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

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.

Functional analysis of picornavirus 2B proteins: effects on calcium homeostasis and intracellular protein trafficking

The family Picornaviridae consists of a large group of plus-strand RNA viruses that share a similar genome organization. The nomenclature of the picornavirus proteins is based on their position in the viral RNA genome but does not necessarily imply a conserved function of proteins of different genera. The enterovirus 2B protein is a small hydrophobic protein that, upon individual expression, is localized to the endoplasmic reticulum (ER) and the Golgi complex, reduces ER and Golgi complex Ca(2+) levels, most likely by forming transmembrane pores, and inhibits protein trafficking through the Golgi complex. At present, little is known about the function of the other picornavirus 2B proteins. Here we show that rhinovirus 2B, which is phylogenetically closely related to enterovirus 2B, shows a similar subcellular localization and function to those of enterovirus 2B. In contrast, 2B proteins of hepatitis A virus, foot-and-mouth disease virus, and encephalomyocarditis virus, all of which are more distantly related to enteroviruses, show a different localization and have little, if any, effects on Ca(2+) homeostasis and intracellular protein trafficking. Our data suggest that the 2B proteins of enterovirus and rhinovirus share the same function in virus replication, while the other picornavirus 2B proteins support the viral life cycle in a different manner. Moreover, we show that an enterovirus 2B protein that is retained in the ER is unable to modify Ca(2+) homeostasis and inhibit protein trafficking, demonstrating the importance of Golgi complex localization for its functioning.

Cytopathic mechanisms of HIV-1

The human immunodeficiency virus type 1 (HIV-1) has been intensely investigated since its discovery in 1983 as the cause of acquired immune deficiency syndrome (AIDS). With relatively few proteins made by the virus, it is able to accomplish many tasks, with each protein serving multiple functions. The Envelope glycoprotein, composed of the two noncovalently linked subunits, SU (surface glycoprotein) and TM (transmembrane glycoprotein) is largely responsible for host cell recognition and entry respectively. While the roles of the N-terminal residues of TM is well established as a fusion pore and anchor for Env into cell membranes, the role of the C-terminus of the protein is not well understood and is fiercely debated. This review gathers information on TM in an attempt to shed some light on the functional regions of this protein.

Molecular determinants of the interaction between coxsackievirus protein 3A and guanine nucleotide exchange factor GBF1

The 3A protein of coxsackievirus B3 (CVB3), a small membrane protein that forms homodimers, inhibits endoplasmic reticulum-to-Golgi complex transport. Recently, we described the underlying mechanism by showing that the CVB3 3A protein binds to and inhibits the function of GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factor 1 (Arf1), thereby interfering with Arf1-mediated COP-I recruitment. This study was undertaken to gain more insight into the molecular determinants underlying the interaction between 3A and GBF1. Here we show that 3A mutants that have lost the ability to dimerize are no longer able to bind to GBF1 and trap it on membranes. Moreover, we identify a conserved region in the N terminus of 3A that is crucial for GBF1 binding but not for 3A dimerization. Analysis of the binding domain in GBF1 showed that the extreme N terminus, the dimerization/cyclophilin binding domain, and the homology upstream of Sec7 domain are required for the interaction with 3A. In contrast to that of full-length GBF1, overexpression of a GBF1 mutant lacking its extreme N terminus failed to rescue the effects of 3A. Together, these data provide insight into the molecular requirements of the interaction between 3A and GBF1.

