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

African swine fever virus modulates the endoplasmic reticulum stress-ATF6-calcium axis to facilitate viral replication

African swine fever (ASF), caused by African swine fever virus (ASFV), is a devastating infectious disease of domestic pigs and wild boar, which threatens the global pig industry. Endoplasmic reticulum (ER) is a multifunctional signaling organelle in eukaryotic cells that is involved in protein synthesis, processing, posttranslational modification and quality control. As intracellular parasitic organisms, viruses have evolved several strategies to modulate ER functions to favor their life cycles. We have previously demonstrated that the differentially expressed genes associated with unfolded protein response (UPR), which represents a response to ER stress, are significantly enriched upon ASFV infection. However, the correlation between the ER stress or UPR and ASFV replication has not been illuminated yet. Here, we demonstrated that ASFV infection induces ER stress both in target cells and in vivo, and subsequently activates the activating transcription factor 6 (ATF6) branch of the UPR to facilitate viral replication. Mechanistically, ASFV infection disrupts intracellular calcium (Ca2+) homeostasis, while the ATF6 pathway facilitates ASFV replication by increasing the cytoplasmic Ca2+ level. More specifically, we demonstrated that ASFV infection triggers ER-dependent Ca2+ release via the inositol triphosphate receptor (IP3R) channel. Notably, we showed that the ASFV B117L protein plays crucial roles in ER stress and the downstream activation of the ATF6 branch, as well as the disruption of Ca2+ homeostasis. Taken together, our findings reveal for the first time that ASFV modulates the ER stress-ATF6-Ca2+ axis to facilitate viral replication, which provides novel insights into the development of antiviral strategies for ASFV.

Insights into the Role of VPS39 and Its Interaction with CP204L and A137R in ASFV Infection

The African swine fever virus (ASFV) is a large and complex DNA virus that causes a highly lethal disease in swine, for which no antiviral drugs or vaccines are currently available. Studying viral-host protein-protein interactions advances our understanding of the molecular mechanisms underlying viral replication and pathogenesis and can facilitate the discovery of antiviral therapeutics. In this study, we employed affinity tagging and purification mass spectrometry to characterize the interactome of VPS39, an important cellular factor during the early phase of ASFV replication. The interaction network of VPS39 revealed associations with mitochondrial proteins involved in membrane contact sites formation and cellular respiration. We show that the ASFV proteins CP204L and A137R target VPS39 by interacting with its clathrin heavy-chain functional domain. Furthermore, we elaborate on the potential mechanisms by which VPS39 may contribute to ASFV replication and prioritize interactions for further investigation into mitochondrial protein function in the context of ASFV infection.

African Swine Fever Virus Host-Pathogen Interactions

African swine fever virus is a complex double-stranded DNA virus that exhibits tropism for cells of the mononuclear phagocytic system. Virus replication is a multi-step process that involves the nucleus of the host cell as well the formation of large perinuclear sites where progeny virions are assembled prior to transport to, and budding through, the plasma membrane. Like many viruses, African swine fever virus reorganises the cellular architecture to facilitate its replication and has evolved multiple mechanisms to avoid the potential deleterious effects of host cell stress response pathways. However, how viral proteins and virus-induced structures trigger cellular stress pathways and manipulate the subsequent responses is still relatively poorly understood. African swine fever virus alters nuclear substructures, modulates autophagy, apoptosis and the endoplasmic reticulum stress response pathways. The viral genome encodes for at least 150 genes, of which approximately 70 are incorporated into the virion. Many of the non-structural genes have not been fully characterised and likely play a role in host range and modifying immune responses. As the field moves towards approaches that take a broader view of the effect of expression of individual African swine fever genes, we summarise how the different steps in virus replication interact with the host cell and the current state of knowledge on how it modulates the resulting stress responses.

African Swine Fever Virus: A Review

African swine fever (ASF) is a viral disease with a high fatality rate in both domestic pigs and wild boars. ASF has greatly challenged pig-raising countries and also negatively impacted regional and national trade of pork products. To date, ASF has spread throughout Africa, Europe, and Asia. The development of safe and effective ASF vaccines is urgently required for the control of ASF outbreaks. The ASF virus (ASFV), the causative agent of ASF, has a large genome and a complex structure. The functions of nearly half of its viral genes still remain to be explored. Knowledge on the structure and function of ASFV proteins, the mechanism underlying ASFV infection and immunity, and the identification of major immunogenicity genes will contribute to the development of an ASF vaccine. In this context, this paper reviews the available knowledge on the structure, replication, protein function, virulence genes, immune evasion, inactivation, vaccines, control, and diagnosis of ASFV.

