Plasma membrane-porating domain in poliovirus 2B protein. A short peptide mimics viroporin activity

African Swine Fever Virus Gene B117L Encodes a Small Protein Endowed with Low-pH-Dependent Membrane Permeabilizing Activity

African swine fever virus (ASFV) is causing a devastating pandemic in domestic and wild swine in Central Europe to East Asia, resulting in economic losses for the swine industry. The virus contains a large double-stranded DNA genome that contains more than 150 genes, most with no experimentally characterized function. In this study, we evaluate the potential function of the product of ASFV gene B117L, a 115-amino-acid integral membrane protein transcribed at late times during the virus replication cycle and showing no homology to any previously published protein. Hydrophobicity distribution along B117L confirmed the presence of a single transmembrane helix, which, in combination with flanking amphipathic sequences, composes a potential membrane-associated C-terminal domain of ca. 50 amino acids. Ectopic transient cell expression of the B117L gene as a green fluorescent protein (GFP) fusion protein revealed the colocalization with markers of the endoplasmic reticulum (ER). Intracellular localization of various B117L constructs also displayed a pattern for the formation of organized smooth ER (OSER) structures compatible with the presence of a single transmembrane helix with a cytoplasmic carboxy terminus. Using partially overlapping peptides, we further demonstrated that the B117L transmembrane helix has the capacity to establish spores and ion channels in membranes at low pH. Furthermore, our evolutionary analysis showed the high conservation of the transmembrane domain during the evolution of the B117L gene, indicating that the integrity of this domain is preserved by the action of the purifying selection. Collectively our data support a viroporin-like assistant role for the B117L gene-encoded product in ASFV entry. IMPORTANCE ASFV is responsible for an extensively distributed pandemic causing important economic losses in the pork industry in Eurasia. The development of countermeasures is partially limited by the insufficient knowledge regarding the function of the majority of the more than 150 genes present on the virus genome. Here, we provide data regarding the functional experimental evaluation of a previously uncharacterized ASFV gene, B117L. Our data suggest that the B117L gene encodes a small membrane protein that assists in the permeabilization of the ER-derived envelope during ASFV infection.

Zika M—A Potential Viroporin: Mutational Study and Drug Repurposing

Genus Flavivirus contains several important human pathogens. Among these, the Zika virus is an emerging etiological agent that merits concern. One of its structural proteins, prM, plays an essential role in viral maturation and assembly, making it an attractive drug and vaccine development target. Herein, we have characterized ZikV-M as a potential viroporin candidate using three different bacteria-based assays. These assays were subsequently employed to screen a library of repurposed drugs from which ten compounds were identified as ZikV-M blockers. Mutational analyses of conserved amino acids in the transmembrane domain of other flaviviruses, including West Nile and Dengue virus, were performed to study their role in ion channel activity. In conclusion, our data show that ZikV-M is a potential ion channel that can be used as a drug target for high throughput screening and drug repurposing.

PepFect14 Signaling and Transfection

PepFect14 is a cell-penetrating peptide (CPP) derived from stearylated transportan-10 (strearil-TP10) with which it shares the stearic acid residue on C' terminus and the amino acid sequence except for lysines that in PepFect14 are substituted with ornithines. Being non-proteinogenic amino acids, ornithines make PepFect14 less sensitive to serum proteases and due to its positive charges the CPP can form complexes with negatively charged cargos, such as splice correcting oligonucleotides (SCOs), plasmid DNA (pDNA), and proteins. It has been reported that PepFect14/SCO complexes enter the cells mainly through endocytosis, in particular: macopinocitosys and caveolae-mediated endocytosis through the interaction with two receptors of the scavenger receptors class A family (SCARAs). PepFect14 and its complexes trigger the chaperone-mediated autophagy response involving the heat shock protein family (HSP70) whose inhibition leads to an increase of PepFect14 transfection efficacy. Exploiting the interaction between HSP70 and PepFect14 and their ability to form nanoparticle. HSP70 has been delivered in Bomirsky Hamster Melanoma cells (BHM) using PepFect14 of which a protocol is described at the end of this chapter.

