Disassociation of the SV40 genome from capsid proteins prior to nuclear entry

Components of the LINC and NPC complexes coordinately target and translocate a virus into the nucleus to promote infection

Nuclear entry represents the final and decisive infection step for most DNA viruses, although how this is accomplished by some viruses is unclear. Polyomavirus SV40 transports from the cell surface through the endosome, the endoplasmic reticulum, and the cytosol from where it enters the nucleus to cause infection. Here we elucidate the nuclear entry mechanism of SV40. Our results show that cytosol-localized SV40 is targeted to the nuclear envelope by directly engaging Nesprin-2 of the linker of nucleoskeleton and cytoskeleton (LINC) nuclear membrane complex. Additionally, we identify the NUP188 subunit of the nuclear pore complex (NPC) as a new Nesprin-2-interacting partner. This physical proximity positions the NPC to capture SV40 upon release from Nesprin-2, enabling the channel to facilitate nuclear translocation of the virus. Strikingly, SV40 disassembles during nuclear entry, generating a viral genome-VP1-VP3 subcomplex that efficiently crosses the NPC to enter the nucleus. Our results reveal how two major nuclear membrane protein complexes are exploited to promote targeting and translocation of a virus into the nucleus.

The Portal Vertex of KSHV Promotes Docking of Capsids at the Nuclear Pores

Kaposi's sarcoma-associated herpesvirus (KSHV) is a cancer-related herpesvirus. Like other herpesviruses, the KSHV icosahedral capsid includes a portal vertex, composed of 12 protein subunits encoded by open reading frame (ORF) 43, which enables packaging and release of the viral genome into the nucleus through the nuclear pore complex (NPC). Capsid vertex-specific component (CVSC) tegument proteins, which directly mediate docking at the NPCs, are organized on the capsid vertices and are enriched on the portal vertex. Whether and how the portal vertex is selected for docking at the NPC is unknown. Here, we investigated the docking of incoming ORF43-null KSHV capsids at the NPCs, and describe a significantly lower fraction of capsids attached to the nuclear envelope compared to wild-type (WT) capsids. Like WT capsids, nuclear envelope-associated ORF43-null capsids co-localized with different nucleoporins (Nups) and did not detach upon salt treatment. Inhibition of nuclear export did not alter WT capsid docking. As ORF43-null capsids exhibit lower extent of association with the NPCs, we conclude that although not essential, the portal has a role in mediating the interaction of the CVSC proteins with Nups, and suggest a model whereby WT capsids can dock at the nuclear envelope through a non-portal penton vertex, resulting in an infection 'dead end'.

Confined no more: Viral mechanisms of nuclear entry and egress

Viruses are obligatory intracellular parasites. For their efficient replication, many require access to the nuclear interior. Yet, only few viral particles are small enough to passively diffuse through the nuclear pore complexes, calling for alternative strategies to bypass the nuclear envelope barrier. Some viruses will await mitotic nuclear envelope breakdown to gain access, whereas others will exploit more active means, for instance by hijacking nuclear pore transport or by directly targeting constituents of the nuclear envelope so as to remodel and temporarily perturb its integrity. After replication, newly produced viral DNA complexes need to cross the same barrier to exit the nucleus and enter the cytoplasm, where the final stages of virion maturation take place. There are also different flavours to the feat of nuclear egress that vary in delicacy and intensity. In this review, we define the major entry and egress strategies that are exploited by different viruses and describe the molecular details thereof. Ultimately, a deeper understanding of these pathways may help identifying molecular targets for blocking viral reproduction or spreading.

Evidence of the Mechanism by Which Polyomaviruses Exploit the Extracellular Vesicle Delivery System during Infection

Increasing evidence suggests that human viruses can hijack extracellular vesicles (EVs) to deliver proteins, mRNAs, microRNAs (miRNAs) and whole viral particles during viral persistence in the host. Human polyomavirus (PyV) miRNAs, which downregulate large T-antigen expression and target host factors, help the virus escape immune elimination and may have roles in the success of viral persistence/replication and the development of diseases. In this context, several investigations have detected PyV miRNAs in EVs obtained from cell culture supernatants after viral infection, demonstrating the ability of these vesicles to deliver miRNAs to uninfected cells, potentially counteracting new viral infection. Additionally, PyV miRNAs have been identified in EVs derived from the biological fluids of clinical samples obtained from patients with or at risk of severe PyV-associated diseases and from asymptomatic control healthy subjects. Interestingly, PyV miRNAs were found to be circulating in blood, urine, cerebrospinal fluid, and saliva samples from patients despite their PyV DNA status. Recently, the association between EVs and PyV viral particles was reported, demonstrating the ability of PyV viral particles to enter the cell without natural receptor-mediated entry and evade antibody-mediated neutralization or to be neutralized at a step different from that of the neutralization of naked whole viral particles. All these data point toward a potential role of the association between PyVs with EVs in viral persistence, suggesting that further work to define the implication of this interaction in viral reactivation is warranted.

