Influenza virus M2 protein slows traffic along the secretory pathway. pH perturbation of acidified compartments affects early Golgi transport steps

MARCH8 inhibits influenza A virus infection by targeting viral M2 protein for ubiquitination-dependent degradation in lysosomes

The membrane-associated RING-CH (MARCH) proteins are E3 ligases that regulate the stability of various cellular membrane proteins. MARCH8 has been reported to inhibit the infection of HIV-1 and a few other viruses, thus plays an important role in host antiviral defense. However, the antiviral spectrum and the underlying mechanisms of MARCH8 are incompletely defined. Here, we demonstrate that MARCH8 profoundly inhibits influenza A virus (IAV) replication both in vitro and in mice. Mechanistically, MARCH8 suppresses IAV release through redirecting viral M2 protein from the plasma membrane to lysosomes for degradation. Specifically, MARCH8 catalyzes the K63-linked polyubiquitination of M2 at lysine residue 78 (K78). A recombinant A/Puerto Rico/8/34 virus carrying the K78R M2 protein shows greater replication and more severe pathogenicity in cells and mice. More importantly, we found that the M2 protein of the H1N1 IAV has evolved to acquire non-lysine amino acids at positions 78/79 to resist MARCH8-mediated ubiquitination and degradation. Together, our data support the important role of MARCH8 in host anti-IAV intrinsic immune defense by targeting M2, and suggest the inhibitory pressure of MARCH8 on H1N1 IAV transmission in the human population.

The SARS-CoV-2 envelope and membrane proteins modulate maturation and retention of the spike protein, allowing assembly of virus-like particles

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a β-coronavirus, is the causative agent of the COVID-19 pandemic. Like for other coronaviruses, its particles are composed of four structural proteins: spike (S), envelope (E), membrane (M), and nucleoprotein (N) proteins. The involvement of each of these proteins and their interactions are critical for assembly and production of β-coronavirus particles. Here, we sought to characterize the interplay of SARS-CoV-2 structural proteins during the viral assembly process. By combining biochemical and imaging assays in infected versus transfected cells, we show that E and M regulate intracellular trafficking of S as well as its intracellular processing. Indeed, the imaging data reveal that S is relocalized at endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) or Golgi compartments upon coexpression of E or M, as observed in SARS-CoV-2-infected cells, which prevents syncytia formation. We show that a C-terminal retrieval motif in the cytoplasmic tail of S is required for its M-mediated retention in the ERGIC, whereas E induces S retention by modulating the cell secretory pathway. We also highlight that E and M induce a specific maturation of N-glycosylation of S, independently of the regulation of its localization, with a profile that is observed both in infected cells and in purified viral particles. Finally, we show that E, M, and N are required for optimal production of virus-like-particles. Altogether, these results highlight how E and M proteins may influence the properties of S proteins and promote the assembly of SARS-CoV-2 viral particles.

Site-directed M2 proton channel inhibitors enable synergistic combination therapy for rimantadine-resistant pandemic influenza

Pandemic influenza A virus (IAV) remains a significant threat to global health. Preparedness relies primarily upon a single class of neuraminidase (NA) targeted antivirals, against which resistance is steadily growing. The M2 proton channel is an alternative clinically proven antiviral target, yet a near-ubiquitous S31N polymorphism in M2 evokes resistance to licensed adamantane drugs. Hence, inhibitors capable of targeting N31 containing M2 (M2-N31) are highly desirable. Rational in silico design and in vitro screens delineated compounds favouring either lumenal or peripheral M2 binding, yielding effective M2-N31 inhibitors in both cases. Hits included adamantanes as well as novel compounds, with some showing low micromolar potency versus pandemic “swine” H1N1 influenza (Eng195) in culture. Interestingly, a published adamantane-based M2-N31 inhibitor rapidly selected a resistant V27A polymorphism (M2-A27/N31), whereas this was not the case for non-adamantane compounds. Nevertheless, combinations of adamantanes and novel compounds achieved synergistic antiviral effects, and the latter synergised with the neuraminidase inhibitor (NAi), Zanamivir. Thus, site-directed drug combinations show potential to rejuvenate M2 as an antiviral target whilst reducing the risk of drug resistance.

"Swine flu" illustrated that the spread of influenza pandemics in the modern era is rapid, making antiviral drugs the best way of limiting disease. One proven influenza drug target is the M2 proton channel, which plays an essential role during virus entry. However, resistance against licensed drugs targeting this protein is now ubiquitous, largely due to an S31N change in the M2 sequence. Understandably, considerable effort has focused on developing M2-N31 inhibitors, yet this has been hampered by controversy surrounding two potential drug binding sites. Here, we show that both sites can in fact be targeted by new M2-N31 inhibitors, generating synergistic antiviral effects. Developing such drug combinations should improve patient outcomes and minimise the emergence of future drug resistance.

pH Gradient Mitigation in the Leaf Cell Secretory Pathway Attenuates the Defense Response of Nicotiana benthamiana to Agroinfiltration

Partial neutralization of the Golgi lumen pH by the ectopic expression of influenza virus M2 proton channel is useful to stabilize acid-labile recombinant proteins in plant cells, but the impact of pH gradient mitigation on host cellular functions has not been investigated. Here, we assessed the unintended effects of M2 expression on the leaf proteome of Nicotiana benthamiana infiltrated with the bacterial gene vector Agrobacterium tumefaciens. An isobaric tags for relative and absolute quantification quantitative proteomics procedure was followed to compare the leaf proteomes of plants agroinfiltrated with either an "empty" vector or an M2-encoding vector. Leaves infiltrated with the empty vector had a low soluble protein content compared to noninfiltrated control leaves, associated with increased levels of stress-related proteins but decreased levels of photosynthesis-associated proteins. M2 expression partly compromised these effects of agroinfiltration to restore soluble protein content in the leaf tissue, associated with restored levels of photosynthesis-associated proteins and reduced levels of stress-related proteins in the apoplast. These data illustrate the cell-wide influence of the Golgi lumen pH homeostasis on the leaf proteome of N. benthamiana responding to microbial challenge. They also underline the relevance of assessing the eventual unintended effects of accessory proteins used to modulate specific cellular or metabolic functions in plant protein biofactories.

Enhancing the Yield and Quality of Influenza Virus-like Particles (VLPs) Produced in Insect Cells by Inhibiting Cytopathic Effects of Matrix Protein M2

To provide broader protection and eliminate the need for annual update of influenza vaccines, biomolecular engineering of influenza virus-like particles (VLPs) to display more conserved influenza proteins such as the matrix protein M2 has been explored. However, achieving high surface density of full-length M2 in influenza VLPs has been left unrealized. In this study, we show that the ion channel activity of M2 induces significant cytopathic effects in Spodoptera frugiperda (Sf9) insect cells when expressed using M2-encoding baculovirus. These effects include altered Sf9 cell morphology and reduced baculovirus replication, resulting in impaired influenza protein expression and thus VLP production. On the basis of the function of M2, we hypothesized that blocking its ion channel activity could potentially relieve these cytopathic effects, and thus restore influenza protein expression to improve VLP production. The use of the M2 inhibitor amantadine indeed improves Sf9 cellular expression not only of M2 (∼3-fold), but also of hemagglutinin (HA) (∼7-fold) and of matrix protein M1 (∼3-fold) when coexpressed to produce influenza VLPs. This increased cellular expression of all three influenza proteins further leads to ∼2-fold greater VLP yield. More importantly, the quality of the resulting influenza VLPs is significantly improved, as demonstrated by the ∼2-fold, ∼50-fold, and ∼2-fold increase in the antigen density to approximately 53 HA, 48 M1, and 156 M2 per influenza VLP, respectively. Taken together, this study represents a novel approach to enable the efficient incorporation of full-length M2 while enhancing both the yield and quality of influenza VLPs produced by Sf9 cells.

