Nuclear Export of the Nonenveloped Parvovirus Virion Is Directed by an Unordered Protein Signal Exposed on the Capsid Surface

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To investigate whether a DNA virus can evade passive immunotherapy with a polyclonal antiserum, we analyzed the protection of a neutralizing capsid antiserum against a lethal infection of the immunosuppressive strain of the parvovirus minute virus of mice (MVMi) in 42 immunodeficient mice over a period of 200 days. A few mice were effectively protected, but most developed a delayed lethal leukopenic syndrome during the treatment or weeks afterwards. Unexpectedly, viruses isolated from treated but also from control leukopenic mice showed no amino acid changes throughout the entire capsid coding region, although the viral populations were genetically heterogeneous, mainly in the second exon of the coding sequence of the NS2 nonstructural protein. The NS2 point amino acid changes (T88A, K96E, L103P, and L153 M) that were consistently selected in several mice clustered within the nuclear exportin CRM1 binding domain, in a reading frame that did not alter the overlapping NS1 coding region. These mutations endowed emerging viruses with an increased fitness that was demonstrable by their relative resistance to the neutralizing capsid antiserum in a postentry plaqueforming assay, the rapid overgrowth of a competing wild-type (wt) population in culture, and a larger yield of infectious particles. Mutant NS2 proteins interacted with a higher affinity and sequestered CRM1 in the perinuclear region of the cytoplasm more efficiently than the wt. Correspondingly this phenomenon, as well as the following timely ordered release of the NS1 nonstructural protein and the empty capsid from the nucleus to the cytoplasm, occurred markedly earlier in the infection cycle of the mutant viruses. We hypothesize that the enhanced cytoplasmic sequestration of CRM1 by the NS2 mutations selected in mice may trigger pleiotropic effects leading to an accelerated MVMi life cycle and thus to increased fitness. These results strengthen our earlier report on the rapid evolutionary capacity of this mammalian-specific DNA virus in vivo and indicate that the NS2-CRM1 interaction is an important determinant of parvovirus virulence that can be modulated in nature, hampering the effectiveness of passive antibody therapies in the long term.

Section: Discussionsupporting

confidence: 63%

Enhanced Cytoplasmic Sequestration of the Nuclear Export Receptor CRM1 by NS2 Mutations Developed in the Host Regulates Parvovirus Fitness

López-Bueno

Valle

Gallego

et al. 2004

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“…The Cys273Ser mutation resulted in a nonviable virus that could not be passaged successfully, and those capsids appeared to be primarily nuclear localized, in contrast to the wild-type CPV or other mutations, where capsids were distributed in both the cytoplasm and the nucleus. Similar results were seen in MVM when serine residues on the N terminus of VP2 were mutated to prevent phosphorylation and the capsids could no longer be recognized by the nuclear export machinery (67). Residue 273 is in the interior of the capsid and distant from the 5-fold pores, so the Cys273Ser change seems unlikely to act through such a mechanism, and additional experiments will be needed to determine its role in the CPV infection and replication process.…”

Section: Internal Capsid Structures Controlling Infectionsupporting

confidence: 53%

Parvovirus Capsid Structures Required for Infection: Mutations Controlling Receptor Recognition and Protease Cleavages