Research progress on the proteins involved in African swine fever virus infection and replication

African swine fever (ASF) is an acute, hemorrhagic and highly contagious infectious disease caused by African swine fever virus (ASFV), which infects domestic pigs or wild boars. It is characterized by short course of disease, high fever and hemorrhagic lesions, with mortality of up to 100% from acute infection. Up to now, the lack of commercial vaccines and effective drugs has seriously threatened the healthy economic development of the global pig industry. ASFV is a double-stranded DNA virus and genome varies between about 170-194 kb, which encodes 150-200 viral proteins, including 68 structural proteins and more than 100 non-structural proteins. In recent years, although the research on structure and function of ASFV-encoded proteins has been deepened, the structure and infection process of ASFV are still not clear. This review summarizes the main process of ASFV infection, replication and functions of related viral proteins to provide scientific basis and theoretical basis for ASFV research and vaccine development.

Regulation of antiviral immune response by African swine fever virus (ASFV)

African swine fever (ASF) is a highly contagious and acute hemorrhagic viral disease with a high mortality approaching 100% in domestic pigs. ASF is an endemic in countries in sub-Saharan Africa. Now, it has been spreading to many countries, especially in Asia and Europe. Due to the fact that there is no commercial vaccine available for ASF to provide sustainable prevention, the disease has spread rapidly worldwide and caused great economic losses in swine industry. The knowledge gap of ASF virus (ASFV) pathogenesis and immune evasion is the main factor to limit the development of safe and effective ASF vaccines. Here, we will summarize the molecular mechanisms of how ASFV interferes with the host innate and adaptive immune responses. An in-depth understanding of ASFV immune evasion strategies will provide us with rational design of ASF vaccines.

Structure of African Swine Fever Virus and Associated Molecular Mechanisms Underlying Infection and Immunosuppression: A Review

African swine fever (ASF) is an acute, highly contagious, and deadly infectious disease. The mortality rate of the most acute and acute ASF infection is almost 100%. The World Organization for Animal Health [Office International des épizooties (OIE)] lists it as a legally reported animal disease and China lists it as class I animal epidemic. Since the first diagnosed ASF case in China on August 3, 2018, it has caused huge economic losses to animal husbandry. ASF is caused by the African swine fever virus (ASFV), which is the only member of Asfarviridae family. ASFV is and the only insect-borne DNA virus belonging to the Nucleocytoplasmic Large DNA Viruses (NCLDV) family with an icosahedral structure and an envelope. Till date, there are still no effective vaccines or antiviral drugs for the prevention or treatment of ASF. The complex viral genome and its sophisticated ability to regulate the host immune response may be the reason for the difficulty in developing an effective vaccine. This review summarizes the recent findings on ASFV structure, the molecular mechanism of ASFV infection and immunosuppression, and ASFV-encoded proteins to provide comprehensive proteomic information for basic research on ASFV. In addition, it also analyzes the results of previous studies and speculations on the molecular mechanism of ASFV infection, which aids the study of the mechanism of clinical pathological phenomena, and provides a possible direction for an intensive study of ASFV infection mechanism. By summarizing the findings on molecular mechanism of ASFV- regulated host cell immune response, this review provides orientations and ideas for fundamental research on ASFV and provides a theoretical basis for the development of protective vaccines against ASFV.

Morphology Remodeling and Selective Autophagy of Intracellular Organelles during Viral Infections

Viruses have evolved different strategies to hijack subcellular organelles during their life cycle to produce robust infectious progeny. Successful viral reproduction requires the precise assembly of progeny virions from viral genomes, structural proteins, and membrane components. Such spatial and temporal separation of assembly reactions depends on accurate coordination among intracellular compartmentalization in multiple organelles. Here, we overview the rearrangement and morphology remodeling of virus-triggered intracellular organelles. Focus is given to the quality control of intracellular organelles, the hijacking of the modified organelle membranes by viruses, morphology remodeling for viral replication, and degradation of intracellular organelles by virus-triggered selective autophagy. Understanding the functional reprogram and morphological remodeling in the virus-organelle interplay can provide new insights into the development of broad-spectrum antiviral strategies.

Only a subpopulation of mouse sperm displays a rapid increase in intracellular calcium during capacitation

Mammalian sperm must undergo a functionally defined process called capacitation to be able to fertilize oocytes. They become capacitated in vivo by interacting with the female reproductive tract or in vitro in a defined capacitation medium that contains bovine serum albumin, calcium (Ca²⁺ ), and bicarbonate (HCO₃ ^- ). In this work, sperm were double stained with propidium iodide and the Ca²⁺ dye Fluo-4 AM and analyzed by flow cytometry to determine changes in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in individual live sperm. An increase in [Ca²⁺ ]_i was observed in a subpopulation of capacitated live sperm when compared with noncapacitated ones. Sperm exposed to the capacitating medium displayed a rapid increase in [Ca²⁺ ]_i within 1 min of incubation, which remained sustained for 90 min. These rise in [Ca²⁺ ]_i after 90 min of incubation in the capacitating medium was evidenced by an increase in the normalized median fluorescence intensity. This increase was dependent on the presence of extracellular Ca²⁺ and, at least in part, reflected the contribution of a new subpopulation of sperm with higher [Ca²⁺ ]_i . In addition, it was determined that the capacitation-associated [Ca²⁺ ]_i increase was dependent of CatSper channels, as sperm derived from CatSper knockout (CatSper KO) or incubated in the presence of CatSper inhibitors failed to increase [Ca²⁺ ]_i . Surprisingly, a minimum increase in [Ca²⁺ ]_i was also observed in CatSper KO sperm suggesting the existence of other Ca²⁺ transport systems. Altogether, these results indicate that a subpopulation of sperm increases [Ca²⁺ ]_i very rapidly during capacitation mainly due to a CatSper-mediated influx of extracellular Ca²⁺ .

