Penetration of nonenveloped viruses into the cytoplasm

Rotaviruses reach late endosomes and require the cation-dependent mannose-6-phosphate receptor and the activity of cathepsin proteases to enter the cell

Rotaviruses (RVs) enter cells through different endocytic pathways. Bovine rotavirus (BRV) UK uses clathrin-mediated endocytosis, while rhesus rotavirus (RRV) employs an endocytic process independent of clathrin and caveolin. Given the differences in the cell internalization pathway used by these viruses, we tested if the intracellular trafficking of BRV UK was the same as that of RRV, which is known to reach maturing endosomes (MEs) to infect the cell. We found that BRV UK also reaches MEs, since its infectivity depends on the function of Rab5, the endosomal sorting complex required for transport (ESCRT), and the formation of endosomal intraluminal vesicles (ILVs). However, unlike RRV, the infectivity of BRV UK was inhibited by knocking down the expression of Rab7, indicating that it has to traffic to late endosomes (LEs) to infect the cell. The requirement for Rab7 was also shared by other RV strains of human and porcine origin. Of interest, most RV strains that reach LEs were also found to depend on the activities of Rab9, the cation-dependent mannose-6-phosphate receptor (CD-M6PR), and cathepsins B, L, and S, suggesting that cellular factors from the trans-Golgi network (TGN) need to be transported by the CD-M6PR to LEs to facilitate RV cell infection. Furthermore, using a collection of UK × RRV reassortant viruses, we found that the dependence of BRV UK on Rab7, Rab9, and CD-M6PR is associated with the spike protein VP4. These findings illustrate the elaborate pathway of RV entry and reveal a new process (Rab9/CD-M6PR/cathepsins) that could be targeted for drug intervention.

IMPORTANCE

Rotavirus is an important etiological agent of severe gastroenteritis in children. In most instances, viruses enter cells through an endocytic pathway that delivers the viral particle to vesicular organelles known as early endosomes (EEs). Some viruses reach the cytoplasm from EEs, where they start to replicate their genome. However, other viruses go deeper into the cell, trafficking from EEs to late endosomes (LEs) to disassemble and reach the cytoplasm. In this work, we show that most RV strains have to traffic to LEs, and the transport of endolysosomal proteases from the Golgi complex to LEs, mediated by the mannose-6-phosphate receptor, is necessary for the virus to exit the vesicular compartment and efficiently start viral replication. We also show that this deep journey into the cell is associated with the virus spike protein VP4. These findings illustrate the elaborate pathway of RV entry that could be used for drug intervention.

Antiviral mechanisms of human defensins

Defensins are an effector component of the innate immune system with broad antimicrobial activity. Humans express two types of defensins, α- and β-defensins, which have antiviral activity against both enveloped and non-enveloped viruses. The diversity of defensin-sensitive viral species reflects a multitude of antiviral mechanisms. These include direct defensin targeting of viral envelopes, glycoproteins, and capsids in addition to inhibition of viral fusion and post-entry neutralization. Binding and modulation of host cell surface receptors and disruption of intracellular signaling by defensins can also inhibit viral replication. In addition, defensins can function as chemokines to augment and alter adaptive immune responses, revealing an indirect antiviral mechanism. Nonetheless, many questions regarding the antiviral activities of defensins remain. Although significant mechanistic data are known for α-defensins, molecular details for β-defensin inhibition are mostly lacking. Importantly, the role of defensin antiviral activity in vivo has not been addressed due to the lack of a complete defensin knockout model. Overall, the antiviral activity of defensins is well established as are the variety of mechanisms by which defensins achieve this inhibition; however, additional research is needed to fully understand the role of defensins in viral pathogenesis.

Exploiting endocytosis for nanomedicines

In this article, we briefly review the endocytic pathways used by cells, pointing out their defining characteristics and highlighting physical limitations that may direct the internalization of nanoparticles to a subset of these pathways. A more detailed description of these pathways is presented in the literature. We then focus on the endocytosis of nanomedicines and present how various nanomaterial parameters impact these endocytic processes. This topic is an area of active research, motivated by the recognition that an improved understanding of how nanomaterials interact at the molecular, cellular, and whole-organism level will lead to the design of better nanomedicines in the future. Next, we briefly review some of the important nanomedicines already on the market or in clinical development that serve to exemplify how endocytosis can be exploited for medical benefit. Finally, we present some key unanswered questions and remaining challenges to be addressed by the field.

Open membranes are the precursors for assembly of large DNA viruses

Nucleo cytoplasmic large DNA viruses (NCLDVs) are a group of double-stranded DNA viruses that replicate their DNA partly or entirely in the cytoplasm in association with viral factories (VFs). They share about 50 genes suggesting that they are derived from a common ancestor. Using transmission electron microscopy (TEM) and electron tomography (ET) we showed that the NCLDV vaccinia virus (VACV) acquires its membrane from open membrane intermediates, derived from the ER. These open membranes contribute to the formation of a single open membrane of the immature virion, shaped into a sphere by the assembly of the viral scaffold protein on its convex side. We now compare VACV with the NCLDV Mimivirus by TEM and ET and show that the latter also acquires its membrane from open membrane intermediates that accumulate at the periphery of the cytoplasmic VF. In analogy to VACV this membrane is shaped by the assembly of a layer on the convexside of its membrane, likely representing the Mimivirus capsid protein. By quantitative ET we show for both viruses that the open membrane intermediates of assembly adopt an 'open-eight' conformation with a characteristic diameter of 90 nm for Mimi- and 50 nm for VACV. We discuss these results with respect to the common ancestry of NCLDVs and propose a hypothesis on the possible origin of this unusual membrane biogenesis.

Interferon-inducible transmembrane protein 3 (IFITM3) restricts reovirus cell entry

Reoviruses are double-stranded RNA viruses that infect the mammalian respiratory and gastrointestinal tract. Reovirus infection elicits production of type I interferons (IFNs), which trigger antiviral pathways through the induction of interferon-stimulated genes (ISGs). Although hundreds of ISGs have been identified, the functions of many of these genes are unknown. The interferon-inducible transmembrane (IFITM) proteins are one class of ISGs that restrict the cell entry of some enveloped viruses, including influenza A virus. One family member, IFITM3, localizes to late endosomes, where reoviruses undergo proteolytic disassembly; therefore, we sought to determine whether IFITM3 also restricts reovirus entry. IFITM3-expressing cell lines were less susceptible to infection by reovirus, as they exhibited significantly lower percentages of infected cells in comparison to control cells. Reovirus replication was also significantly reduced in IFITM3-expressing cells. Additionally, cells expressing an shRNA targeting IFITM3 exhibited a smaller decrease in infection after IFN treatment than the control cells, indicating that endogenous IFITM3 restricts reovirus infection. However, IFITM3 did not restrict entry of reovirus infectious subvirion particles (ISVPs), which do not require endosomal proteolysis, indicating that restriction occurs in the endocytic pathway. Proteolysis of outer capsid protein μ1 was delayed in IFITM3-expressing cells in comparison to control cells, suggesting that IFITM3 modulates the function of late endosomal compartments either by reducing the activity of endosomal proteases or delaying the proteolytic processing of virions. These data provide the first evidence that IFITM3 restricts infection by a nonenveloped virus and suggest that IFITM3 targets an increasing number of viruses through a shared requirement for endosomes during cell entry.