Coxsackievirus B3 proteins directionally complement each other to downregulate surface major histocompatibility complex class I

Picornaviruses carry a small number of proteins with diverse functions that subvert and exploit the host cell. We have previously shown that three coxsackievirus B3 (CVB3) proteins (2B, 2BC, and 3A) target the Golgi complex and inhibit protein transit. Here we investigate these effects in more detail and evaluate the distribution of major histocompatibility complex (MHC) class I molecules, which are critical mediators of the CD8(+) T-cell response. We report that concomitant with viral protein synthesis, MHC class I surface expression is rapidly downregulated during infection. However, this phenomenon may not result solely from inhibition of anterograde trafficking; we propose a new mechanism whereby the CVB3 2B and 2BC proteins upregulate the internalization of MHC class I (and possibly other surface proteins), perhaps by focusing of endocytic vesicles at the Golgi complex. Thus, our findings indicate that CVB3 carries at least three nonstructural proteins that directionally complement one another; 3A disrupts the Golgi complex to inhibit anterograde transport, while 2B and/or 2BC upregulates endocytosis, rapidly removing proteins from the cell surface. Taken together, these effects may render CVB3-infected cells invisible to CD8(+) T cells and untouchable by many antiviral effector molecules. This has important implications for immune evasion by CVB3.

Intracellular localization and effects of individually expressed human parechovirus 1 non-structural proteins

Human parechovirus 1 (HPEV-1) has many unique features compared with other picornaviruses and it has been shown that the replication complex formed during HPEV-1 infection is different from that of other picornaviruses. Here, the intracellular localization and functional effects of individually expressed HPEV-1 non-structural proteins were studied. The 2A and 3D proteins were found diffusely in the cytoplasm and nucleus of the cell. The 3A and 3AB proteins were observed to co-localize with the markers for the Golgi apparatus, whereas 2B co-localized with markers for the endoplasmic reticulum and the 2C and 2BC proteins were observed mainly on the surface of lipid droplets. The 2C protein, which has been implicated in replication-complex formation in enterovirus-infected cells, was not able to induce vesicles similar to those seen in HPEV-1-infected cells when expressed individually. However, in superinfected cells, the fusion protein was able to relocate to the virus replication complexes. Similar to other picornaviruses, HPEV-1 was found to interfere with cellular secretion, but this function could not be ascribed to any of the individually expressed non-structural proteins.

Dectin-1 Interaction with Tetraspanin CD37 Inhibits IL-6 Production

C-type lectins are pattern-recognition receptors important for pathogen binding and uptake by APCs. Evidence is accumulating that integration of incoming cellular signals in APCs is regulated by grouping of receptors and signaling molecules into organized membrane complexes, such as lipid rafts and tetraspanin microdomains. In this study, we demonstrate that C-type lectin dectin-1 functionally interacts with leukocyte-specific tetraspanin CD37. Dectin-1 and CD37 colocalize on the surface of human APCs. Importantly, macrophages of CD37-deficient (CD37(-/-)) mice express decreased dectin-1 membrane levels, due to increased dectin-1 internalization. Furthermore, transfection of CD37 into a macrophage cell line elevated endogenous dectin-1 surface expression. Although CD37 deficiency does not affect dectin-1-mediated phagocytosis, we observed a striking 10-fold increase of dectin-1-induced IL-6 production in CD37(-/-) macrophages compared with wild-type cells, despite reduced dectin-1 cell surface expression. Importantly, the observed increase in IL-6 production was specific for dectin-1, because signaling via other pattern-recognition receptors was unaffected in CD37(-/-) macrophages and because the dectin-1 ligand curdlan was used. Taken together, these findings show that tetraspanin CD37 is important for dectin-1 stabilization in APC membranes and controls dectin-1-mediated IL-6 production.

Role of the amphipathic peptide of Semliki forest virus replicase protein nsP1 in membrane association and virus replication