Dynein Engages and Disassembles Cytosol-Localized Simian Virus 40 To Promote Infection

During entry, polyomavirus (PyV) is endocytosed and sorts to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol. From the cytosol, the virus moves to the nucleus to cause infection. How PyV is transported from the cytosol into the nucleus, a crucial infection step, is unclear. We found that upon reaching the cytosol, the archetypal PyV simian virus 40 (SV40) recruits the cytoplasmic dynein motor, which disassembles the viral particle. This reaction enables the resulting disassembled virus to enter the nucleus to promote infection. Our findings reveal how a cytosolic motor can be hijacked to impart conformational changes to a viral particle, a process essential for successful infection.IMPORTANCE How a nonenveloped virus successfully traffics from the cell surface to the nucleus to cause infection remains enigmatic in many instances. In the case of the nonenveloped PyV, the viral particle is sorted from the plasma membrane to the ER and then the cytosol, from which it enters the nucleus to promote infection. The molecular mechanism by which PyV reaches the nucleus from the cytosol is not entirely clear. Here we demonstrate that the prototype PyV SV40 recruits dynein upon reaching the cytosol. Importantly, this cellular motor disassembles the viral particle during cytosol-to-nucleus transport to cause infection.

How Simian Virus 40 Hijacks the Intracellular Protein Trafficking Pathway to Its Own Benefit … and Ours

Viruses efficiently transfer and express their genes in host cells and evolve to evade the host's defense responses. These properties render them highly attractive for use as gene delivery vectors in vaccines, gene, and immunotherapies. Among the viruses used as gene delivery vectors, the macaque polyomavirus Simian Virus 40 (SV40) is unique in its capacity to evade intracellular antiviral defense responses upon cell entry. We here describe the unique way by which SV40 particles deliver their genomes in the nucleus of permissive cells and how they prevent presentation of viral antigens to the host's immune system. The non-immunogenicity in its natural host is not only of benefit to the virus but also to us in developing effective SV40 vector-based treatments for today's major human diseases.

Interaction of the Mouse Polyomavirus Capsid Proteins with Importins Is Required for Efficient Import of Viral DNA into the Cell Nucleus

The mechanism used by mouse polyomavirus (MPyV) to overcome the crowded cytosol to reach the nucleus has not been fully elucidated. Here, we investigated the involvement of importin α/β1 mediated transport in the delivery of MPyV genomes into the nucleus. Interactions of the virus with importin β1 were studied by co-immunoprecipitation and proximity ligation assay. For infectivity and nucleus delivery assays, the virus and its capsid proteins mutated in the nuclear localization signals (NLSs) were prepared and produced. We found that at early times post infection, virions bound importin β1 in a time dependent manner with a peak of interactions at 6 h post infection. Mutation analysis revealed that only when the NLSs of both VP1 and VP2/3 were disrupted, virus did not bind efficiently to importin β1 and its infectivity remarkably decreased (by 80%). Nuclear targeting of capsid proteins was improved when VP1 and VP2 were co-expressed. VP1 and VP2 were effectively delivered into the nucleus, even when one of the NLS, either VP1 or VP2, was disrupted. Altogether, our results showed that MPyV virions can use VP1 and/or VP2/VP3 NLSs in concert or individually to bind importins to deliver their genomes into the cell nucleus.

Viral mechanisms for docking and delivering at nuclear pore complexes

Some viruses possess the remarkable ability to transport their genomes across nuclear pore complexes (NPCs) for replication inside the host cell's intact nuclear compartment. Viral mechanisms for crossing the restrictive NPC passageway are highly complex and astonishingly diverse, requiring in each case stepwise interaction between incoming virus particles and components of the nuclear transport machinery. Exactly how a large viral genome loaded with accessory proteins is able to pass through the relatively narrow central channel of the NPC without causing catastrophic structural damage is not yet fully understood. It appears likely, however, that the overall structure of the NPC changes in response to the cargo. Translocation may result in nucleic acids being misdelivered to the cytoplasm. Here we consider in detail the diverse strategies that viruses have evolved to target and subvert NPCs during infection. For decades, this process has both captivated and confounded researchers in the fields of virology, cell biology, and structural biology.

Entry, infection, replication, and egress of human polyomaviruses: an update

Polyomaviruses (PyVs), belonging to the family Polyomaviridae, are a group of small, nonenveloped, double-stranded, circular DNA viruses widely distributed in the vertebrates. PyVs cause no apparent disease in adult laboratory mice but cause a wide variety of tumors when artificially inoculated into neonates or semipermissive animals. A few human PyVs, such as BK, JC, and Merkel cell PyVs, have been unequivocally linked to pathogenesis under conditions of immunosuppression. Infection is thought to occur early in life and persists for the lifespan of the host. Over evolutionary time scales, it appears that PyVs have slowly co-evolved with specific host animal lineages. Host cell surface glycoproteins and glycolipids seem to play a decisive role in the entry stage of viral infection and in channeling the virions to specific intracellular membrane-bound compartments and ultimately to the nucleus, where the genomes are replicated and packaged for release. Therefore the transport of the infecting virion or viral genome to this site of multiplication is an essential process in productive viral infection as well as in latent infection and transformation. This review summarizes the major findings related to the characterization of the nature of the interactions between PyV and host protein and their impact in host cell invasion.

