In vitro reconstitution of an intermediate assembly stage of vaccinia virus

Glutamine and glutaminolysis are required for efficient replication of infectious spleen and kidney necrosis virus in Chinese perch brain cells

Viruses rely on host cellular metabolism for energy and macromolecule synthesis during their replication. Infectious spleen and kidney necrosis virus (ISKNV) causes significant economic losses in the Chinese perch (Siniperca chuatsi) industry worldwide. However, little is known about the relationship between ISKNV replication and cellular metabolism. Using transcriptomic analysis, we observed that glutamine metabolism in Chinese perch brain (CPB) cells is altered during ISKNV infection. Moreover, ISKNV replication was decreased in CPB cells cultured in the glutamine-depleted medium. ISKNV replication was also inhibited in CPB cells cultured in the presence of bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl) ethyl sulfide (an inhibitor of glutaminase), (-)-epigallocatechinmo nogallate (an inhibitor of glutamate dehydrogenase) or L-buthionine sulfoximine (an inhibitor of glutathione synthesis). However, virus replication was rescued by the addition of multiple tricarboxylic acid cycle intermediates, ATP, or glutathione reduced ethyl ester. ATP and reduced glutathione/oxidized glutathione levels were increased in CPB cells infected with ISKNV, but were decreased in CPB cells cultured in glutamine-depleted medium. These results indicate ISKNV infection induces glutaminolysis to accommodate the biosynthetic and energy needs for its efficient virus replication.

Increased ATP generation in the host cell is required for efficient vaccinia virus production

To search for cellular genes up-regulated by vaccinia virus (VV) infection, differential display-reverse transcription-polymerase chain reaction (ddRT-PCR) assays were used to examine the expression of mRNAs from mock-infected and VV-infected HeLa cells. Two mitochondrial genes for proteins that are part of the electron transport chain that generates ATP, ND4 and CO II, were up-regulated after VV infection. Up-regulation of ND4 level by VV infection was confirmed by Western blotting analysis. Up-regulation of ND4 was reduced by the MAPK inhibitor, apigenin, which has been demonstrated elsewhere to inhibit VV replication. The induction of ND4 expression occurred after viral DNA replication since ara C, an inhibitor of poxviral DNA replication, could block this induction. ATP production was increased in the host cells after VV infection. Moreover, 4.5 μM oligomycin, an inhibitor of ATP production, reduced the ATP level 13 hr after virus infection to that of mock-infected cells and inhibited viral protein expression and virus production, suggesting that increased ATP production is required for efficient VV production. Our results further suggest that induction of ND4 expression is through a Bcl-2 independent pathway.

Sequence-independent targeting of transmembrane proteins synthesized within vaccinia virus factories to nascent viral membranes

The primary membrane of vaccinia virus, as well as those of other poxviruses, forms within a discrete cytoplasmic factory region. We recently determined the existence of an operative pathway from the endoplasmic reticulum within the virus factory to nascent viral membranes and demonstrated that a viral protein could be diverted from this pathway to Golgi membranes by the addition of COPII-binding sites (M. Husain, A. S. Weisberg, and B. Moss, Proc. Natl. Acad. Sci. USA, 103:19506-19511, 2006). Here we describe an investigation of the structural features that are required for transit of proteins to the viral membrane. Deletion of either the N-terminal domain or the C-terminal cytoplasmic tail from the conserved A9 protein did not prevent its incorporation into viral membranes, whereas deletion of the transmembrane domain resulted in its distribution throughout the cytoplasm. Nevertheless, replacement of the A9 transmembrane domain with the corresponding region of a nonpoxvirus transmembrane protein or of a vaccinia virus extracellular envelope protein allowed viral membrane targeting, indicating no requirement for a specific amino acid sequence. Remarkably, the epitope-tagged A9 transmembrane domain alone, as well as a heterologous transmembrane domain lacking a poxvirus sequence, was sufficient for viral membrane association. The data are consistent with a sequence-independent pathway in which transmembrane proteins that are synthesized within the virus factory and lack COPII or other binding sites that enable conventional endoplasmic reticulum exiting are incorporated into nascent viral membranes.

