 times. Of the 4,404 patients screened for these trials, 3,202 had evaluable biopsy data: 940 with F0‐F2 fibrosis and 2,262 with F3‐F4 fibrosis. Significant differences between median values of NITs for patients with F0‐F2 versus F3‐F4 fibrosis were observed: −0.972 versus 0.318 for NFS, 1.18 versus 2.20 for FIB‐4, 9.22 versus 10.39 for ELF, and 8.8 versus 16.5 kPa for LS by VCTE (all P < 0.001). AUROCs ranged from 0.75 to 0.80 to discriminate advanced fibrosis. FIB‐4 followed by an LS by VCTE or ELF test in those with indeterminate values (FIB‐4 between 1.3 and 2.67) maintained an acceptable performance while reducing the rate of indeterminate results. Conclusion: Among patients being considered for enrollment into clinical trials, NITs alone or in combination can reduce the need for liver biopsy to discriminate advanced fibrosis caused by NASH. The predictive value of these tests for general screening will require confirmation in a real‐world population.
The ring-shaped cohesin complex brings together distant DNA domains to maintain, express, and segregate the genome. Establishing specific chromosomal linkages depends on cohesin recruitment to ...defined loci. One such locus is the budding yeast centromere, which is a paradigm for targeted cohesin loading. The kinetochore, a multiprotein complex that connects centromeres to microtubules, drives the recruitment of high levels of cohesin to link sister chromatids together. We have exploited this system to determine the mechanism of specific cohesin recruitment. We show that phosphorylation of the Ctf19 kinetochore protein by a conserved kinase, DDK, provides a binding site for the Scc2/4 cohesin loading complex, thereby directing cohesin loading to centromeres. A similar mechanism targets cohesin to chromosomes in vertebrates. These findings represent a complete molecular description of targeted cohesin loading, a phenomenon with wide-ranging importance in chromosome segregation and, in multicellular organisms, transcription regulation.
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•Reconstitution of a phosphorylation-dependent pathway for targeted cohesin loading•The mechanism by which kinetochores enhance sister centromere cohesion
The mechanism by which cohesin is recruited and loaded in yeast and vertebrates is elucidated with implications for our understanding of chromosome segregation.
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