Sorting of progeny coronavirus from condensed secretory proteins at the exit from the trans-Golgi network of AtT20 cells

Murine hepatitis virus (strain A59), (MHV-A59) is a coronavirus that buds into pre-Golgi compartments and then exploits the exocytic pathway of the host cell to reach the exterior. The fibroblastic cells in which replication of this virus is usually studied have only a constitutive exocytic pathway that the virus uses. MHV-A59 also infects, albeit inefficiently, AtT20 cells, murine pituitary tumor cells with a regulated as well as a constitutive exocytic pathway. Here we examine AtT20 cells at early times after the infection, when the Golgi apparatus retains its morphological and biochemical integrity. We observe that progeny coronavirus and secretory protein destined for the secretory granules of the regulated exocytic pathway traverse the same Golgi stacks and accumulate in the trans-Golgi network. Their pathways diverge at this site, the condensed secretory proteins including the ACTH going to the secretory granules and the coronavirus to post-Golgi transport vesicles devoid of ACTH. On very rare occasions there is missorting such that aggregates of condensed secretory proteins and viruses occur together in post-Golgi vesicles. We conclude that the constitutive and regulated exocytic pathways, identified respectively by the progeny virions and the secretory protein ACTH, diverge at the exit from the trans-Golgi network.     

Legionella phagosomes intercept vesicular traffic from endoplasmic reticulum exit sites

It is unknown how Legionella pneumophila cells escape the degradative lysosomal pathway after phagocytosis by macrophages and replicate in an organelle derived from the endoplasmic reticulum. Here we show that, after internalization, L. pneumophila-containing phagosomes recruit early secretory vesicles. Once L. pneumophila phagosomes have intercepted early secretory vesicles they begin to acquire proteins residing in transitional and rough endoplasmic reticulum. The functions of Sar1 and ADP-ribosylation factor-1 are important for biogenesis of the L. pneumophila replicative organelle. These data indicate that L. pneumophila intercepts vesicular traffic from endoplasmic-reticulum exit sites to create an organelle that permits intracellular replication and prevents destruction by the host cell.     

Inhibition of cellular protein secretion by poliovirus proteins 2B and 3A.

Poliovirus RNA replication occurs on the surface of membranous vesicles that proliferate throughout the cytoplasm of the infected cell. Since at least some of these vesicles are thought to originate within the secretory pathway of the host cell, we examined the effect of poliovirus infection on protein transport through the secretory pathway. We found that transport of both plasma membrane and secretory proteins was inhibited by poliovirus infection early in the infectious cycle. Transport inhibition did not require viral RNA replication or the inhibition of host cell translation by poliovirus. The viral proteins 2B and 3A were each sufficient to inhibit transport in the absence of viral infection. The intracellular localization of a secreted protein in the presence of 3A with the endoplasmic reticulum suggested that 3A directly blocks transport from the endoplasmic reticulum to the Golgi apparatus.     

Co-opted Oxysterol-Binding ORP and VAP Proteins Channel Sterols to RNA Virus Replication Sites via Membrane Contact Sites

Viruses recruit cellular membranes and subvert cellular proteins involved in lipid biosynthesis to build viral replicase complexes and replication organelles. Among the lipids, sterols are important components of membranes, affecting the shape and curvature of membranes. In this paper, the tombusvirus replication protein is shown to co-opt cellular Oxysterol-binding protein related proteins (ORPs), whose deletion in yeast model host leads to decreased tombusvirus replication. In addition, tombusviruses also subvert Scs2p VAP protein to facilitate the formation of membrane contact sites (MCSs), where membranes are juxtaposed, likely channeling lipids to the replication sites. In all, these events result in redistribution and enrichment of sterols at the sites of viral replication in yeast and plant cells. Using in vitro viral replication assay with artificial vesicles, we show stimulation of tombusvirus replication by sterols. Thus, co-opting cellular ORP and VAP proteins to form MCSs serves the virus need to generate abundant sterol-rich membrane surfaces for tombusvirus replication.