Dysregulation of M segment gene expression contributes to influenza A virus host restriction

The M segment of the 2009 pandemic influenza A virus (IAV) has been implicated in its emergence into human populations. To elucidate the genetic contributions of the M segment to host adaptation, and the underlying mechanisms, we examined a panel of isogenic viruses that carry avian- or human-derived M segments. Avian, but not human, M segments restricted viral growth and transmission in mammalian model systems, and the restricted growth correlated with increased expression of M2 relative to M1. M2 overexpression was associated with intracellular accumulation of autophagosomes, which was alleviated by interference of the viral proton channel activity by amantadine treatment. As M1 and M2 are expressed from the M mRNA through alternative splicing, we separated synonymous and non-synonymous changes that differentiate human and avian M segments and found that dysregulation of gene expression leading to M2 overexpression diminished replication, irrespective of amino acid composition of M1 or M2. Moreover, in spite of efficient replication, virus possessing a human M segment that expressed avian M2 protein at low level did not transmit efficiently. We conclude that (i) determinants of transmission reside in the IAV M2 protein, and that (ii) control of M segment gene expression is a critical aspect of IAV host adaptation needed to prevent M2-mediated dysregulation of vesicular homeostasis.

Symmetric dimeric adamantanes for exploring the structure of two viroporins: influenza virus M2 and hepatitis C virus p7

Background

Adamantane-based compounds have been identified to interfere with the ion-channel activity of viroporins and thereby inhibit viral infection. To better understand the difference in the inhibition mechanism of viroporins, we synthesized symmetric dimeric adamantane analogs of various alkyl-spacer lengths.

Methods

Symmetric dimeric adamantane derivatives were synthesized where two amantadine or rimantadine molecules were linked by various alkyl-spacers. The inhibitory activity of the compounds was studied on two viroporins: the influenza virus M2 protein, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique, and the hepatitis C virus (HCV) p7 channels for five different genotypes (1a, 1b, 2a, 3a, and 4a) expressed in HEK293 cells using whole-cell patch-clamp recording techniques.

Results

Upon testing on M2 protein, dimeric compounds showed significantly lower inhibitory activity relative to the monomeric amantadine. The lack of channel blockage of the dimeric amantadine and rimantadine analogs against M2 wild type and M2-S31N mutant was consistent with previously proposed drug-binding mechanisms and further confirmed that the pore-binding model is the pharmacologically relevant drug-binding model. On the other hand, these dimers showed similar potency to their respective monomeric analogs when tested on p7 protein in HCV genotypes 1a, 1b, and 4a while being 700-fold and 150-fold more potent than amantadine in genotypes 2a and 3a, respectively. An amino group appears to be important for inhibiting the ion-channel activity of p7 protein in genotype 2a, while its importance was minimal in all other genotypes.

Conclusion

Symmetric dimeric adamantanes can be considered a prospective class of p7 inhibitors that are able to address the differences in adamantane sensitivity among the various genotypes of HCV.

The amino-terminus of the hepatitis C virus (HCV) p7 viroporin and its cleavage from glycoprotein E2-p7 precursor determine specific infectivity and secretion levels of HCV particle types

Viroporins are small transmembrane proteins with ion channel activities modulating properties of intracellular membranes that have diverse proviral functions. Hepatitis C virus (HCV) encodes a viroporin, p7, acting during assembly, envelopment and secretion of viral particles (VP). HCV p7 is released from the viral polyprotein through cleavage at E2-p7 and p7-NS2 junctions by signal peptidase, but also exists as an E2p7 precursor, of poorly defined properties. Here, we found that ectopic p7 expression in HCVcc-infected cells reduced secretion of particle-associated E2 glycoproteins. Using biochemical assays, we show that p7 dose-dependently slows down the ER-to-Golgi traffic, leading to intracellular retention of E2, which suggested that timely E2p7 cleavage and p7 liberation are critical events to control E2 levels. By studying HCV mutants with accelerated E2p7 processing, we demonstrate that E2p7 cleavage controls E2 intracellular expression and secretion levels of nucleocapsid-free subviral particles and infectious virions. In addition, our imaging data reveal that, following p7 liberation, the amino-terminus of p7 is exposed towards the cytosol and coordinates the encounter between NS5A and NS2-based assembly sites loaded with E1E2 glycoproteins, which subsequently leads to nucleocapsid envelopment. We identify punctual mutants at p7 membrane interface that, by abrogating NS2/NS5A interaction, are defective for transmission of infectivity owing to decreased secretion of core and RNA and to increased secretion of non/partially-enveloped particles. Altogether, our results indicate that the retarded E2p7 precursor cleavage is essential to regulate the intracellular and secreted levels of E2 through p7-mediated modulation of the cell secretory pathway and to unmask critical novel assembly functions located at p7 amino-terminus.

Influenza A Virus M2 Protein: Roles from Ingress to Egress

Influenza A virus (IAV) matrix protein 2 (M2) is among the smallest bona fide, hence extensively studied, ion channel proteins. The M2 ion channel activity is not only essential for virus replication, but also involved in modulation of cellular homeostasis in a variety of ways. It is also the target for ion channel inhibitors, i.e., anti-influenza drugs. Thus far, several studies have been conducted to elucidate its biophysical characteristics, structure-function relationships of the ion channel, and the M2-host interactome. In this review, we discuss M2 protein synthesis and assembly into an ion channel, its roles in IAV replication, and the pathophysiological impact on the host cell.

Cell Cycle-independent Role of Cyclin D3 in Host Restriction of Influenza Virus Infection

To identify new host factors that modulate the replication of influenza A virus, we performed a yeast two-hybrid screen using the cytoplasmic tail of matrix protein 2 from the highly pathogenic H5N1 strain. The screen revealed a high-score interaction with cyclin D3, a key regulator of cell cycle early G₁ phase. M2-cyclin D3 interaction was validated through GST pull-down and recapitulated in influenza A/WSN/33-infected cells. Knockdown of Ccnd3 by small interfering RNA significantly enhanced virus progeny titers in cell culture supernatants. Interestingly, the increase in virus production was due to cyclin D3 deficiency per se and not merely a consequence of cell cycle deregulation. A combined knockdown of Ccnd3 and Rb1, which rescued cell cycle progression into S phase, failed to normalize virus production. Infection by influenza A virus triggered redistribution of cyclin D3 from the nucleus to the cytoplasm, followed by its proteasomal degradation. When overexpressed in HEK 293T cells, cyclin D3 impaired binding of M2 with M1, which is essential for proper assembly of progeny virions, lending further support to its role as a putative restriction factor. Our study describes the identification and characterization of cyclin D3 as a novel interactor of influenza A virus M2 protein. We hypothesize that competitive inhibition of M1-M2 interaction by cyclin D3 impairs infectious virion formation and results in attenuated virus production. In addition, we provide mechanistic insights into the dynamic interplay of influenza virus with the host cell cycle machinery during infection.

Host Cellular Protein TRAPPC6AΔ Interacts with Influenza A Virus M2 Protein and Regulates Viral Propagation by Modulating M2 Trafficking

Influenza A virus (IAV) matrix protein 2 (M2) plays multiple roles in the early and late phases of viral infection. Once synthesized, M2 is translocated to the endoplasmic reticulum (ER), travels to the Golgi apparatus, and is sorted at the trans-Golgi network (TGN) for transport to the apical plasma membrane, where it functions in virus budding. We hypothesized that M2 trafficking along with its secretory pathway must be finely regulated, and host factors could be involved in this process. However, no studies examining the role of host factors in M2 posttranslational transport have been reported. Here, we used a yeast two-hybrid (Y2H) system to screen for host proteins that interact with the M2 protein and identified transport protein particle complex 6A (TRAPPC6A) as a potential binding partner. We found that both TRAPPC6A and its N-terminal internal-deletion isoform, TRAPPC6A delta (TRAPPC6AΔ), interact with M2. Truncation and mutation analyses showed that the highly conserved leucine residue at position 96 of M2 is critical for mediating this interaction. The role of TRAPPC6AΔ in the viral life cycle was investigated by the knockdown of endogenous TRAPPC6AΔ with small interfering RNA (siRNA) and by generating a recombinant virus that was unable to interact with TRAPPC6A/TRAPPC6AΔ. The results indicated that TRAPPC6AΔ, through its interaction with M2, slows M2 trafficking to the apical plasma membrane, favors viral replication in vitro, and positively modulates virus virulence in mice.