How viruses use the endoplasmic reticulum for entry, replication, and assembly

To cause infection, a virus enters a host cell, replicates, and assembles, with the resulting new viral progeny typically released into the extracellular environment to initiate a new infection round. Virus entry, replication, and assembly are dynamic and coordinated processes that require precise interactions with host components, often within and surrounding a defined subcellular compartment. Accumulating evidence pinpoints the endoplasmic reticulum (ER) as a crucial organelle supporting viral entry, replication, and assembly. This review focuses on the molecular mechanism by which different viruses co-opt the ER to accomplish these crucial infection steps. Certain bacterial toxins also hijack the ER for entry. An interdisciplinary approach, using rigorous biochemical and cell biological assays coupled with advanced microscopy strategies, will push to the next level our understanding of the virus-ER interaction during infection.

High temporal resolution imaging reveals endosomal membrane penetration and escape of adenoviruses in real time

Imaging host-pathogen interactions in real time can provide significant insight into dynamic processes and provide information about time and space of their occurences. Here, we present detailed experimental instructions on how to image the membrane penetration process of the non-enveloped adenovirus in real time. The system is based on a cell line stably expressing the lectin galectin-3 fused to a fluorophore. Membrane-lytic events during adenovirus cell entry can be monitored by the recruitment of galectin-3 to galactose-containing membrane glycoproteins on the exo-surface of ruptured membranes. The simultaneous use of fluorescently labeled adenoviral capsids allows to image the events in unmatched temporal resolution.

Poxvirus membrane biogenesis: rupture not disruption

Enveloped viruses acquire their membrane from the host by budding at, or wrapping by, cellular membranes. Transmission electron microscopy (TEM) images, however, suggested that the prototype member of the poxviridae, vaccinia virus (VACV), may create its membrane 'de novo' with free open ends exposed in the cytosol. Within the frame of the German-wide priority programme we re-addressed the biogenesis and origin of the VACV membrane using electron tomography (ET), cryo-EM and lipid analysis of purified VACV using mass spectrometry (MS). This review discussed how our data led to a model of unconventional membrane biogenesis involving membrane rupture and the generation of a single open membrane from open membrane intermediates. Lipid analyses of purified virus by MS suggest an ER origin with a relatively low cholesterol content compared with whole cells, confirming published data. Unlike previous reports using thin-layer chromatography, no depletion of phosphatidylethanolamine was detected. We did detect, however, an enrichment for phosphatidic acid, diacylglycerol and phosphatidylinositol in the virion. Our data are discussed in the light of other pathogens that may requirecellular membrane rupture during their intracellular life cycle.

Investigating endocytic pathways to the endoplasmic reticulum and to the cytosol using SNAP-trap

Cholera toxin enters cells via an unusual pathway that involves trafficking through endosomes to the endoplasmic reticulum (ER). Whether the toxin induces its own pathway or travels along a physiological retrograde route is not known. To study its trafficking, we labeled cholera toxin B (CTB) or endogenous plasma membrane proteins with a small chemical compound, benzylguanine, which covalently reacts with the protein SNAP-tag. Using ER-targeted SNAP-tag as reporter, we found that transport of CTB to the ER depends on dynamin-2 and syntaxin 5. Plasma membrane proteins and a fluid-phase marker added to the medium were also transported to the ER. This flux was not affected by exposing cells to CTB but was inhibited by depleting syntaxin 5 and increased by depleting dynamin-2. As a control for confined intracellular localization of ER-targeted SNAP-tag we used adenovirus-5, which traffics to endosomes and then escapes into the cytosol. The virus did not react with ER-targeted SNAP but with cytosolic SNAP. Together, our results establish a new method (SNAP-trap) to study trafficking of different cargo to the ER and the cytosol and provide evidence for the existence of a constitutive pathway from the cell surface to the ER.

A transmembrane domain and GxxxG motifs within L2 are essential for papillomavirus infection

During cellular invasion, human papillomavirus type 16 (HPV16) must transfer its viral genome (vDNA) across the endosomal membrane prior to its accumulation at nuclear PML bodies for the establishment of infection. After cellular uptake, the capsid likely undergoes pH-dependent disassembly within the endo-/lysosomal compartment, thereby exposing hidden domains in L2 that facilitate membrane penetration of L2/vDNA complexes. In an effort to identify regions of L2 that might physically interact with membranes, we have subjected the L2 sequence to multiple transmembrane (TM) domain prediction algorithms. Here, we describe a conserved TM domain within L2 (residues 45 to 67) and investigate its role in HPV16 infection. In vitro, the predicted TM domain adopts an alpha-helical structure in lipid environments and can function as a real TM domain, although not as efficiently as the bona fide TM domain of PDGFR. An L2 double point mutant renders the TM domain nonfunctional and blocks HPV16 infection by preventing endosomal translocation of vDNA. The TM domain contains three highly conserved GxxxG motifs. These motifs can facilitate homotypic and heterotypic interactions between TM helices, activities that may be important for vDNA translocation. Disruption of some of these GxxxG motifs resulted in noninfectious viruses, indicating a critical role in infection. Using a ToxR-based homo-oligomerization assay, we show a propensity for this TM domain to self-associate in a GxxxG-dependent manner. These data suggest an important role for the self-associating L2 TM domain and the conserved GxxxG motifs in the transfer of vDNA across the endo-/lysosomal membrane.

Nucleotide-resolution profiling of RNA recombination in the encapsidated genome of a eukaryotic RNA virus by next-generation sequencing

Next-generation sequencing has been used in numerous investigations to characterize and quantify the genetic diversity of a virus sample through the mapping of polymorphisms and measurement of mutation frequencies. Next-generation sequencing has also been employed to identify recombination events occurring within the genomes of higher organisms, for example, detecting alternative RNA splicing events and oncogenic chromosomal rearrangements. Here, we combine these two approaches to profile RNA recombination within the encapsidated genome of a eukaryotic RNA virus, flock house virus. We detect hundreds of thousands of recombination events, with single-nucleotide resolution, which result in diversity in the encapsidated genome rivaling that due to mismatch mutation. We detect previously identified defective RNAs as well as many other abundant and novel defective RNAs. Our approach is exceptionally sensitive and unbiased and requires no prior knowledge beyond the virus genome sequence. RNA recombination is a powerful driving force behind the evolution and adaptation of RNA viruses. The strategy implemented here is widely applicable and provides a highly detailed description of the complex mutational landscape of the transmissible viral genome.