Gut Susceptibility to Viral Invasion: Contributing Roles of Diet, Microbiota and Enteric Nervous System to Mucosal Barrier Preservation

The gastrointestinal lumen is a rich source of eukaryotic and prokaryotic viruses which, together with bacteria, fungi and other microorganisms comprise the gut microbiota. Pathogenic viruses inhabiting this niche have the potential to induce local as well as systemic complications; among them, the viral ability to disrupt the mucosal barrier is one mechanism associated with the promotion of diarrhea and tissue invasion. This review gathers recent evidence showing the contributing effects of diet, gut microbiota and the enteric nervous system to either support or impair the mucosal barrier in the context of viral attack.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

Recent Progress on Functional Genomics Research of Enterovirus 71

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

Structural and Functional Properties of Viral Membrane Proteins

Viruses have developed a large variety of transmembrane proteins to carry out their infectious cycles. Some of these proteins are simply anchored to membrane via transmembrane helices. Others, however, adopt more interesting structures to perform tasks such as mediating membrane fusion and forming ion-permeating channels. Due to the dynamic or plastic nature shown by many of the viral membrane proteins, structural and mechanistic understanding of these proteins has lagged behind their counterparts in prokaryotes and eukaryotes. This chapter provides an overview of the use of NMR spectroscopy to unveil the transmembrane and membrane-proximal regions of viral membrane proteins, as well as their interactions with potential therapeutics.

Studying the Mechanism of Membrane Permeabilization Induced by Antimicrobial Peptides Using Patch-Clamp Techniques

Many short peptides selectively permeabilize the bacteria plasma membrane, leading to their lyses and death: they are therefore a source of antibacterial molecules and inspiration for novel and more selective drugs, which may have wider application in many other fields, as selective anticancer drugs. In this chapter, it is presented a new method to investigate the permeabilization properties of antimicrobial peptides under strict physiological conditions, employing the patch-clamp technique coupled to a fast perfusion system.

Mutation-induced changes of transmembrane pore size revealed by combined ion-channel conductance and single vesicle permeabilization analyses

Permeabilization of the Endoplasmic Reticulum (ER) is instrumental in the progression of host-cell infection by many viral pathogens. We have described that permeabilization of ER model membranes by the pore-forming domain of the Classical Swine Fever Virus (CSFV) p7 protein depends on two sequence determinants: the C-terminal transmembrane helix, and the preceding polar loop that regulates its activity. Here, by combining ion-channel activity measurements in planar lipid bilayers with imaging of single Giant Unilamellar Vesicles (GUVs), we demonstrate that point substitutions directed to conserved residues within these regions affect ER-like membrane permeabilization following distinct mechanisms. Whereas the polar loop appeared to be involved in protein insertion and oligomerization, substitution of residues predicted to face the lumen of the pore inhibited large conducting channels (>1 nS) over smaller ones (120 pS). Quantitative analyses of the ER-GUV distribution as a function of the solute size revealed a selective inhibition for the permeation of solutes with sizes larger than 4 kDa, further demonstrating that the mutation targeting the transmembrane helix prevented formation of the large pores. Collectively, our data support the idea that the pore-forming domain of p7 may assemble into finite pores with approximate diameters of 1 and 5 nm. Moreover, the observation that the mutation interfering with formation of the larger pores can hamper virus production without affecting ER localization or homo-oligomerization, suggests prospective strategies to block/attenuate pestiviruses.

Organelle luminal dependence of (+)strand RNA virus replication reveals a hidden druggable target

Positive-strand RNA viruses replicate their genomes in membrane-bounded cytoplasmic complexes. We show that endoplasmic reticulum (ER)-linked genomic RNA replication by brome mosaic virus (BMV), a well-studied member of the alphavirus superfamily, depends on the ER luminal thiol oxidase ERO1. We further show that BMV RNA replication protein 1a, a key protein for the formation and function of vesicular BMV RNA replication compartments on ER membranes, permeabilizes these membranes to release oxidizing potential from the ER lumen. Conserved amphipathic sequences in 1a are sufficient to permeabilize liposomes, and mutations in these sequences simultaneously block membrane permeabilization, formation of a disulfide-linked, oxidized 1a multimer, 1a's RNA capping function, and productive genome replication. These results reveal new transmembrane complexities in positive-strand RNA virus replication, show that-as previously reported for certain picornaviruses and flaviviruses-some alphavirus superfamily members encode viroporins, identify roles for such viroporins in genome replication, and provide a potential new foundation for broad-spectrum antivirals.