IMPORTANCE

The influenza A virus M2 protein regulates the trafficking of not only other proteins but also itself along the secretory pathway. However, the host factors involved in the regulation of the posttranslational transport of M2 are largely unknown. In this study, we identified TRAPPC6A and its N-terminal internal-deletion isoform, TRAPPC6AΔ, as interacting partners of M2. We found that the leucine (L) residue at position 96 of M2 is critical for mediating this interaction, which leads us to propose that the high level of conservation of 96L is a consequence of M2 adaptation to its interacting host factor TRAPPC6A/TRAPPC6AΔ. Importantly, we discovered that TRAPPC6AΔ can positively regulate viral replication in vitro by modulating M2 trafficking to the plasma membrane.

Modulating secretory pathway pH by proton channel co-expression can increase recombinant protein stability in plants

Eukaryotic expression systems are used for the production of complex secreted proteins. However, recombinant proteins face considerable biochemical challenges along the secretory pathway, including proteolysis and pH variation between organelles. As the use of synthetic biology matures into solutions for protein production, various host-cell engineering approaches are being developed to ameliorate host-cell factors that can limit recombinant protein quality and yield. We report the potential of the influenza M2 ion channel as a novel tool to neutralize the pH in acidic subcellular compartments. Using transient expression in the plant host, Nicotiana benthamiana, we show that ion channel expression can significantly raise pH in the Golgi apparatus and that this can have a strong stabilizing effect on a fusion protein separated by an acid-susceptible linker peptide. We exemplify the utility of this effect in recombinant protein production using influenza hemagglutinin subtypes differentially stable at low pH; the expression of hemagglutinins prone to conformational change in mildly acidic conditions is considerably enhanced by M2 co-expression. The co-expression of a heterologous ion channel to stabilize acid-labile proteins and peptides represents a novel approach to increasing the yield and quality of secreted recombinant proteins in plants and, possibly, in other eukaryotic expression hosts.

Viral Membrane Channels: Role and Function in the Virus Life Cycle

Viroporins are small, hydrophobic trans-membrane viral proteins that oligomerize to form hydrophilic pores in the host cell membranes. These proteins are crucial for the pathogenicity and replication of viruses as they aid in various stages of the viral life cycle, from genome uncoating to viral release. In addition, the ion channel activity of viroporin causes disruption in the cellular ion homeostasis, in particular the calcium ion. Fluctuation in the calcium level triggers the activation of the host defensive programmed cell death pathways as well as the inflammasome, which in turn are being subverted for the viruses’ replication benefits. This review article summarizes recent developments in the functional investigation of viroporins from various viruses and their contributions to viral replication and virulence.

Influenza A virus hemagglutinin and neuraminidase mutually accelerate their apical targeting through clustering of lipid rafts

In polarized epithelial cells, influenza A virus hemagglutinin (HA) and neuraminidase (NA) are intrinsically associated with lipid rafts and target the apical plasma membrane for viral assembly and budding. Previous studies have indicated that the transmembrane domain (TMD) and cytoplasmic tail (CT) of HA and NA are required for association with lipid rafts, but the raft dependencies of their apical targeting are controversial. Here, we show that coexpression of HA with NA accelerated their apical targeting through accumulation in lipid rafts. HA was targeted to the apical plasma membrane even when expressed alone, but the kinetics was much slower than that of HA in infected cells. Coexpression experiments revealed that apical targeting of HA and NA was accelerated by their coexpression. The apical targeting of HA was also accelerated by coexpression with M1 but not M2. The mutations in the outer leaflet of the TMD and the deletion of the CT in HA and NA that reduced their association with lipid rafts abolished the acceleration of their apical transport, indicating that the lipid raft association is essential for efficient apical trafficking of HA and NA. An in situ proximity ligation assay (PLA) revealed that HA and NA were accumulated and clustered in the cytoplasmic compartments only when both were associated with lipid rafts. Analysis with mutant viruses containing nonraft HA/NA confirmed these findings. We further analyzed lipid raft markers by in situ PLA and suggest a possible mechanism of the accelerated apical transport of HA and NA via clustering of lipid rafts.

IMPORTANCE

Lipid rafts serve as sites for viral entry, particle assembly, and budding, leading to efficient viral replication. The influenza A virus utilizes lipid rafts for apical plasma membrane targeting and particle budding. The hemagglutinin (HA) and neuraminidase (NA) of influenza virus, key players for particle assembly, contain determinants for apical sorting and lipid raft association. However, it remains to be elucidated how lipid rafts contribute to the apical trafficking and budding. We investigated the relation of lipid raft association of HA and NA to the efficiency of apical trafficking. We show that coexpression of HA and NA induces their accumulation in lipid rafts and accelerates their apical targeting, and we suggest that the accelerated apical transport likely occurs by clustering of lipid rafts at the TGN. This finding provides the first evidence that two different raft-associated viral proteins induce lipid raft clustering, thereby accelerating apical trafficking of the viral proteins.

VAMP7 modulates ciliary biogenesis in kidney cells

Epithelial cells elaborate specialized domains that have distinct protein and lipid compositions, including the apical and basolateral surfaces and primary cilia. Maintaining the identity of these domains is required for proper cell function, and requires the efficient and selective SNARE-mediated fusion of vesicles containing newly synthesized and recycling proteins with the proper target membrane. Multiple pathways exist to deliver newly synthesized proteins to the apical surface of kidney cells, and the post-Golgi SNAREs, or VAMPs, involved in these distinct pathways have not been identified. VAMP7 has been implicated in apical protein delivery in other cell types, and we hypothesized that this SNARE would have differential effects on the trafficking of apical proteins known to take distinct routes to the apical surface in kidney cells. VAMP7 expressed in polarized Madin Darby canine kidney cells colocalized primarily with LAMP2-positive compartments, and siRNA-mediated knockdown modulated lysosome size, consistent with the known function of VAMP7 in lysosomal delivery. Surprisingly, VAMP7 knockdown had no effect on apical delivery of numerous cargoes tested, but did decrease the length and frequency of primary cilia. Additionally, VAMP7 knockdown disrupted cystogenesis in cells grown in a three-dimensional basement membrane matrix. The effects of VAMP7 depletion on ciliogenesis and cystogenesis are not directly linked to the disruption of lysosomal function, as cilia lengths and cyst morphology were unaffected in an MDCK lysosomal storage disorder model. Together, our data suggest that VAMP7 plays an essential role in ciliogenesis and lumen formation. To our knowledge, this is the first study implicating an R-SNARE in ciliogenesis and cystogenesis.

Sialylation of N-linked glycans mediates apical delivery of endolyn in MDCK cells via a galectin-9-dependent mechanism

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Apical delivery of endolyn requires recognition of sialic acids on its N-glycans possibly (or likely) mediated by galectin-9.

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Along rat kidney tubules, endolyn is variously localized to the apical surface and endosomal/lysosomal compartments. Apical delivery of newly synthesized rat endolyn predominates over direct lysosomal delivery in polarized Madin–Darby canine kidney cells. Apical sorting depends on terminal processing of a subset of lumenal N-glycans. Here we dissect the requirements of N-glycan processing for apical targeting and investigate the underlying mechanism. Modulation of glycan branching and subsequent polylactosamine elongation by knockdown of N-acetylglucosaminyltransferase III or V had no effect on apical delivery of endolyn. In contrast, combined but not individual knockdown of sialyltransferases ST3Gal-III, ST3Gal-IV, and ST6Gal-I, which together are responsible for addition of α2,3- and α2,6-linked sialic acids on N-glycans, dramatically decreased endolyn surface polarity. Endolyn synthesized in the presence of kifunensine, which blocks terminal N-glycan processing, reduced its interaction with several recombinant canine galectins, and knockdown of galectin-9 (but not galectin-3, -4, or -8) selectively disrupted endolyn polarity. Our data suggest that sialylation enables recognition of endolyn by galectin-9 to mediate efficient apical sorting. They raise the intriguing possibility that changes in glycosyltransferase expression patterns and/or galectin-9 distribution may acutely modulate endolyn trafficking in the kidney.