Authors Summary

Cellular proteins and cellular membranes are usurped by positive-stranded RNA viruses to assemble viral replicase complexes required for their replication. Tombusviruses, which are small RNA viruses of plants, depend on sterol-rich membranes for replication. The authors show that the tombusviral replication protein binds to cellular oxysterol-binding ORP proteins. Moreover, the endoplasmic reticulum resident cellular VAP proteins also co-localize with viral replication proteins. These protein interactions likely facilitate the formation of membrane contact sites that are visible in cells replicating tombusvirus RNA. The authors also show that sterols are recruited and enriched to the sites of viral replication. In vitro replication assay was used to show that sterols indeed stimulate tombusvirus replication. In summary, tombusviruses use subverted cellular proteins to build sterol-rich membrane microdomain to promote the assembly of the viral replicase complex. The paper connects efficient virus replication with cellular lipid transport and membrane structures.     

Spatial segregation of the regulated and constitutive secretory pathways

Recent experiments using DNA transfection have shown that secretory proteins in AtT-20 cells are sorted into two biochemically distinct secretory pathways. These two pathways differ in the temporal regulation of exocytosis. Proteins secreted by the regulated pathway are stored in dense-core granules until release is stimulated by secretagogues. In contrast, proteins secreted by the constitutive pathway are exported continuously, without storage. It is not known whether there are mechanisms to segregate regulated and constitutive secretory vesicles spatially. In this study, we examined the site of insertion of constitutive vesicles and compared it with that of regulated secretory granules. Regulated granules accumulate at tips of processes in these cells. To determine whether constitutively externalized membrane proteins are inserted into plasma membrane at the cell body or at process tips, AtT-20 cells were infected with ts-O45, a temperature-sensitive mutant of vesicular stomatitis virus in which transport of the surface glycoprotein G is conditionally blocked in the ER. After switching to the permissive temperature, insertion of G protein was detected at the cell body, not at process tips. Targeting of constitutive and regulated secretory vesicles to distinct areas of the plasma membrane appears to be mediated by microtubules. We found that while disruption of microtubules by colchicine had no effect on constitutive secretion, it completely blocked the accumulation of regulated granules at special release sites. Colchicine also affected the proper packaging of regulated secretory proteins. We conclude that regulated and constitutive secretory vesicles are targeted to different areas of the plasma membrane, most probably by differential interactions with microtubules. These results imply that regulated secretory granules may have unique membrane receptors for selective attachment to microtubules.     

Tubule-guided cell-to-cell movement of a plant virus requires class XI myosin motors

Cell-to-cell movement of plant viruses occurs via plasmodesmata (PD), organelles that evolved to facilitate intercellular communications. Viral movement proteins (MP) modify PD to allow passage of the virus particles or nucleoproteins. This passage occurs via several distinct mechanisms one of which is MP-dependent formation of the tubules that traverse PD and provide a conduit for virion translocation. The MP of tubule-forming viruses including Grapevine fanleaf virus (GFLV) recruit the plant PD receptors called Plasmodesmata Located Proteins (PDLP) to mediate tubule assembly and virus movement. Here we show that PDLP1 is transported to PD through a specific route within the secretory pathway in a myosin-dependent manner. This transport relies primarily on the class XI myosins XI-K and XI-2. Inactivation of these myosins using dominant negative inhibition results in mislocalization of PDLP and MP and suppression of GFLV movement. We also found that the proper targeting of specific markers of the Golgi apparatus, the plasma membrane, PD, lipid raft subdomains within the plasma membrane, and the tonoplast was not affected by myosin XI-K inhibition. However, the normal tonoplast dynamics required myosin XI-K activity. These results reveal a new pathway of the myosin-dependent protein trafficking to PD that is hijacked by GFLV to promote tubule-guided transport of this virus between plant cells.     

