Determinants for membrane association and permeabilization of the coxsackievirus 2B protein and the identification of the Golgi complex as the target organelle

Enterovirus A71 2B Inhibits Interferon-Activated JAK/STAT Signaling by Inducing Caspase-3-Dependent Karyopherin-α1 Degradation

Enterovirus A71 (EV-A71) is a major pathogen that causes the hand, foot, and mouth disease, which could be fatal with neurological complications in children. The underlying mechanism for the severe pathogenicity remains obscure, but impaired or aberrant innate immunity is considered to play a key role in viral pathogenesis. We reported previously that EV-A71 suppressed type I interferon (IFN) responses by inducing degradation of karyopherin-α1 (KPNA1), a component of the p-STAT1/2 complex. In this report, we showed that 2B, a non-structural protein of EV-A71, was critical to the suppression of the IFN-α-induced type I response in infected cells. Among viral proteins, 2B was the only one that was involved in the degradation of KPNA1, which impeded the formation of the p-STAT1/2/KPNA1 complex and blocked the translocation of p-STAT1/2 into the nucleus upon IFN-α stimulation. Degradation of KPNA1 induced by 2B can be inhibited in the cells pre-treated with Z-DEVD-FMK, a caspase-3 inhibitor, or siRNA targeting caspase-3, indicating that 2B-induced degradation of KPNA1 was caspase-3 dependent. The mechanism by which 2B functioned in the dysregulation of the IFN signaling was analyzed and a putative hydrophilic domain (H1) in the N-terminus of 2B was characterized to be critical for the release of cytochrome c into the cytosol for the activation of pro-caspase-3. We generated an EV-A71 infectious clone (rD1), which was deficient of the H1 domain. In rD1-infected cells, degradation of KPNA1 was relieved and the infected cells were more sensitive to IFN-α, leading to decreased viral replication, in comparison to the cells infected with the virus carrying a full length 2B. Our findings demonstrate that EV-A71 2B protein plays an important role in dysregulating JAK-STAT signaling through its involvement in promoting caspase-3 dependent degradation of KPNA1, which represents a novel strategy employed by EV-A71 to evade host antiviral innate immunity.

Enterovirus D: A Small but Versatile Species

Enteroviruses (EVs) from the D species are the causative agents of a diverse range of infectious diseases in spite of comprising only five known members. This small clade has a diverse host range and tissue tropism. It contains types infecting non-human primates and/or humans, and for the latter, they preferentially infect the eye, respiratory tract, gastrointestinal tract, and nervous system. Although several Enterovirus D members, in particular EV-D68, have been associated with neurological complications, including acute myelitis, there is currently no effective treatment or vaccine against any of them. This review highlights the peculiarities of this viral species, focusing on genome organization, functional elements, receptor usage, and pathogenesis.

A comparative analysis of parechovirus protein structures with other picornaviruses

Parechoviruses belong to the genus Parechovirus within the family Picornaviridae and are non-enveloped icosahedral viruses with a single-stranded RNA genome. Parechoviruses include human and animal pathogens classified into six species. Those that infect humans belong to the Parechovirus A species and can cause infections ranging from mild gastrointestinal or respiratory illness to severe neonatal sepsis. There are no approved antivirals available to treat parechovirus (nor any other picornavirus) infections. In this parechovirus review, we focus on the cleaved protein products resulting from the polyprotein processing after translation comparing and contrasting their known or predicted structures and functions to those of other picornaviruses. The review also includes our original analysis from sequence and structure prediction. This review highlights significant structural differences between parechoviral and other picornaviral proteins, suggesting that parechovirus drug development should specifically be directed to parechoviral targets.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

Coxsackievirus B3-Its Potential as an Oncolytic Virus

Oncolytic virotherapy represents one of the most advanced strategies to treat otherwise untreatable types of cancer. Despite encouraging developments in recent years, the limited fraction of patients responding to therapy has demonstrated the need to search for new suitable viruses. Coxsackievirus B3 (CVB3) is a promising novel candidate with particularly valuable features. Its entry receptor, the coxsackievirus and adenovirus receptor (CAR), and heparan sulfate, which is used for cellular entry by some CVB3 variants, are highly expressed on various cancer types. Consequently, CVB3 has broad anti-tumor activity, as shown in various xenograft and syngeneic mouse tumor models. In addition to direct tumor cell killing the virus induces a strong immune response against the tumor, which contributes to a substantial increase in the efficiency of the treatment. The toxicity of oncolytic CVB3 in healthy tissues is variable and depends on the virus strain. It can be abrogated by genetic engineering the virus with target sites of microRNAs. In this review, we present an overview of the current status of the development of CVB3 as an oncolytic virus and outline which steps still need to be accomplished to develop CVB3 as a therapeutic agent for clinical use in cancer treatment.

Enterovirus Infection Induces Massive Recruitment of All Isoforms of Small Cellular Arf GTPases to the Replication Organelles

Enterovirus replication requires the cellular protein GBF1, a guanine nucleotide exchange factor for small Arf GTPases. When activated, Arfs associate with membranes, where they regulate numerous steps of membrane homeostasis. The requirement for GBF1 implies that Arfs are important for replication, but which of the different Arfs function(s) during replication remains poorly understood. Here, we established cell lines expressing each of the human Arfs fused to a fluorescent tag and investigated their behavior during enterovirus infection. Arf1 was the first to be recruited to the replication organelles, where it strongly colocalized with the viral antigen 2B and mature virions but not double-stranded RNA. By the end of the infectious cycle, Arf3, Arf4, Arf5, and Arf6 were also concentrated on the replication organelles. Once on the replication membranes, all Arfs except Arf3 were no longer sensitive to inhibition of GBF1, suggesting that in infected cells they do not actively cycle between GTP- and GDP-bound states. Only the depletion of Arf1, but not other class 1 and 2 Arfs, significantly increased the sensitivity of replication to GBF1 inhibition. Surprisingly, depletion of Arf6, a class 3 Arf, normally implicated in plasma membrane events, also increased the sensitivity to GBF1 inhibition. Together, our results suggest that GBF1-dependent Arf1 activation directly supports the development and/or functioning of the replication complexes and that Arf6 plays a previously unappreciated role in viral replication. Our data reveal a complex pattern of Arf activation in enterovirus-infected cells that may contribute to the resilience of viral replication in different cellular environments.IMPORTANCE Enteroviruses include many known and emerging pathogens, such as poliovirus, enteroviruses 71 and D68, and others. However, licensed vaccines are available only against poliovirus and enterovirus 71, and specific anti-enterovirus therapeutics are lacking. Enterovirus infection induces the massive remodeling of intracellular membranes and the development of specialized domains harboring viral replication complexes, replication organelles. Here, we investigated the roles of small Arf GTPases during enterovirus infection. Arfs control distinct steps in intracellular membrane traffic, and one of the Arf-activating proteins, GBF1, is a cellular factor required for enterovirus replication. We found that all Arfs expressed in human cells, including Arf6, normally associated with the plasma membrane, are recruited to the replication organelles and that Arf1 appears to be the most important Arf for enterovirus replication. These results document the rewiring of the cellular membrane pathways in infected cells and may provide new ways of controlling enterovirus infections.