Ectromelia Virus Affects Mitochondrial Network Morphology, Distribution, and Physiology in Murine Fibroblasts and Macrophage Cell Line

Mitochondria are multifunctional organelles that participate in numerous processes in response to viral infection, but they are also a target for viruses. The aim of this study was to define subcellular events leading to alterations in mitochondrial morphology and function during infection with ectromelia virus (ECTV). We used two different cell lines and a combination of immunofluorescence techniques, confocal and electron microscopy, and flow cytometry to address subcellular changes following infection. Early in infection of L929 fibroblasts and RAW 264.7 macrophages, mitochondria gathered around viral factories. Later, the mitochondrial network became fragmented, forming punctate mitochondria that co-localized with the progeny virions. ECTV-co-localized mitochondria associated with the cytoskeleton components. Mitochondrial membrane potential, mitochondrial fission–fusion, mitochondrial mass, and generation of reactive oxygen species (ROS) were severely altered later in ECTV infection leading to damage of mitochondria. These results suggest an important role of mitochondria in supplying energy for virus replication and morphogenesis. Presumably, mitochondria participate in transport of viral particles inside and outside of the cell and/or they are a source of membranes for viral envelope formation. We speculate that the observed changes in the mitochondrial network organization and physiology in ECTV-infected cells provide suitable conditions for viral replication and morphogenesis.

Viral capsid assembly as a model for protein aggregation diseases: Active processes catalyzed by cellular assembly machines comprising novel drug targets

Viruses can be conceptualized as self-replicating multiprotein assemblies, containing coding nucleic acids. Viruses have evolved to exploit host cellular components including enzymes to ensure their replicative life cycle. New findings indicate that also viral capsid proteins recruit host factors to accelerate their assembly. These assembly machines are RNA-containing multiprotein complexes whose composition is governed by allosteric sites. In the event of viral infection, the assembly machines are recruited to support the virus over the host and are modified to achieve that goal. Stress granules and processing bodies may represent collections of such assembly machines, readily visible by microscopy but biochemically labile and difficult to isolate by fractionation. We hypothesize that the assembly of protein multimers such as encountered in neurodegenerative or other protein conformational diseases, is also catalyzed by assembly machines. In the case of viral infection, the assembly machines have been modified by the virus to meet the virus' need for rapid capsid assembly rather than host homeostasis. In the case of the neurodegenerative diseases, it is the monomers and/or low n oligomers of the so-called aggregated proteins that are substrates of assembly machines. Examples for substrates are amyloid β peptide (Aβ) and tau in Alzheimer's disease, α-synuclein in Parkinson's disease, prions in the prion diseases, Disrupted-in-schizophrenia 1 (DISC1) in subsets of chronic mental illnesses, and others. A likely continuum between virus capsid assembly and cell-to-cell transmissibility of aggregated proteins is remarkable. Protein aggregation diseases may represent dysfunction and dysregulation of these assembly machines analogous to the aberrations induced by viral infection in which cellular homeostasis is pathologically reprogrammed. In this view, as for viral infection, reset of assembly machines to normal homeostasis should be the goal of protein aggregation therapeutics. A key basis for the commonality between viral and neurodegenerative disease aggregation is a broader definition of assembly as more than just simple aggregation, particularly suited for the crowded cytoplasm. The assembly machines are collections of proteins that catalytically accelerate an assembly reaction that would occur spontaneously but too slowly to be relevant in vivo. Being an enzyme complex with a functional allosteric site, appropriated for a non-physiological purpose (e.g. viral infection or conformational disease), these assembly machines present a superior pharmacological target because inhibition of their active site will amplify an effect on their substrate reaction. Here, we present this hypothesis based on recent proof-of-principle studies against Aβ assembly relevant in Alzheimer's disease.