Reovirus uses multiple endocytic pathways for cell entry

Entry of reovirus virions has been well studied in several tissue culture systems. After attachment to junctional adhesion molecule A (JAM-A), virions undergo clathrin-mediated endocytosis followed by proteolytic disassembly of the capsid and penetration to the cytoplasm. However, during in vivo infection of the intestinal tract, and likely in the tumor microenvironment, capsid proteolysis (uncoating) is initiated extracellularly. We used multiple approaches to determine if uncoated reovirus particles, called intermediate subviral particles (ISVPs), enter cells by directly penetrating the limiting membrane or if they take advantage of endocytic pathways to establish productive infection. We found that entry and infection by reovirus ISVPs was inhibited by dynasore, an inhibitor of dynamin-dependent endocytosis, as well as by genistein and dominant-negative caveolin-1, which block caveolar endocytosis. Inhibition of caveolar endocytosis also reduced infection by reovirus virions. Extraction of membrane cholesterol with methyl-β-cyclodextrin inhibited infection by virions but had no effect when infection was initiated with ISVPs. We found this pathway to be independent of both clathrin and caveolin. Together, these data suggest that reovirus virions can use both dynamin-dependent and dynamin-independent endocytic pathways during cell entry, and they reveal that reovirus ISVPs can take advantage of caveolar endocytosis to establish productive infection.

Dissecting quasi-equivalence in nonenveloped viruses: membrane disruption is promoted by lytic peptides released from subunit pentamers, not hexamers

Nonenveloped viruses often invade membranes by exposing hydrophobic or amphipathic peptides generated by a proteolytic maturation step that leaves a lytic peptide noncovalently associated with the viral capsid. Since multiple copies of the same protein form many nonenveloped virus capsids, it is unclear if lytic peptides derived from subunits occupying different positions in a quasi-equivalent icosahedral capsid play different roles in host infection. We addressed this question with Nudaurelia capensis omega virus (NωV), an insect RNA virus with an icosahedral capsid formed by protein α, which undergoes autocleavage during maturation, producing the lytic γ peptide. NωV is a unique model because autocatalysis can be precisely initiated in vitro and is sufficiently slow to correlate lytic activity with γ peptide production. Using liposome-based assays, we observed that autocatalysis is essential for the potent membrane disruption caused by NωV. We observed that lytic activity is acquired rapidly during the maturation program, reaching 100% activity with less than 50% of the subunits cleaved. Previous time-resolved structural studies of partially mature NωV particles showed that, during this time frame, γ peptides derived from the pentamer subunits are produced and are organized in a vertical helical bundle that is projected toward the particle surface, while identical polypeptides in quasi-equivalent subunits are produced later or are in positions inappropriate for release. Our functional data provide experimental support for the hypothesis that pentamers containing a central helical bundle, observed in different nonenveloped virus families, are a specialized lytic motif.

Essential role of the unordered VP2 n-terminal domain of the parvovirus MVM capsid in nuclear assembly and endosomal enlargement of the virion fivefold channel for cell entry

The unordered N-termini of parvovirus capsid proteins (Nt) are translocated through a channel at the icosahedral five-fold axis to serve for virus traffick. Heterologous peptides were genetically inserted at the Nt of MVM to study their functional tolerance to manipulations. Insertion of a 5T4-single-chain antibody at VP2-Nt (2Nt) yielded chimeric capsid subunits failing to enter the nucleus. The VEGFR2-binding peptide (V1) inserted at both 2Nt and VP1-Nt efficiently assembled in virions, but V1 disrupted VP1 and VP2 entry functions. The VP2 defect correlated with restricted externalization of V1-2Nt out of the coat. The specific infectivity of MVM and wtVP-pseudotyped mosaic MVM-V1 virions, upon heating and/or partial 2Nt cleavage, demonstrated that some 2Nt domains become intracellularly translocated out of the virus shell and cleaved to initiate entry. The V1 insertion defines a VP2-driven endosomal enlargement of the channel as an essential structural rearrangement performed by the MVM virion to infect.

Acid-dependent viral entry

Virus infection of host cells requires that entry into the cell results in efficient genome release leading to translation and replication. These initial steps revolving around the entry and genomic release processes are crucial for viral progeny generation. Despite the variety of receptors used by viruses to initiate entry, evidence from both enveloped and non-enveloped viral infections is highlighting the important role played by intracellular acidic compartments in the entry of many viruses. These compartments provide connecting nodes within the endocytic network, presenting multiple viral internalization pathways. Endosomal compartments employing an internal acidic pH can trigger molecular mechanisms leading to disassembly of viral particles, thus providing appropriate genome delivery. Accordingly, viruses have evolved to select optimal intracellular conditions for promoting efficient genome release, leading to propagation of the infectious agent. This review will address the implications of cellular compartment involvement in virus infectious processes, and the roles played by the viruses' own machinery, including pH sensing mechanisms and the methodologies applied for studying acid-dependent viral entry into host cells.

Cytoplasmic fungal lipases release fungicides from ultra-deformable vesicular drug carriers

The Transfersome® is a lipid vesicle that contains membrane softeners, such as Tween 80, to make it ultra-deformable. This feature makes the Transfersome® an efficient carrier for delivery of therapeutic drugs across the skin barrier. It was reported that TDT 067 (a topical formulation of 15 mg/ml terbinafine in Transfersome® vesicles) has a much more potent antifungal activity in vitro compared with conventional terbinafine, which is a water-insoluble fungicide. Here we use ultra-structural studies and live imaging in a model fungus to describe the underlying mode of action. We show that terbinafine causes local collapse of the fungal endoplasmic reticulum, which was more efficient when terbinafine was delivered in Transfersome® vesicles (TFVs). When applied in liquid culture, fluorescently labeled TFVs rapidly entered the fungal cells (T_1/2∼2 min). Entry was F-actin- and ATP-independent, indicating that it is a passive process. Ultra-structural studies showed that passage through the cell wall involves significant deformation of the vesicles, and depends on a high concentration of the surfactant Tween 80 in their membrane. Surprisingly, the TFVs collapsed into lipid droplets after entry into the cell and the terbinafine was released from their interior. With time, the lipid bodies were metabolized in an ATP-dependent fashion, suggesting that cytosolic lipases attack and degrade intruding TFVs. Indeed, the specific monoacylglycerol lipase inhibitor URB602 prevented Transfersome® degradation and neutralized the cytotoxic effect of Transfersome®-delivered terbinafine. These data suggest that (a) Transfersomes deliver the lipophilic fungicide Terbinafine to the fungal cell wall, (b) the membrane softener Tween 80 allows the passage of the Transfersomes into the fungal cell, and (c) fungal lipases digest the invading Transfersome® vesicles thereby releasing their cytotoxic content. As this mode of action of Transfersomes is independent of the drug cargo, these results demonstrate the potential of Transfersomes in the treatment of all fungal diseases.