Inhibition of the integrated stress response by viral proteins that block p-eIF2-eIF2B association

Eukaryotic cells, when exposed to environmental or internal stress, activate the integrated stress response (ISR) to restore homeostasis and promote cell survival. Specific stress stimuli prompt dedicated stress kinases to phosphorylate eukaryotic initiation factor 2 (eIF2). Phosphorylated eIF2 (p-eIF2) in turn sequesters the eIF2-specific guanine exchange factor eIF2B to block eIF2 recycling, thereby halting translation initiation and reducing global protein synthesis. To circumvent stress-induced translational shutdown, viruses encode ISR antagonists. Those identified so far prevent or reverse eIF2 phosphorylation. We now describe two viral proteins-one from a coronavirus and the other from a picornavirus-that have independently acquired the ability to counteract the ISR at its very core by acting as a competitive inhibitor of p-eIF2-eIF2B interaction. This allows continued formation of the eIF2-GTP-Met-tRNAi ternary complex and unabated global translation at high p-eIF2 levels that would otherwise cause translational arrest. We conclude that eIF2 and p-eIF2 differ in their interaction with eIF2B to such effect that p-eIF2-eIF2B association can be selectively inhibited.

Viral Generated Inter-Organelle Contacts Redirect Lipid Flux for Genome Replication

Positive-stranded RNA viruses extensively remodel host cell architecture to enable viral replication. Here, we examined the poorly understood formation of specialized membrane compartments that are critical sites for the synthesis of the viral genome. We show that the replication compartments (RCs) of enteroviruses are created through novel membrane contact sites that recruit host lipid droplets (LDs) to the RCs. Viral proteins tether the RCs to the LDs and interact with the host lipolysis machinery to enable transfer of fatty acids from LDs, thereby providing lipids essential for RC biogenesis. Inhibiting the formation of the membrane contact sites between LDs and RCs or inhibition of the lipolysis pathway disrupts RC biogenesis and enterovirus replication. Our data illuminate mechanistic and functional aspects of organelle remodeling in viral infection and establish that pharmacological targeting of contact sites linking viral and host compartments is a potential strategy for antiviral development.

Biological Function and Application of Picornaviral 2B Protein: A New Target for Antiviral Drug Development

Picornaviruses are associated with acute and chronic diseases. The clinical manifestations of infections are often mild, but infections may also lead to respiratory symptoms, gastroenteritis, myocarditis, meningitis, hepatitis, and poliomyelitis, with serious impacts on human health and economic losses in animal husbandry. Thus far, research on picornaviruses has mainly focused on structural proteins such as VP1, whereas the non-structural protein 2B, which plays vital roles in the life cycle of the viruses and exhibits a viroporin or viroporin-like activity, has been overlooked. Viroporins are viral proteins containing at least one amphipathic α-helical structure, which oligomerizes to form transmembrane hydrophilic pores. In this review, we mainly summarize recent research data on the viroporin or viroporin-like activity of 2B proteins, which affects the biological function of the membrane, regulates cell death, and affects the host immune response. Considering these mechanisms, the potential application of the 2B protein as a candidate target for antiviral drug development is discussed, along with research challenges and prospects toward realizing a novel treatment strategy for picornavirus infections.

The Saffold Virus-Penang 2B and 3C Proteins, but not the L Protein, Induce Apoptosis in HEp-2 and Vero Cells

Our previous work has shown that Saffold virus (SAFV) induced several rodent and primate cell lines to undergo apoptosis (Xu et al. in Emerg Microb Infect 3:1-8, 2014), but the essential viral proteins of SAFV involved in apoptotic activity lack study. In this study, we individually transfected the viral proteins of SAFV into HEp-2 and Vero cells to assess their ability to induce apoptosis, and found that the 2B and 3C proteins are proapoptotic. Further investigation indicated the transmembrane domain of the 2B protein is essential for the apoptotic activity and tetramer formation of the 2B protein. Our research provides clues for the possible mechanisms of apoptosis induced by SAFV in different cell lines. It also opens up new directions to study viral proteins (the 2B, 3C protein), and sets the stage for future exploration of any possible link between SAFV, inclusive of its related uncultivable genotypes, and multiple sclerosis.

Non-Structural Protein 2B of Human Rhinovirus 16 Activates Both PERK and ATF6 Rather Than IRE1 to Trigger ER Stress

To understand the underlying mechanisms of endoplasmic reticulum (ER) stress caused by human rhinovirus (HRV) 16 and non-structural transmembrane protein 2B, the expressions of ER chaperone glucose-regulated protein 78 (GRP78) and three signal transduction pathways, including protein kinase RNA-like ER kinase (PERK), activating transcription factor 6 (ATF6) and inositol-requiring enzyme 1 (IRE1), were evaluated after HRV16 infection and 2B gene transfection. Our results showed that both HRV16 infection and 2B gene transfection increased the expression of ER chaperone GRP78, and induced phosphorylation of PERK and cleavage of ATF6 in a time-dependent manner. Our data also revealed that the HRV16 2B protein was localized to the ER membrane. However, both HRV16 infection and HRV16 2B gene transfection did not induce ER stress through the IRE1 pathway. Moreover, our results showed that apoptosis occurred in H1-HeLa cells infected with HRV16 or transfected with 2B gene accompanied with increased expression of CHOP and cleaved caspase-3. Taken together, non-structural protein 2B of HRV16 induced an ER stress response through the PERK and ATF6 pathways rather than the IRE1 pathway.