Semliki Forest virus RNA replication takes place in association with specific cytoplasmic vacuoles, derived from the endosomal apparatus. Of the four virus-encoded replicase proteins, nsP1 serves as the membrane anchor of the replication complex. An amphipathic peptide segment, G245STLYTESRKLLRSWHLPSV264, has been implicated in the membrane binding of nsP1. nsP1 variants with changes within the peptide were studied after protein expression and in the context of virus infection. Proteins with mutations R253E and W259A accumulated in the cytoplasm and were very poorly palmitoylated. The same mutations also drastically affected the localization of the precursor polyprotein P123, and they were lethal when introduced into the virus genome. Mutations R253A and L255A+L256A partially changed the localization of nsP1, and the respective viruses acquired compensatory changes. L255A+L256A only yielded virus encoding L255A+L256V, indicating the importance of a hydrophobic residue in the central 256 position. When fused to green fluorescent protein, the peptide was required in at least two tandem copies to effect a change in localization, but even then the fusion protein was associated with membranes in a nonspecific manner. Thus, the amphipathic peptide is a crucial element for the membrane association of nsP1 and the replication complex. It provides essential affinity for membranes, and other regions of nsP1 also appear to contribute to the localization of the protein.

Effects of picornavirus 3A Proteins on Protein Transport and GBF1-dependent COP-I recruitment

The 3A protein of the coxsackievirus B3 (CVB3), an enterovirus that belongs to the family of the picornaviruses, inhibits endoplasmic reticulum-to-Golgi transport. Recently, we elucidated the underlying mechanism by showing that CVB3 3A interferes with ADP-ribosylation factor 1 (Arf1)-dependent COP-I recruitment to membranes by binding and inhibiting the function of GBF1, a guanine nucleotide exchange factor that is required for the activation of Arf1 (E. Wessels et al., Dev. Cell 11:191-201, 2006). Here, we show that the 3A protein of poliovirus, another enterovirus, is also able to interfere with COP-I recruitment through the same mechanism. No interference with protein transport or COP-I recruitment was observed for the 3A proteins of any of the other picornaviruses tested here (human rhinovirus [HRV], encephalomyocarditis virus, foot-and-mouth disease virus, and hepatitis A virus). We show that the 3A proteins of HRV, which are the most closely related to the enteroviruses, are unable to inhibit COP-I recruitment, due to a reduced ability to bind GBF1. When the N-terminal residues of the HRV 3A proteins are replaced by those of CVB3 3A, chimeric proteins are produced that have gained the ability to bind GBF1 and, by consequence, to inhibit protein transport. These results show that the N terminus of the CVB3 3A protein is important for binding of GBF1 and its transport-inhibiting function. Taken together, our data demonstrate that the activity of the enterovirus 3A protein to inhibit GBF1-dependent COP-I recruitment is unique among the picornaviruses.

Cytobiological consequences of calcium-signaling alterations induced by human viral proteins

Since calcium-signaling regulates specific and fundamental cellular processes, it represents the ideal target of viral proteins, in order for the virus to control cellular functions and favour its persistence, multiplication and spread. A detailed analysis of reports focused on the impact of viral proteins on calcium-signaling has shown that virus-related elevations of cytosolic calcium levels allow increased viral protein expression (HIV-1, HSV-1/2), viral replication (HBx, enterovirus 2B, HTLV-1 p12(I), HHV-8, EBV), viral maturation (rotavirus), viral release (enterovirus 2B) and cell immortalization (EBV). Interestingly, virus-induced decreased cytosolic calcium levels have been found to be associated with inhibition of immune cells functions (HIV-1 Tat, HHV-8 K15, EBV LMP2A). Finally, several viral proteins are able to modulate intracellular calcium-signaling to control cell viability (HIV-1 Tat, HTLV-1 p13(II), HCV core, HBx, enterovirus 2B, HHV-8 K7). These data point out calcium-signaling as a key cellular target for viral infection and should stimulate further studies exploring new calcium-related therapeutic strategies.