Enterovirus 71 2B Induces Cell Apoptosis by Directly Inducing the Conformational Activation of the Proapoptotic Protein Bax

To survive and replicate within a host, many viruses have evolved strategies that target crucial components within the apoptotic cascade, leading to either inhibition or induction of cell apoptosis. Enterovirus 71 (EV71) infections have been demonstrated to impact the mitochondrial apoptotic pathway and induce apoptosis in many cell lines. However, the detailed mechanism of EV71-induced apoptosis remains to be elucidated. In this study, we report that EV71 2B protein (2B) localized to the mitochondria and induced cell apoptosis by interacting directly with and activating the proapoptotic protein Bax. 2B recruited Bax to the mitochondria and induced Bax conformational activation. In addition, mitochondria isolated from 2B-expressing cells that were treated with a recombinant Bax showed increased Bax interaction and cytochrome c (Cyt c) release. Importantly, apoptosis in cells with either EV71 infection or 2B expression was dramatically reduced in Bax knockdown cells but not in Bak knockdown cells, suggesting that Bax played a pivotal role in EV71- or 2B-induced apoptosis. Further studies indicate that a hydrophobic region of 18 amino acids (aa) in the C-terminal region of 2B (aa 63 to 80) was responsible for the location of 2B in the mitochondria. A hydrophilic region of 14 aa in the N-terminal region of 2B was functional in Bax interaction and its subsequent activation. Moreover, overexpression of the antiapoptotic protein Bcl-X_L abrogates 2B-induced release of Cyt c and caspase activation. Therefore, this study provides direct evidence that EV71 2B induces cell apoptosis and impacts the mitochondrial apoptotic pathway by directly modulating the redistribution and activation of proapoptotic protein Bax.

IMPORTANCE

EV71 infections are usually accompanied by severe neurological complications. It has also been postulated that the induction of cell apoptosis resulting from tissue damage is a possible process of EV71-related pathogenesis. In this study, we report that EV71 2B protein (2B) localized to the mitochondria and induced cell apoptosis by interacting directly with and activating the proapoptotic protein Bax. This study provides evidence that EV71 induces cell apoptosis by modulating Bax activation and reveals important clues regarding the mechanism of Cyt c release and mitochondrial permeabilization during EV71 infection.

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

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

Viroporins, Examples of the Two-Stage Membrane Protein Folding Model

Viroporins are small, α-helical, hydrophobic virus encoded proteins, engineered to form homo-oligomeric hydrophilic pores in the host membrane. Viroporins participate in multiple steps of the viral life cycle, from entry to budding. As any other membrane protein, viroporins have to find the way to bury their hydrophobic regions into the lipid bilayer. Once within the membrane, the hydrophobic helices of viroporins interact with each other to form higher ordered structures required to correctly perform their porating activities. This two-step process resembles the two-stage model proposed for membrane protein folding by Engelman and Poppot. In this review we use the membrane protein folding model as a leading thread to analyze the mechanism and forces behind the membrane insertion and folding of viroporins. We start by describing the transmembrane segment architecture of viroporins, including the number and sequence characteristics of their membrane-spanning domains. Next, we connect the differences found among viroporin families to their viral genome organization, and finalize focusing on the pathways used by viroporins in their way to the membrane and on the transmembrane helix-helix interactions required to achieve proper folding and assembly.