Endocytic processing of adeno-associated virus type 8 vectors for transduction of target cells

We investigated the transduction of HEK293T cells permissive to adeno-associated virus serotype 8 (AAV8) to understand the mechanisms underlying its endocytic processing. Results showed that AAV8 enters cells through clathrin-mediated endocytosis followed by trafficking through various endosomal compartments. Interestingly, compared to the relatively well-characterized AAV2, a distinct involvement of late endosomes was observed for AAV8 trafficking within the target cell. AAV8 particles were also shown to exploit the cytoskeleton network to facilitate their transport within cells. Moreover, the cellular factors involved during endosomal escape were examined by an in vitro membrane permeabilization assay. Our data demonstrated that an acidic endosomal environment was required for AAV2 penetration through endosomal membranes and that the cellular endoprotease furin could promote AAV2 escape from the early endosomes. In contrast, these factors were not sufficient for AAV8 penetration through endosomal membranes. We further found that the ubiquitin-proteasome system is likely involved in the intracellular transport of AAV8 to nucleus. Taken together, our data have shed some light on the intracellular trafficking pathways of AAV8, which, in turn, could provide insight for potentializing AAV-mediated gene delivery.

Disulfide-bond formation by a single cysteine mutation in adenovirus protein VI impairs capsid release and membrane lysis

The internal capsid protein VI mediates adenovirus (AdV) endosome penetration during cell entry. Essential to this process is the release of protein VI from the AdV capsid and subsequent membrane targeting and insertion by the liberated VI molecules within the endocytic vesicle. In this study, we describe a human AdV (HAdV) substitution mutant (AdV VI-G48C) within the critical N-terminal amphipathic α-helical domain of protein VI. The VI-G48C virus displays altered capsid stability that impacts protein VI release, membrane disruption and virus infectivity. This is due in part to aberrant disulfide-bonding of protein VI molecules within the AdV particle. Our results provide insight into the structural organization of protein VI in the virus particle, as well as highlight the role of protein VI in cell entry.

Host RNAs, including transposons, are encapsidated by a eukaryotic single-stranded RNA virus

Next-generation sequencing is a valuable tool in our growing understanding of the genetic diversity of viral populations. Using this technology, we have investigated the RNA content of a purified nonenveloped single-stranded RNA virus, flock house virus (FHV). We have also investigated the RNA content of virus-like particles (VLPs) of FHV and the related Nudaurelia capensis omega virus. VLPs predominantly package ribosomal RNA and transcripts of their baculoviral expression vectors. In addition, we find that 5.3% of the packaged RNAs are transposable elements derived from the Sf21 genome. This observation may be important when considering the therapeutic use of VLPs. We find that authentic FHV virions also package a variety of host RNAs, accounting for 1% of the packaged nucleic acid. Significant quantities of host messenger RNAs, ribosomal RNA, noncoding RNAs, and transposable elements are readily detected. The packaging of these host RNAs elicits the possibility of horizontal gene transfer between eukaryotic hosts that share a viral pathogen. We conclude that the genetic content of nonenveloped RNA viruses is variable, not just by genome mutation, but also in the diversity of RNA transcripts that are packaged.

How viruses and toxins disassemble to enter host cells

Many viruses and toxins disassemble to enter host cells and cause disease. These conformational changes must be orchestrated temporally and spatially during entry to avoid premature disassembly leading to nonproductive pathways. Although viruses and toxins are evolutionarily distinct toxic agents, emerging findings in their respective fields have revealed that the cellular locations supporting disassembly, the host factors co-opted during disassembly, the nature of the conformational changes, and the physiological function served by disassembly are strikingly conserved. Here, we examine some of the shared disassembly principles observed in model viruses and toxins. Where appropriate, we also underscore their differences. Our major intention is to draw together the fields of viral and toxin cell entry by using lessons gleaned from each field to inform and benefit one another.

Role of lipids in virus replication

Viruses intricately interact with and modulate cellular membranes at several stages of their replication, but much less is known about the role of viral lipids compared to proteins and nucleic acids. All animal viruses have to cross membranes for cell entry and exit, which occurs by membrane fusion (in enveloped viruses), by transient local disruption of membrane integrity, or by cell lysis. Furthermore, many viruses interact with cellular membrane compartments during their replication and often induce cytoplasmic membrane structures, in which genome replication and assembly occurs. Recent studies revealed details of membrane interaction, membrane bending, fission, and fusion for a number of viruses and unraveled the lipid composition of raft-dependent and -independent viruses. Alterations of membrane lipid composition can block viral release and entry, and certain lipids act as fusion inhibitors, suggesting a potential as antiviral drugs. Here, we review viral interactions with cellular membranes important for virus entry, cytoplasmic genome replication, and virus egress.

The SV40 late protein VP4 is a viroporin that forms pores to disrupt membranes for viral release

Nonenveloped viruses are generally released by the timely lysis of the host cell by a poorly understood process. For the nonenveloped virus SV40, virions assemble in the nucleus and then must be released from the host cell without being encapsulated by cellular membranes. This process appears to involve the well-controlled insertion of viral proteins into host cellular membranes rendering them permeable to large molecules. VP4 is a newly identified SV40 gene product that is expressed at late times during the viral life cycle that corresponds to the time of cell lysis. To investigate the role of this late expressed protein in viral release, water-soluble VP4 was expressed and purified as a GST fusion protein from bacteria. Purified VP4 was found to efficiently bind biological membranes and support their disruption. VP4 perforated membranes by directly interacting with the membrane bilayer as demonstrated by flotation assays and the release of fluorescent markers encapsulated into large unilamellar vesicles or liposomes. The central hydrophobic domain of VP4 was essential for membrane binding and disruption. VP4 displayed a preference for membranes comprised of lipids that replicated the composition of the plasma membranes over that of nuclear membranes. Phosphatidylethanolamine, a lipid found at high levels in bacterial membranes, was inhibitory against the membrane perforation activity of VP4. The disruption of membranes by VP4 involved the formation of pores of ∼3 nm inner diameter in mammalian cells including permissive SV40 host cells. Altogether, these results support a central role of VP4 acting as a viroporin in the perforation of cellular membranes to trigger SV40 viral release.

Viruses exploit host cells for their propagation. Once an adequate number of viral particles have been assembled within the cell through the aid of cellular machinery of the host cell, the virus must be released from the cell for the virus to spread. For nonenveloped viruses or viruses that are solely encapsulated by a protein shell, this step most commonly involves the perforation of cellular membranes resulting in the lysis or death of the host cell. The mechanism for how this key terminal step in the viral life cycle is performed is poorly understood. We demonstrated that for the model nonenveloped virus SV40, the newly discovered virally encoded protein, termed VP4, perforates membranes by forming pores with a diameter of ∼3 nm in host cell membranes. While these pores are not of a sufficient size to provide a conduit that permits the movement of the virus through the membrane, they support membrane destabilization that leads to the disintegration of the membrane of the host cell and viral release.