ERdj5 Reductase Cooperates with Protein Disulfide Isomerase To Promote Simian Virus 40 Endoplasmic Reticulum Membrane Translocation

The nonenveloped polyomavirus (PyV) simian virus 40 (SV40) traffics from the cell surface to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol before mobilizing into the nucleus to cause infection. Prior to ER membrane penetration, ER lumenal factors impart structural rearrangements to the virus, generating a translocation-competent virion capable of crossing the ER membrane. Here we identify ERdj5 as an ER enzyme that reduces SV40's disulfide bonds, a reaction important for its ER membrane transport and infection. ERdj5 also mediates human BK PyV infection. This enzyme cooperates with protein disulfide isomerase (PDI), a redox chaperone previously implicated in the unfolding of SV40, to fully stimulate membrane penetration. Negative-stain electron microscopy of ER-localized SV40 suggests that ERdj5 and PDI impart structural rearrangements to the virus. These conformational changes enable SV40 to engage BAP31, an ER membrane protein essential for supporting membrane penetration of the virus. Uncoupling of SV40 from BAP31 traps the virus in ER subdomains called foci, which likely serve as depots from where SV40 gains access to the cytosol. Our study thus pinpoints two ER lumenal factors that coordinately prime SV40 for ER membrane translocation and establishes a functional connection between lumenal and membrane events driving this process.

IMPORTANCE

PyVs are established etiologic agents of many debilitating human diseases, especially in immunocompromised individuals. To infect cells at the cellular level, this virus family must penetrate the host ER membrane to reach the cytosol, a critical entry step. In this report, we identify two ER lumenal factors that prepare the virus for ER membrane translocation and connect these lumenal events with events on the ER membrane. Pinpointing cellular components necessary for supporting PyV infection should lead to rational therapeutic strategies for preventing and treating PyV-related diseases.

Nuclear entry of DNA viruses

DNA viruses undertake their replication within the cell nucleus, and therefore they must first deliver their genome into the nucleus of their host cells. Thus, trafficking across the nuclear envelope is at the basis of DNA virus infections. Nuclear transport of molecules with diameters up to 39 nm is a tightly regulated process that occurs through the nuclear pore complex (NPC). Due to the enormous diversity of virus size and structure, each virus has developed its own strategy for entering the nucleus of their host cells, with no two strategies alike. For example, baculoviruses target their DNA-containing capsid to the NPC and subsequently enter the nucleus intact, while the hepatitis B virus capsid crosses the NPC but disassembles at the nuclear side of the NPC. For other viruses such as herpes simplex virus and adenovirus, although both dock at the NPC, they have each developed a distinct mechanism for the subsequent delivery of their genome into the nucleus. Remarkably, other DNA viruses, such as parvoviruses and human papillomaviruses, access the nucleus through an NPC-independent mechanism. This review discusses our current understanding of the mechanisms used by DNA viruses to deliver their genome into the nucleus, and further presents the experimental evidence for such mechanisms.

Old foes, new understandings: nuclear entry of small non-enveloped DNA viruses

The nuclear import of viral genomes is an important step of the infectious cycle for viruses that replicate in the nucleus of their host cells. Although most viruses use the cellular nuclear import machinery or some components of this machinery, others have developed sophisticated ways to reach the nucleus. Some of these have been known for some time; however, recent studies have changed our understanding of how some non-enveloped DNA viruses access the nucleus. For example, parvoviruses enter the nucleus through small disruptions of the nuclear membranes and nuclear lamina, and adenovirus tugs at the nuclear pore complex, using kinesin-1, to disassemble their capsids and deliver viral proteins and genomes into the nucleus. Here we review recent findings of the nuclear import strategies of three small non-enveloped DNA viruses, including adenovirus, parvovirus, and the polyomavirus simian virus 40.

DNA virus uncoating

Virus genomes are condensed and packaged inside stable proteinaceous capsids that serve to protect them during transit from one cell or host organism, to the next. During virus entry, capsid shells are primed and disassembled in a complex, tightly-regulated, multi-step process termed uncoating. Here we compare the uncoating-programs of DNA viruses of the pox-, herpes-, adeno-, polyoma-, and papillomavirus families. Highlighting the chemical and mechanical cues virus capsids respond to, we review the conformational changes that occur during stepwise disassembly of virus capsids and how these culminate in the release of viral genomes at the right time and cellular location to assure successful replication.

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.

Production and biomedical applications of virus-like particles derived from polyomaviruses

Virus-like particles (VLPs), aggregates of capsid proteins devoid of viral genetic material, show great promise in the fields of vaccine development and gene therapy. These particles spontaneously self-assemble after heterologous expression of viral structural proteins. This review will focus on the use of virus-like particles derived from polyomavirus capsid proteins. Since their first recombinant production 27 years ago these particles have been investigated for a myriad of biomedical applications. These virus-like particles are safe, easy to produce, can be loaded with a broad range of diverse cargoes and can be tailored for specific delivery or epitope presentation. We will highlight the structural characteristics of polyomavirus-derived VLPs and give an overview of their applications in diagnostics, vaccine development and gene delivery.