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

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

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

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

Mechanistic insight into CM18-Tat11 peptide membrane-perturbing action by whole-cell patch-clamp recording

The membrane-destabilization properties of the recently-introduced endosomolytic CM₁₈-Tat₁₁ hybrid peptide (KWKLFKKIGAVLKVLTTG-YGRKKRRQRRR, residues 1–7 of cecropin-A, 2–12 of melittin, and 47–57 of HIV-1 Tat protein) are investigated in CHO-K1 cells by using the whole-cell configuration of the patch-clamp technique. CM₁₈-Tat₁₁, CM₁₈, and Tat₁₁ peptides are administered to the cell membrane with a computer-controlled micro-perfusion system. CM₁₈-Tat₁₁ induces irreversible cell-membrane permeabilization at concentrations (≥4 µM) at which CM₁₈ triggers transient pore formation, and Tat₁₁ does not affect membrane integrity. We argue that the addition of the Tat₁₁ module to CM₁₈ is able to trigger a shift in the mechanism of membrane destabilization from “toroidal” to “carpet”, promoting a detergent-like membrane disruption. Collectively, these results rationalize previous observations on CM₁₈-Tat₁₁ delivery properties that we believe can guide the engineering of new modular peptides tailored to specific cargo-delivery applications.

The minimalist architectures of viroporins and their therapeutic implications

Many viral genomes encode small, integral membrane proteins that form homo-oligomeric channels in membrane, and they transport protons, cations, and other molecules across the membrane barrier to aid various steps of viral entry and maturation. These viral proteins, collectively named viroporins, are crucial for viral pathogenicity. In the past five years, structures obtained by nuclear magnetic resonance (NMR), X-ray crystallography, and electron microscopy (EM) showed that viroporins often adopt minimalist architectures to achieve their functions. A number of small molecules have been identified to interfere with their channel activities and thereby inhibit viral infection, making viroporins potential drug targets for therapeutic intervention. The known architectures and inhibition mechanisms of viroporins differ significantly from each other, but some common principles are shared between them. This review article summarizes the recent developments in the structural investigation of viroporins and their inhibition by antiviral compounds. This article is part of a Special Issue entitled: Viral Membrane Proteins-Channels for Cellular Networking.

Detection of antibodies specific for foot-and-mouth disease virus infection using indirect ELISA based on recombinant nonstructural protein 2B

Foot-and-mouth disease (FMD) is a highly contagious viral disease of transboundary importance. In India, since the launch of the FMD control programme, there has been a substantial increase in the vaccinated bovine population. In this scenario, there is a need for additional locally developed non-structural protein (NSP)-based immnoassays for efficient identification of FMD virus (FMDV)-infected animals in the vaccinated population. The 2B NSP of FMDV, lacking the transmembrane domain (Δ2B), was expressed successfully in a prokaryotic system, and an indirect ELISA (I-ELISA) was developed and validated in this study. The diagnostic sensitivity and specificity of the Δ2B I-ELISA were found to be 95.3 % and 94.6 %, respectively. In experimentally infected cattle, the assay could consistently detect Δ2B-NSP-specific antibodies from 10 to approximately 400 days postinfection. The assay was further validated with bovine serum samples collected randomly from different parts of the country. The performance of the Δ2B I-ELISA was compared with the in-house r3AB3 I-ELISA, and the overall concordance in test results was found to be 86.49 %. The Δ2B I-ELISA could be useful as a screening or confirmatory assay in the surveillance of FMD irrespective of vaccination.

Pore-forming activity of pestivirus p7 in a minimal model system supports genus-specific viroporin function

Viroporins are small integral membrane proteins functional in viral assembly and egress by promoting permeabilization. Blocking of viroporin function therefore constitutes a target for antiviral development. Classical swine fever virus (CSFV) protein p7 has been recently regarded as a class II viroporin. Here, we sought to establish the determinants of the CSFV p7 permeabilizing activity in a minimal model system. Assessment of an overlapping peptide library mapped the porating domain to the C-terminal hydrophobic stretch (residues 39-67). Pore-opening dependence on pH or sensitivity to channel blockers observed for the full protein required the inclusion of a preceding polar sequence (residues 33-38). Effects of lipid composition and structural data further support that the resulting peptide (residues 33-67), may comprise a bona fide surrogate to assay p7 activity in model membranes. Our observations imply that CSFV p7 relies on genus-specific structures-mechanisms to perform its viroporin function.