Liposomal nanocontainers as models for viral infection: monitoring viral genomic RNA transfer through lipid membranes

After uptake into target cells, many nonenveloped viruses undergo conformational changes in the low-pH environment of the endocytic compartment. This results in exposure of amphipathic viral peptides and/or hydrophobic protein domains that are inserted into and either disrupt or perforate the vesicular membranes. The viral nucleic acids thereby gain access to the cytosol and initiate replication. We here demonstrate the in vitro transfer of the single-stranded positive-sense RNA genome of human rhinovirus 2 into liposomes decorated with recombinant very-low-density lipoprotein receptor fragments. Membrane-attached virions were exposed to pH 5.4, mimicking the in vivo pH environment of late endosomes. This triggered the release of the RNA whose arrival in the liposomal lumen was detected via in situ cDNA synthesis by encapsulated reverse transcriptase. Subsequently, cDNA was PCR amplified. At a low ratio between virions and lipids, RNA transfer was positively correlated with virus concentration. However, membranes became leaky at higher virus concentrations, which resulted in decreased cDNA synthesis. In accordance with earlier in vivo data, the RNA passes through the lipid membrane without causing gross damage to vesicles at physiologically relevant virus concentrations.

Late-penetrating viruses

Many enveloped and non-enveloped animal viruses delay the penetration into the cytosol of host cells until they have arrived to endocytic vacuoles deep in the cytoplasm. The late timing is generally determined by a low pH-threshold for the acid-activated penetration process (pH 6.2-4.9), but there can be a combination of other reasons for a delay. Since late-penetrating viruses (L-PVs) must be sorted into the degradative pathway, they are particularly sensitive to perturbations that interfere with molecular sorting and proper maturation of endosomes, including switching of Rabs, formation of intraluminal vesicles, and microtubule-mediated transport. In this short review, we focus on L-PVs from several virus families, and their interactions with the endocytic machinery.

A large and intact viral particle penetrates the endoplasmic reticulum membrane to reach the cytosol

Non-enveloped viruses penetrate host membranes to infect cells. A cell-based assay was used to probe the endoplasmic reticulum (ER)-to-cytosol membrane transport of the non-enveloped SV40. We found that, upon ER arrival, SV40 is released into the lumen and undergoes sequential disulfide bond disruptions to reach the cytosol. However, despite these ER-dependent conformational changes, SV40 crosses the ER membrane as a large and intact particle consisting of the VP1 coat, the internal components VP2, VP3, and the genome. This large particle subsequently disassembles in the cytosol. Mutant virus and inhibitor studies demonstrate VP3 and likely the viral genome, as well as cellular proteasome, control ER-to-cytosol transport. Our results identify the sequence of events, as well as virus and host components, that regulate ER membrane penetration. They also suggest that the ER membrane supports passage of a large particle, potentially through either a sizeable protein-conducting channel or the lipid bilayer.

Biological membranes represent a major barrier during viral infection. While the mechanism by which an enveloped virus breaches the limiting membrane of a host cell is well-characterized, this membrane penetration process is poorly understood for non-enveloped viruses. Indeed, most available insights on membrane transport of non-enveloped viruses are built upon in vitro studies. Here we established a cell-based assay to elucidate the molecular mechanism by which the non-enveloped SV40 penetrates the endoplasmic reticulum (ER) membrane to access the cytosol, a critical step in infection. Strikingly, we uncovered SV40 breaches the ER membrane as a large and intact viral particle, despite the conformational changes it experiences in the ER lumen. This result suggests that the ER membrane can accommodate translocation of a large protein complex, possibly through either a sizeable protein channel or the ER membrane bilayer. In addition to this finding, we also pinpoint viral and host components that control the ER-to-cytosol membrane transport event. Together, our data illuminate the cellular mechanism by which a non-enveloped virus penetrates the limiting membrane of a target cell during infection.

Functional genetic and biophysical analyses of membrane disruption by human adenovirus

The identification of the adenovirus (AdV) protein that mediates endosome penetration during infection has remained elusive. Several lines of evidence from previous studies suggest that the membrane lytic factor of AdV is the internal capsid protein VI. While these earlier results imply a role for protein VI in endosome disruption, direct evidence during cell entry has not been demonstrated. To acquire more definitive proof, we engineered random mutations in a critical N-terminal amphipathic α-helix of VI in an attempt to generate AdV mutants that lack efficient membrane penetration and infection. Random mutagenesis within the context of the AdV genome was achieved via the development of a novel technique that incorporates both error-prone PCR and recombineering. Using this system, we identified a single mutation, L40Q, that significantly reduced infectivity and selectively impaired endosome penetration. Furthermore, we obtained biophysical data showing that the lack of efficient endosomalysis is associated with reduced insertion of the L40Q mutation in protein VI (VI-L40Q) into membranes. Our studies indicate that protein VI is the critical membrane lytic factor of AdV during cellular entry and reveal the biochemical basis for its membrane interactions.

Molecular virology of hepatitis E virus

This review details the molecular virology of the hepatitis E virus (HEV). While replicons and in vitro infection systems have recently become available, a lot of information on HEV has been generated through comparisons with better-studied positive-strand RNA viruses and through subgenomic expression of viral open reading frames. These models are now being verified with replicon and infection systems. We provide here the current knowledge on the HEV genome and its constituent proteins--ORF1, ORF2 and ORF3. Based on the available information, we also modify the existing model of the HEV life cycle.

Lysosomal localization and mechanism of membrane penetration influence nonenveloped virus activation of the NLRP3 inflammasome

Adenovirus (Ad) endosomal membrane penetration activates the NLRP3 inflammasome by releasing lysosomal cathepsin B (catB) into the cytoplasm. We therefore examined the extent to which inflammasome activation correlates with Ad colocalization with catB-enriched lysosomes. Inflammasome activation, is greater during infections with Ad5 possessing an Ad16 fiber (Ad5F16gfp), or Ad5gfp neutralized by human serum, than Ad5gfp alone. Enhanced IL-1β release by Ad5F16gfp is partially due to increased TLR9 signaling but also correlates with greater release of catB into the cytoplasm. This increased TLR9 signaling and catB release correlates with a greater localization of Ad5F16gfp to lysosomes prior to endosomal escape. Another nonenveloped virus, reovirus, requires catB to penetrate cell membranes. However, reovirus did not release catB into the cytoplasm despite significantly greater colocalization with lysosomes compared to Ad5gfp and efficient membrane penetration. Thus, not only lysosomal localization, but the mechanism of membrane penetration influences viral activation of the NLRP3 inflammasome.

Enter the kill zone: initiation of death signaling during virus entry

Infection of host cells by a variety of viruses results in programmed cell death or apoptosis. In many cases, early events in virus replication that occur prior to synthesis of viral proteins and replication of viral genomes directly or indirectly activate signaling pathways that culminate in cell death. Using examples of viruses for which prodeath signaling is better defined, this review will describe how cell entry steps including virus attachment to receptors, virus uncoating in endosomes, and events that occur following membrane penetration lead to apoptosis. The relevance and physiologic consequences of early induction of prodeath signaling to viral pathogenesis also will be discussed.