Recent Progress on Functional Genomics Research of Enterovirus 71

Enterovirus 71 (EV71) is one of the main pathogens that causes hand-foot-and-mouth disease (HFMD). HFMD caused by EV71 infection is mostly self-limited; however, some infections can cause severe neurological diseases, such as aseptic meningitis, brain stem encephalitis, and even death. There are still no effective clinical drugs used for the prevention and treatment of HFMD. Studying EV71 protein function is essential for elucidating the EV71 replication process and developing anti-EV71 drugs and vaccines. In this review, we summarized the recent progress in the studies of EV71 non-coding regions (5' UTR and 3' UTR) and all structural and nonstructural proteins, especially the key motifs involving in viral infection, replication, and immune regulation. This review will promote our understanding of EV71 virus replication and pathogenesis, and will facilitate the development of novel drugs or vaccines to treat EV71.

Molecular Characterization of the Viroporin Function of Foot-and-Mouth Disease Virus Nonstructural Protein 2B

Nonstructural protein 2B of foot-and-mouth disease (FMD) virus (FMDV) is comprised of a small, hydrophobic, 154-amino-acid protein. Structure-function analyses demonstrated that FMDV 2B is an ion channel-forming protein. Infrared spectroscopy measurements using partially overlapping peptides that spanned regions between amino acids 28 and 147 demonstrated the adoption of helical conformations in two putative transmembrane regions between residues 60 and 78 and between residues 119 and 147 and a third transmembrane region between residues 79 and 106, adopting a mainly extended structure. Using synthetic peptides, ion channel activity measurements in planar lipid bilayers and imaging of single giant unilamellar vesicles (GUVs) revealed the existence of two sequences endowed with membrane-porating activity: one spanning FMDV 2B residues 55 to 82 and the other spanning the C-terminal region of 2B from residues 99 to 147. Mapping the latter sequence identified residues 119 to 147 as being responsible for the activity. Experiments to assess the degree of insertion of the synthetic peptides in bilayers and the inclination angle adopted by each peptide regarding the membrane plane normal confirm that residues 55 to 82 and 119 to 147 of 2B actively insert as transmembrane helices. Using reverse genetics, a panel of 13 FMD recombinant mutant viruses was designed, which harbored nonconservative as well as alanine substitutions in critical amino acid residues in the area between amino acid residues 28 and 147. Alterations to any of these structures interfered with pore channel activity and the capacity of the protein to permeabilize the endoplasmic reticulum (ER) to calcium and were lethal for virus replication. Thus, FMDV 2B emerges as the first member of the viroporin family containing two distinct pore domains.IMPORTANCE FMDV nonstructural protein 2B is able to insert itself into cellular membranes to form a pore. This pore allows the passage of ions and small molecules through the membrane. In this study, we were able to show that both current and small molecules are able to pass though the pore made by 2B. We also discovered for the first time a virus with a pore-forming protein that contains two independent functional pores. By making mutations in our infectious clone of FMDV, we determined that mutations in either pore resulted in nonviable virus. This suggests that both pore-forming functions are independently required during FMDV infection.

Identification and Molecular Characterization of the Chloroplast Targeting Domain of Turnip yellow mosaic virus Replication Proteins

Turnip yellow mosaic virus (TYMV) is a positive-strand RNA virus infecting plants. The TYMV 140K replication protein is a key organizer of viral replication complex (VRC) assembly, being responsible for recruitment of the viral polymerase and for targeting the VRCs to the chloroplast envelope where viral replication takes place. However, the structural requirements determining the subcellular localization and membrane association of this essential viral protein have not yet been defined. In this study, we investigated determinants for the in vivo chloroplast targeting of the TYMV 140K replication protein. Subcellular localization studies of deletion mutants identified a 41-residue internal sequence as the chloroplast targeting domain (CTD) of TYMV 140K; this sequence is sufficient to target GFP to the chloroplast envelope. The CTD appears to be located in the C-terminal extension of the methyltransferase domain-a region shared by 140K and its mature cleavage product 98K, which behaves as an integral membrane protein during infection. We predicted the CTD to fold into two amphipathic α-helices-a folding that was confirmed in vitro by circular dichroism spectroscopy analyses of a synthetic peptide. The importance for subcellular localization of the integrity of these amphipathic helices, and the function of 140K/98K, was demonstrated by performing amino acid substitutions that affected chloroplast targeting, membrane association and viral replication. These results establish a short internal α-helical peptide as an unusual signal for targeting proteins to the chloroplast envelope membrane, and provide new insights into membrane targeting of viral replication proteins-a universal feature of positive-strand RNA viruses.

Protein 2B of Coxsackievirus B3 Induces Autophagy Relying on Its Transmembrane Hydrophobic Sequences

Coxsackievirus B (CVB) belongs to Enterovirus genus within the Picornaviridae family, and it is one of the most common causative pathogens of viral myocarditis in young adults. The pathogenesis of myocarditis caused by CVB has not been completely elucidated. In CVB infection, autophagy is manipulated to facilitate viral replication. Here we report that protein 2B, one of the non-structural proteins of CVB3, possesses autophagy-inducing capability. The autophagy-inducing motif of protein 2B was identified by the generation of truncated 2B and site-directed mutagenesis. The expression of 2B alone was sufficient to induce the formation of autophagosomes in HeLa cells, while truncated 2B containing the two hydrophobic regions of the protein also induced autophagy. In addition, we demonstrated that a single amino acid substitution (56V→A) in the stem loop in between the two hydrophobic regions of protein 2B abolished the formation of autophagosomes. Moreover, we found that 2B and truncated 2B with autophagy-inducting capability were co-localized with LC3-II. This study indicates that protein 2B relies on its transmembrane hydrophobic regions to induce the formation of autophagosomes, while 56 valine residue in the stem loop of protein 2B might exert critical structural influence on its two hydrophobic regions. These results may provide new insight for understanding the molecular mechanism of autophagy triggered by CVB infection.