Export of beta-lactamase is independent of the signal recognition particle

In Escherichia coli, three different types of proteins engage the SecY translocon of the inner bacterial membrane for translocation or insertion: 1) polytopic membrane proteins that prior to their insertion into the membrane are targeted to the translocon using the bacterial signal recognition particle (SRP) and its receptor; 2) secretory proteins that are targeted to and translocated across the SecY translocon in a SecA- and SecB-dependent reaction; and 3) membrane proteins with large periplasmic domains, requiring SRP for targeting and SecA for the translocation of the periplasmic moiety. In addition to its role as a targeting device for membrane proteins, a function of the bacterial SRP in the export of SecB-independent secretory proteins has also been postulated. In particular, beta-lactamase, a hydrolytic enzyme responsible for cleavage of the beta-lactam ring containing antibiotics, is considered to be recognized and targeted by SRP. To examine the role of the SRP pathway in beta-lactamase targeting and export, we performed a detailed in vitro analysis. Chemical cross-linking and membrane binding assays did not reveal any significant interaction between SRP and beta-lactamase nascent chains. More importantly, membrane vesicles prepared from mutants lacking a functional SRP pathway did block the integration of SRP-dependent membrane proteins but supported the export of beta-lactamase in the same way as that of the SRP-independent protein OmpA. These data demonstrate that in contrast to previous results, the bacterial SRP is not involved in the export of beta-lactamase and further suggest that secretory proteins of Gram-negative bacteria in general are not substrates of SRP.     

Turnip mosaic virus co-opts the vacuolar sorting receptor VSR4 to promote viral genome replication in plants by targeting viral replication vesicles to the endosome

Accumulated experimental evidence has shown that viruses recruit the host intracellular machinery to establish infection. It has recently been shown that the potyvirus Turnip mosaic virus (TuMV) transits through the late endosome (LE) for viral genome replication, but it is still largely unknown how the viral replication vesicles labelled by the TuMV membrane protein 6K2 target LE. To further understand the underlying mechanism, we studied the involvement of the vacuolar sorting receptor (VSR) family proteins from Arabidopsis in this process. We now report the identification of VSR4 as a new host factor required for TuMV infection. VSR4 interacted specifically with TuMV 6K2 and was required for targeting of 6K2 to enlarged LE. Following overexpression of VSR4 or its recycling-defective mutant that accumulates in the early endosome (EE), 6K2 did not employ the conventional VSR-mediated EE to LE pathway, but targeted enlarged LE directly from cis-Golgi and viral replication was enhanced. In addition, VSR4 can be N-glycosylated and this is required for its stability and for monitoring 6K2 trafficking to enlarged LE. A non-glycosylated VSR4 mutant enhanced the dissociation of 6K2 from cis-Golgi, leading to the formation of punctate bodies that targeted enlarged LE and to more robust viral replication than with glycosylated VSR4. Finally, TuMV hijacks N-glycosylated VSR4 and protects VSR4 from degradation via the autophagy pathway to assist infection. Taken together, our results have identified a host factor VSR4 required for viral replication vesicles to target endosomes for optimal viral infection and shed new light on the role of N-glycosylation of a host factor in regulating viral infection.     

Vesicle-mediated trafficking of parasite proteins to the host cell cytosol and erythrocyte surface membrane in Plasmodium falciparum infected erythrocytes

During the development of the asexual stage of the malaria parasite, Plasmodium falciparum, the composition, structure and function of the host cell membrane is dramatically altered, including the ability to adhere to vascular endothelium. Crucial to these changes is the transport of parasite proteins, which become associated with or inserted into the erythrocyte membrane. Protein and membrane targeting beyond the parasite plasma membrane must require unique pathways, given the parasites intracellular location within a parasitophorous vacuolar membrane and the lack of organelles and biosynthetic machinery in the host cell necessary to support a secretory system. It is not clear how these proteins cross the parasitophorous vacuolar membrane or how they traverse the erythrocyte cytosol to reach their final destinations. The identification of: (1) a P. falciparum homologue of the protein Sar1p, which is an essential component of the COPII-based secretory system in mammalian cells and yeast and (2) electron-dense, possibly coated, secretory vesicles bearing P. falciparum erythrocyte membrane protein 1 and P. falciparum erythrocyte membrane protein 3 in the host cell cytosol of P. falciparum infected erythrocytes recently provided the first direct evidence of a vesicle-mediated pathway for the trafficking of some parasite proteins to the erythrocyte membrane. The major advance in uncovering the parasite-induced secretory pathway was made by incubating infected erythrocytes with aluminium tetrafluoride, an activator of guanidine triphosphate-binding proteins, which resulted in the accumulation of the vesicles into multiple vesicle strings. These vesicle complexes were often associated with and closely abutted the erythrocyte membrane, but were apparently prevented from fusing by the aluminium fluoride treatment, making their capture by electron microscopy possible. It appears that malaria parasites export proteins into the host cell cytosol to support a vesicle-mediated protein trafficking pathway.     