Liposomal leakage induced by virus-derived peptides, viral proteins, and entire virions: rapid analysis by chip electrophoresis

Permeabilization of model lipid membranes by virus-derived peptides, viral proteins, and entire virions of human rhinovirus was assessed by quantifying the release of a fluorescent dye from liposomes via a novel chip electrophoretic assay. Liposomal leakage readily occurred upon incubation with the pH-sensitive synthetic fusogenic peptide GALA and, less efficiently, with a 24mer peptide (P1-N) derived from the N-terminus of the capsid protein VP1 of human rhinovirus 2 (HRV2) at acidic pH. Negative stain transmission electron microscopy showed that liposomes incubated with the rhinovirus-derived peptide remained largely intact. At similar concentrations, the GALA peptide caused gross morphological changes of the liposomes. On a molar basis, the leakage-inducing efficiency of the P1 peptide was by about 2 orders of magnitude inferior to that of recombinant VP1 (from HRV89) and entire HRV2. This underscores the role in membrane destabilization of VP1 domains remote from the N-terminus and the arrangement of the peptide in the context of the icosahedral virion. Our method is rapid, requires tiny amounts of sample, and allows for the parallel determination of released and retained liposomal cargo.

A PDI family network acts distinctly and coordinately with ERp29 to facilitate polyomavirus infection

Endoplasmic reticulum (ER)-to-cytosol membrane transport is a decisive infection step for the murine polyomavirus (Py). We previously determined that ERp29, a protein disulfide isomerase (PDI) member, extrudes the Py VP1 C-terminal arm to initiate ER membrane penetration. This reaction requires disruption of Py's disulfide bonds. Here, we found that the PDI family members ERp57, PDI, and ERp72 facilitate virus infection. However, while all three proteins disrupt Py's disulfide bonds in vitro, only ERp57 and PDI operate in concert with ERp29 to unfold the VP1 C-terminal arm. An alkylated Py cannot stimulate infection, implying a pivotal role of viral free cysteines during infection. Consistent with this, we found that although PDI and ERp72 reduce Py, ERp57 principally isomerizes the virus in vitro, a reaction that requires viral free cysteines. Our mutagenesis study subsequently identified VP1 C11 and C15 as important for infection, suggesting a role for these residues during isomerization. C11 and C15 also act together to stabilize interpentamer interactions for a subset of the virus pentamers, likely because some of these residues form interpentamer disulfide bonds. This study reveals how a PDI family functions coordinately and distinctly to promote Py infection and pinpoints a role of viral cysteines in this process.

Uncoating of human rhinoviruses

Human rhinoviruses (HRVs) are a major cause of the common cold. The more than one hundred serotypes, divided into species HRV-A and HRV-B, either bind intercellular adhesion molecule 1 (major group viruses) or members of the low-density lipoprotein receptor (minor group viruses) for cell entry. Some major group HRVs can also access the host cell via heparan sulphate proteoglycans. The cell attachment protein(s) of the recently discovered phylogenetic clade HRV-C is unknown. The respective receptors direct virus uptake via clathrin-dependent or independent endocytosis or via macropinocytosis. Triggered by ICAM-1 and/or the low pH environment in endosomes the virions undergo conformational alterations giving rise to hydrophobic subviral particles. These are handed over from the receptors to the endosomal membrane. According to the current view, the RNA genome is released through an opening at one of the fivefold axes of the icosahedral capsid and crosses the membrane through a pore presumably formed by viral proteins. Alternatively, the membrane may be ruptured allowing subviral particles and RNA to enter the cytosol. Whether a channel is formed or the membrane is disrupted most probably depends on the respective HRV receptor.

Determinants of strain-specific differences in efficiency of reovirus entry

Cell entry of reovirus requires a series of ordered steps, which include conformational changes in outer capsid protein μ1 and its autocleavage. The μ1N fragment released as a consequence of these events interacts with host cell membranes and mediates their disruption, leading to delivery of the viral core into the cytoplasm. The prototype reovirus strains T1L and T3D exhibit differences in the efficiency of autocleavage, in the propensity to undergo conformational changes required for membrane penetration, and in the capacity for penetrating host cell membranes. To better understand how polymorphic differences in μ1 influence reovirus entry events, we generated recombinant viruses that express chimeric T1L-T3D μ1 proteins and characterized them for the capacity to efficiently complete each step required for membrane penetration. Our studies revealed two important functions for the central δ region of μ1. First, we found that μ1 autocleavage is regulated by the N-terminal portion of δ, which forms an α-helical pedestal structure. Second, we observed that the C-terminal portion of δ, which forms a jelly-roll β barrel structure, regulates membrane penetration by influencing the efficiency of ISVP* formation. Thus, our studies highlight the molecular basis for differences in the membrane penetration efficiency displayed by prototype reovirus strains and suggest that distinct portions of the reovirus δ domain influence different steps during entry.

Lipids and proteins act in opposing manners to regulate polyomavirus infection

How receptors control virus infection is poorly understood. Polyomavirus (Py) binds to the sialic acid-galactose moiety on receptors to gain entry into host cells and cause infection. We previously demonstrated that the sialic acid-galactose-containing glycolipids called gangliosides GD1a and GT1b promote Py infection, in part, by sorting the virus from the endolysosomes to the endoplasmic reticulum (ER), a critical infection route. Whether these glycolipids act as Py entry receptors, however, is not clear. Additionally, as the majority of glycoproteins also harbor terminal sialic acid-galactose residues, their roles in Py infection are also not well established. Using a ganglioside-deficient cell line, we show that GD1a is the functional entry receptor for Py. GD1a binds to Py on the plasma membrane, and the receptor-virus complex is internalized and transported to the late endosomes and then the ER to initiate infection. In contrast, our findings indicate that glycoproteins act as decoy receptors, restricting the ER transport and infection of Py. Thus, glycolipids and glycoproteins, two major constituents of the plasma membrane, execute opposing functions in regulating infection by a defined virus.

Subunits fold at position-dependent rates during maturation of a eukaryotic RNA virus

Effective antiviral agents are difficult to develop because of the close relationship between the cell biology of the virus and host. However, viral capsid maturation, the in vivo process where the particle transitions from a noninfectious provirion to an infectious virion, is an ideal process to interrupt because the provirion is usually fragile and the conversion to the virion often involves large conformational changes and autocatalytic chemistry that can be hampered by small molecules. The Nudaurelia capensis omega virus (N omegaV) is one of the few eukaryotic viruses where this process can be investigated in vitro with a variety of biophysical methods, allowing fundamental chemical and structural principles of the maturation to be established. It has a T = 4 quasi-equivalent capsid with a dramatic maturation pathway that includes a particle size reduction of 100 A and an autocatalytic cleavage. Here we use cryo-EM and difference maps, computed at three time points following maturation initiation, to show that regions of N omegaV subunit folding are maturation dependent and occur at rates determined by their quasi-equivalent position in the capsid, explaining the unusual kinetics of the maturation cleavage. This study shows that folding is rapid and peptide chain self-cleavage occurs early for subunits adjacent to 3-fold and 5-fold icosahedral symmetry elements and that folding is slower in regions where molecular switches are required for the formation of the proper interfacial contacts. The results connect viral maturation to the well-studied assembly-dependent folding that occurs in the formation of cellular complexes.

