African Swine Fever Virus Is Wrapped by the Endoplasmic Reticulum

African swine fever (ASF) virus is a large DNA virus that shares the striking icosahedral symmetry of iridoviruses and the genomic organization of poxviruses. Both groups of viruses have a complex envelope structure. In this study, the mechanism of formation of the inner envelope of ASF virus was investigated. Examination of thin cryosections by electron microscopy showed two internal membranes in mature intracellular virions and all structural intermediates. These membranes were in continuity with intracellular membrane compartments, suggesting that the virus gained two membranes from intracellular membrane cisternae. Immunogold electron microscopy showed the viral structural protein p17 and resident membrane proteins of the endoplasmic reticulum (ER) within virus assembly sites, virus assembly intermediates, and mature virions. Resident ER proteins were also detected by Western blotting of isolated virions. The data suggested the ASF virus was wrapped by the ER. Analysis of the published sequence of ASF virus (R. J. Yanez et al., Virology 208:249–278, 1995) revealed a reading frame, XP124L, that encoded a protein predicted to translocate into the lumen of the ER. Pulse-chase immunoprecipitation and glycosylation analysis of pXP124L, the product of the XP124L gene, showed that pXP124L was retained in the ER lumen after synthesis. When analyzed by immunogold electron microscopy, pXP124L localized to virus assembly intermediates and fully assembled virions. Western blot analysis detected pXP124L in virions isolated from Percoll gradients. The packaging of pXP124L from the lumen of the ER into the virion is consistent with ASF virus being wrapped by ER cisternae: a mechanism which explains the presence of two membranes in the viral envelope.African swine fever (ASF) virus is a large icosahedral enveloped DNA virus that causes a lethal hemorrhagic disease in domestic pigs. The virus is endemic in areas of southern Europe and in Africa where it causes major problems for the development of pig industries. At present there are no vaccines, and the disease is controlled through the slaughter of infected animals. The economic importance of ASF virus has made the virus the focus of much research since it was first described in 1921 (32). ASF virus is unique among animal viruses, and its classification has been controversial. ASF virus shares the striking icosahedral symmetry of iridoviruses (5, 8, 13, 34), while the presence of inverted terminal repeats and covalently linked ends in the 170-kDa genome suggests similarities with poxviruses (16). The ASF virus genome encoding at least 150 proteins has been sequenced (17, 51), and the amino acid sequences of at least 11 structural proteins are known. p73 is the major structural protein (14, 28) and has sequence similarities to the capsid protein of iridoviruses (39). The ordered proteolysis of pp220 produces p150, p37/p34 and p14 (40), which together comprise 25% of the viral proteins (3). These proteins localize to the interior of the virion (3). Three proteins, J13L/p54, I1L/p17, and p22, with membrane-spanning domains localize to the viral envelope (10, 37, 41, 43). Three other structural proteins, p14.5 encoded by E120R (30), p10 encoded by K78R (35), and p5AR encoded by A104R (7), have DNA-binding properties (51) and may be involved in DNA packaging. The virus has been the subject of several detailed electron microscopy studies (2–4, 8, 9, 11, 13, 34, 47). Electron micrographs of sections taken through ASF virus assembly sites reveal fully assembled virions as 200-nm hexagons and an ordered series of assembly intermediates with one to six sides of a hexagon. Close inspection of intracellular virions identifies multiple concentric layers of differing electron densities. According to recent models, the layers represent a central electron-dense nucleocapsid core, surrounded by an inner core shell, an inner envelope, and an outer capsid layer (3). The mechanism of formation of the inner envelope of ASF virus has not been resolved.Most viruses gain a single membrane envelope by budding into intracellular membrane compartments or from the plasma membrane, as reviewed in reference 21. When viruses bud into an intracellular compartment, the domains of the membrane proteins that are initially located in the lumen of membrane compartments are exposed on the outside of the virion after release from the cell (Fig. ​(Fig.1a).1a). A second mechanism of envelopment, described recently for poxviruses and herpesviruses (18, 20, 24, 38, 42, 46, 50), is more complex and involves the wrapping of virions by membrane cisternae derived from specific membrane compartments. Wrapping provides two membrane envelopes in one step and leaves the virion free in the cytoplasm. When compared with budding, wrapping reverses the orientation of membrane proteins within the virus such that the domains of membrane proteins located in the lumen of the wrapping organelle are confined to the interior of the virus after release from the cell, whereas cytoplasmic tails are exposed on the outside of the virus (Fig. ​(Fig.1b).1b). Given these important consequences for understanding the mechanism of assembly of the virus and for determining the final orientation of membrane proteins in virions, we have set out to determine whether ASF virus acquires its membranes by the conventional budding mechanism or whether the virus is wrapped by intracellular membrane compartments before release from the cell. Open in a separate windowFIG. 1Schematic comparison of budding and wrapping mechanisms of virus envelopment. (a) Budding. Viral nucleoprotein complexes bind to the cytoplasmic domains of virally encoded integral membrane proteins (|, membrane glycoproteins). Interactions between viral proteins lead to membrane curvature, and the virion gains a single membrane by budding into the lumen of the membrane compartment. When the virion is released from the cell, oligosaccharides () are exposed on the surface of the virus, and the cytoplasmic tail of the membrane glycoprotein is buried within the virion. (b) Wrapping. Viral nucleoprotein complexes bind to the cytoplasmic domains of virally encoded integral membrane proteins. The nucleoprotein complex is then wrapped by the membrane cisternae, and the virus gains two membranes. The particle remains in the cytosol. When the virion is released from the cell by cell lysis, oligosaccharides () are buried within the two membranes of the virion while the cytoplasmic tail of the membrane glycoprotein is exposed on the surface of the virus.In this study we have taken advantage of thin cryoelectron microscopic sections to enhance the definition of viral membranes. The micrographs show two membranes within mature intracellular virions and all structural intermediates. They also show assembly intermediates in continuity with cellular membrane compartments. Consistent with our earlier study showing that p73 was enveloped by the endoplasmic reticulum (ER) (15), immunogold labelling experiments show resident proteins of the ER within membranes found at assembly sites, in virus assembly intermediates, and in mature virions. Importantly, we have identified a protein (pXP124L) encoded by ASF virus that translocates completely into the lumen of the ER and is incorporated as a structural protein of the virus. The presence of two membranes within intracellular virions and structural intermediates and the packaging of a structural protein from the lumen of the ER into the virus, strongly suggest that ASF virus is wrapped by the ER.