Targeting the Channel Activity of Viroporins

Since the discovery that certain small viral membrane proteins, collectively termed as viroporins, can permeabilize host cellular membranes and also behave as ion channels, attempts have been made to link this feature to specific biological roles. In parallel, most viroporins identified so far are virulence factors, and interest has focused toward the discovery of channel inhibitors that would have a therapeutic effect, or be used as research tools to understand the biological roles of viroporin ion channel activity. However, this paradigm is being shifted by the difficulties inherent to small viral membrane proteins, and by the realization that protein-protein interactions and other diverse roles in the virus life cycle may represent an equal, if not, more important target. Therefore, although targeting the channel activity of viroporins can probably be therapeutically useful in some cases, the focus may shift to their other functions in following years. Small-molecule inhibitors have been mostly developed against the influenza A M2 (IAV M2 or AM2). This is not surprising since AM2 is the best characterized viroporin to date, with a well-established biological role in viral pathogenesis combined the most extensive structural investigations conducted, and has emerged as a validated drug target. For other viroporins, these studies are still mostly in their infancy, and together with those for AM2, are the subject of the present review.

Enterovirus infection of human islets of Langerhans affects β-cell function resulting in disintegrated islets, decreased glucose stimulated insulin secretion and loss of Golgi structure

AIMS/HYPOTHESIS

In type 1 diabetes (T1D), most insulin-producing β cells are destroyed, but the trigger is unknown. One of the possible triggers is a virus infection and the aim of this study was to test if enterovirus infection affects glucose stimulated insulin secretion and the effect of virus replication on cellular macromolecules and organelles involved in insulin secretion.

METHODS

Isolated human islets were infected with different strains of coxsackievirus B (CVB) virus and the glucose-stimulated insulin release (GSIS) was measured in a dynamic perifusion system. Classical morphological electron microscopy, large-scale electron microscopy, so-called nanotomy, and immunohistochemistry were used to study to what extent virus-infected β cells contained insulin, and real-time PCR was used to analyze virus induced changes of islet specific genes.

RESULTS

In islets infected with CVB, GSIS was reduced in correlation with the degree of virus-induced islet disintegration. The expression of the gene encoding insulin was decreased in infected islets, whereas the expression of glucagon was not affected. Also, in islets that were somewhat disintegrated, there were uninfected β cells. Ultrastructural analysis revealed that virus particles and virus replication complexes were only present in β cells. There was a significant number of insulin granules remaining in the virus-infected β cells, despite decreased expression of insulin mRNA. In addition, no typical Golgi apparatus was detected in these cells. Exposure of islets to synthetic dsRNA potentiated glucose-stimulated insulin secretion.

CONCLUSIONS/INTERPRETATION

Glucose-stimulated insulin secretion; organelles involved in insulin secretion and gene expression were all affected by CVB replication in β cells.

Engineering Enhanced Vaccine Cell Lines To Eradicate Vaccine-Preventable Diseases: the Polio End Game

Vaccine manufacturing costs prevent a significant portion of the world's population from accessing protection from vaccine-preventable diseases. To enhance vaccine production at reduced costs, a genome-wide RNA interference (RNAi) screen was performed to identify gene knockdown events that enhanced poliovirus replication. Primary screen hits were validated in a Vero vaccine manufacturing cell line using attenuated and wild-type poliovirus strains. Multiple single and dual gene silencing events increased poliovirus titers >20-fold and >50-fold, respectively. Host gene knockdown events did not affect virus antigenicity, and clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9-mediated knockout of the top candidates dramatically improved viral vaccine strain production. Interestingly, silencing of several genes that enhanced poliovirus replication also enhanced replication of enterovirus 71, a clinically relevant virus to which vaccines are being targeted. The discovery that host gene modulation can markedly increase virus vaccine production dramatically alters mammalian cell-based vaccine manufacturing possibilities and should facilitate polio eradication using the inactivated poliovirus vaccine.

IMPORTANCE

Using a genome-wide RNAi screen, a collection of host virus resistance genes was identified that, upon silencing, increased poliovirus and enterovirus 71 production by from 10-fold to >50-fold in a Vero vaccine manufacturing cell line. This report provides novel insights into enterovirus-host interactions and describes an approach to developing the next generation of vaccine manufacturing through engineered vaccine cell lines. The results show that specific gene silencing and knockout events can enhance viral titers of both attenuated (Sabin strain) and wild-type polioviruses, a finding that should greatly facilitate global implementation of inactivated polio vaccine as well as further reduce costs for live-attenuated oral polio vaccines. This work describes a platform-enabling technology applicable to most vaccine-preventable diseases.

The C-terminal region of the non-structural protein 2B from Hepatitis A Virus demonstrates lipid-specific viroporin-like activity

Viroporins are virally encoded, membrane-active proteins, which enhance viral replication and assist in egress of viruses from host cells. The 2B proteins in the picornaviridae family are known to have viroporin-like properties, and play critical roles during virus replication. The 2B protein of Hepatitis A Virus (2B), an unusual picornavirus, is somewhat dissimilar from its analogues in several respects. HAV 2B is approximately 2.5 times the length of other 2B proteins, and does not disrupt calcium homeostasis or glycoprotein trafficking. Additionally, its membrane penetrating properties are not yet clearly established. Here we show that the membrane interacting activity of HAV 2B is localized in its C-terminal region, which contains an alpha-helical hairpin motif. We show that this region is capable of forming small pores in membranes and demonstrates lipid specific activity, which partially rationalizes the intracellular localization of full-length 2B. Using a combination of biochemical assays and molecular dynamics simulation studies, we also show that HAV 2B demonstrates a marked propensity to dimerize in a crowded environment, and probably interacts with membranes in a multimeric form, a hallmark of other picornavirus viroporins. In sum, our study clearly establishes HAV 2B as a bona fide viroporin in the picornaviridae family.

Mechanisms of Beta Cell Dysfunction Associated With Viral Infection

Type 1 diabetes (T1D) results from genetic predisposition and environmental factors leading to the autoimmune destruction of pancreatic beta cells. Recently, a rapid increase in the incidence of childhood T1D has been observed worldwide; this is too fast to be explained by genetic factors alone, pointing to the spreading of environmental factors linked to the disease. Enteroviruses (EVs) are perhaps the most investigated environmental agents in relationship to the pathogenesis of T1D. While several studies point to the likelihood of such correlation, epidemiological evidence in its support is inconclusive or in some instances even against it. Hence, it is still unknown if and how EVs are involved in the development of T1D. Here we review recent findings concerning the biology of EV in beta cells and the potential implications of this knowledge for the understanding of beta cell dysfunction and autoimmune destruction in T1D.