African swine fever virus organelle rearrangements

Like most viruses African swine fever virus (ASFV) subsumes the host cell apparatus in order to facilitate its replication. ASFV replication is a highly orchestrated process with a least four stages of transcription, immediate-early, early, intermediate and late. As the infective cycle progresses through these stages most if not all of the organelles that comprise a nucleated cell are modified, adapted or in some cases destroyed. The entry of the virus is receptor-mediated, but the precise mechanism of endocytosis is a matter of keen, current debate. Once ASFV has exited from the endosomal-lysosomal complex the virus life-cycle enters into an intimate relationship with the microtubular network. Genome replication is believed to be initiated within the nucleus and ASFV infection completely reorders the structure of this organelle. The majority of replication and assembly occurs in discrete, perinuclear regions of the cell called virus factories and finally progeny virions are transported to the plasma membrane along microtubules where they bud out or are propelled away along actin projections to infect new cells. The generation of ASFV replication sites induces profound reorganisation of the organelles that comprise the secretory pathway and may contribute to the induction of cellular stress responses that ASFV modulates. The level of organisation and complexity of virus factories are not dissimilar to those seen in cellular organelles. Like their cellular counterparts the formation of virus factories, as well as virus entry and exit, are dependent on the various components of the cytoskeleton. This review will summarise these rearrangements, the viral proteins involved and their functional consequences.

Mitochondria and viruses

Mitochondria are involved in a variety of cellular metabolic processes, and their functions are regulated by extrinsic and intrinsic stimuli including viruses. Recent studies have shown that mitochondria play a central role in the primary host defense mechanisms against viral infections, and a number of novel viral and mitochondrial proteins are involved in these processes. Some viral proteins localize in mitochondria and interact with mitochondrial proteins to regulate cellular responses. This review summarizes recent findings on the functions and roles of these molecules as well as mitochondrial responses to viral infections.

African swine fever virus protein p17 is essential for the progression of viral membrane precursors toward icosahedral intermediates

The first morphological evidence of African swine fever virus (ASFV) assembly is the appearance of precursor viral membranes, thought to derive from the endoplasmic reticulum, within the assembly sites. We have shown previously that protein p54, a viral structural integral membrane protein, is essential for the generation of the viral precursor membranes. In this report, we study the role of protein p17, an abundant transmembrane protein localized at the viral internal envelope, in these processes. Using an inducible virus for this protein, we show that p17 is essential for virus viability and that its repression blocks the proteolytic processing of polyproteins pp220 and pp62. Electron microscopy analyses demonstrate that when the infection occurs under restrictive conditions, viral morphogenesis is blocked at an early stage, immediately posterior to the formation of the viral precursor membranes, indicating that protein p17 is required to allow their progression toward icosahedral particles. Thus, the absence of this protein leads to an accumulation of these precursors and to the delocalization of the major components of the capsid and core shell domains. The study of ultrathin serial sections from cells infected with BA71V or the inducible virus under permissive conditions revealed the presence of large helicoidal structures from which immature particles are produced, suggesting that these helicoidal structures represent a previously undetected viral intermediate.

The African swine fever virus virion membrane protein pE248R is required for virus infectivity and an early postentry event

The African swine fever virus (ASFV) protein pE248R, encoded by the gene E248R, is a late structural component of the virus particle. The protein contains intramolecular disulfide bonds and has been previously identified as a substrate of the ASFV-encoded redox system. Its amino acid sequence contains a putative myristoylation site and a hydrophobic transmembrane region near its carboxy terminus. We show here that the protein pE248R is myristoylated during infection and associates with the membrane fraction in infected cells, behaving as an integral membrane protein. Furthermore, the protein localizes at the inner envelope of the virus particles in the cytoplasmic factories. The function of the protein pE248R in ASFV replication was investigated by using a recombinant virus that inducibly expresses the gene E248R. Under repressive conditions, the ASFV polyproteins pp220 and pp62 are normally processed and virus particles with morphology indistinguishable from that of those produced in a wild-type infection or under permissive conditions are generated. Moreover, the mutant virus particles can exit the cell as does the parental virus. However, the infectivity of the pE248R-deficient virions was reduced at least 100-fold. An investigation of the defect of the mutant virus indicated that neither virus binding nor internalization was affected by the absence of the protein pE248R, but a cytopathic effect was not induced and early and late gene expression was impaired, indicating that the protein is required for some early postentry event.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

The envelope of intracellular African swine fever virus is composed of a single lipid bilayer

African swine fever virus (ASFV) is a member of a family of large nucleocytoplasmic DNA viruses that include poxviruses, iridoviruses, and phycodnaviruses. Previous ultrastructural studies of ASFV using chemical fixation and cryosectioning for electron microscopy (EM) have produced uncertainty over whether the inner viral envelope is composed of a single or double lipid bilayer. In this study we prepared ASFV-infected cells for EM using chemical fixation, cryosectioning, and high-pressure freezing. The appearance of the intracellular viral envelope was determined and compared to that of mitochondrial membranes in each sample. The best resolution of membrane structure was obtained with samples prepared by high-pressure freezing, and images suggested that the envelope of ASFV consisted of a single lipid membrane. It was less easy to interpret virus structure in chemically fixed or cryosectioned material, and in the latter case the virus envelope could be interpreted as having two membranes. Comparison of membrane widths in all three preparations indicated that the intracellular viral envelope of ASFV was not significantly different from the outer mitochondrial membrane (P < 0.05). The results support the hypothesis that the intracellular ASFV viral envelope is composed of a single lipid bilayer.