A single polar residue and distinct membrane topologies impact the function of the infectious bronchitis coronavirus E protein

The coronavirus E protein is a small membrane protein with a single predicted hydrophobic domain (HD), and has a poorly defined role in infection. The E protein is thought to promote virion assembly, which occurs in the Golgi region of infected cells. It has also been implicated in the release of infectious particles after budding. The E protein has ion channel activity in vitro, although a role for channel activity in infection has not been established. Furthermore, the membrane topology of the E protein is of considerable debate, and the protein may adopt more than one topology during infection. We previously showed that the HD of the infectious bronchitis virus (IBV) E protein is required for the efficient release of infectious virus, an activity that correlated with disruption of the secretory pathway. Here we report that a single residue within the hydrophobic domain, Thr16, is required for secretory pathway disruption. Substitutions of other residues for Thr16 were not tolerated. Mutations of Thr16 did not impact virus assembly as judged by virus-like particle production, suggesting that alteration of secretory pathway and assembly are independent activities. We also examined how the membrane topology of IBV E affected its function by generating mutant versions that adopted either a transmembrane or membrane hairpin topology. We found that a transmembrane topology was required for disrupting the secretory pathway, but was less efficient for virus-like particle production. The hairpin version of E was unable to disrupt the secretory pathway or produce particles. The findings reported here identify properties of the E protein that are important for its function, and provide insight into how the E protein may perform multiple roles during infection.

Coronaviruses are enveloped viruses that bud and assemble intracellularly, and therefore must use the host secretory pathway for release. Coronavirus E is a small protein that contains a single predicted hydrophobic domain and is targeted to the Golgi region. The E protein has been implicated in the assembly of coronavirus particles, as well as in virus release after assembly. The mechanism of action is not understood, but may involve ion channel activity. The membrane topology of the E protein is also unclear, and the protein may adopt distinct topologies that have different functions. We previously showed that the E protein from the infectious bronchitis virus could disrupt the secretory pathway to the apparent advantage of the virus. Here we have mapped this activity to a single, essential residue within the hydrophobic domain. Additionally, we developed mutant versions of IBV E that adopt a single membrane topology, and showed that a transmembrane topology is required for disruption of the secretory pathway. Our results broaden the understanding of E protein function and will impact the development of antiviral strategies.

The coronavirus E protein: assembly and beyond

The coronavirus E protein is a small membrane protein that has an important role in the assembly of virions. Recent studies have indicated that the E protein has functions during infection beyond assembly, including in virus egress and in the host stress response. Additionally, the E protein has ion channel activity, interacts with host proteins, and may have multiple membrane topologies. The goal of this review is to highlight the properties and functions of the E protein, and speculate on how they may be related.

Human annexin A6 interacts with influenza a virus protein M2 and negatively modulates infection

The influenza A virus M2 ion channel protein has the longest cytoplasmic tail (CT) among the three viral envelope proteins and is well conserved between different viral strains. It is accessible to the host cellular machinery after fusion with the endosomal membrane and during the trafficking, assembly, and budding processes. We hypothesized that identification of host cellular interactants of M2 CT could help us to better understand the molecular mechanisms regulating the M2-dependent stages of the virus life cycle. Using yeast two-hybrid screening with M2 CT as bait, a novel interaction with the human annexin A6 (AnxA6) protein was identified, and their physical interaction was confirmed by coimmunoprecipitation assay and a colocalization study of virus-infected human cells. We found that small interfering RNA (siRNA)-mediated knockdown of AnxA6 expression significantly increased virus production, while its overexpression could reduce the titer of virus progeny, suggesting a negative regulatory role for AnxA6 during influenza A virus infection. Further characterization revealed that AnxA6 depletion or overexpression had no effect on the early stages of the virus life cycle or on viral RNA replication but impaired the release of progeny virus, as suggested by delayed or defective budding events observed at the plasma membrane of virus-infected cells by transmission electron microscopy. Collectively, this work identifies AnxA6 as a novel cellular regulator that targets and impairs the virus budding and release stages of the influenza A virus life cycle.

BST-2 is rapidly down-regulated from the cell surface by the HIV-1 protein Vpu: evidence for a post-ER mechanism of Vpu-action

Recent evidence suggests that transmembrane domain (TMD) interactions are essential for HIV-1 Vpu-mediated antagonism of the restriction factor BST-2/tetherin. We made Vpu TMD mutants to study the mechanism of BST-2 antagonism. Vpu-I17A, -A18F, -W22L, and -S23L co-localized with BST-2 within endosomal membranes while effectively enhancing virion release and down-regulating surface BST-2. However, Vpu-A18H was confined to an endoplasmic reticulum (ER)-like distribution, resulting in impaired down-regulation of BST-2 and reduced virion release. Brefeldin A confined wild type Vpu to the ER, resulting in a similarly impaired phenotype, as did the addition of a C-terminal ER-retention signal to Vpu. We determined the half-life of cell-surface BST-2 to be ~8 hours, whereas Vpu mediated an ~80% reduction of surface BST-2 within 6 hours, suggesting that TMD interactions between Vpu and BST-2 occur within post-ER membranes to directly and rapidly remove BST-2 from the cell surface and relieve restricted virion release.

Nucleofection disrupts tight junction fence function to alter membrane polarity of renal epithelial cells

Here, we compared the effects of nucleofection and lipid-based approaches to introduce siRNA duplexes on the subsequent development of membrane polarity in kidney cells. Nucleofection of Madin-Darby canine kidney (MDCK) cells, even with control siRNA duplexes, disrupted the initial surface polarity as well as the steady-state distribution of membrane proteins. Transfection using lipofectamine yielded slightly less efficient knockdown but did not disrupt membrane polarity. Polarized secretion was unaffected by nucleofection, suggesting a selective defect in the development of membrane polarity. Cilia frequency and length were not altered by nucleofection. However, the basolateral appearance of a fluorescent lipid tracer added to the apical surface of nucleofected cells was dramatically enhanced relative to untransfected controls or lipofectamine-treated cells. In contrast, [(3)H]inulin diffusion and transepithelial electrical resistance were not altered in nucleofected cells compared with untransfected ones. We conclude that nucleofection selectively hinders development of the tight junction fence function in MDCK cells.

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.

Different degrees of somatotroph ablation compromise pituitary growth hormone cell network structure and other pituitary endocrine cell types

We have generated transgenic mice with somatotroph-specific expression of a modified influenza virus ion channel, (H37A)M2, leading to ablation of GH cells with three levels of severity, dependent on transgene copy number. GH-M2(low) mice grow normally and have normal-size pituitaries but 40-50% reduction in pituitary GH content in adult animals. GH-M2(med) mice have male-specific transient growth retardation and a reduction in pituitary GH content by 75% at 42 d and 97% by 100 d. GH-M2(high) mice are severely dwarfed with undetectable pituitary GH. The GH secretory response of GH-M2(low) and GH-M2(med) mice to GH-releasing peptide-6 and GHRH was markedly attenuated. The content of other pituitary hormones was affected depending on transgene copy number: no effect in GH-M2(low) mice, prolactin and TSH reduced in GH-M2(med) mice, and all hormones reduced in GH-M2(high) mice. The effect on non-GH hormone content was associated with increased macrophage invasion of the pituitary. Somatotroph ablation affected GH cell network organization with limited disruption in GH-M2(low) mice but more severe disruption in GH-M2(med) mice. The remaining somatotrophs formed tight clusters after puberty, which contrasts with GHRH-M2 mice with a secondary reduction in somatotrophs that do not form clusters. A reduction in pituitary beta-catenin staining was correlated with GH-M2 transgene copy number, suggesting M2 expression has an effect on cell-cell communication in somatotrophs and other pituitary cell types. GH-M2 transgenic mice demonstrate that differing degrees of somatotroph ablation lead to correlated secondary effects on cell populations and cellular network organization.