Different rotavirus strains enter MA104 cells through different endocytic pathways: the role of clathrin-mediated endocytosis

Rotaviruses, the single most important agents of acute severe gastroenteritis in children, are nonenveloped viruses formed by a three-layered capsid that encloses a genome formed by 11 segments of double-stranded RNA. The mechanism of entry of these viruses into the host cell is not well understood. The best-studied strain, RRV, which is sensitive to neuraminidase (NA) treatment of the cells, uses integrins alpha2 beta1 and alphav beta3 and the heat shock protein hsc70 as receptors and enters MA104 cells through a non-clathrin-, non-caveolin-mediated pathway that depends on a functional dynamin and on the presence of cholesterol on the cell surface. In this work, using a combination of pharmacological, biochemical, and genetic approaches, we compared the entry characteristics of four rotavirus strains known to have different receptor requirements. We chose four rotavirus strains that represent all phenotypic combinations of NA resistance or sensitivity and integrin dependence or independence. We found that even though all the strains share their requirements for hsc70, dynamin, and cholesterol, three of them differ from the simian strain RRV in the endocytic pathway used. The human strain Wa, porcine strain TFR-41, and bovine strain UK seem to enter the cell through clathrin-mediated endocytosis, since treatments that inhibit this pathway block their infectivity; consistent with this entry route, these strains were sensitive to changes in the endosomal pH. The inhibition of other endocytic mechanisms, such as macropinocytosis or caveola-mediated uptake, had no effect on the internalization of the rotavirus strains tested here.

Endocytosis of hepatitis C virus non-enveloped capsid-like particles induces MAPK-ERK1/2 signaling events

Although HCV is an enveloped virus, naked nucleocapsids have been reported in the serum of infected patients. The HCV core particle serves as a protective capsid shell for the viral genome and recombinant in vitro assembled HCV core particles induce strong specific immunity. We investigated the post-binding mechanism of recombinant core particle uptake and its intracellular fate. In hepatic cells, these particles are internalized, most likely in a clathrin-dependent pathway, reaching early to late endosomes and finally lysosomes. The endocytic acidic milieu is implicated in trafficking process. Using specific phosphoantibodies, signaling pathway inhibitors and chemical agents, ERK(1/2) was found to be activated in a sustained way after endocytosis, followed by downstream immediate early genes (c-fos and egr-1) modulation. We propose that the intriguing properties of cellular internalization of HCV non-enveloped particles can induce specific ERK(1/2)-MAPKs events that could be important in HCV life cycle and pathogenesis of HCV infection.

The propagation of prion-like protein inclusions in neurodegenerative diseases

The most common neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, are characterized by the misfolding of a small number of proteins that assemble into ordered aggregates in affected brain cells. For many years, the events leading to aggregate formation were believed to be entirely cell-autonomous, with protein misfolding occurring independently in many cells. Recent research has now shown that cell non-autonomous mechanisms are also important for the pathogenesis of neurodegenerative diseases with intracellular filamentous inclusions. The intercellular transfer of inclusions made of tau, alpha-synuclein, huntingtin and superoxide dismutase 1 has been demonstrated, revealing the existence of mechanisms reminiscent of those by which prions spread through the nervous system.

An N-terminal domain of adenovirus protein VI fragments membranes by inducing positive membrane curvature

Adenovirus (Ad) membrane penetration during cell entry is poorly understood. Here we show that antibodies which neutralize the membrane lytic activity of the Ad capsid protein VI interfere with Ad endosomal membrane penetration. In vitro studies using a peptide corresponding to an N-terminal amphipathic alpha-helix of protein VI (VI-Phi), as well as other truncated forms of protein VI suggest that VI-Phi is largely responsible for protein VI binding to and lysing of membranes. Additional studies suggest that VI-Phi lies nearly parallel to the membrane surface. Protein VI fragments membranes and induces highly curved structures. Further studies suggest that protein VI induces positive membrane curvature. These data support a model in which protein VI binds membranes, inducing positive curvature strain which ultimately leads to membrane fragmentation. These results agree with previous observations of Ad membrane permeabilization during cell entry and provide an initial mechanistic description of a nonenveloped virus membrane lytic protein.

3.3 A cryo-EM structure of a nonenveloped virus reveals a priming mechanism for cell entry

To achieve cell entry, many nonenveloped viruses must transform from a dormant to a primed state. In contrast to the membrane fusion mechanism of enveloped viruses (e.g., influenza virus), this membrane penetration mechanism is poorly understood. Here, using single-particle cryo-electron microscopy, we report a 3.3 A structure of the primed, infectious subvirion particle of aquareovirus. The density map reveals side-chain densities of all types of amino acids (except glycine), enabling construction of a full-atom model of the viral particle. Our structure and biochemical results show that priming involves autocleavage of the membrane penetration protein and suggest that Lys84 and Glu76 may facilitate this autocleavage in a nucleophilic attack. We observe a myristoyl group, covalently linked to the N terminus of the penetration protein and embedded in a hydrophobic pocket. These results suggest a well-orchestrated process of nonenveloped virus entry involving autocleavage of the penetration protein prior to exposure of its membrane-insertion finger.

Effect of mutations in VP5 hydrophobic loops on rotavirus cell entry

Experiments in cell-free systems have demonstrated that the VP5 cleavage fragment of the rotavirus spike protein, VP4, undergoes a foldback rearrangement that translocates three clustered hydrophobic loops from one end of the molecule to the other. This conformational change resembles the foldback rearrangements of enveloped virus fusion proteins. By recoating rotavirus subviral particles with recombinant VP4 and VP7, we tested the effects on cell entry of substituting hydrophilic for hydrophobic residues in the clustered VP5 loops. Several of these mutations decreased the infectivity of recoated particles without preventing either recoating or folding back. In particular, the V391D mutant had a diminished capacity to interact with liposomes when triggered to fold back by serial protease digestion in solution, and particles recoated with this mutant VP4 were 10,000-fold less infectious than particles recoated with wild-type VP4. Particles with V391D mutant VP4 attached normally to cells and internalized efficiently, but they failed in the permeabilization step that allows coentry of the toxin alpha-sarcin. These findings indicate that the hydrophobicity of the VP5 apex is required for membrane disruption during rotavirus cell entry.