Permeabilization of the plasma membrane by Ebola virus GP2

The glycoprotein (GP) of Ebola virus (EBOV) is a multifunctional protein known to play a role in virus attachment and entry, cell rounding and cytotoxicity, down-regulation of host surface proteins, and enhancement of virus assembly and budding. EBOV GP is synthesized as a precursor which is subsequently cleaved to yield two disulfide-linked subunits: GP1 (surface-exposed [SU] subunit) and GP2 (membrane-anchored [TM] subunit). We sought to determine the effect of membrane-anchored GP2 protein expression on the integrity of host cell lipid membranes. Our findings indicated that: (i) expression of GP2 enhanced membrane permeability to hygromycin-B (hyg-B), (ii) the transmembrane (TM) domain of GP2 was essential for enhanced membrane permeability, (iii) amino acids (aa) 667ALF669 within the TM region of GP2 were important for enhanced membrane permeability, and (iv) EBOV infected cells were more permeable to hyg-B than mock infected cells. Together, these data suggest that the TM region of GP2 modifies the permeability of the plasma membrane. These findings may have important implications for GP-induced cell damage and pathogenesis of EBOV infection.

The small envelope protein of porcine reproductive and respiratory syndrome virus possesses ion channel protein-like properties

The small envelope (E) protein of porcine reproductive and respiratory syndrome virus (PRRSV) is a hydrophobic 73 amino acid protein encoded in the internal open reading frame (ORF) of the bicistronic mRNA2. As a first step towards understanding the biological role of E protein during PRRSV replication, E gene expression was blocked in a full-length infectious clone by mutating the ATG translational initiation to GTG, such that the full-length mutant genomic clone was unable to synthesize the E protein. DNA transfection of PRRSV-susceptible cells with the E gene knocked-out genomic clone showed the absence of virus infectivity. P129-ΔE-transfected cells however produced virion particles in the culture supernatant, and these particles contained viral genomic RNA, demonstrating that the E protein is essential for PRRSV infection but dispensable for virion assembly. Electron microscopy suggests that the P129-ΔE virions assembled in the absence of E had a similar appearance to the wild-type particles. Strand-specific RT-PCR demonstrated that the E protein-negative, non-infectious P129-ΔE virus particles were able to enter cells but further steps of replication were interrupted. The entry of PRRSV has been suggested to be via receptor-mediated endocytosis, and lysomotropic basic compounds and known ion-channel blocking agents both inhibited PRRSV replication effectively during the uncoating process. The expression of E protein in Escherichia coli-mediated cell growth arrests and increased the membrane permeability. Cross-linking experiments in cells infected with PRRSV or transfected with E gene showed that the E protein was able to form homo-oligomers. Taken together, our data suggest that the PRRSV E protein is likely an ion-channel protein embedded in the viral envelope and facilitates uncoating of virus and release of the genome in the cytoplasm.

The coxsackievirus 2B protein increases efflux of ions from the endoplasmic reticulum and Golgi, thereby inhibiting protein trafficking through the Golgi

Coxsackievirus infection leads to a rapid reduction of the filling state of the endoplasmic reticulum (ER) and Golgi Ca2+ stores. The coxsackievirus 2B protein, a small membrane protein that localizes to the Golgi and to a lesser extent to the ER, has been proposed to play an important role in this effect by forming membrane-integral pores, thereby increasing the efflux of Ca2+ from the stores. Here, evidence is presented that supports this idea and that excludes the possibility that 2B reduces the uptake of Ca2+ into the stores. Measurement of intra-organelle-free Ca2+ in permeabilized cells revealed that the ability of 2B to reduce the Ca2+ filling state of the stores was preserved at steady ATP. Biochemical analysis in a cell-free system further showed that 2B had no adverse effect on the activity of the sarco/endoplasmic reticulum calcium ATPase, the Ca2+-ATPase that transports Ca2+ from the cytosol into the stores. To investigate whether 2B specifically affects Ca2+ homeostasis or other ion gradients, we measured the lumenal Golgi pH. Expression of 2B resulted in an increased Golgi pH, indicative for the efflux of H+ from the Golgi lumen. Together, these data support a model that 2B increases the efflux of ions from the ER and Golgi by forming membrane-integral pores. We have demonstrated that a major consequence of this activity is the inhibition of protein trafficking through the Golgi complex.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Stress-activated protein kinases are involved in coxsackievirus B3 viral progeny release