Cholesterol-binding viral proteins in virus entry and morphogenesis

Up to now less than a handful of viral cholesterol-binding proteins have been characterized, in HIV, influenza virus and Semliki Forest virus. These are proteins with roles in virus entry or morphogenesis. In the case of the HIV fusion protein gp41 cholesterol binding is attributed to a cholesterol recognition consensus (CRAC) motif in a flexible domain of the ectodomain preceding the trans-membrane segment. This specific CRAC sequence mediates gp41 binding to a cholesterol affinity column. Mutations in this motif arrest virus fusion at the hemifusion stage and modify the ability of the isolated CRAC peptide to induce segregation of cholesterol in artificial membranes.Influenza A virus M2 protein co-purifies with cholesterol. Its proton translocation activity, responsible for virus uncoating, is not cholesterol-dependent, and the transmembrane channel appears too short for integral raft insertion. Cholesterol binding may be mediated by CRAC motifs in the flexible post-TM domain, which harbours three determinants of binding to membrane rafts. Mutation of the CRAC motif of the WSN strain attenuates virulence for mice. Its affinity to the raft-non-raft interface is predicted to target M2 protein to the periphery of lipid raft microdomains, the sites of virus assembly. Its influence on the morphology of budding virus implicates M2 as factor in virus fission at the raft boundary. Moreover, M2 is an essential factor in sorting the segmented genome into virus particles, indicating that M2 also has a role in priming the outgrowth of virus buds.SFV E1 protein is the first viral type-II fusion protein demonstrated to directly bind cholesterol when the fusion peptide loop locks into the target membrane. Cholesterol binding is modulated by another, proximal loop, which is also important during virus budding and as a host range determinant, as shown by mutational studies.

Neutralization of endomembrane compartments in epithelial MDCK cells affects proteoglycan synthesis in the apical secretory pathway

PGs (proteoglycans) are proteins acquiring long, linear and sulfated GAG (glycosaminoglycan) chains during Golgi passage. In MDCK cells (Madin–Darby canine kidney cells), most of the CS (chondroitin sulfate) PGs are secreted apically, whereas most of the HS (heparan sulfate) PGs are secreted basolaterally. The apical and basolateral secretory routes differ in their GAG synthesis, since a protein core that traverses both routes acquires shorter chains, but more sulfate, in the basolateral pathway than in the apical counterpart [Tveit, Dick, Skibeli and Prydz (2005) J. Biol. Chem. 280, 29596–29603]. Golgi cisternae and the trans-Golgi network have slightly acidic lumens. We therefore investigated how neutralization of endomembrane compartments with the vacuolar H+-ATPase inhibitor Baf A1 (bafilomycin A1) affected GAG synthesis and PG sorting in MDCK cells. Baf A1 induced a slight reduction in basolateral secretion of macromolecules, which was compensated by an apical increase. More dramatic changes occurred to PG synthesis in the apical pathway on neutralization. The difference in apical and basolateral PG sulfation levels observed for control cells was abolished, due to enhanced sulfation of apical CS-GAGs. In addition, a large fraction of apical HS-GAGs was elongated to longer chain lengths. The differential sensitivity of the apical and basolateral secretory pathways to Baf A1 indicates that the apical pathway is more acidic than the basolateral counterpart in untreated MDCK cells. Neutralization gave an apical GAG output that was more similar to that of the basolateral pathway, suggesting that neutralization made the luminal environments of the two pathways more similar.

Differential sorting and Golgi export requirements for raft-associated and raft-independent apical proteins along the biosynthetic pathway

Sorting signals for apically destined proteins are highly diverse and can be present within the luminal, membrane-associated, and cytoplasmic domains of these proteins. A subset of apical proteins partition into detergent-resistant membranes, and the association of these proteins with glycolipid-enriched microdomains or lipid rafts may be important for their proper targeting. Recently, we observed that raft-associated and raft-independent apical proteins take different routes to the apical surface of polarized Madin-Darby canine kidney cells (Cresawn, K. O., Potter, B. A., Oztan, A., Guerriero, C. J., Ihrke, G., Goldenring, J. R., Apodaca, G., and Weisz, O. A. (2007) EMBO J. 26, 3737-3748). Here we reconstituted in vitro the export of raft-associated and raft-independent markers staged intracellularly at 19 degrees C. Surprisingly, whereas release of the raft-associated protein influenza hemagglutinin was dependent on the addition of an ATP-regenerating system and cytosol, release of a yellow fluorescent protein (YFP)-tagged raft-independent protein (the 75-kDa neurotrophin receptor; YFP-p75) was efficient even in the absence of these constituents. Subsequent studies suggested that YFP-p75 is released from the trans-Golgi network in fragile tubules that do not withstand isolation procedures. Moreover, immunofluorescence analysis revealed that hemagglutinin and YFP-p75 segregate into distinct subdomains of the Golgi complex at 19 degrees C. Our data suggest that raft-associated and raft-independent proteins accumulate at distinct intracellular sites upon low temperature staging, and that upon warming, they exit these compartments in transport carriers that have very different membrane characteristics and morphologies.

Biology of influenza a virus

The outbreaks of avian influenza A virus in poultry and humans over the last decade posed a pandemic threat to human. Here, we discuss the basic classification and the structure of influenza A virus. The viral genome contains eight RNA viral segments and the functions of viral proteins encoded by this genome are described. In addition, the RNA transcription and replication of this virus are reviewed.

Triple therapy of initial high-dose interferon with ribavirin and amantadine for patients with chronic hepatitis C

AIMS

We previously reported the potential effect of combination therapy of an initial high-dose interferon (IFN) and amantadine on the eradication of HCV-RNA in patients with chronic hepatitis C. The additive effects of amantadine on interferon and ribavirin combination therapy remain controversial. In this study we investigated the efficacy of initial high-dose IFN with ribavirin and amantadine on the virological response in patients with chronic hepatitis C with a high viral load of genotype 1b.

METHODS

Twenty-two patients with high viral loads of genotype 1b hepatitis C virus were enrolled in this pilot study. Patients were administered IFN-beta for four weeks and then IFN-alpha2b for 22 weeks with daily oral administration of ribavirin and amantadine.

RESULTS

A sustained virological response (SVR) was shown in 31.8% (seven of 22 patients). With the naïve patients, the SVR rate was 21.4% (three of 14 patients). In patients who could not eradicate HCV-RNA by previous IFN monotherapy, the SVR rate was 50% (four of eight patients).

CONCLUSION

Triple therapy with an initial high dose of IFN with ribavirin and amantadine may be effective, especially for chronic hepatitis C IFN-retreatment patients with a high viral load of genotype 1b.

Influenza M2 envelope protein augments avian influenza hemagglutinin pseudotyping of lentiviral vectors

Lentivirus-based gene transfer has the potential to efficiently deliver DNA-based therapies into non-dividing epithelial cells of the airway for the treatment of lung diseases such as cystic fibrosis. However, significant barriers both to lung-specific gene transfer and to production of lentivirus vectors must be overcome before these vectors can be routinely used for applications to the lung. In this study, we investigated whether the ability to produce lentiviral vectors pseudotyped with fowl plague virus hemagglutinin (HA) could be improved by co-expression of influenza virus M2 in vector-producing cells. We found that M2 expression led to a 10-30-fold increase in production of HA-pseudotyped lentivirus vectors based upon equine infectious anemia virus (EIAV) or human immunodeficiency virus type 1 (HIV-1). Experiments using the M2 inhibitor amantadine and a drug-resistant mutant of M2 established that the ion channel activity of M2 was important for M2-dependent augmentation of vector production. Furthermore, the neuraminidase activity necessary for particle release from producer cells could also be incorporated into producer cells by co-expression of influenza NA cDNA. Lentiviral vectors pseudotyped with influenza envelope proteins were able to efficiently transduce via the apical membrane of polarized mouse tracheal cultures in vitro as well as mouse tracheal epithelia in vivo.