Conformational changes in the capsid of a calicivirus upon interaction with its functional receptor

Nonenveloped viral capsids are metastable structures that undergo conformational changes during virus entry that lead to interactions of the capsid or capsid fragments with the cell membrane. For members of the Caliciviridae, neither the nature of these structural changes in the capsid nor the factor(s) responsible for inducing these changes is known. Feline junctional adhesion molecule A (fJAM-A) mediates the attachment and infectious viral entry of feline calicivirus (FCV). Here, we show that the infectivity of some FCV isolates is neutralized following incubation with the soluble receptor at 37 degrees C. We used this property to select mutants resistant to preincubation with the soluble receptor. We isolated and sequenced 24 soluble receptor-resistant (srr) mutants and characterized the growth properties and receptor-binding activities of eight mutants. The location of the mutations within the capsid structure of FCV was mapped using a new 3.6-A structure of native FCV. The srr mutations mapped to the surface of the P2 domain were buried at the protruding domain dimer interface or were present in inaccessible regions of the capsid protein. Coupled with data showing that both the parental FCV and the srr mutants underwent increases in hydrophobicity upon incubation with the soluble receptor at 37 degrees C, these findings indicate that FCV likely undergoes conformational change upon interaction with its receptor. Changes in FCV capsid conformation following its interaction with fJAM-A may be important for subsequent interactions of the capsid with cellular membranes, membrane penetration, and genome delivery.

Cellular entry of polyomaviruses

Polyomaviruses (Pys) are nonenveloped DNA tumor viruses that include the murine polyomavirus (mPy), simian virus 40 (SV40), and the human BK, JC, KI, WU, and Merkel Cell viruses. To cause infection, Pys must enter host cells and navigate through various intracellular compartments, where they undergo sequential conformational changes enabling them to uncoat and deliver the DNA genome into the nucleus. The ensuing transcription and replication of the genome leads to lytic infection or cell transformation. In recent years, a more coherent understanding of how Pys are transported from the plasma membrane to the nucleus is starting to emerge. This review will focus on the decisive steps of Py entry, including engagement of the host cell receptor, targeting to the endoplasmic reticulum (ER), penetration across the ER membrane, nuclear entry, and genome release. Strikingly, a number of these steps resemble the intoxication pathway of the AB(5) bacterial toxins. Thus, as Pys and bacterial toxins hijack similar cellular machineries during infection, a general principle appears to guide their entry into host cells.

Depletion of virion-associated divalent cations induces parvovirus minute virus of mice to eject its genome in a 3'-to-5' direction from an otherwise intact viral particle

We describe a structural rearrangement that can occur in parvovirus minute virus of mice (MVMp) virions following prolonged exposure to buffers containing 0.5 mM EDTA. Such particles remain stable at 4 degrees C but undergo a conformational shift upon heating to 37 degrees C at pH 7.2 that leads to the ejection of much of the viral genome in a 3'-to-5' direction, leaving the DNA tightly associated with the otherwise intact capsid. This rearrangement can be prevented by the addition of 1 mM CaCl(2) or MgCl(2) prior to incubation at 37 degrees C, suggesting that readily accessible divalent cation binding sites in the particle are critical for genome retention. Uncoating was not seen following the incubation of virions at pH 5.5 and 37 degrees C or at pH 7.2 and 37 degrees C in particles with subgenomic DNA, suggesting that pressure exerted by the full-length genome may influence this process. Uncoated genomes support complementary-strand synthesis by T7 DNA polymerase, but synthesis aborts upstream of the right-hand end, which remains capsid associated. We conclude that viral genomes are positioned so that their 3' termini and coding sequences can be released from intact particles at physiological temperatures by a limited conformational rearrangement. In the presence of divalent cations, incremental heating between 45 degrees C and 65 degrees C induces structural transitions that first lead to the extrusion of VP1 N termini, followed by genome exposure. However, in cation-depleted virions, the sequence of these shifts is blurred. Moreover, cation-depleted particles that have been induced to eject their genomes at 37 degrees C continue to sequester their VP1 N termini within the intact capsid, suggesting that these two extrusion events represent separable processes.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

Low endocytic pH and capsid protein autocleavage are critical components of Flock House virus cell entry

The process by which nonenveloped viruses cross cell membranes during host cell entry remains poorly defined; however, common themes are emerging. Here, we use correlated in vivo and in vitro studies to understand the mechanism of Flock House virus (FHV) entry and membrane penetration. We demonstrate that low endocytic pH is required for FHV infection, that exposure to acidic pH promotes FHV-mediated disruption of model membranes (liposomes), and particles exposed to low pH in vitro exhibit increased hydrophobicity. In addition, FHV particles perturbed by heating displayed a marked increase in liposome disruption, indicating that membrane-active regions of the capsid are exposed or released under these conditions. We also provide evidence that autoproteolytic cleavage, to generate the lipophilic gamma peptide (4.4 kDa), is required for membrane penetration. Mutant, cleavage-defective particles failed to mediate liposome lysis, regardless of pH or heat treatment, suggesting that these particles are not able to expose or release the requisite membrane-active regions of the capsid, namely, the gamma peptides. Based on these results, we propose an updated model for FHV entry in which (i) the virus enters the host cell by endocytosis, (ii) low pH within the endocytic pathway triggers the irreversible exposure or release of gamma peptides from the virus particle, and (iii) the exposed/released gamma peptides disrupt the endosomal membrane, facilitating translocation of viral RNA into the cytoplasm.

A lipid receptor sorts polyomavirus from the endolysosome to the endoplasmic reticulum to cause infection

The mechanisms by which receptors guide intracellular virus transport are poorly characterized. The murine polyomavirus (Py) binds to the lipid receptor ganglioside GD1a and traffics to the endoplasmic reticulum (ER) where it enters the cytosol and then the nucleus to initiate infection. How Py reaches the ER is unclear. We show that Py is transported initially to the endolysosome where the low pH imparts a conformational change that enhances its subsequent ER-to-cytosol membrane penetration. GD1a stimulates not viral binding or entry, but rather sorting of Py from late endosomes and/or lysosomes to the ER, suggesting that GD1a binding is responsible for ER targeting. Consistent with this, an artificial particle coated with a GD1a antibody is transported to the ER. Our results provide a rationale for transport of Py through the endolysosome, demonstrate a novel endolysosome-to-ER transport pathway that is regulated by a lipid, and implicate ganglioside binding as a general ER targeting mechanism.

To cause infection, viruses must reach appropriate compartments within the cell where they undergo a programmed series of conformational changes that enable the viral genome to be exposed and released. The mechanisms that target viruses to these compartments are often not clear. Here we study the infectious pathway of the murine polyomavirus (Py). Py is transported from the cell surface to the intracellular organelle called the endoplasmic reticulum (ER), where it breaches the ER membrane to reach the nucleus to stimulate infection. How Py is transported from the cell surface to the ER is poorly characterized. Our studies show that Py first enters the endolysosome compartments before reaching the ER. The low pH of the endolysosome imparts a structural change on the virus that facilitates its subsequent ER membrane penetration. Importantly, transport of Py from the endolysosome to the ER is guided by the lipid receptor ganglioside GD1a. We also demonstrate that an artificial bead capable of binding to GD1a is transported to the ER as well. Collectively, our data identify a lipid-dependent mechanism that targets a virus to its appropriate organelle during infection.