Identification of the interaction of VP1 with GM130 which may implicate in the pathogenesis of CVB3-induced acute pancreatitis

Coxsackievirus B3 (CVB3) is a causative agent of viral myocarditis, pancreatitis, and meningitis in humans. Although the susceptibility of CVB3-induced acute pancreatitis is age-dependent, the underlying mechanisms remain unclear. Here we identified the host factor Golgi matrix protein 130 (GM130) as a novel target of CVB3 during CVB3-induced acute pancreatitis. The viral protein VP1 interacted with GM130, disrupted GM130-GRASP65 complexes, and caused GM130 degradation, which may lead to disruption of the Golgi ribbon and development of acute pancreatitis in mice. Interestingly, the expression level of GM130 in mouse pancreas was age-dependent, which was nicely correlated with the age-associated susceptibility of CVB3-induced acute pancreatitis. Furthermore, interference RNA-mediated knockdown of GM130 significantly reduced CVB3 replication in HeLa cells. Taken together, the study identified GM130 as a novel target of CVB3, which may implicate in the pathogenesis of CVB3-induced acute pancreatitis.

Replication and Inhibitors of Enteroviruses and Parechoviruses

The Enterovirus (EV) and Parechovirus genera of the picornavirus family include many important human pathogens, including poliovirus, rhinovirus, EV-A71, EV-D68, and human parechoviruses (HPeV). They cause a wide variety of diseases, ranging from a simple common cold to life-threatening diseases such as encephalitis and myocarditis. At the moment, no antiviral therapy is available against these viruses and it is not feasible to develop vaccines against all EVs and HPeVs due to the great number of serotypes. Therefore, a lot of effort is being invested in the development of antiviral drugs. Both viral proteins and host proteins essential for virus replication can be used as targets for virus inhibitors. As such, a good understanding of the complex process of virus replication is pivotal in the design of antiviral strategies goes hand in hand with a good understanding of the complex process of virus replication. In this review, we will give an overview of the current state of knowledge of EV and HPeV replication and how this can be inhibited by small-molecule inhibitors.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Viroporin Activity of the Foot-and-Mouth Disease Virus Non-Structural 2B Protein

Viroporins are a family of low-molecular-weight hydrophobic transmembrane proteins that are encoded by various animal viruses. Viroporins form transmembrane pores in host cells via oligomerization, thereby destroying cellular homeostasis and inducing cytopathy for virus replication and virion release. Among the Picornaviridae family of viruses, the 2B protein encoded by enteroviruses is well understood, whereas the viroporin activity of the 2B protein encoded by the foot-and-mouth disease virus (FMDV) has not yet been described. An analysis of the FMDV 2B protein domains by computer-aided programs conducted in this study revealed that this protein may contain two transmembrane regions. Further biochemical, biophysical and functional studies revealed that the protein possesses a number of features typical of a viroporin when it is overexpressed in bacterial and mammalian cells as well as in FMDV-infected cells. The protein was found to be mainly localized in the endoplasmic reticulum (ER), with both the N- and C-terminal domains stretched into the cytosol. It exhibited cytotoxicity in Escherichia coli, which attenuated 2B protein expression. The release of virions from cells infected with FMDV was inhibited by amantadine, a viroporin inhibitor. The 2B protein monomers interacted with each other to form both intracellular and extracellular oligomers. The Ca(2+) concentration in the cells increased, and the integrity of the cytoplasmic membrane was disrupted in cells that expressed the 2B protein. Moreover, the 2B protein induced intense autophagy in host cells. All of the results of this study demonstrate that the FMDV 2B protein has properties that are also found in other viroporins and may be involved in the infection mechanism of FMDV.

Ultrastructure of the replication sites of positive-strand RNA viruses

Positive strand RNA viruses replicate in the cytoplasm of infected cells and induce intracellular membranous compartments harboring the sites of viral RNA synthesis. These replication factories are supposed to concentrate the components of the replicase and to shield replication intermediates from the host cell innate immune defense. Virus induced membrane alterations are often generated in coordination with host factors and can be grouped into different morphotypes. Recent advances in conventional and electron microscopy have contributed greatly to our understanding of their biogenesis, but still many questions remain how viral proteins capture membranes and subvert host factors for their need. In this review, we will discuss different representatives of positive strand RNA viruses and their ways of hijacking cellular membranes to establish replication complexes. We will further focus on host cell factors that are critically involved in formation of these membranes and how they contribute to viral replication.

Ion flux in the lung: virus-induced inflammasome activation

Innate immunity has a primary role in lung antimicrobial defenses. The inflammasome has evolved for this purpose and is an important surveillance system that, when triggered, fights infection and eliminates pathogens. However, there is growing evidence that the inflammasome also plays a role in the pathogenesis of acute and chronic respiratory disease. Inflammasomes contribute to both the clearance of the pathogen as well as its pathogenesis - depending on the amount of inflammation triggered. How respiratory viruses trigger inflammasome activation remains unclear. Emerging evidence shows that ion flux is responsible for triggering inflammasome activation in the lung, causing lung pathology and disease exacerbations. Viroporins, encoded by all common respiratory viruses, are responsible for the changes in intracellular ion homeostasis that modulate inflammasome activation. This is a novel mechanism by which respiratory viral infection activates inflammasomes, and identifies sensing of disturbances in intracellular ionic concentrations as a novel pathogen-recognition pathway in the lung.

Modulation of lipid synthesis and trafficking pathways by picornaviruses

Picornaviruses include rapidly replicating viruses that may complete their infectious cycle within a few hours. During this short time the massive development of viral replication organelles completely transforms the cellular membrane landscape. The origin of these structures and mechanism(s) underlying their rapid expansion are still poorly understood. Recent studies revealed profound reorganization of synthesis and distribution of major structural lipids in infected cells. These data show that the lipid composition of the replication organelles is significantly different from that of preexisting cellular membranes. The apparently universal activation of specific lipid syntheses by diverse picornaviruses and at least some other (+)RNA viruses suggests that the mechanism(s) of replication organelle development may be conserved among distantly related viruses.