A single amino acid substitution within the transmembrane domain of the human immunodeficiency virus type 1 Vpu protein renders simian–human immunodeficiency virus (SHIVKU-1bMC33) susceptible to rimantadine

Previous studies from our laboratory have shown that the transmembrane domain (TM) of the Vpu protein of human immunodeficiency virus type 1 (HIV-1) contributes to the pathogenesis of SHIV(KU-1bMC33) in macaques and that the TM domain of Vpu could be replaced with the M2 protein viroporin from influenza A virus. Recently, we showed that the replacement of the TM domain of Vpu with that of the M2 protein of influenza A virus resulted in a virus (SHIV(M2)) that was sensitive to rimantadine [Hout, D.R., Gomez, M.L., Pacyniak, E., Gomez, L.M., Inbody, S.H., Mulcahy, E.R., Culley, N., Pinson, D.M., Powers, M.F., Wong, S.W., Stephens, E.B., 2006. Substitution of the transmembrane domain of Vpu in simian human immunodeficiency virus (SHIV(KU-1bMC33)) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques. Virology 344, 541-558]. Based on previous studies of the M2 protein which have shown that the His-X-X-X-Trp motif within the M2 is essential to the function of the M2 proton channel, we have constructed a novel SHIV in which the alanine at position 19 of the TM domain was replaced with a histidine residue resulting in the motif His-Ile-Leu-Val-Trp. The SHIV(VpuA19H) replicated with similar kinetics as the parental SHIV(KU-1bMC33) and pulse-chase analysis revealed that the processing of viral proteins was similar to SHIV(KU-1bMC33). This SHIV(VpuA19H) virus was found to be more sensitive to the M2 ion channel blocker rimantadine than SHIV(M2). Electron microscopic examination of SHIV(VpuA19H)-infected cells treated with rimantadine revealed an accumulation of viral particles at the cell surface and within intracellular vesicles, which was similar to that previously observed to SHIV(M2)-infected cells treated with rimantadine. These data indicate that the Vpu protein of HIV-1 can be converted into a rimantadine-sensitive ion channel with the alteration of one amino acid and provide additional evidence that drugs targeting the Vpu TM/ion channel can be effective anti-HIV-1 drugs.

Sialidase activity of influenza A virus in an endocytic pathway enhances viral replication

N2 neuraminidase (NA) genes of the 1957 and 1968 pandemic influenza virus strains possessed avian-like low-pH stability of sialidase activity, unlike most epidemic strains. We generated four reverse-genetics viruses from a genetic background of A/WSN/33 (H1N1) that included parental N2 NAs of 1968 pandemic (H3N2) and epidemic (H2N2) strains or their counterpart N2 NAs in which the low-pH stability of the sialidase activity was changed by substitutions of one or two amino acid residues. We found that the transfectant viruses bearing low-pH-stable sialidase (WSN/Stable-NAs) showed 25- to 80-times-greater ability to replicate in Madin-Darby canine kidney (MDCK) cells than did the transfectant viruses bearing low-pH-unstable sialidase (WSN/Unstable-NAs). Enzymatic activities of WSN/Stable-NAs were detected in endosomes of MDCK cells after 90 min of virus internalization by in situ fluorescent detection with 5-bromo-4-chloro-indole-3-yl-alpha-N-acetylneuraminic acid and Fast Red Violet LB. Inhibition of sialidase activity of WSN/Stable-NAs on the endocytic pathway by pretreatment with 4-guanidino-2,4-dideoxy-N-acetylneuraminic acid (zanamivir) resulted in a significant decrease in progeny viruses. In contrast, the enzymatic activities of WSN/Unstable-NAs, the replication of which had no effect on pretreatment with zanamivir, were undetectable in cells under the same conditions. Hemadsorption assays of transfectant-virus-infected cells revealed that the low-pH stability of the sialidase had no effect on the process of removal of sialic acid from hemagglutinin in the Golgi regions. Moreover, high titers of viruses were recovered from the lungs of mice infected with WSN/Stable-NAs on day 3 after intranasal inoculation, but WSN/Unstable-NAs were cleared from the lungs of the mice. These results indicate that sialidase activity in late endosome/lysosome traffic enhances influenza A virus replication.

Substitution of the transmembrane domain of Vpu in simian-human immunodeficiency virus (SHIVKU1bMC33) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques

The Vpu protein of human immunodeficiency virus type 1 has been shown to shunt the CD4 receptor molecule to the proteasome for degradation and to enhance virus release from infected cells. The exact mechanism by which the Vpu protein enhances virus release is currently unknown but some investigators have shown that this function is associated with the transmembrane domain and potential ion channel properties. In this study, we determined if the transmembrane domain of Vpu could be functionally substituted with that of the prototypical viroporin, the M2 protein of influenza A virus. We constructed chimeric vpu gene in which the transmembrane domain of Vpu was replaced with that of the M2 protein of influenza. This chimeric vpu gene was substituted for the vpu gene in the genome of a pathogenic simian human immunodeficiency virus, SHIVKU-1bMC33. The resulting virus, SHIVM2, synthesized a Vpu protein that had a slightly different Mr compared to the parental SHIVKU-1bMC33, reflecting the different sizes of the two Vpu proteins. The SHIVM2 was shown to replicate with slightly reduced kinetics when compared to the parental SHIVKU-1bMC33 but electron microscopy revealed that the site of maturation was similar to the parental virus SHIVKU1bMC33. We show that the replication and spread of SHIVM2 could be blocked with the antiviral drug rimantadine, which is known to target the M2 ion channel. Our results indicate a dose dependent inhibition of SHIVM2 with 100 microM rimantadine resulting in a >95% decrease in p27 released into the culture medium. Rimantadine did not affect the replication of the parental SHIVKU-1bMC33. Examination of SHIVM2-infected cells treated with 50 microM rimantadine revealed numerous viral particles associated with the cell plasma membrane and within intracytoplasmic vesicles, which is similar to HIV-1 mutants lacking a functional vpu. To determine if SHIVM2 was as pathogenic as the parental SHIVKU-1bMC33 virus, two pig-tailed macaques were inoculated and followed for up to 8 months. Both pig-tailed macaques developed severe CD4+ T cell loss within 1 month of inoculation, high viral loads, and histological lesions consistent with lymphoid depletion similar to the parental SHIVKU-1bMC33. Taken together, these results indicate for the first time that the TM domain of the Vpu protein can be functionally substituted with the TM of M2 of influenza A virus, and shows that compounds that target the TM domain of Vpu protein of HIV-1 could serve as novel anti-HIV-1 drugs.

The influenza A virus M2 cytoplasmic tail is required for infectious virus production and efficient genome packaging

The M2 integral membrane protein encoded by influenza A virus possesses an ion channel activity that is required for efficient virus entry into host cells. The role of the M2 protein cytoplasmic tail in virus replication was examined by generating influenza A viruses encoding M2 proteins with truncated C termini. Deletion of 28 amino acids (M2Stop70) resulted in a virus that produced fourfold-fewer particles but >1,000-fold-fewer infectious particles than wild-type virus. Expression of the full-length M2 protein in trans restored the replication of the M2 truncated virus. Although the M2Stop70 virus particles were similar to wild-type virus in morphology, the M2Stop70 virions contained reduced amounts of viral nucleoprotein and genomic RNA, indicating a defect in vRNP packaging. The data presented indicate the M2 cytoplasmic tail plays a role in infectious virus production by coordinating the efficient packaging of genome segments into influenza virus particles.

ADP-ribosylation factor 1-independent protein sorting and export from the trans-Golgi network

Polarized epithelial cells efficiently sort newly synthesized apical and basolateral proteins into distinct transport carriers that emerge from the trans-Golgi network (TGN), and this sorting is recapitulated in nonpolarized cells. While the targeting signals of basolaterally destined proteins are generally cytoplasmically disposed, apical sorting signals are not typically accessible to the cytosol, and the transport machinery required for segregation and export of apical cargo remains largely unknown. Here we investigated the molecular requirements for TGN export of the apical marker influenza hemagglutinin (HA) in HeLa cells using an in vitro reconstitution assay. HA was released from the TGN in intact membrane-bound compartments, and export was dependent on addition of an ATP-regenerating system and exogenous cytosol. HA release was inhibited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as well as under conditions known to negatively regulate apical transport in vivo, including expression of the acid-activated proton channel influenza M2. Interestingly, release of HA was unaffected by depletion of ADP-ribosylation factor 1, a small GTPase that has been implicated in the recruitment of all known adaptors and coat proteins to the Golgi complex. Furthermore, regulation of HA release by GTPgammaS or M2 expression was unaffected by cytosolic depletion of ADP-ribosylation factor 1, suggesting that HA sorting remains functionally intact in the absence of the small GTPase. These data suggest that TGN sorting and export of influenza HA does not require classical adaptors involved in the formation of other classes of exocytic carriers and thus appears to proceed via a novel mechanism.