Sequential Recruitment of the Endoplasmic Reticulum and Chloroplasts for Plant Potyvirus Replication

The replication of positive-strand RNA viruses occurs in cytoplasmic membrane-bound virus replication complexes (VRCs). Depending on the virus, distinct cellular organelles such as the endoplasmic reticulum (ER), chloroplast, mitochondrion, endosome, and peroxisome are recruited for the formation of VRC-associated membranous structures. Previously, the 6,000-molecular-weight protein (6K) of plant potyviruses was shown to be an integral membrane protein that induces the formation of 6K-containing membranous vesicles at endoplasmic reticulum (ER) exit sites for potyvirus genome replication. Here, we present evidence that the 6K-induced vesicles predominantly target chloroplasts, where they amalgamate and induce chloroplast membrane invaginations. The vesicular transport pathway and actomyosin motility system are involved in the trafficking of the 6K vesicles from the ER to chloroplasts. Viral RNA, double-stranded RNA, and viral replicase components are concentrated at the 6K vesicles that associate with chloroplasts in infected cells, suggesting that these chloroplast-bound 6K vesicles are the site for potyvirus replication. Taken together, these results suggest that plant potyviruses sequentially recruit the ER and chloroplasts for their genome replication.The replication of eukaryotic positive-strand RNA viruses in infected cells is closely associated with unique virus-induced intracellular membranous vesicles (22). These membranous vesicles have been proposed to provide a scaffold for anchoring the virus replication complex (VRC), confine the process of RNA replication to a specific safeguarded cytoplasmic location, and prevent the activation of certain host defense mechanisms that can be triggered by double-stranded RNA (dsRNA) intermediates during virus replication (33, 47). Depending on the type of virus, the virus-induced membranous vesicles are derived from various intracellular organelles in the host. Many plant and animal viruses remodel and utilize the endoplasmic reticulum (ER) in VRCs (1, 6, 17, 33, 34, 36, 38, 39, 46). Other cellular organelles such as endosomes, lysomes, chloroplasts, peroxisomes, and mitochondria have also been suggest to be the replication site for togaviruses, tymoviruses, and tombusviruses, respectively (25, 27, 31). Given that the ER appears to be the site where the host cell translation machinery is hijacked for the biosynthesis of the first set of viral proteins, the subcellular location of virus replication (either in the vicinity of the ER or elsewhere) and the mechanism of transport to locations other than the ER are poorly understood.Plant potyviruses, accounting for ∼30% of known plant viruses including many agriculturally important viruses, e.g., Turnip mosaic virus (TuMV), Maize dwarf mosaic virus (MDMV), Tobacco etch virus (TEV), and Potato virus Y (PVY), are related to picornaviruses and picorna-like viruses (20, 21, 43). The potyviral genome is a single-stranded positive-sense RNA of about 10 kb in length and encodes at least 11 mature viral proteins (8, 43). Of these 11 proteins, the 6-kDa protein (designated 6K or 6K2) contains a central hydrophobic domain (35). In seminal work, Carrington and colleagues determined that 6K induces the formation of the ER-derived vesicles for TEV replication (35, 38). More recently, viral proteins required for replication and several host factors, namely, eukaryotic initiation factor (isoform) 4E, poly(A)-binding protein, eukaryotic elongation factor 1A, and heat shock cognate 70-3 protein, have been shown to associate with the TuMV 6K-induced vesicles (9, 41), raising the possibility that the potyviral 6K vesicles represent sites of viral genome replication. Furthermore, we have demonstrated that the biogenesis of the potyviral 6K vesicles occurs at COPII-accumulating ER exit sites (ERES) on the ER membrane (45). In this study, we further studied the trafficking of 6K-induced vesicles and found that the 6K-induced mobile vesicles trafficked predominantly from the ER to the periphery of chloroplasts. We show that these 6K vesicles docked on the outer chloroplast envelope and induced chloroplast invaginations. The chloroplast-associated 6K vesicles contained viral replicase components and dsRNA and were concentrated with viral RNA. We provide evidence that the early secretory pathway and actomyosin motility system were required for the trafficking of 6K vesicles from the ER to chloroplasts. These results suggest that plant potyviruses sequentially recruit the ER and chloroplasts for their genome replication.     