Stress-activated protein kinases (SAPKs), consisting of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), are activated upon various environmental stimuli, including viral infections. Cellular survival and death signaling events following coxsackievirus B3 (CVB3) infection have been studied in relationship to viral replication, but the role of SAPKs has not been scrutinized. In this study, we found that the phosphorylation of JNK1/2 and p38 MAPK was increased during active replication of CVB3 and that their phosphorylation was independent of CVB3-induced caspase activation or production of reactive oxygen species. The roles of these kinases in CVB3 infection were further evaluated using specific inhibitors: SP600125 for JNK1/2 and SB203580 for p38 MAPK. JNK1/2 inhibitors reduced CVB3-induced phosphorylation of activating transcription factor 2, and the p38 MAPK inhibitor reduced CVB3-induced phosphorylation of heat shock protein 27. Although inhibition of these kinases by specific inhibitors did not affect CVB3 viral protein synthesis, inhibition of p38 MAPK but not of JNK1/2 resulted in significant reduction of viral progeny release, suppression of CVB3-induced cell death, and blockage of CVB3-induced caspase-3 activation in infected cells. We conclude that SAPK pathways play critical roles in the life cycle of CVB3, particularly in viral progeny release.

Subcellular localization and membrane association of SARS-CoV 3a protein

SARS-CoV 3a protein was a unique protein of SARS coronavirus (SARS-CoV), which was identified in SARS-CoV infected cells and SARS patients’ specimen. Recent studies revealed that 3a could interact specifically with many SARS-CoV structural proteins, such as M, E and S protein. Expressed 3a protein was reported to localize to Golgi complex in SARS-CoV infected cells. In this study, it was shown that 3a protein was mainly located in Golgi apparatus with different tags at N- or C-terminus. The localization pattern was similar in different transfected cells. With the assay of truncated 3a protein, it was shown that 3a might contain three transmembrane regions, and the second or third region was properly responsible for Golgi localization. By ultra-centrifugation experiment with different extraction buffers, it was confirmed that 3a was an integral membrane protein and embedded in the phospholipid bilayer. Immunofluorescence assay indicated that 3a was co-localized with M protein in Golgi complex in co-transfected cells. These results provide a new insight for further study of the 3a protein on the pathogenesis of SARS-CoV.

Amino acid changes in proteins 2B and 3A mediate rhinovirus type 39 growth in mouse cells

Many steps of viral replication are dependent on the interaction of viral proteins with host cell components. To identify rhinovirus proteins involved in such interactions, human rhinovirus 39 (HRV39), a virus unable to replicate in mouse cells, was adapted to efficient growth in mouse cells producing the viral receptor ICAM-1 (ICAM-L cells). Amino acid changes were identified in the 2B and 3A proteins of the adapted virus, RV39/L. Changes in 2B were sufficient to permit viral growth in mouse cells; however, changes in both 2B and 3A were required for maximal viral RNA synthesis in mouse cells. Examination of infected HeLa cells by electron microscopy demonstrated that human rhinoviruses induced the formation of cytoplasmic membranous vesicles, similar to those observed in cells infected with other picornaviruses. Vesicles were also observed in the cytoplasm of HRV39-infected mouse cells despite the absence of viral RNA replication. Synthesis of picornaviral nonstructural proteins 2C, 2BC, and 3A is known to be required for formation of membranous vesicles. We suggest that productive HRV39 infection is blocked in ICAM-L cells at a step posttranslation and prior to the formation of a functional replication complex. The observation that changes in HRV39 2B and 3A proteins lead to viral growth in mouse cells suggests that one or both of these proteins interact with host cell proteins to promote viral replication.