The transmembrane domain of hepatitis C virus E1 glycoprotein induces cell death

The E1 protein of hepatitis C virus (HCV) shows the ability to induce cell lysis by the alteration of membrane permeability when expressed in Escherichia coli cells. This function seems to be an intrinsic property of a C-terminal hydrophobic region of E1 as permeability changes and cell lysis can be blocked by mutagenesis of specific amino acids in this domain. To establish whether the expression of E1 protein and its C-terminal domain was able to induce cell death also in eukaryotic cell, we cloned HCV sequences expressing the full-length E1 (E383), the C-terminal domain (SVP) and a mutant lacking the C-terminal region (E340) in the pRC/CMV expression vector. HepG2 cell line was co-transfected with empty vector or HCV expression plasmids and a reporter vector that expressed beta-galactosidase (beta-gal) to visualize co-transfected blue cells. At 60 h after transfection, the loss of blue cells, considered as a measure of cell death, was 31.5 and 64.3% for the E1 and SVP clones. On the contrary, the number of blue cells after transfection with E340 plasmid was similar to that observed with the control vector. The analysis by the terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL) assay revealed an increased number of apoptotic cells at 48 h after transfection with E1 and SVP clones. Furthermore, cells transfected with SVP revealed a typical internucleosomal DNA fragmentation and the activation of caspase-3-like proteases as the specific inhibitor Ac-DEVD-CHO peptide partially blocked SVP apoptosis. These data indicate that the intracellular expression of HCV E1 protein and its C-terminal domain induces an apoptotic response in human hepatoma cell line.

The influenza virus ion channel and maturation cofactor M2 is a cholesterol-binding protein

The influenza-virus M2 protein has proton channel activity required for virus uncoating and maturation of hemagglutinin (HA) through low-pH compartments. The proton channel is cytotoxic in heterologous expression systems and can be blocked with rimantadine. In an independent, rimantadine-resistant function, M2, interacting with the M1 protein, controls the shape of virus particles. These bud from cholesterol-rich membrane rafts where viral glycoproteins and matrix (M1)/RNP complexes assemble. We demonstrate that M2 preparations from influenza virus-infected cells and from a baculovirus expression system contain 0.5-0.9 molecules of cholesterol per monomer. Sequence analyses of the membrane-proximal M2 endodomain reveal interfacial hydrophobicity, a cholesterol-binding motif first identified in peripheral benzodiazepine receptor and human immunodeficiency virus gp41, and an overlapping phosphatidylinositol 4,5-bisphosphate-binding motif. M2 induced rimantadine-reversible cytotoxicity in intrinsically cholesterol-free E. coli, and purified E. coli-expressed M2 functionally reconstituted into cholesterol-free liposomes supported rimantadine-sensitive proton translocation. Therefore, cholesterol was nonessential for M2 ion-channel function and cytotoxicity and for the effect of rimantadine. Only about 5-8% of both M2 preparations, regardless of cholesterol content, associated with detergent-resistant membranes. Cholesterol affinity and palmitoylation, in combination with a short transmembrane segment suggest M2 is a peripheral raft protein. Preference for the raft/non-raft interface may determine colocalization with HA during apical transport, the low level of M2 incorporated into the viral envelope and its undisclosed role in virus budding for which a model is presented. M2 may promote clustering and merger of rafts and the pinching-off (fission) of virus particles.

The small hydrophobic (SH) protein accumulates within lipid-raft structures of the Golgi complex during respiratory syncytial virus infection

The cellular distribution of the small hydrophobic (SH) protein in respiratory syncytial virus (RSV)-infected cells was examined. Although the SH protein was distributed throughout the cytoplasm, it appeared to accumulate in the Golgi complex within membrane structures that were enriched in the raft lipid, GM1. The ability of the SH protein to interact with lipid-raft membranes was further confirmed by examining its detergent-solubility properties in Triton X-100 at 4 degrees C. This analysis showed that a large proportion of the SH protein exhibited detergent-solubility characteristics that were consistent with an association with lipid-raft membranes. Analysis of virus-infected cells by immuno-transmission electron microscopy revealed SH protein clusters on the cell surface, but only very low levels of the protein appeared to be associated with mature virus filaments and inclusion bodies. These data suggest that during virus infection, the compartments in the secretory pathway, such as the endoplasmic reticulum (ER) and Golgi complex, are major sites of accumulation of the SH protein. Furthermore, although a significant amount of this protein interacts with lipid-raft membranes within the Golgi complex, its presence within mature virus filaments is minimal.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

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.

Construction of yellow fever-influenza A chimeric virus particles

In order to obtain a better understanding of the functional mechanisms involved in the fusogenesis of enveloped viruses, the influenza A (X31) and the yellow fever (17DD) virus particles were used to construct a chimeric structure based on their distinct pH requirements for fusion, and the distinct malleability of their nucleocapsids. The malleable nucleocapsid of the influenza A virus particle is characterized by a pleomorphic configuration when observed by electron microscopy. A heat inactivated preparation of X31 virus was used as a lectin to interact with the sialic acid domains present in the 17DD virus envelope. The E spikes of 17DD virus were induced to promote fusion of both envelopes, creating a double genome enveloped structure, the chimeric yellow fever-influenza A virus particle. These chimeric viral particles, originally denominated 'partículas virais quiméricas' (PVQ), were characterized by their infectious capacity for different biological systems. Cell inoculation with PVQ resulted in viral products that showed similar characteristics to those obtained after 17DD virus infections. Our findings open new opportunities towards the understanding of both virus particles and aspects of cellular physiologic quality control. The yellow fever-influenza A chimeric particles, by means of their hybrid composition, should be a valuable tool in the study of cell biology and the function of viral components.

Abnormal glycosylation and altered Golgi structure in colorectal cancer: dependence on intra-Golgi pH

Abnormal glycosylation of cellular glycoconjugates is a common phenotypic change in many human tumors. Here, we explore the possibility that an altered Golgi pH may also be responsible for these cancer-associated glycosylation abnormalities. We show that a mere dissipation of the acidic Golgi pH results both in increased expression of some cancer-associated carbohydrate antigens and in structural disorganization of the Golgi apparatus in otherwise normally glycosylating cells. pH dependence of these alterations was confirmed by showing that an acidification-defective breast cancer cell line (MCF-7) also displayed a fragmented Golgi apparatus, whereas the Golgi apparatus was structurally normal in its acidification-competent subline (MCF-7/AdrR). Acidification competence was also found to rescue normal glycosylation potential in MCF-7/AdrR cells. Finally, we show that abnormal glycosylation is also accompanied by similar structural disorganization and fragmentation of the Golgi apparatus in colorectal cancer cells in vitro and in vivo. These results suggest that an inappropriate Golgi pH may indeed be responsible for the abnormal Golgi structure and lowered glycosylation potential of the Golgi apparatus in malignant cells.

Multiple roles for phosphatidylinositol 4-kinase in biosynthetic transport in polarized Madin-Darby canine kidney cells

Phosphatidylinositols (PI) play important roles in regulating numerous cellular processes including cytoskeletal organization and membrane trafficking. The control of PI metabolism by phosphatidylinositol kinases has been the subject of extensive investigation; however, little is known about how phosphatidylinositol kinases regulate traffic in polarized epithelial cells. Because phosphatidylinositol 4-kinase (PI4K)-mediated phosphatidylinositol 4-phosphate (PI(4)P) production has been suggested to regulate biosynthetic traffic in yeast and mammalian cells, we have examined the role of PI4Kbeta in protein delivery in polarized MDCK cells, at different levels of the biosynthetic pathway. Expression of wild type PI4Kbeta had no effect on the rate of transport of influenza hemagglutinin (HA) through the Golgi complex, but inhibited the rate of trans-Golgi network (TGN)-to-cell surface delivery of this protein. By contrast, expression of dominant-negative, kinase-dead PI4Kbeta (PI4Kbeta(D656A)) inhibited intra-Golgi transport but stimulated TGN-to-cell surface delivery of HA. Moreover, expression of PI4Kbeta(D656A) significantly increased the solubility in cold Triton X-100 of HA staged in the TGN, suggesting that altered association of HA with lipid rafts may be responsible for the enhanced transport rate. Both wild type and kinase-dead PI4Kbeta inhibited basolateral delivery of vesicular stomatitis virus G protein, suggesting an effector function for PI4Kbeta in the regulation of basolateral traffic. Thus, by contrast with the observed requirement for PI4Kbeta activity and PI(4)P for efficient transport in yeast, our data suggest that changes in PI(4)P levels can stimulate and inhibit Golgi to cell surface delivery in mammalian cells.