Aggresomes and pericentriolar sites of virus assembly: cellular defense or viral design?

Virus replication and virus assembly often occur in virus inclusions or virus factories that form at pericentriolar sites close to the microtubule organizing center or in specialized nuclear domains called ND10/PML bodies. Similar inclusions called aggresomes form in response to protein aggregation. Protein aggregates are toxic to cells and are transported along microtubules to aggresomes for immobilization and subsequent degradation by proteasomes and/or autophagy. The similarity between aggresomes and virus inclusions raises the possibility that viruses use aggresome pathways to concentrate cellular and viral proteins to facilitate replication and assembly. Alternatively, aggresomes may be part of an innate cellular response that recognizes virus components as foreign or misfolded and targets them for storage and degradation. Insights into the possible roles played by aggresomes during virus assembly are emerging from an understanding of how virus inclusions form and how viral proteins are targeted to them.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Reduced redox potential of the cytosol is important for African swine fever virus capsid assembly and maturation

Assembly of African swine fever virus (ASFV) involves the transfer of the major capsid protein, p73, from the cytosol onto the cytoplasmic face of endoplasmic reticulum-derived membranes. During this process, the folding of p73 is dependent upon transient association with a specific viral chaperone, CAP80. The cell cytoplasm maintains high concentrations of reduced glutathione, leading to a reducing environment. Here, the effects of redox environment on the assembly of ASFV have been studied. Diamide, which oxidizes the cell cytosol, slowed the folding of p73 and prevented release from CAP80 and subsequent binding of p73 to membranes. Similarly, cell oxidation slowed the assembly of p73 molecules already bound to membranes into virus capsid precursors. Interestingly, addition of oxidized glutathione to newly assembled virus capsid precursors in vitro led to disassembly; however, virus particles released from cells were resistant to oxidized glutathione. These data show that assembly of ASFV requires the reducing environment that prevails in the cytosol, but as the virus matures, it becomes resistant to oxidation, possibly indicating preparation for release from the cell.

The African swine fever virus nonstructural protein pB602L is required for formation of the icosahedral capsid of the virus particle

African swine fever virus (ASFV) protein pB602L has been described as a molecular chaperone for the correct folding of the major capsid protein p72. We have studied the function of protein pB602L during the viral assembly process by using a recombinant ASFV, vB602Li, which inducibly expresses the gene coding for this protein. We show that protein pB602L is a late nonstructural protein, which, in contrast with protein p72, is excluded from the viral factory. Repression of protein pB602L synthesis inhibits the proteolytic processing of the two viral polyproteins pp220 and pp62 and leads to a decrease in the levels of protein p72 and a delocalization of the capsid protein pE120R. As shown by electron microscopy analysis of cells infected with the recombinant virus vB602Li, the viral assembly process is severely altered in the absence of protein pB602L, with the generation of aberrant "zipper-like" structures instead of icosahedral virus particles. These "zipper-like" structures are similar to those found in cells infected under restrictive conditions with the recombinant virus vA72 inducibly expressing protein p72. Immunoelectron microscopy studies show that the abnormal forms generated in the absence of protein pB602L contain the inner envelope protein p17 and the two polyproteins but lack the capsid proteins p72 and pE120R. These findings indicate that protein pB602L is essential for the assembly of the icosahedral capsid of the virus particle.

Production of infectious human cytomegalovirus virions is inhibited by drugs that disrupt calcium homeostasis in the endoplasmic reticulum

We previously reported that human cytomegalovirus (HCMV) infection induces endoplasmic reticulum (ER) stress, resulting in activation of the unfolded protein response (UPR). Although some normal consequences of UPR activation (e.g., translation attenuation) are detrimental to viral infection, we have previously shown that HCMV infection adapts the UPR to benefit the viral infection (14). For example, UPR-induced translation attenuation is inhibited by viral infection, while potentially beneficial aspects of the UPR are maintained. In the present work, we tested the ability of HCMV to overcome a robust induction of the UPR by the drugs thapsigargin and clotrimazole (CLT), which disrupt ER calcium homeostasis. A 24-h treatment with these drugs beginning at 48, 72, or 96 h postinfection (hpi) completely inhibited further production of infectious virions. HCMV could not overcome the inhibition of global translation by CLT; however, between 48 and 72 hpi, HCMV overcame translational inhibition caused by thapsigargin. Despite the restoration of translation in thapsigargin, the accumulation of immediate-early and early gene products was modestly retarded (50% or less), whereas the accumulation of an early-late and late gene product was significantly retarded. Electron microscopic analysis shows that the drugs severely disrupt the maturation of HCMV virions. This can be accounted for by both the retarded accumulation of late gene products and the drug-induced depletion of ER calcium, which disrupts critical cellular functions needed for maturation.