Callaway

Feng

Lee

et al. 2017

Parvovirus capsids are small but complex molecular machines responsible for undertaking many of the steps of cell infection, genome packing, and cell-tocell as well as host-to-host transfer. The details of parvovirus infection of cells are still not fully understood, but the processes must involve small changes in the capsid structure that allow the endocytosed virus to escape from the endosome, pass through the cell cytoplasm, and deliver the single-stranded DNA (ssDNA) genome to the nucleus, where viral replication occurs. Here, we examine capsid substitutions that eliminate canine parvovirus (CPV) infectivity and identify how those mutations changed the capsid structure or altered interactions with the infectious pathway. Amino acid substitutions on the exterior surface of the capsid (Gly299Lys/Ala300Lys) altered the binding of the capsid to transferrin receptor type 1 (TfR), particularly during virus dissociation from the receptor, but still allowed efficient entry into both feline and canine cells without successful infection. These substitutions likely control specific capsid structural changes resulting from TfR binding required for infection. A second set of changes on the interior surface of the capsid reduced viral infectivity by Ͼ100-fold and included two cysteine residues and neighboring residues. One of these substitutions, Cys270Ser, modulates a VP2 cleavage event found in ϳ10% of the capsid proteins that also was shown to alter capsid stability. A neighboring substitution, Pro272Lys, significantly reduced capsid assembly, while a Cys273Ser change appeared to alter capsid transport from the nucleus. These mutants reveal additional structural details that explain cell infection processes of parvovirus capsids.IMPORTANCE Parvoviruses are commonly found in both vertebrate and invertebrate animals and cause widespread disease. They are also being developed as oncolytic therapeutics and as gene therapy vectors. Most functions involved in infection or transduction are mediated by the viral capsid, but the structure-function correlates of the capsids and their constituent proteins are still incompletely understood, especially in relation to identifying capsid processes responsible for infection and release from the cell. Here, we characterize the functional effects of capsid protein mutations that result in the loss of virus infectivity, giving a better understanding of the portions of the capsid that mediate essential steps in successful infection pathways and how they contribute to viral infectivity.

“…Rabbit anti-VPs (polyclonal against MVM structural proteins), rabbit anti-N-VP2 (polyclonal against the N terminus of VP2), and mouse anticapsid (monoclonal against intact capsids; clone B7) antibodies have been described elsewhere (23,24,28). A rabbit polyclonal antibody against the N terminus of VP1 was kindly provided by P. Tattersall (7).…”

Section: Cells and Virusesmentioning

confidence: 99%

“…The N-VP2 has been found to function as a nuclear export signal for the maturing virions with encapsidated genome (28). However, so far, the intracellular site, the mechanism, and the role of the VP2 cleavage during the entry process have not been elucidated.…”

mentioning

confidence: 99%

Low pH-Dependent Endosomal Processing of the Incoming Parvovirus Minute Virus of Mice Virion Leads to Externalization of the VP1 N-Terminal Sequence (N-VP1), N-VP2 Cleavage, and Uncoating of the Full-Length Genome

Mani

Baltzer

Valle

et al. 2006

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100

118

Minute virus of mice (MVM) enters the host cell via receptor-mediated endocytosis. Although endosomal processing is required, its role remains uncertain. In particular, the effect of low endosomal pH on capsid configuration and nuclear delivery of the viral genome is unclear. We have followed the progression and structural transitions of DNA full-virus capsids (FC) and empty capsids (EC) containing the VP1 and VP2 structural proteins and of VP2-only virus-like particles (VLP) during the endosomal trafficking. Three capsid rearrangements were detected in FC: externalization of the VP1 N-terminal sequence (N-VP1), cleavage of the exposed VP2 N-terminal sequence (N-VP2), and uncoating of the full-length genome. All three capsid modifications occurred simultaneously, starting as early as 30 min after internalization, and all of them were blocked by raising the endosomal pH. In particles lacking viral single-stranded DNA (EC and VLP), the N-VP2 was not exposed and thus it was not cleaved. However, the EC did externalize N-VP1 with kinetics similar to those of FC. The bulk of all the incoming particles (FC, EC, and VLP) accumulated in lysosomes without signs of lysosomal membrane destabilization. Inside lysosomes, capsid degradation was not detected, although the uncoated DNA of FC was slowly degraded. Interestingly, at any time postinfection, the amount of structural proteins of the incoming virions accumulating in the nuclear fraction was negligible. These results indicate that during the early endosomal trafficking, the MVM particles are structurally modified by low-pH-dependent mechanisms. Regardless of the structural transitions and protein composition, the majority of the entering viral particles and genomes end in lysosomes, limiting the efficiency of MVM nuclear translocation.