A proline-rich region in the coxsackievirus 3A protein is required for the protein to inhibit endoplasmic reticulum-to-golgi transport

The ability of the 3A protein of coxsackievirus B (CVB) to inhibit protein secretion was investigated for this study. Here we show that the ectopic expression of CVB 3A blocked the transport of both the glycoprotein of vesicular stomatitis virus, a membrane-bound secretory marker, and the alpha-1 protease inhibitor, a luminal secretory protein, at a step between the endoplasmic reticulum (ER) and the Golgi complex. CVB 3A contains a conserved proline-rich region in its N terminus. The importance of this proline-rich region was investigated by introducing Pro-to-Ala substitutions. The mutation of Pro19 completely abolished the ability of 3A to inhibit ER-to-Golgi transport. The mutation of Pro14, Pro17, or Pro20 also impaired this ability, but to a lesser extent. The mutation of Pro18 had no effect. We also investigated the possible importance of this proline-rich region for the function of 3A in viral RNA replication. To this end, we introduced the Pro-to-Ala mutations into an infectious cDNA clone of CVB3. The transfection of cells with in vitro-transcribed RNAs of these clones gave rise to mutant viruses that replicated with wild-type characteristics. We concluded that the proline-rich region in CVB 3A is required for its ability to inhibit ER-to-Golgi transport, but not for its function in viral RNA replication. The functional relevance of the proline-rich region is discussed in light of the proposed structural model of 3A.

Requirement of the vesicular system for membrane permeabilization by Sindbis virus

The vast majority of animal viruses enhance membrane permeability at two moments of infection. Herein, we describe that the entry of Sindbis virus (SV) in BHK cells promotes the co-entry of the macromolecule alpha-sarcin into the cytoplasm, thereby blocking translation. At a later stage, this protein toxin cannot enter the cell, while low molecular weight compounds, such as hygromycin B, readily pass through the plasma membrane of Sindbis virus-infected BHK cells. To unveil the participation of the different Sindbis virus structural proteins in late permeabilization, transfection experiments with each late gene by separate have been carried out. Our findings indicate that 6K is the main determinant that enhances membrane permeabilization. The co-expression of both viral glycoproteins employing a Sindbis virus variant that lacks the entire 6K gene partly modifies membrane permeability. Brefeldin A, a macrolide antibiotic that interferes with the proper functioning of the vesicular system, hampers the induction of membrane leakiness without significantly affecting viral protein synthesis. On the other hand, the flavone compound Ro-090179 also diminishes the entry of hygromycin B, while bafilomycin A1 or nocodazole have no effect. These data reveal the requirement of the vesicular system for late viral membrane permeabilization.

Enterovirus protein 2B po(u)res out the calcium: a viral strategy to survive?

Enteroviruses modify several cellular functions to ensure efficient replication. However, some of these alterations can trigger a defensive apoptotic host-cell program. To prevent premature abortion of their productive cycle, enteroviruses have developed anti-apoptotic countermeasures. Here, we discuss recent evidence that the enterovirus 2B protein exerts an anti-apoptotic activity that is related to its ability to form pores in endoplasmic reticulum (ER) and Golgi membranes, thereby reducing their Ca(2+) content and perturbing ER-mitochondrial Ca(2+) signaling.