Evidence against the acidification hypothesis in cystic fibrosis

The pleiotropic effects of cystic fibrosis (CF) result from the mislocalization or inactivity of an apical membrane chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR may also modulate intracellular chloride conductances and thus affect organelle pH. To test the role of CFTR in organelle pH regulation, we developed a model system to selectively perturb the pH of a subset of acidified compartments in polarized cells and determined the effects on various protein trafficking steps. We then tested whether these effects were observed in cells lacking wild-type CFTR and whether reintroduction of CFTR affected trafficking in these cells. Our model system involves adenovirus-mediated expression of the influenza virus M2 protein, an acid-activated ion channel. M2 expression selectively slows traffic through the trans-Golgi network (TGN) and apical endocytic compartments in polarized Madin-Darby canine kidney (MDCK) cells. Expression of M2 or treatment with other pH perturbants also slowed protein traffic in the CF cell line CFPAC, suggesting that the TGN in this cell line is normally acidified. Expression of functional CFTR had no effect on traffic and failed to rescue the effect of M2. Our results argue against a role for CFTR in the regulation of organelle pH and protein trafficking in epithelial cells.

Overexpression of frequenin, a modulator of phosphatidylinositol 4-kinase, inhibits biosynthetic delivery of an apical protein in polarized madin-darby canine kidney cells

Polyphosphoinositides regulate numerous steps in membrane transport. The levels of individual phosphatidylinositols are controlled by specific lipid kinases, whose activities and localization are in turn regulated by a variety of effectors. Here we have examined the effect of overexpression of frequenin, a modulator of phosphatidylinositol 4-kinase activity, on biosynthetic and postendocytic traffic in polarized Madin-Darby canine kidney cells. Endogenous frequenin was identified in these cells by polymerase chain reaction, Western blotting, and indirect immunofluorescence. Adenoviral-mediated overexpression of frequenin had no effect on early Golgi transport of membrane proteins, as assessed by acquisition of resistance to endoglycosidase H. However, delivery of newly synthesized influenza hemagglutinin from the trans-Golgi network to the apical cell surface was severely inhibited in cells overexpressing frequenin, whereas basolateral delivery of the polymeric immunoglobulin receptor was unaffected. Overexpression of frequenin did not affect postendocytic trafficking steps including apical and basolateral recycling and basal-to-apical transcytosis. We conclude that frequenin, and by inference, phosphatidylinositol 4-kinase, plays an important and selective role in apical delivery in polarized cells.

Influenza M2 proton channel activity selectively inhibits trans-Golgi network release of apical membrane and secreted proteins in polarized Madin-Darby canine kidney cells

The function of acidification in protein sorting along the biosynthetic pathway has been difficult to elucidate, in part because reagents used to alter organelle pH affect all acidified compartments and are poorly reversible. We have used a novel approach to examine the role of acidification in protein sorting in polarized Madin-Darby canine kidney (MDCK) cells. We expressed the influenza virus M2 protein, an acid-activated ion channel that equilibrates lumenal and cytosolic pH, in polarized MDCK cells and examined the consequences on the targeting and delivery of apical and basolateral proteins. M2 activity affects the pH of only a subset of acidified organelles, and its activity can be rapidly reversed using ion channel blockers (Henkel, J.R., G. Apodaca, Y. Altschuler, S. Hardy, and O.A. Weisz. 1998. Mol. Biol. Cell. 8:2477-2490; Henkel, J.R., J.L. Popovich, G.A. Gibson, S.C. Watkins, and O.A. Weisz. 1999. J. Biol. Chem. 274:9854-9860). M2 expression significantly decreased the kinetics of cell surface delivery of the apical membrane protein influenza hemagglutinin, but not of the basolaterally delivered polymeric immunoglobulin receptor. Similarly, the kinetics of apical secretion of a soluble form of gamma-glutamyltranspeptidase were reduced with no effect on the basolaterally secreted fraction. Interestingly, M2 activity had no effect on the rate of secretion of a nonglycosylated protein (human growth hormone [hGH]) that was secreted equally from both surfaces. However, M2 slowed apical secretion of a glycosylated mutant of hGH that was secreted predominantly apically. Our results suggest a role for acidic trans-Golgi network pH in signal-mediated loading of apical cargo into forming vesicles.

Selective perturbation of early endosome and/or trans-Golgi network pH but not lysosome pH by dose-dependent expression of influenza M2 protein

Many sorting stations along the biosynthetic and endocytic pathways are acidified, suggesting a role for pH regulation in protein traffic. However, the function of acidification in individual compartments has been difficult to examine because global pH perturbants affect all acidified organelles in the cell and also have numerous side effects. To circumvent this problem, we have developed a method to selectively perturb the pH of a subset of acidified compartments. We infected HeLa cells with a recombinant adenovirus encoding influenza virus M2 protein (an acid-activated ion channel that dissipates proton gradients across membranes) and measured the effects on various steps in protein transport. At low multiplicity of infection (m.o.i.), delivery of influenza hemagglutinin from the trans-Golgi network to the cell surface was blocked, but there was almost no effect on the rate of recycling of internalized transferrin. At higher m.o.i., transferrin recycling was inhibited, suggesting increased accumulation of M2 in endosomes. Interestingly, even at the higher m.o.i., M2 expression had no effect on lysosome morphology or on EGF degradation, suggesting that lysosomal pH was not compromised by M2 expression. However, delivery of newly synthesized cathepsin D to lysosomes was slowed in cells expressing active M2, suggesting that acidification of the TGN and endosomes is important for efficient delivery of lysosomal hydrolases. Fluorescence labeling using a pH-sensitive dye confirmed the reversible effect of M2 on the pH of a subset of acidified compartments in the cell. The ability to dissect the role of acidification in individual steps of a complex pathway should be useful for numerous other studies on protein processing and transport.

Selective perturbation of apical membrane traffic by expression of influenza M2, an acid-activated ion channel, in polarized madin-darby canine kidney cells

The function of acidification along the endocytic pathway is not well understood, in part because the perturbants used to modify compartmental pH have global effects and in some cases alter cytoplasmic pH. We have used a new approach to study the effect of pH perturbation on postendocytic traffic in polarized Madin-Darby canine kidney (MDCK) cells. Influenza M2 is a small membrane protein that functions as an acid-activated ion channel and can elevate the pH of the trans-Golgi network and endosomes. We used recombinant adenoviruses to express the M2 protein of influenza virus in polarized MDCK cells stably transfected with the polymeric immunoglobulin (Ig) receptor. Using indirect immunofluorescence and immunoelectron microscopy, M2 was found to be concentrated at the apical plasma membrane and in subapical vesicles; intracellular M2 colocalized partly with internalized IgA in apical recycling endosomes as well as with the trans-Golgi network marker TGN-38. Expression of M2 slowed the rate of IgA transcytosis across polarized MDCK monolayers. The delay in transport occurred after IgA reached the apical recycling endosome, consistent with the localization of intracellular M2. Apical recycling of IgA was also slowed in the presence of M2, whereas basolateral recycling of transferrin and degradation of IgA were unaffected. By contrast, ammonium chloride affected both apical IgA and basolateral transferrin release. Together, our data suggest that M2 expression selectively perturbs acidification in compartments involved in apical delivery without disrupting other postendocytic transport steps.