Existence of an operative pathway from the endoplasmic reticulum to the immature poxvirus membrane

In thin sections of cells infected with vaccinia virus or other poxviruses, the viral membrane is first discerned as a crescent or circle lacking obvious continuity with a cellular organelle, presenting an appearance of de novo membrane biogenesis. This notion, which many consider heretical, is nevertheless consistent with the absence of a signature of endoplasmic reticulum (ER) trafficking, such as signal peptide cleavage or glycosylation, in any of the numerous viral membrane proteins. The purpose of this study was to determine whether an operative pathway exists between the ER and the immature virion membrane. We showed that the highly conserved A9 viral membrane protein was inserted into the ER of uninfected cells with the same topology as in viral membranes. Next, we found that replacement of the nonessential cytoplasmic tail of A9 with one containing COPII-binding sites reduced incorporation of the modified A9 into viral membranes and led to its accumulation in the Golgi apparatus, implying that A9 was inserted into the ER and then diverted from its natural path. Most importantly, we demonstrated cleavage of a heterologous signal peptide fused to the N-terminal region of A9 and localized the truncated protein in immature and mature virions. Additionally, immunoelectron micrographs showed A9 in tubules containing protein disulfide isomerase, an ER lumenal protein, near immature viral membranes. The present data provide strong evidence for an operative pathway from ER domains within the virus factory to the viral membrane.

In a nutshell: structure and assembly of the vaccinia virion

Poxviruses comprise a large family of viruses characterized by a large, linear dsDNA genome, a cytoplasmic site of replication and a complex virion morphology. The most notorious member of the poxvirus family is variola, the causative agent of smallpox. The laboratory prototype virus used for the study of poxviruses is vaccinia, the virus that was used as a live, naturally attenuated vaccine for the eradication of smallpox. Both the morphogenesis and structure of poxvirus virions are unique among viruses. Poxvirus virions apparently lack any of the symmetry features common to other viruses such as helical or icosahedral capsids or nucleocapsids. Instead poxvirus virions appear as "brick shaped" or "ovoid" membrane-bound particles with a complex internal structure featuring a walled, biconcave core flanked by "lateral bodies." The virion assembly pathway involves a remarkable fabrication of membrane-containing crescents and immature virions, which evolve into mature virions in a process that is unparalleled in virology. As a result of significant advances in poxvirus genetics and molecular biology during the past 15 years, we can now positively identify over 70 specific gene products contained in poxvirus virions, and we can describe the effects of mutations in over 50 specific genes on poxvirus assembly. This review summarizes these advances and attempts to assemble them into a comprehensible and thoughtful picture of poxvirus structure and assembly.

Deep-etch EM reveals that the early poxvirus envelope is a single membrane bilayer stabilized by a geodetic "honeycomb" surface coat

Three-dimensional "deep-etch" electron microscopy (DEEM) resolves a longstanding controversy concerning poxvirus morphogenesis. By avoiding fixative-induced membrane distortions that confounded earlier studies, DEEM shows that the primary poxvirus envelope is a single membrane bilayer coated on its external surface by a continuous honeycomb lattice. Freeze fracture of quick-frozen poxvirus-infected cells further shows that there is only one fracture plane through this primary envelope, confirming that it consists of a single lipid bilayer. DEEM also illustrates that the honeycomb coating on this envelope is completely replaced by a different paracrystalline coat as the poxvirus matures. Correlative thin section images of infected cells freeze substituted after quick-freezing, plus DEEM imaging of Tokuyasu-type cryo-thin sections of infected cells (a new application introduced here) all indicate that the honeycomb network on immature poxvirus virions is sufficiently continuous and organized, and tightly associated with the envelope throughout development, to explain how its single lipid bilayer could remain stable in the cytoplasm even before it closes into a complete sphere.

Assembly of vaccinia virus revisited: de novo membrane synthesis or acquisition from the host?