Viral RNA replication in association with cellular membranes

All plus-strand RNA viruses replicate in association with cytoplasmic membranes of infected cells. The RNA replication complex of many virus families is associated with the endoplasmic reticulum membranes, for example, picorna-, flavi-, arteri-, and bromoviruses. However, endosomes and lysosomes (togaviruses), peroxisomes and chloroplasts (tombusviruses), and mitochondria (nodaviruses) are also used as sites for RNA replication. Studies of individual nonstructural proteins, the virus-specific components of the RNA replicase, have revealed that the replication complexes are associated with the membranes and targeted to the respective organelle by the ns proteins rather than RNA. Many ns proteins have hydrophobic sequences and may transverse the membrane like polytopic integral membrane proteins, whereas others interact with membranes monotopically. Hepatitis C virus ns proteins offer examples of polytopic transmembrane proteins (NS2, NS4B), a “tip-anchored” protein attached to the membrane by an amphipathic α-helix (NS5A) and a “tail-anchored” posttranslationally inserted protein (NS5B). Semliki Forest virus nsP1 is attached to the plasma membrane by a specific binding peptide in the middle of the protein, which forms an amphipathic α-helix. Interaction of nsP1 with membrane lipids is essential for its capping enzyme activities. The other soluble replicase proteins are directed to the endo-lysosomal membranes only as part of the initial polyprotein. Poliovirus ns proteins utilize endoplasmic reticulum membranes from which vesicles are released in COPII coats. However, these vesicles are not directed to the normal secretory pathway, but accumulate in the cytoplasm. In many cases the replicase proteins induce membrane invaginations or vesicles, which function as protective environments for RNA replication.

The NS3 protein of bluetongue virus exhibits viroporin-like properties

Viroporins compose a group of small hydrophobic transmembrane proteins that can form hydrophilic pores through lipid bilayers. Viroporins have been implicated in promoting virus release from infected cells and in affecting cellular functions including protein trafficking and membrane permeability. Nonstructural protein 3 (NS3) of bluetongue virus has been shown previously to be important for efficient release of newly made virions from infected cells. In this report, we demonstrate that NS3 possesses properties commonly associated with viroporins. Our findings indicate that: (i) NS3 localizes to the Golgi apparatus and plasma membrane in transfected cells, (ii) NS3 can homo-oligomerize in transfected cells, (iii) targeting of NS3 to the Golgi apparatus and plasma membrane correlates with the enhanced permeability of cells to the translation inhibitor hygromycin B (hyg-B), (iv) amino acids 118-148 comprising transmembrane region 1 (TM1) of NS3 are critical for Golgi targeting and hyg-B permeability, and (v) deletion of amino acids 156-181 comprising transmembrane region 2 (TM2) of NS3 has little to no affect on Golgi targeting and hyg-B permeability. These viroporin-like properties may contribute to the role of NS3 in virus release and may have important implications for pathogenicity of bluetongue virus infections.

The coxsackievirus 2B protein suppresses apoptotic host cell responses by manipulating intracellular Ca2+ homeostasis

Enteroviruses, small cytolytic RNA viruses, confer an antiapoptotic state to infected cells in order to suppress infection-limiting apoptotic host cell responses. This antiapoptotic state also lends protection against cell death induced by metabolic inhibitors like actinomycin D and cycloheximide. The identity of the viral antiapoptotic protein and the underlying mechanism are unknown. Here, we provide evidence that the coxsackievirus 2B protein modulates apoptosis by manipulating intracellular Ca(2+) homeostasis. Using fluorescent Ca(2+) indicators and organelle-targeted aequorins, we demonstrate that ectopic expression of 2B in HeLa cells decreases the Ca(2+) content of both the endoplasmic reticulum and the Golgi, resulting in down-regulation of Ca(2+) signaling between these stores and the mitochondria, and increases the influx of extracellular Ca(2+). In our studies of the physiological importance of the 2B-induced alterations in Ca(2+) signaling, we found that the expression of 2B suppressed caspase activation and apoptotic cell death induced by various stimuli, including actinomycin D and cycloheximide. Mutants of 2B that were defective in reducing the Ca(2+) content of the stores failed to suppress apoptosis. These data implicate a functional role of the perturbation of intracellular Ca(2+) compartmentalization in the enteroviral strategy to suppress intrinsic apoptotic host cell responses. The putative down-regulation of an endoplasmic reticulum-dependent apoptotic pathway is discussed.