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.

Virus factories: associations of cell organelles for viral replication and morphogenesis

Genome replication and assembly of viruses often takes place in specific intracellular compartments where viral components concentrate, thereby increasing the efficiency of the processes. For a number of viruses the formation of 'factories' has been described, which consist of perinuclear or cytoplasmic foci that mostly exclude host proteins and organelles but recruit specific cell organelles, building a unique structure. The formation of the viral factory involves a number of complex interactions and signalling events between viral and cell factors. Mitochondria, cytoplasmic membranes and cytoskeletal components frequently participate in the formation of viral factories, supplying basic and common needs for key steps in the viral replication cycle.

African swine fever virus structural protein p54 is essential for the recruitment of envelope precursors to assembly sites

The assembly of African swine fever virus (ASFV) at the cytoplasmic virus factories commences with the formation of precursor membranous structures, which are thought to be collapsed cisternal domains recruited from the surrounding endoplasmic reticulum (ER). This report analyzes the role in virus morphogenesis of the structural protein p54, a 25-kDa polypeptide encoded by the E183L gene that contains a putative transmembrane domain and localizes at the ER-derived envelope precursors. We show that protein p54 behaves in vitro and in infected cells as a type I membrane-anchored protein that forms disulfide-linked homodimers through its unique luminal cysteine. Moreover, p54 is targeted to the ER membranes when it is transiently expressed in transfected cells. Using a lethal conditional recombinant, vE183Li, we also demonstrate that the repression of p54 synthesis arrests virus morphogenesis at a very early stage, even prior to the formation of the precursor membranes. Under restrictive conditions, the virus factories appeared as discrete electron-lucent areas essentially free of viral structures. In contrast, outside the assembly sites, large amounts of aberrant zipper-like structures formed by the unprocessed core polyproteins pp220 and pp62 were produced in close association to ER cisternae. Altogether, these results indicate that the transmembrane structural protein p54 is critical for the recruitment and transformation of the ER membranes into the precursors of the viral envelope.

African Swine Fever virus proteinase is essential for core maturation and infectivity

African swine fever virus (ASFV) encodes two polyprotein precursors named pp220 and pp62 that are sequentially processed during viral infection, giving rise to six major structural proteins. These reside at the core shell, a matrix domain located between the endoplasmic reticulum-derived inner envelope and the DNA-containing nucleoid. Proteolytic processing of the polyprotein precursors is catalyzed by the viral proteinase pS273R, a cysteine proteinase that shares sequence similarity with the SUMO1-processing peptidases. We describe here the construction and characterization of an ASFV recombinant, vS273Ri, that inducibly expresses the ASFV proteinase. Using vS273Ri, we show that repression of proteinase expression inhibits polyprotein processing and strongly impairs infective virus production. Electron microscopic examination of vS273Ri-infected cells showed that inhibition of proteolytic processing leads to the assembly of defective icosahedral particles containing a noncentered electron-dense nucleoid surrounded by an abnormal core shell of irregular thickness. The analysis of purified extracellular defective particles revealed that they contain the unprocessed pp220 and pp62 precursors, as well as the major DNA-binding nucleoid proteins p10 and pA104R. Altogether, these results indicate that the proteolytic processing of the polyproteins is not required for their incorporation into the assembling particles nor for the incorporation of the DNA-containing nucleoid. Instead, the ASFV proteinase is involved in a late maturational step that is essential for proper core assembly and infectivity.

Membrane association facilitates the correct processing of pp220 during production of the major matrix proteins of African swine fever virus

The African swine fever (ASF) virus polyprotein pp220 is processed at Gly-Gly-X sites by a virally encoded SUMO-like protease to produce matrix proteins p150, p37, p34, and p14. Four Gly-Gly-X sites are used to produce the matrix proteins, but the polyprotein contains an additional 15 sites potentially recognized by the protease. This study shows that cleavage occurs at many, if not all, Gly-Gly-X sites, and at steady state, p150 and p34 are minor products of processing. Significantly, only the final structural proteins, p150 and p34, were found in mature virions, suggesting that there is a mechanism for excluding incorrectly processed forms. ASF virus is assembled on the cytoplasmic face of the endoplasmic reticulum, and the distribution of pp220 products between membranes and cytosol was studied. Incorrectly processed forms of p34 were recovered from both the cytosol and membrane fractions. Interestingly, p34 was only detected in the membrane fraction, and of the many processed forms bound to membranes, only p34 was protected from trypsin, suggesting envelopment. The majority of the incorrectly processed forms of p150 were recovered from the cytosol. Again, the correct product of processing, p150, was selectively recruited to membranes. Sucrose density centrifugation showed that membrane-associated forms of p34 and p150 assembled into large structures suggestive of a viral matrix, while cytosolic and/or incorrectly processed forms of pp220 did not. Taken together, these results suggest that association with cellular membranes is important for regulating the correct processing of pp220 and the packaging of matrix proteins into virions.

