Electron microscopy study of the pox-like virus of Melolontha melolontha L(Coleoptera, Scarabeidae). Virus morphogenesis

Engineered Promoter-Switched Viruses Reveal the Role of Poxvirus Maturation Protein A26 as a Negative Regulator of Viral Spread

Vaccinia virus produces two types of virions known as single-membraned intracellular mature virus (MV) and double-membraned extracellular enveloped virus (EV). EV production peaks earlier when initial MVs are further wrapped and secreted to spread infection within the host. However, late during infection, MVs accumulate intracellularly and become important for host-to-host transmission. The process that regulates this switch remains elusive and is thought to be influenced by host factors. Here, we examined the hypothesis that EV and MV production are regulated by the virus through expression of F13 and the MV-specific protein A26. By switching the promoters and altering the expression kinetics of F13 and A26, we demonstrate that A26 expression downregulates EV production and plaque size, thus limiting viral spread. This process correlates with A26 association with the MV surface protein A27 and exclusion of F13, thus reducing EV titers. Thus, MV maturation is controlled by the abundance of the viral A26 protein, independently of other factors, and is rate limiting for EV production. The A26 gene is conserved within vertebrate poxviruses but is strikingly lost in poxviruses known to be transmitted exclusively by biting arthropods. A26-mediated virus maturation thus has the appearance to be an ancient evolutionary adaptation to enhance transmission of poxviruses that has subsequently been lost from vector-adapted species, for which it may serve as a genetic signature. The existence of virus-regulated mechanisms to produce virions adapted to fulfill different functions represents a novel level of complexity in mammalian viruses with major impacts on evolution, adaptation, and transmission. IMPORTANCE Chordopoxviruses are mammalian viruses that uniquely produce a first type of virion adapted to spread within the host and a second type that enhances transmission between hosts, which can take place by multiple ways, including direct contact, respiratory droplets, oral/fecal routes, or via vectors. Both virion types are important to balance intrahost dissemination and interhost transmission, so virus maturation pathways must be tightly controlled. Here, we provide evidence that the abundance and kinetics of expression of the viral protein A26 regulates this process by preventing formation of the first form and shifting maturation toward the second form. A26 is expressed late after the initial wave of progeny virions is produced, so sufficient viral dissemination is ensured, and A26 provides virions with enhanced environmental stability. Conservation of A26 in all vertebrate poxviruses, but not in those transmitted exclusively via biting arthropods, reveals the importance of A26-controlled virus maturation for transmission routes involving environmental exposure.

Budded baculovirus particle structure revisited

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Baculovirus budded virion ultrastructure has been revisited using cryo-electron microscopy.

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The now well-preserved virions have a remarkable elongated, ovoid shape and large lateral space between nucleocapsid and the intact envelope.

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Consistent with previous findings using classical electron microscopy the nucleocapsid has a distinctive cap and base structure interacting tightly with the envelope.

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Most spikes are densely clustered at the two apical ends of the virion. Using cryo-electron microscopy the viral envelope appeared to contain two layers with a total thickness of ≈6–7 nm, which is significantly thicker than a usual biological membrane (<4 nm).

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The spikes on the surface of AcMNPV BVs appear distinctly different from those of SeMNPV. Based on our observations we propose a new structural model of baculovirus budded virions.

Baculoviruses are a group of enveloped, double-stranded DNA insect viruses with budded (BV) and occlusion-derived (ODV) virions produced during their infection cycle. BVs are commonly described as rod shaped particles with a high apical density of protein extensions (spikes) on the lipid envelope surface. However, due to the fragility of BVs the conventional purification and electron microscopy (EM) staining methods considerably distort the native viral structure. Here, we use cryo-EM analysis to reveal the near-native morphology of two intensively studied baculoviruses, Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Spodoptera exigua MNPV (SeMNPV), as models for BVs carrying GP64 and F as envelope fusion protein on the surface. The now well-preserved AcMNPV and SeMNPV BV particles have a remarkable elongated, ovoid shape leaving a large, lateral space between nucleocapsid (NC) and envelope. Consistent with previous findings the NC has a distinctive cap and base structure interacting tightly with the envelope. This tight interaction may explain the partial retaining of the envelope on both ends of the NC and the disappearance of the remainder of the BV envelope in the negative-staining EM images. Cryo-EM also reveals that the viral envelope contains two layers with a total thickness of ≈6–7 nm, which is significantly thicker than a usual biological membrane (<4 nm) as measured by X-ray scanning. Most spikes are densely clustered at the two apical ends of the virion although some envelope proteins are also found more sparsely on the lateral regions. The spikes on the surface of AcMNPV BVs appear distinctly different from those of SeMNPV. Based on our observations we propose a new near-native structural model of baculovirus BVs.

Morphogenesis of salmonid gill poxvirus associated with proliferative gill disease in farmed Atlantic salmon (Salmo salar) in Norway

Proliferative gill disease (PGD) is an emerging problem in Norwegian culture of Atlantic salmon (Salmo salar). Parasites (Ichthyobodo spp.) and bacteria (Flexibacter/Flavobacterium) may cause PGD, but for most cases of PGD in farmed salmon in Norway, no specific pathogen has been identified as the causative agent. However, Neoparamoeba sp. and several bacteria and viruses have been associated with this disease. In the spring of 2006, a new poxvirus, salmon gill poxvirus (SGPV), was discovered on the gills of salmon suffering from PGD in fresh water in northern Norway. Later the same year, this virus was also found on gills of salmon at two marine sites in western Norway. All farms suffered high losses associated with the presence of this virus. In this study, we describe the entry and morphogenesis of the SGP virus in epithelial gill cells from Atlantic salmon. Intracellular mature virions (IMVs) are the only infective particles that seem to be produced. These are spread by cell lysis and by "budding" of virus packages, containing more that 100 IMVs, from the apical surface of infected cells. Entry of the IMVs appears to occur by attachment to microridges on the cell surface and fusion of the viral and cell membranes, delivering the cores into the cytoplasm. The morphogenesis starts with the emergence of crescents in viroplasm foci in perinuclear areas of infected cells. These crescents consist of two tightly apposed unit membranes (each 5 nm thick) that seem to be derived from membranes of the endoplasmic reticulum. The crescents develop into spheres, immature virions (IVs), that are 350 nm in diameter and surrounded by two unit membranes. The maturation of the IVs occurs by condensation of the core material and a change from spherical to boat-shaped particles, intracellular mature virions (IMVs), that are about 300 nm long. Hence, the IMVs from the SGP virus have a different morphology compared to other vertebrate poxviruses that are members of the subfamily Chordopoxvirinae, and they are more similar to members of subfamily Entomopoxvirinae, genus Alphaentomopoxvirus. However, it is premature to make a taxonomic assignment until the genome of the SGP virus has been sequenced, but morphogenesis clearly shows that this virus is a member of family Poxviridae.