Topology and biological function of enterovirus non-structural protein 2B as a member of the viroporin family

Viroporins are a group of transmembrane proteins with low molecular weight that are encoded by many animal viruses. Generally, viroporins are composed of 50–120 amino acid residues and possess a minimum of one hydrophobic region that interacts with the lipid bilayer and leads to dispersion. Viroporins are involved in destroying the morphology of host cells and disturbing their biological functions to complete the life cycle of the virus. The 2B proteins encoded by enteroviruses, which belong to the family Picornaviridae, can form transmembrane pores by oligomerization, increase the permeability of plasma membranes, disturb the homeostasis of calcium in cells, induce apoptosis, and cause autophagy; these abilities are shared among viroporins. The present paper introduces the structure and biological characteristics of various 2B proteins encoded by enteroviruses of the family Picornaviridae and may provide a novel idea for developing antiviral drugs.

Rhinovirus-induced calcium flux triggers NLRP3 and NLRC5 activation in bronchial cells

Human rhinoviruses have been linked with underlying lung disorders, such as asthma and chronic obstructive pulmonary disease, in children and adults. However, the mechanism of virus-induced airway inflammation is poorly understood. In this study, using virus deletion mutants and silencing for nucleotide-binding oligomerization domain-like receptors (NLRs), we show that the rhinovirus ion channel protein 2B triggers NLRP3 and NLRC5 inflammasome activation and IL-1β secretion in bronchial cells. 2B protein targets the endoplasmic reticulum and Golgi and induces Ca(2+) reduction in these organelles, thereby disturbing the intracellular calcium homeostasis. NLRP3 and NLRC5 act in a cooperative manner during the inflammasome assembly by sensing intracellular Ca(2+) fluxes and trigger IL-1β secretion. These results reveal for the first time that human rhinovirus infection in primary bronchial cells triggers inflammasome activation.

Bovine ephemeral fever rhabdovirus α1 protein has viroporin-like properties and binds importin β1 and importin 7

Bovine ephemeral fever virus (BEFV) is an arthropod-borne rhabdovirus that is classified as the type species of the genus Ephemerovirus. In addition to the five canonical rhabdovirus structural proteins (N, P, M, G, and L), the large and complex BEFV genome contains several open reading frames (ORFs) between the G and L genes (α1, α2/α3, β, and γ) encoding proteins of unknown function. We show that the 10.5-kDa BEFV α1 protein is expressed in infected cells and, consistent with previous predictions based on its structure, has the properties of a viroporin. Expression of a BEFV α1-maltose binding protein (MBP) fusion protein in Escherichia coli was observed to inhibit cell growth and increase membrane permeability to hygromycin B. Increased membrane permeability was also observed in BEFV-infected mammalian cells (but not cells infected with an α1-deficient BEFV strain) and in cells expressing a BEFV α1-green fluorescent protein (GFP) fusion protein, which was shown by confocal microscopy to localize to the Golgi complex. Furthermore, the predicted C-terminal cytoplasmic domain of α1, which contains a strong nuclear localization signal (NLS), was translocated to the nucleus when expressed independently, and in an affinity chromatography assay employing a GFP trap, the full-length α1 was observed to interact specifically with importin β1 and importin 7 but not with importin α3. These data suggest that, in addition to its function as a viroporin, BEFV α1 may modulate components of nuclear trafficking pathways, but the specific role thereof remains unclear. Although rhabdovirus accessory genes occur commonly among arthropod-borne rhabdoviruses, little is known of their functions. Here, we demonstrate that the BEFV α1 ORF encodes a protein which has the structural and functional characteristics of a viroporin. We show that α1 localizes in the Golgi complex and increases cellular permeability. We also show that BEFV α1 binds importin β1 and importin 7, suggesting that it may have a yet unknown role in modulating nuclear trafficking. This is the first functional analysis of an ephemerovirus accessory protein and of a rhabdovirus viroporin.

Antiviral drug discovery for the treatment of enterovirus 71 infections

Enterovirus 71 (EV71) is a small, positive-sense, single-stranded RNA virus in the genus Enterovirus, family Picornavirus. It causes hand, foot and mouth disease in infants and children, which in a small percentage of cases progresses to central nervous system infection, ranging from aseptic meningitis to fatal encephalitis. Sporadic cases of EV71 infection occur throughout the world, but large epidemics have occurred recently in Southeast Asia and China. There are currently no approved vaccines or antiviral therapies for the prevention or treatment of EV71 infection. This paper reviews efforts to develop antiviral therapies against EV71.

Viroporins: structure and biological functions

Viroporins are small virally encoded hydrophobic proteins that oligomerize in the membrane of host cells, leading to the formation of hydrophilic pores. This activity modifies several cellular functions, including membrane permeability, Ca²⁺ homeostasis, membrane remodelling and glycoprotein trafficking.

A classification scheme for viroporins is proposed on the basis of their structure and membrane topology. Thus, class I and class II viroporins are defined according to the number of transmembrane domains in the protein (one and two, respectively), and subclasses are defined according to their orientation in the membrane.

The main function of viroporins during viral replication is to participate in virion morphogenesis and release from host cells. In addition, some viroporins are involved in viral entry and genome replication.

The structure and activity of several viroporins, such as picornavirus protein 2B (P2B), influenza A virus matrix protein 2 (M2), hepatitis C virus p7 and HIV-1 viral protein U (Vpu), have been analysed in detail.

New members of this expanding family of viral proteins have been described, from both RNA and DNA viruses. In addition to having a common general structure, all of these new viroporins have the ability to increase membrane permeability.

Viroporins represent ideal targets to block viral replication and the spread of infection. Although a number of selective inhibitors of viroporin ion channels have been analysed in detail, optimized screening systems promise to provide new and more potent antiviral compounds in the near future.

Viroporins belong to a growing family of virally encoded proteins that form aqueous channels in the membranes of host cells. Here, Carrasco and colleagues review the structure and diverse biological functions of these proteins during the viral life cycle, as well as their potential as antiviral therapeutic targets.

Viroporins are small, hydrophobic proteins that are encoded by a wide range of clinically relevant animal viruses. When these proteins oligomerize in host cell membranes, they form hydrophilic pores that disrupt a number of physiological properties of the cell. Viroporins are crucial for viral pathogenicity owing to their involvement in several diverse steps of the viral life cycle. Thus, these viral proteins, which include influenza A virus matrix protein 2 (M2), HIV-1 viral protein U (Vpu) and hepatitis C virus p7, represent ideal targets for therapeutic intervention, and several compounds that block their pore-forming activity have been identified. Here, we review recent studies in the field that have advanced our knowledge of the structure and function of this expanding family of viral proteins.