Interaction of HIV-1 gag and membranes in a cell-free system

A coupled transcription-translation (TNT) reticulocyte lysate system was used to examine posttranslational alterations in HIV-1 Gag upon addition of Jurkat T cell membranes. Incubation of the Gag precursor protein, Pr55gag, with membranes resulted in a time-dependent alteration in Gag resulting in partial resistance to trypsin treatment. Treatment of membranes and TNT extract with apyrase or pretreatment of membranes with trypsin prevented this posttranslational alteration of Gag. In contrast, this activity was not disrupted by pretreatment of membranes with Triton X-100 at 4 degrees C, under conditions which do not solubilize raft-associated proteins. Flotation studies revealed that acquisition of trypsin-resistance was accompanied by Gag binding to membranes. The myristylation signal and nucleocapsid domain were found to mediate Gag binding to membranes. The posttranslational alteration of Gag accompanying membrane interaction may represent a conformational change, oligomerization, and/or association with or envelopment by membranes. These findings provide new clues to the stepwise process of HIV-1 assembly.

Interaction of hepatitis C virus-like particles and cells: a model system for studying viral binding and entry

Hepatitis C virus-like particles (HCV-LPs) containing the structural proteins of HCV H77 strain (1a genotype) was used as a model for HCV virion to study virus-cell interaction. HCV-LPs showed a buoyant density of 1.17 to 1.22 g/cm(3) in a sucrose gradient and formed double-shelled particles 35 to 49 nm in diameter. Flow cytometry analysis by an indirect method (detection with anti-E2 antibody) and a direct method (use of dye-labeled HCV-LPs) showed that HCV-LPs binds to several human hepatic (primary hepatocytes, HepG2, HuH7, and NKNT-3) and T-cell (Molt-4) lines. HCV-LPs binding to cells occurred in a dose- and calcium-dependent manner and was not mediated by CD81. Scatchard plot analysis suggests the presence of two binding sites for HCV-LPs with high (K(d) approximately 1 microg/ml) and low (K(d) approximately 50 to 60 microg/ml) affinities of binding. Anti-E1 and -E2 antibodies inhibited HCV-LPs binding to cells. While preincubation of HCV-LPs with very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), or high-density lipoprotein (HDL) blocked its binding to cells, preincubation of cells with VLDL, LDL, HDL, or anti-LDL-R antibody did not. Confocal microscopy analysis showed that, after binding to cells, dye-labeled HCV-LPs were internalized into the cytoplasm. This process could be inhibited with anti-E1 or anti-E2 antibodies, suggesting that E1 and E2 proteins mediate HCV-LPs binding and, subsequently, their entry into cells. Altogether, our results indicate that HCV-LPs can be used to further characterize the mechanisms involved in the early steps of HCV infection.

The g5R (D250) gene of African swine fever virus encodes a Nudix hydrolase that preferentially degrades diphosphoinositol polyphosphates

The African swine fever virus (ASFV) g5R gene encodes a protein containing a Nudix hydrolase motif which in terms of sequence appears most closely related to the mammalian diadenosine tetraphosphate (Ap4A) hydrolases. However, purified recombinant g5R protein (g5Rp) showed a much wider range of nucleotide substrate specificity compared to eukaryotic Ap4A hydrolases, having highest activity with GTP, followed by adenosine 5'-pentaphosphate (p5A) and dGTP. Diadenosine and diguanosine nucleotides were substrates, but the enzyme showed no activity with cap analogues such as 7mGp3A. In common with eukaryotic diadenosine hexaphosphate (Ap6A) hydrolases, which prefer higher-order polyphosphates as substrates, g5Rp also hydrolyzes the diphosphoinositol polyphosphates PP-InsP5 and [PP]2-InsP4. A comparison of the kinetics of substrate utilization showed that the k(cat)/K(m) ratio for PP-InsP5 is 60-fold higher than that for GTP, which allows classification of g5R as a novel diphosphoinositol polyphosphate phosphohydrolase (DIPP). Unlike mammalian DIPP, g5Rp appeared to preferentially remove the 5-beta-phosphate from both PP-InsP5 and [PP]2-InsP4. ASFV infection led to a reduction in the levels of PP-InsP5, ATP and GTP by ca. 50% at late times postinfection. The measured intracellular concentrations of these compounds were comparable to the respective K(m) values of g5Rp, suggesting that one or all of these may be substrates for g5Rp during ASFV infection. Transfection of ASFV-infected Vero cells with a plasmid encoding epitope-tagged g5Rp suggested localization of this protein in the rough endoplasmic reticulum. These results suggest a possible role for g5Rp in regulating a stage of viral morphogenesis involving diphosphoinositol polyphosphate-mediated membrane trafficking.