Viroporins: structure and biological functions

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

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

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

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

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

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

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

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

Mechanism of function of viral channel proteins and implications for drug development

Viral channel-forming proteins comprise a class of viral proteins which, similar to their host companions, are made to alter electrochemical or substrate gradients across lipid membranes. These proteins are active during all stages of the cellular life cycle of viruses. An increasing number of proteins are identified as channel proteins, but the precise role in the viral life cycle is yet unknown for the majority of them. This review presents an overview about these proteins with an emphasis on those with available structural information. A concept is introduced which aligns the transmembrane domains of viral channel proteins with those of host channels and toxins to give insights into the mechanism of function of the viral proteins from potential sequence identities. A summary of to date investigations on drugs targeting these proteins is given and discussed in respect of their mode of action in vivo.

Membrane integration of poliovirus 2B viroporin

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

Membrane potential depolarization as a triggering mechanism for Vpu-mediated HIV-1 release

Vpu, a component unique to HIV-1, greatly enhances the efficiency of viral particle release by unclear mechanisms. This Vpu function is intrinsically linked to its channel-like structure, which enables it to interfere with homologous transmembrane structures in infected cells. Because Vpu interacts destructively with host background K⁺ channels that set the cell resting potential, we hypothesized that Vpu might trigger viral release by destabilizing the electric field across a budding membrane. Here, we found that the efficiency of Vpu-mediated viral release is inversely correlated with membrane potential polarization. By inhibiting the background K⁺ currents, Vpu dissipates the voltage constraint on viral particle discharge. As a proof of concept, we show that HIV-1 release can be accelerated by externally imposed depolarization alone. Our findings identify the trigger of Vpu-mediated release as a manifestation of the general principle of depolarization-stimulated exocytosis.

An inside job: subversion of the host secretory pathway by intestinal pathogens

PURPOSE OF REVIEW

The cellular secretory pathway, composed of the endoplasmic reticulum, Golgi apparatus, and cellular vesicles, mediates the intracellular trafficking of proteins and lipids. Gastrointestinal pathogens frequently affect the functions of enterocytes, the differentiated cells involved in secretion and absorption of extracellular molecules. Microbial pathogenesis can be enhanced by altering the trafficking of key molecules such as brush border enzymes, soluble immune mediators such as cytokines and chemokines, and MHC Class I molecules, all of which rely on the secretory pathway for their appropriate cellular localization. This review focuses on our current understanding of the distinct mechanisms employed by enteric pathogens to antagonize the secretory pathway.

RECENT FINDINGS

Many pathogens encode individual or multiple proteins to antagonize the secretory pathway, including disrupting the trafficking of vesicles between the endoplasmic reticulum, Golgi, and plasma membrane. This antagonism allows for increased pathogenesis and can assist, directly or indirectly, in microbial replication. Virtually all arms of the secretory pathway are targeted by intestinal pathogens, supporting the pathogenic significance of shutting this pathway down.

SUMMARY

This review summarizes the mechanisms utilized by gut pathogens to disrupt the cellular secretory pathway and addresses potential therapeutic targets to combat these highly prevalent and burdensome microbes.

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.

Cell permeabilization by poliovirus 2B viroporin triggers bystander permeabilization in neighbouring cells through a mechanism involving gap junctions