Mutational analysis of different regions in the coxsackievirus 2B protein: requirements for homo-multimerization, membrane permeabilization, subcellular localization, and virus replication

The coxsackievirus 2B protein is a small hydrophobic protein (99 amino acids) that increases host cell membrane permeability, possibly by forming homo-multimers that build membrane-integral pores. Previously, we defined the functional role of the two hydrophobic regions HR1 and HR2. Here, we investigated the importance of regions outside HR1 and HR2 for multimerization, increasing membrane permeability, subcellular localization, and virus replication through analysis of linker insertion and substitution mutants. From these studies, the following conclusions could be drawn. (i) The hydrophilic region ((58)RNHDD(62)) between HR1 and HR2 is critical for multimerization and increasing membrane permeability. Substitution analysis of Asn(61) and Asn(62) demonstrated the preference for short polar side chains (Asp, Asn), residues that are often present in turns, over long polar side chains (Glu, Gln). This finding supports the idea that the hydrophilic region is involved in pore formation by facilitating a turn between HR1 and HR2 to reverse chain direction. (ii) Studies undertaken to define the downstream boundary of HR2 demonstrated that the aromatic residues Trp(80) and Trp(82), but not the positively charged residues Arg(81), Lys(84), and Lys(86) are important for increasing membrane permeability. (iii) The N terminus is not required for multimerization but does contribute to the membrane-active character of 2B. (iv) The subcellular localization of 2B does not rely on regions outside HR1 and HR2 and does not require multimerization. (v) Virus replication requires both the membrane-active character and an additional function of 2B that is not connected to this activity.

Viroporins

Viroporins are a group of proteins that participate in several viral functions, including the promotion of release of viral particles from cells. These proteins also affect cellular functions, including the cell vesicle system, glycoprotein trafficking and membrane permeability. Viroporins are not essential for the replication of viruses, but their presence enhances virus growth. Comprising some 60-120 amino acids, viroporins have a hydrophobic transmembrane domain that interacts with and expands the lipid bilayer. Some viroporins also contain other motifs, such as basic amino acid residues or a domain rich in aromatic amino acids that confers on the protein the ability to interact with the interfacial lipid bilayer. Viroporin oligomerization gives rise to hydrophilic pores at the membranes of virus-infected cells. As the list of known viroporins steadily grows, recent research efforts focus on deciphering the actions of the viroporins poliovirus 2B, alphavirus 6K, HIV-1 Vpu and influenza virus M2. All these proteins can enhance the passage of ions and small molecules through membranes depending on their concentration gradient. Future work will lengthen the list of viroporins and will provide a deeper understanding of their mechanisms of action.

Mechanisms of membrane permeabilization by picornavirus 2B viroporin

Cell infection by picornaviruses leads to membrane permeabilization. Recent evidence suggests the involvement of the non-structural protein 2B in this process. We have recently reported the detection of 2B porin-like activity in isolated membrane-protein systems that lack other cell components. According to data derived from these model membranes, four self-aggregated 2B monomers (i.e. tetramers) would be sufficient to permeabilize a single lipid vesicle, allowing the free diffusion of solutes under ca. 1000 Da. Our findings also support a role for lipids in protein oligomerization and subsequent pore opening. The lipid dependence of these processes points to negatively charged cytofacial surfaces as 2B cell membrane targets.

Changes in rhinovirus protein 2C allow efficient replication in mouse cells

Rhinovirus type 16 was found to replicate in mouse L cells that express the viral receptor, human intercellular adhesion molecule 1 (ICAM-1). However, infection of these cells at a low multiplicity of infection leads to no discernible cytopathic effect, and low virus titers are produced. A variant virus, 16/L, was isolated after alternate passage of rhinovirus 16 between HeLa and ICAM-1 L cells. Infection of mouse cells with 16/L leads to higher virus titers, increased production of RNA, and total cytopathic effect. Three amino acid changes were identified in the P2 region of virus 16/L, and the adaptation phenotype mapped to two changes in protein 2C. The characterization of a rhinovirus host range mutant will facilitate the investigation of cellular proteins required for efficient viral growth and the development of a murine model for rhinovirus infection.