The trans Golgi network is lost from cells infected with African swine fever virus

The cellular secretory pathway is important during the assembly and envelopment of viruses and also controls the transport of host proteins, such as cytokines and major histocompatibility proteins, that function during the elimination of viruses by the immune system. African swine fever virus (ASFV) encodes at least 26 proteins with stretches of hydrophobic amino acids suggesting entry into the secretory pathway (R. J. Yanez, J. M. Rodriguez, M. L. Nogal, L. Yuste, C. Enriquez, J. F. Rodriguez, and E. Vinuela, Virology 208:249-278, 1995). To predict how and where these potential membrane proteins function, we have studied the integrity of the secretory pathway in cells infected with ASFV. Remarkably, ASFV caused complete loss of immunofluorescence signal for the trans Golgi network (TGN) marker protein TGN46 and dispersed the AP1 TGN adapter complex. Loss of TGN46 signal was not due to degradation of TGN46, suggesting redistribution of TGN46 to other membrane compartments. ASFV markedly slowed transport of cathepsin D to lysosomes, demonstrating that loss of TGN structure correlated with loss of TGN function. ASFV shows a tropism for macrophages, and it is possible that ASFV compromises TGN function to augment the activity of viral membrane proteins or to suppress the function of host immunoregulatory proteins.

A virally encoded chaperone specialized for folding of the major capsid protein of African swine fever virus

It is generally believed that cellular chaperones facilitate the folding of virus capsid proteins, or that capsid proteins fold spontaneously. Here we show that p73, the major capsid protein of African swine fever virus (ASFV) failed to fold and aggregated when expressed alone in cells. This demonstrated that cellular chaperones were unable to aid the folding of p73 and suggested that ASFV may encode a chaperone. An 80-kDa protein encoded by ASFV, termed the capsid-associated protein (CAP) 80, bound to the newly synthesized capsid protein in infected cells. The 80-kDa protein was released following conformational maturation of p73 and dissociated before capsid assembly. Coexpression of the 80-kDa protein with p73 prevented aggregation and allowed the capsid protein to fold with kinetics identical to those seen in infected cells. CAP80 is, therefore, a virally encoded chaperone that facilitates capsid protein folding by masking domains exposed by the newly synthesized capsid protein, which are susceptible to aggregation, but cannot be accommodated by host chaperones. It is likely that these domains are ultimately buried when newly synthesized capsid proteins are added to the growing capsid shell.

African swine fever virus structural protein pE120R is essential for virus transport from assembly sites to plasma membrane but not for infectivity

This report examines the role of African swine fever virus (ASFV) structural protein pE120R in virus replication. Immunoelectron microscopy revealed that protein pE120R localizes at the surface of the intracellular virions. Consistent with this, coimmunoprecipitation assays showed that protein pE120R binds to the major capsid protein p72. Moreover, it was found that, in cells infected with an ASFV recombinant that inducibly expresses protein p72, the incorporation of pE120R into the virus particle is dependent on p72 expression. Protein pE120R was also studied using an ASFV recombinant in which E120R gene expression is regulated by the Escherichia coli lac repressor-operator system. In the absence of inducer, pE120R expression was reduced about 100-fold compared to that obtained with the parental virus or the recombinant virus grown under permissive conditions. One-step virus growth curves showed that, under conditions that repress pE120R expression, the titer of intracellular progeny was similar to the total virus yield obtained under permissive conditions, whereas the extracellular virus yield was about 100-fold lower than in control infections. Immunofluorescence and electron microscopy demonstrated that, under restrictive conditions, intracellular mature virions are properly assembled but remain confined to the replication areas. Altogether, these results indicate that pE120R is necessary for virus dissemination but not for virus infectivity. The data also suggest that protein pE120R might be involved in the microtubule-mediated transport of ASFV particles from the viral factories to the plasma membrane.

The late stage of human immunodeficiency virus type 1 assembly is an energy-dependent process

Several recent studies have indicated the involvement of host cell factors in human immunodeficiency virus type 1 (HIV-1) assembly. To ascertain whether ATP-dependent factors play a role in this process, we quantified virus-like particle (VLP) production by ATP-depleted cells. Pharmacological ATP depletion abrogated VLP production without affecting cell viability or inducing degradation of HIV-1 Gag protein. This effect occurred even when the ATP-depleting agents were added 1 h into the assembly process, and it was reversed by removal of these agents. ATP depletion did not affect Gag membrane binding or multimerization. Density gradient analysis indicated that HIV-1 assembly intermediates were stalled late in the assembly process. This conclusion was further supported by electron microscopy analysis, which revealed a preponderance of plasma membrane-associated stalk-like structures in the ATP-depleted cells. Since no HIV-1 proteins bind or hydrolyze ATP, these findings indicate that an ATP-requiring cellular factor is an obligatory participant late in the HIV-1 assembly process.