Poliovirus 2B protein is a well‐known viroporin implicated in plasma membrane permeabilization to ions and low‐molecular‐weight compounds during infection. Translation in mammalian cells expressing 2B protein is inhibited by hygromycin B (HB) but remains unaffected in mock cells, which are not permeable to the inhibitor. Here we describe a previously unreported bystander effect in which healthy baby hamster kidney (BHK) cells become sensitive to HB when co‐cultured with a low proportion of cells expressing poliovirus 2B. Viroporins E from mouse hepatitis virus, 6K from Sindbis virus and NS4A protein from hepatitis C virus were also able to permeabilize neighbouring cells to different extents. Expression of 2B induced permeabilization of neighbouring cell lines other than BHK. We found that gap junctions are responsible mediating the observed bystander permeabilization. Gap junctional communication was confirmed in 2B‐expressing co‐cultures by fluorescent dye transfer. Moreover, the presence of connexin 43 was confirmed in both mock and 2B‐transfected cells. Finally, inhibition of HB entry to neighbouring cells was observed with 18α‐glycyrrhethinic acid, an inhibitor of gap junctions. Taken together, these findings support a mechanism involving gap junctional intercellular communication in the bystander permeabilization effect observed in healthy cells co‐cultured with poliovirus 2B‐expressing cells.

A peptide based on the pore-forming domain of pro-apoptotic poliovirus 2B viroporin targets mitochondria

Non-structural poliovirus 2B protein induces plasma membrane permeabilization and has been recently implicated in triggering apoptosis via the mitochondrial pathway. Here we describe that the pore-forming P3 peptide, based on the 2B amphipathic domain, translocates through the plasma membrane of culture cells and targets mitochondria. Cell permeabilization by P3 versions of different lengths, together with peptide uptake analyses supported an internalization mechanism dependent on P3 capacity to interact physically with lipid bilayers and establish permeating pores therein. Internalized P3 was found associated with mitochondria, but contrary to the parental 2B protein, the short peptide did not affect the morphology or cell distribution of these organelles, nor induced apoptosis. We conclude that P3 constitutes a mitochondriotropic sequence, which is however devoid of 2B pro-apoptotic activity.

Foot-and-mouth disease virus, but not bovine enterovirus, targets the host cell cytoskeleton via the nonstructural protein 3Cpro

Foot-and-mouth disease virus (FMDV), a member of the Picornaviridae, is a pathogen of cloven-hoofed animals and causes a disease of major economic importance. Picornavirus-infected cells show changes in cell morphology and rearrangement of cytoplasmic membranes, which are a consequence of virus replication. We show here, by confocal immunofluorescence and electron microscopy, that the changes in morphology of FMDV-infected cells involve changes in the distribution of microtubule and intermediate filament components during infection. Despite the continued presence of centrosomes in infected cells, there is a loss of tethering of microtubules to the microtubule organizing center (MTOC) region. Loss of labeling for gamma-tubulin, but not pericentrin, from the MTOC suggests a targeting of gamma-tubulin (or associated proteins) rather than a total breakdown in MTOC structure. The identity of the FMDV protein(s) responsible was determined by the expression of individual viral nonstructural proteins and their precursors in uninfected cells. We report that the only viral nonstructural protein able to reproduce the loss of gamma-tubulin from the MTOC and the loss of integrity of the microtubule system is FMDV 3C(pro). In contrast, infection of cells with another picornavirus, bovine enterovirus, did not affect gamma-tubulin distribution, and the microtubule network remained relatively unaffected.

Cis-active RNA elements (CREs) and picornavirus RNA replication

Our understanding of picornavirus RNA replication has improved over the past 10 years, due in large part to the discovery of cis-active RNA elements (CREs) within picornavirus RNA genomes. CREs function as templates for the conversion of VPg, the Viral Protein of the genome, into VPgpUpU(OH). These so called CREs are different from the previously recognized cis-active RNA sequences and structures within the 5' and 3' NTRs of picornavirus genomes. Two adenosine residues in the loop of the CRE RNA structures allow the viral RNA-dependent RNA polymerase 3D(Pol) to add two uridine residues to the tyrosine residue of VPg. Because VPg and/or VPgpUpU(OH) prime the initiation of viral RNA replication, the asymmetric replication of viral RNA could not be explained without an understanding of the viral RNA template involved in the conversion of VPg into VPgpUpU(OH) primers. We review the growing body of knowledge regarding picornavirus CREs and discuss how CRE RNAs work coordinately with viral replication proteins and other cis-active RNAs in the 5' and 3' NTRs during RNA replication.