Herpes simplex virus type 1 (HSV-1) is an important human pathogen that infects the majority of the population at an early age and then establishes a life-long latent infection in sensory neurones. Periodic reactivation of latent virus causes episodes of active disease characterized by epithelial lesions at the site of the original primary infection. As with all herpesviruses, the ability of HSV-1 to establish and reactivate from latency is key to its clinical importance and evolutionary success. Therefore, the molecular mechanisms that regulate these processes have been the subject of intensive research (reviewed in reference 15). HSV-1 immediate-early (IE) protein ICP0 is required for efficient reactivation from latency in both mouse models and cultured cell systems of quiescent infection (15). ICP0 is also required to stimulate lytic infection by enhancing the probability that a cell receiving a viral genome will engage in productive infection (reviewed in references 19, 20 and 42). Therefore, a full understanding of the biology of HSV-1 infection requires a definition of the functions and mode of action of ICP0.The basic phenotype of ICP0-null mutant HSV-1 is a low probability of plaque formation, particularly in human diploid fibroblasts, that causes a high particle-to-PFU ratio (reference 20 and references therein). Biochemically, ICP0 is an E3 ubiquitin ligase of the RING finger class (4) that induces the degradation of several cellular proteins, including the promyelocytic leukemia (PML) protein (23), centromere proteins including CENP-C (54, 55), and the catalytic subunit of DNAprotein kinase (53,72). Among the consequences of these activities are the disruption of PML nuclear bodies (herein termed nuclear domain 10 [ND10]) (24, 58) and centromeres (54). ICP0 has also been reported to interact with histone deacetylase enzymes (HDACs) (56) and the CoREST repressor protein, thereby disrupting the CoREST/HDAC-1 complex (37, 39). Evidence has also been presented that expression of ICP0 correlates with increased acetylation of histones on viral chromatin (12). ICP0-null mutant viruses replicate less efficiently than the wild type (wt) in cells pretreated with interferon (IFN) (44,66), and there is evidence that ICP0 is able to impede an IFN-independent induction of IFN-stimulated genes that arises after infection with defective HSV-1 mutants (16,59,60,65,67,76). As a further complication, ICP0-null mutant HSV-1 replicates more efficiently in cells that have been highly stressed by a variety of treatments (5,6,79).On the basis of this evidence, several not necessarily mutually exclusive hypotheses have been put forward to explain the biological effects of ICP0. These include (i) that ICP0 counteracts an intrinsic cellular resistance mechanism that involves PML and other components of ND10, (ii) that ICP0 overcomes the innate cellular antiviral defense based on the IFN pathway, and (iii) that ICP0 counteracts the establishment of a repressed chromatin structure on the viral genome by interfering with histone d...

Ultrastructural analysis of a filovirus assembling within infected eukaryotic cells reveals differences in structure and assembly mechanisms between related RNA viruses.

Lassa virus is an enveloped, bi-segmented RNA virus and the most prevalent and fatal of all Old World arenaviruses. Virus entry into the host cell is mediated by a tripartite surface spike complex, which is composed of two viral glycoprotein subunits, GP1 and GP2, and the stable signal peptide. Of these, GP1 binds to cellular receptors and GP2 catalyzes fusion between the viral envelope and the host cell membrane during endocytosis. The molecular structure of the spike and conformational rearrangements induced by low pH, prior to fusion, remain poorly understood. Here, we analyzed the three-dimensional ultrastructure of Lassa virus using electron cryotomography. Sub-tomogram averaging yielded a structure of the glycoprotein spike at 14-Å resolution. The spikes are trimeric, cover the virion envelope, and connect to the underlying matrix. Structural changes to the spike, following acidification, support a viral entry mechanism dependent on binding to the lysosome-resident receptor LAMP1 and further dissociation of the membrane-distal GP1 subunits.

RbpA is a small non–DNA-binding transcription factor that associates with RNA polymerase holoenzyme and stimulates transcription in actinobacteria, including Streptomyces coelicolor and Mycobacterium tuberculosis. RbpA seems to show specificity for the vegetative form of RNA polymerase as opposed to alternative forms of the enzyme. Here, we explain the basis of this specificity by showing that RbpA binds directly to the principal σ subunit in these organisms, but not to more diverged alternative σ factors. Nuclear magnetic resonance spectroscopy revealed that, although differing in their requirement for structural zinc, the RbpA orthologues from S. coelicolor and M. tuberculosis share a common structural core domain, with extensive, apparently disordered, N- and C-terminal regions. The RbpA–σ interaction is mediated by the C-terminal region of RbpA and σ domain 2, and S. coelicolor RbpA mutants that are defective in binding σ are unable to stimulate transcription in vitro and are inactive in vivo. Given that RbpA is essential in M. tuberculosis and critical for growth in S. coelicolor, these data support a model in which RbpA plays a key role in the σ cycle in actinobacteria.

Guanarito virus (GTOV) is an emergent and deadly pathogen. We present the crystal structure of the glycosylated GTOV fusion glycoprotein to 4.1-Å resolution in the postfusion conformation. Our structure reveals a classical six-helix bundle and presents direct verification that New World arenaviruses exhibit class I viral membrane fusion machinery. The structure provides visualization of an N-linked glycocalyx coat, and consideration of glycan dynamics reveals extensive coverage of the underlying protein surface, following virus-host membrane fusion.

Enveloped viruses utilize membrane glycoproteins on their surface to mediate entry into host cells. Three-dimensional structural analysis of these glycoprotein 'spikes' is often technically challenging but important for understanding viral pathogenesis and in drug design. Here, a protocol is presented for viral spike structure determination through computational averaging of electron cryo-tomography data. Electron cryotomography is a technique in electron microscopy used to derive three-dimensional tomographic volume reconstructions, or tomograms, of pleomorphic biological specimens such as membrane viruses in a near-native, frozen-hydrated state. These tomograms reveal structures of interest in three dimensions, albeit at low resolution. Computational averaging of sub-volumes, or sub-tomograms, is necessary to obtain higher resolution detail of repeating structural motifs, such as viral glycoprotein spikes. A detailed computational approach for aligning and averaging sub-tomograms using the Jsubtomo software package is outlined. This approach enables visualization of the structure of viral glycoprotein spikes to a resolution in the range of 20-40 Å and study of the study of higher order spike-to-spike interactions on the virion membrane. Typical results are presented for Bunyamwera virus, an enveloped virus from the family Bunyaviridae. This family is a structurally diverse group of pathogens posing a threat to human and animal health.

Viruses are a significant threat to both human health and the economy, and there is an urgent need for novel anti-viral drugs and vaccines. High-resolution viral structures inform our understanding of the virosphere, and inspire novel therapies. Here we present a method of obtaining such structural information that avoids potentially disruptive handling, by collecting diffraction data from intact infected cells. We identify a suitable combination of cell type and virus to accumulate particles in the cells, establish a suitable time point where most cells contain virus condensates and use electron microscopy to demonstrate that these are ordered crystalline arrays of empty capsids. We then use an X-ray free electron laser to provide extremely bright illumination of sub-micron intracellular condensates of bacteriophage phiX174 inside living Escherichia coli at room temperature. We have been able to collect low resolution diffraction data. Despite the limited resolution and completeness of these initial data, due to a far from optimal experimental setup, we have used novel methodology to determine a putative space group, unit cell dimensions, particle packing and likely maturation state of the particles.

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