RNA interference analysis of Legionella in Drosophila cells: exploitation of early secretory apparatus dynamics

Legionella pneumophila translocates multiple bacterial effector proteins into host cells to direct formation of a replication vacuole for the bacterium. The emerging consensus is that formation of this compartment involves recruitment of membrane material that traffics between the endoplasmic reticulum (ER) and Golgi. To investigate this model, a targeted approach was used to knock down expression of proteins involved in membrane trafficking, using RNA interference in Drosophila cells. Surprisingly, few single knockdowns of ER-Golgi transport proteins decreased L. pneumophila replication. By analyzing double-stranded RNAs in pairs, combinations were identified that together caused defects in intracellular replication, consistent with the model that membrane traffic funnels into the replication vacuole from multiple sources. In particular, simultaneous depletion of the intermediate compartment and Golgi-tethering factor transport protein particle together with the ER SNARE protein Sec22 reduced replication efficiency, indicating that introduction of lesions at distinct sites in the secretory system reduces replication efficiency. In contrast to knockdowns in secretory traffic, which required multiple simultaneous hits, knockdown of single cytosolic components of ER-associated degradation, including Cdc48/p97 and associated cofactors, was sufficient to inhibit intracellular replication. The requirement for the Cdc48/p97 complex was conserved in mammalian cells, in which replication vacuoles showed intense recruitment of ubiquitinated proteins, the preferred substrates of Cdc48/p97. This complex promoted dislocation of both ubiquitinated proteins and bacterial effectors from the replication vacuole, consistent with the model that maintenance of high-level replication requires surveillance of the vacuole surface. This work demonstrates that L. pneumophila has the ability to gain access to multiple sites in the secretory system and provides the first evidence for a role of the Cdc48/p97 complex in promoting intracellular replication of pathogens and maintenance of replication vacuoles.     

A Critical Role of a Cellular Membrane Traffic Protein in Poliovirus RNA Replication

Replication of many RNA viruses is accompanied by extensive remodeling of intracellular membranes. In poliovirus-infected cells, ER and Golgi stacks disappear, while new clusters of vesicle-like structures form sites for viral RNA synthesis. Virus replication is inhibited by brefeldin A (BFA), implicating some components(s) of the cellular secretory pathway in virus growth. Formation of characteristic vesicles induced by expression of viral proteins was not inhibited by BFA, but they were functionally deficient. GBF1, a guanine nucleotide exchange factor for the small cellular GTPases, Arf, is responsible for the sensitivity of virus infection to BFA, and is required for virus replication. Knockdown of GBF1 expression inhibited virus replication, which was rescued by catalytically active protein with an intact N-terminal sequence. We identified a mutation in GBF1 that allows growth of poliovirus in the presence of BFA. Interaction between GBF1 and viral protein 3A determined the outcome of infection in the presence of BFA.     