The encephalomyocarditis virus

The encephalomyocarditis virus (EMCV) is a small non-enveloped single-strand RNA virus, the causative agent of not only myocarditis and encephalitis, but also neurological diseases, reproductive disorders and diabetes in many mammalian species. EMCV pathogenesis appears to be viral strain- and host-specific, and a better understanding of EMCV virulence factors is increasingly required. Indeed, EMCV is often used as a model for diabetes and viral myocarditis, and is also widely used in immunology as a double-stranded RNA stimulus in the study of Toll-like as well as cytosolic receptors. However, EMCV virulence and properties have often been neglected. Moreover, EMCV is able to infect humans albeit with a low morbidity. Progress on xenografts, such as pig heart transplantation in humans, has raised safety concerns that need to be explored. In this review we will highlight the biology of EMCV and all known and potential virulence factors.

Viral targets of acylguanidines

Acylguanidines are a new class of antiviral compounds with the unique ability to target both RNA polymerase and transmembrane proteins of viruses from different families. Importantly, they inhibit proteins which are not targeted by existing antiviral therapies, for example, Vpu of HIV type 1, p7 of hepatitis C virus, E of severe acute respiratory syndrome coronavirus and RNA-dependent RNA polymerase of coxsackievirus B3. BIT225, developed by Biotron Limited, is the first acylguanidine in clinical trials against HIV type 1 and hepatitis C virus. In this article we focus on the mechanisms of inhibition of viral proteins by acylguanidines.

Mitochondrial Ca(2+) and apoptosis

Mitochondria are key decoding stations of the apoptotic process. In support of this view, a large body of experimental evidence has unambiguously revealed that, in addition to the well-established function of producing most of the cellular ATP, mitochondria play a fundamental role in triggering apoptotic cell death. Various apoptotic stimuli cause the release of specific mitochondrial pro-apoptotic factors into the cytosol. The molecular mechanism of this release is still controversial, but there is no doubt that mitochondrial calcium (Ca(2+)) overload is one of the pro-apoptotic ways to induce the swelling of mitochondria, with perturbation or rupture of the outer membrane, and in turn the release of mitochondrial apoptotic factors into the cytosol. Here, we review as different proteins that participate in mitochondrial Ca(2+) homeostasis and in turn modulate the effectiveness of Ca(2+)-dependent apoptotic stimuli. Strikingly, the final outcome at the cellular level is similar, albeit through completely different molecular mechanisms: a reduced mitochondrial Ca(2+) overload upon pro-apoptotic stimuli that dramatically blunts the apoptotic response.

Membrane integration of poliovirus 2B viroporin

Virus infections can result in a variety of cellular injuries, and these often involve the permeabilization of host membranes by viral proteins of the viroporin family. Prototypical viroporin 2B is responsible for the alterations in host cell membrane permeability that take place in enterovirus-infected cells. 2B protein can be localized at the endoplasmic reticulum (ER) and the Golgi complex, inducing membrane remodeling and the blockade of glycoprotein trafficking. These findings suggest that 2B has the potential to integrate into the ER membrane, but specific information regarding its biogenesis and mechanism of membrane insertion is lacking. Here, we report experimental results of in vitro translation-glycosylation compatible with the translocon-mediated insertion of the 2B product into the ER membrane as a double-spanning integral membrane protein with an N-/C-terminal cytoplasmic orientation. A similar topology was found when 2B was synthesized in cultured cells. In addition, the in vitro translation of several truncated versions of the 2B protein suggests that the two hydrophobic regions cooperate to insert into the ER-derived microsomal membranes.

In silico investigations of possible routes of assembly of ORF 3a from SARS-CoV

ORF 3a of human severe acute respiratory syndrome corona virus (SARS-CoV) has been identified as a 274 amino acid membrane protein. When expressed in Xenopus oocytes the protein forms channels. Based on bioinformatics approaches the topology has been identified to include three transmembrane domains (TMDs). Since structural models from experiments are still lacking, computational methods can be challenged to generate such models. In this study, a ‘sequential approach’ for the assembly is proposed in which the individual TMDs are assembled one by one. This protocol is compared with a concerted protocol in which all TMDs are assembled simultaneously. The role of the loops between the TMDs during assembly of the monomers into a bundle is investigated. Molecular dynamics simulations for 20 ns are performed as a short equilibration to assess the bundle stability in a lipid environment. The results suggest that bundles are likely with the second TMD facing the putative pore. All the putative bundles show water molecules trapped within the lumen of the pore with only occasional events of complete crossing.

Viral channel forming proteins - modeling the target

The cellular and subcellular membranes encounter an important playground for the activity of membrane proteins encoded by viruses. Viral membrane proteins, similar to their host companions, can be integral or attached to the membrane. They are involved in directing the cellular and viral reproduction, the fusion and budding processes. This review focuses especially on those integral viral membrane proteins which form channels or pores, the classification to be so, modeling by in silico methods and potential drug candidates. The sequence of an isolate of Vpu from HIV-1 is aligned with host ion channels and a toxin. The focus is on the alignment of the transmembrane domains. The results of the alignment are mapped onto the 3D structures of the respective channels and toxin. The results of the mapping support the idea of a 'channel-pore dualism' for Vpu.

Ion channels as antivirus targets

Ion channels are membrane proteins that are found in a number of viruses and which are of crucial physiological importance in the viral life cycle. They have one common feature in that their action mode involves a change of electrochemical or proton gradient across the bilayer lipid membrane which modulates viral or cellular activity. We will discuss a group of viral channel proteins that belong to the viroproin family, and which participate in a number of viral functions including promoting the release of viral particles from cells. Blocking these channel-forming proteins may be "lethal", which can be a suitable and potential therapeutic strategy. In this review we discuss seven ion channels of viruses which can lead serious infections in human beings: M2 of influenza A, NB and BM2 of influenza B, CM2 of influenza C, Vpu of HIV-1, p7 of HCV and 2B of picornaviruses.