In 1968 it was proposed that the first membrane structures that assemble in vaccinia virus-infected cells, the crescents, are formed by a unique viral mechanism in which a single membrane bilayer is synthesized de novo. 25 years later it was suggested that the vaccinia membranes are derived from an organelle that is part of the host cell's secretory pathway, the intermediate compartment (IC), and that the viral crescents are made of two tightly apposed membranes rather than a single bilayer. Several independent studies have subsequently shown that membrane proteins of the intracellular mature virus (IMV) insert co-translationally into endoplasmic reticulum (ER) membranes, and are targeted to and retained in the IC, the compartment from which the virus acquires its membranes. Furthermore, a recent study on the entry of both the IMV and extracellular enveloped virus (EEV) suggests that these viruses do not enter by a simple fusion mechanism, consistent with the idea that both are surrounded by more than one lipid bilayer.

Role of ATP in influenza virus budding

Influenza viruses bud from the plasma membrane of virus-infected cells. Although budding is a critical step in virus replication, little is known about the requirements of the budding process. In this report, we have investigated the role of ATP in influenza virus budding by treating influenza virus infected Madin-Darby canine kidney (MDCK) cells with a number of metabolic inhibitors. When WSN virus-infected MDCK cells were exposed to antimycin A, carbonyl cyanide m-chlorophenylhydrazone, carbonyl cyanide p-trifluoromethoxy-phenylhydrazone, or oligomycin for a short time (15 min or 1 h) late in the infectious cycle, the rate of virus budding decreased. This inhibitory effect was reversible upon removal of the inhibitors. The role of ATP hydrolysis was analyzed by treating lysophosphatidylcholine (LPC)-permeabilized live filter-grown virus-infected MDCK cells with nonpermeable ATP analogues from the basal side and assaying virus budding from the apical side. In LPC-permeabilized cells, membrane-impermeable ATP analogues such as adenosine 5'-O-(3-thiotriphosphate) or 5'-adenylylimidodiphosphate caused reduction of virus budding which could be partially restored by adding excess ATP. These data demonstrated that ATP hydrolysis and not just ATP binding was required for virus budding. However, inhibitors of ion channel (ATPases) and protein ubiquitinylation, which also required the ATP as energy source, did not affect influenza virus budding, suggesting that neither ion channel nor protein ubiquitinylation activity was involved in influenza virus budding. On the other hand, treatment with dimethyl sulfoxide (DMSO), which decreases membrane viscosity, reduced the rate of virus budding, demonstrating that the physical state of membrane viscosity and membrane fluidity had an important effect on virus budding. Data presented in the report indicate that influenza virus budding is an active ATP-dependent process and suggest that reduced virus budding by ATP depletion and DMSO treatment may be partly due to decreased membrane viscosity.

Biochemical requirements of virus wrapping by the endoplasmic reticulum: involvement of ATP and endoplasmic reticulum calcium store during envelopment of African swine fever virus

Enwrapment by membrane cisternae has emerged recently as a mechanism of envelopment for large enveloped DNA viruses, such as herpesviruses, poxviruses, and African swine fever (ASF) virus. For both ASF virus and the poxviruses, wrapping is a multistage process initiated by the recruitment of capsid proteins onto membrane cisternae of the endoplasmic reticulum (ER) or associated ER-Golgi intermediate membrane compartments. Capsid assembly induces progressive bending of membrane cisternae into the characteristic shape of viral particles, and envelopment provides virions with two membranes in one step. We have used biochemical assays for ASF virus capsid recruitment, assembly, and envelopment to define the cellular processes important for the enwrapment of viruses by membrane cisternae. Capsid assembly on the ER membrane, and envelopment by ER cisternae, were inhibited when cells were depleted of ATP or depleted of calcium by incubation with A23187 and EDTA or the ER calcium ATPase inhibitor, thapsigargin. Electron microscopy analysis showed that cells depleted of calcium were unable to assemble icosahedral particles. Instead, assembly sites contained crescent-shaped and bulbous structures and, in rare cases, empty closed five-sided particles. Interestingly, recruitment of the capsid protein from the cytosol onto the ER membrane did not require ATP or an intact ER calcium store. The results show that following recruitment of the virus capsid protein onto the ER membrane, subsequent stages of capsid assembly and enwrapment are dependent on ATP and are regulated by the calcium gradients present across the ER membrane cisternae.