Mechanisms and functions of guanylate‐binding proteins and related interferon‐inducible GTPases: Roles in intracellular lysis of pathogens

Guanylate‐binding proteins (GBPs) are a group interferon‐inducible GTPases within the constellation of the dynamin GTPase superfamily. These proteins restrict the replication of intracellular pathogens in both immune and non‐immune cells. GBPs and their related family members immunity‐related GTPases target and lyse the membrane of the pathogen‐containing vacuole, destroying the residential niche of vacuolar protozoal and bacterial pathogens. They also prevent virion infectivity and target replication complexes of ribonucleic acid viruses. The exciting concept that GBPs and immunity‐related GTPases can directly target the membrane of bacteria and protozoa has emerged. Rupture and lysis of the pathogen membrane mediates liberation of concealed microbial ligands for activation of innate immune sensing pathways and the inflammasome. Further studies have demonstrated a capacity of GBPs to recruit additional antimicrobial factors, highlighting the complexity of the molecular mechanisms involved in pathogen killing. In this mini‐review, we discuss recent advances describing the localisation and functions of GBPs on the host and pathogen membrane. We also highlight unresolved questions related to the regulation of GBPs in cell‐autonomous immunity to intracellular pathogens.     

How Viruses Hijack the ERAD Tuning Machinery

An essential step during the intracellular life cycle of many positive-strand RNA viruses is the rearrangement of host cell membranes to generate membrane-bound replication platforms. For example, Nidovirales and Flaviviridae subvert the membrane of the endoplasmic reticulum (ER) for their replication. However, the absence of conventional ER and secretory pathway markers in virus-induced ER-derived membranes has for a long time hampered a thorough understanding of their biogenesis. Recent reports highlight the analogies between mouse hepatitis virus-, equine arteritis virus-, and Japanese encephalitis virus-induced replication platforms and ER-associated degradation (ERAD) tuning vesicles (or EDEMosomes) that display nonlipidated LC3 at their cytosolic face and segregate the ERAD factors EDEM1, OS-9, and SEL1L from the ER lumen. In this Gem, we briefly summarize the current knowledge on ERAD tuning pathways and how they might be hijacked for viral genome replication. As ERAD tuning components, such as SEL1L and nonlipidated LC3, appear to contribute to viral infection, these cellular pathways represent novel candidate drug targets to combat positive-strand RNA viruses.     

Biogenesis of cytoplasmic membranous vesicles for plant potyvirus replication occurs at endoplasmic reticulum exit sites in a COPI- and COPII-dependent manner

Single-stranded positive-sense RNA viruses induce the biogenesis of cytoplasmic membranous vesicles, where viral replication takes place. However, the mechanism underlying this characteristic vesicular proliferation remains poorly understood. Previously, a 6-kDa potyvirus membrane protein (6K) was shown to interact with the endoplasmic reticulum (ER) and to induce the formation of the membranous vesicles. In this study, the involvement of the early secretory pathway in the formation of the 6K-induced vesicles was investigated in planta. By means of live-cell imaging, it was found that the 6K protein was predominantly colocalized with Sar1, Sec23, and Sec24, which are known markers of ER exit sites (ERES). The localization of 6K at ERES was prevented by the coexpression of a dominant-negative mutant of Sar1 that disables the COPII activity or by the coexpression of a mutant of Arf1 that disrupts the COPI complex. The secretion of a soluble secretory marker targeting the apoplast was arrested at the level of the ER in cells overexpressing 6K or infected by a potyvirus. This blockage of protein trafficking out of the ER by 6K and the distribution of 6K toward the ERES may account for the aggregation of the 6K-bound vesicles. Finally, virus infection was reduced when the accumulation of 6K at ERES was inhibited by impairing either the COPI or COPII complex. Taken together, these results imply that the cellular COPI and COPII coating machineries are involved in the biogenesis of the potyvirus 6K vesicles at the ERES for viral-genome replication.     