Viral proteins function as ion channels

Viral ion channels are short membrane proteins with 50–120 amino acids and play an important role either in regulating virus replication, such as virus entry, assembly and release or modulating the electrochemical balance in the subcellular compartments of host cells. This review summarizes the recent advances in viral encoded ion channel proteins (or viroporins), including PBCV-1 KcV, influenza M2, HIV-1 Vpu, HCV p7, picornavirus 2B, and coronavirus E and 3a. We focus on their function and mechanisms, and also discuss viral ion channel protein serving as a potential drug target.

Amino acid substitutions within the 2C coding sequence of Theiler's Murine Encephalomyelitis virus alter virus growth and affect protein distribution

Theiler's murine encephalomyelitis virus (TMEV) was used to investigate the distribution of P2 proteins in host cells and examine the effect of amino acid substitutions in conserved residues of the 2C protein on virus growth. The distribution of viral proteins 2B, 2C and 2BC with marker proteins of the endoplasmic reticulum (ER) and/or Golgi suggest an association with membranes of the secretory pathway. Similar results were obtained for truncated 2C and 2BC proteins with C-terminal deletions suggesting that the N-terminal region of the 2C protein is important in dictating distribution patterns. The significance of the high degree of conservation of this 2C region throughout the Picornaviridae was investigated by substituting conserved amino acid residues for alanine to create six mutant strains. Substitution mutations E(8)A, W(18)A and W(29)A abolished the ability of the virus to induce cytopathic effect (CPE) in BHK-21 cells. K(14)A, R(4)A and I(23)A delayed the onset and progression of CPE compared to the wild-type (WT) virus, and decreased virus yield. Immunofluorescence analysis of cells transiently expressing mutant 2C proteins revealed that the distribution of 2C was affected by substituting K(14), W(18) and I(23) for alanine indicating that specific conserved residues in 2C dictate protein distribution and virus growth.

Viral and host proteins involved in picornavirus life cycle

Picornaviruses cause several diseases, not only in humans but also in various animal hosts. For instance, human enteroviruses can cause hand-foot-and-mouth disease, herpangina, myocarditis, acute flaccid paralysis, acute hemorrhagic conjunctivitis, severe neurological complications, including brainstem encephalitis, meningitis and poliomyelitis, and even death. The interaction between the virus and the host is important for viral replication, virulence and pathogenicity. This article reviews studies of the functions of viral and host factors that are involved in the life cycle of picornavirus. The interactions of viral capsid proteins with host cell receptors is discussed first, and the mechanisms by which the viral and host cell factors are involved in viral replication, viral translation and the switch from translation to RNA replication are then addressed. Understanding how cellular proteins interact with viral RNA or viral proteins, as well as the roles of each in viral infection, will provide insights for the design of novel antiviral agents based on these interactions.

GBF1, a guanine nucleotide exchange factor for Arf, is crucial for coxsackievirus B3 RNA replication

The replication of enteroviruses is sensitive to brefeldin A (BFA), an inhibitor of endoplasmic reticulum-to-Golgi network transport that blocks activation of guanine exchange factors (GEFs) of the Arf GTPases. Mammalian cells contain three BFA-sensitive Arf GEFs: GBF1, BIG1, and BIG2. Here, we show that coxsackievirus B3 (CVB3) RNA replication is insensitive to BFA in MDCK cells, which contain a BFA-resistant GBF1 due to mutation M832L. Further evidence for a critical role of GBF1 stems from the observations that viral RNA replication is inhibited upon knockdown of GBF1 by RNA interference and that replication in the presence of BFA is rescued upon overexpression of active, but not inactive, GBF1. Overexpression of Arf proteins or Rab1B, a GTPase that induces GBF1 recruitment to membranes, failed to rescue RNA replication in the presence of BFA. Additionally, the importance of the interaction between enterovirus protein 3A and GBF1 for viral RNA replication was investigated. For this, the rescue from BFA inhibition of wild-type (wt) replicons and that of mutant replicons of both CVB3 and poliovirus (PV) carrying a 3A protein that is impaired in binding GBF1 were compared. The BFA-resistant GBF1-M832L protein efficiently rescued RNA replication of both wt and mutant CVB3 and PV replicons in the presence of BFA. However, another BFA-resistant GBF1 protein, GBF1-A795E, also efficiently rescued RNA replication of the wt replicons, but not that of mutant replicons, in the presence of BFA. In conclusion, this study identifies a critical role for GBF1 in CVB3 RNA replication, but the importance of the 3A-GBF1 interaction requires further study.

Origins of membrane vesicles generated during replication of positive-strand RNA viruses

Infection of cells by positive-strand RNA viruses generates large numbers of membrane vesicles that provide sites for genome replication. Vesicle formation is initiated by targeting replicase proteins to the cytosolic face of membrane-bound organelles where protein assembly induces membrane curvature. This can result in invagination into the limiting membrane of membrane compartments or induce vesicle budding into the cytoplasm. The new membranes are thought to provide a platform to concentrate proteins, lipids and nucleotides that are required for genome replication. This article describes how recent advances in cell biology and cellular imaging can reveal these structures in 3D, and begin to define how they are formed in terms of effects of specific viral proteins on specific cellular processes.

Model generation of viral channel forming 2B protein bundles from polio and coxsackie viruses

2B is a 99 amino acid membrane protein encoded by enteroviruses such as polio and coxsackie viruses with two transmembrane domains. The protein is found to make membranes of infected cells permeable. Using a computational approach which positions the models and assesses stability by molecular dynamics (MD) simulations a putative tetrameric bundle model of 2B is generated. The bundles show a pore lining motif of three lysines followed by a serine. The bundle is discussed in terms of different possible orientations of the helices in the membrane and the consequences this has on the in vivo activity of 2B.

Viral channel-forming proteins

Channel-forming proteins are found in a number of viral genomes. In some cases, their role in the viral life cycle is well understood, in some cases it needs still to be elucidated. A common theme is that their mode of action involves a change of electrochemical or proton gradient across the lipid membrane which modulates the viral or cellular activity. Blocking these proteins can be a suitable therapeutic strategy as for some viruses this may be "lethal." Besides the many biological relevant questions still to be answered, there are also many open questions concerning the biophysical side as well as structural information and the mechanism of function on a molecular level. The immanent biophysical issues are addressed and the work in the field is summarized.