Hijacking components of the cellular secretory pathway for replication of poliovirus RNA

Infection of cells with poliovirus induces a massive intracellular membrane reorganization to form vesicle-like structures where viral RNA replication occurs. The mechanism of membrane remodeling remains unknown, although some observations have implicated components of the cellular secretory and/or autophagy pathways. Recently, we showed that some members of the Arf family of small GTPases, which control secretory trafficking, became membrane-bound after the synthesis of poliovirus proteins in vitro and associated with newly formed membranous RNA replication complexes in infected cells. The recruitment of Arfs to specific target membranes is mediated by a group of guanine nucleotide exchange factors (GEFs) that recycle Arf from its inactive, GDP-bound state to an active GTP-bound form. Here we show that two different viral proteins independently recruit different Arf GEFs (GBF1 and BIG1/2) to the new structures that support virus replication. Intracellular Arf-GTP levels increase approximately 4-fold during poliovirus infection. The requirement for these GEFs explains the sensitivity of virus growth to brefeldin A, which can be rescued by the overexpression of GBF1. The recruitment of Arf to membranes via specific GEFs by poliovirus proteins provides an important clue toward identifying cellular pathways utilized by the virus to form its membranous replication complex.     

African swine fever virus causes microtubule-dependent dispersal of the trans-golgi network and slows delivery of membrane protein to the plasma membrane

Viral interference with secretory cargo is a common mechanism for pathogen immune evasion. Selective down regulation of critical immune system molecules such as major histocompatibility complex (MHC) proteins enables pathogens to mask themselves from their host. African swine fever virus (ASFV) disrupts the trans-Golgi network (TGN) by altering the localization of TGN46, an organelle marker for the distal secretory pathway. Reorganization of membrane transport components may provide a mechanism whereby ASFV can disrupt the correct secretion and/or cell surface expression of host proteins. In the study reported here, we used the tsO45 temperature-sensitive mutant of the G protein of vesicular stomatitis virus to show that ASFV significantly reduces the rate at which the protein is delivered to the plasma membrane. This is linked to a general reorganization of the secretory pathway during infection and a specific, microtubule-dependent disruption of structural components of the TGN. Golgin p230 and TGN46 are separated into distinct vesicles, whereupon TGN46 is depleted. These data suggest that disruption of the TGN by ASFV can slow membrane traffic during viral infection. This may be functionally important because infection of macrophages with virulent isolates of ASFV increased the expression of MHC class I genes, but there was no parallel increase in MHC class I molecule delivery to the plasma membrane.     

病毒劫持ESCRT系统促进自身复制的研究进展

内吞体分选转运复合体（endosomal sorting complex required for transport，ESCRT）系统驱动细胞的不同生命进程，包括内体分选、细胞器生物发生、囊泡运输、维持质膜完整性、细胞质分裂期间的膜裂变、有丝分裂后的核膜重组、自噬过程中吞噬孔的封闭以及包膜病毒出芽等。越来越多的证据表明，ESCRT系统能够被不同家族病毒劫持用于自身增殖。在病毒生命周期的不同阶段，病毒可以通过各种方式干扰或利用ESCRT系统介导的生理过程，最大限度地提高感染宿主的机会。此外，许多逆转录病毒和RNA病毒蛋白具有“晚期结构域”基序，可招募宿主ESCRT亚基蛋白帮助病毒内吞、运输、复制、出芽以及外排。因此，病毒“晚期结构域”基序和ESCRT亚基蛋白可能是病毒感染治疗中具有广泛应用前景的药物靶点。本文重点综述了ESCRT系统的组成及功能，ESCRT亚基和病毒“晚期结构域”基序对病毒复制的影响以及ESCRT介导的抗病毒作用，以期为抗病毒药物的开发和利用提供参考。     

Traffic COPs and the formation of vesicle coats

Forward and retrograde trafficking of secretory proteins between the endoplasmic reticulum and the Golgi apparatus is driven by two biochemically distinct vesicle coats, COPI and COPII. Assembly of the coats on their target membranes is thought to provide the driving force for membrane deformation and the selective packaging of cargo and targeting molecules into nascent transport vesicles. This review describes our current knowledge on these issues and discusses how the two coats may be differentially targeted and assembled to achieve protein sorting and transport within the early secretory pathway.