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

VEGF-Virus Interactions: Pathogenic Mechanisms and Therapeutic Applications

Many types of viruses directly or indirectly target the vascular endothelial growth factor (VEGF) system, which is a central regulator of vasculogenesis and angiogenesis in physiological homeostasis, causing diverse pathologies. Other viruses have been developed into effective therapeutic tools for VEGF modulation in conditions such as cancer and eye diseases. Some viruses may alter the levels of VEGF in the pathogenesis of respiratory syndromes, or they may encode VEGF-like factors, promoting vascular disruption and angiogenesis to enable viruses' systemic spread. Oncogenic viruses may express interactive factors that perturb VEGF's functional levels or downstream signaling, which increases the neovascularization and metastasis of tumors. Furthermore, many viruses are being developed as therapeutic vectors for vascular pathologies in clinical trials. Major examples are those viral vectors that inhibit the role of VEGF in the neovascularization required for cancer progression; this is achieved through the induction of immune responses, by exposing specific peptides that block signaling or by expressing anti-VEGF and anti-VEGF receptor-neutralizing antibodies. Other viruses have been engineered into effective pro- or anti-angiogenesis multitarget vectors for neovascular eye diseases, paving the way for therapies with improved safety and minimal side effects. This article critically reviews the large body of literature on these issues, highlighting those contributions that describe the molecular mechanisms, thus expanding our understanding of the VEGF-virus interactions in disease and therapy. This could facilitate the clinical use of therapeutic virus vectors in precision medicine for the VEGF system.

Intracellular virion traffic to the endosome driven by cell type specific sialic acid receptors determines parvovirus tropism

Parvoviruses are promising anticancer and gene therapy agents, but a deep knowledge of the entry process is crucial to exploit their therapeutic potential. We addressed this issue while attempting to retarget the oncolytic parvovirus minute virus of mice (MVMp) to the tumor vasculature. Residues at three functional domains of the icosahedral capsid were substituted by rational design with peptides competing with the vascular endothelial growth factor. Most substitutions impaired virus maturation, though some yielded infectious chimeric virions, and substitutions in a dimple at the twofold axis that allocates sialic acid (SIA) receptors altered viral tropism. One dimple-modified chimeric virion was efficiently attached as MVMp to α2-linked SIA moieties, but the infection was impaired by the binding to some inhibitory α2-3,-6,-8 SIA pseudoreceptors, which hampers intracellular virus traffic to the endosome in a cell type-dependent manner. Infectious from nonproductive traffic could be mechanistically discriminated by an endosomal drastic capsid structural transition comprising the cleavage of some VP2-Nt sequences and its associated VP1-Nt exposure. Correspondingly, neuraminidase removal of inhibitory SIA moieties enhanced the infection quantitatively, correlating to the restored virus traffic to the endosome and the extent of VP2-Nt cleavage/VP1-Nt exposure. This study illustrates (i) structural constraints to retarget parvoviruses with evolutionary adopted narrow grooves allocating small SIA receptors, (ii) the possibility to enhance parvovirus oncolysis by relaxing the glycan network on the cancer cell surface, and (iii) the major role played by the attachment to cell type-specific SIAs in the intracellular virus traffic to the endosome, which may determine parvovirus tropism and host range.

Nuclear entry and egress of parvoviruses

Parvoviruses are small non-enveloped single-stranded DNA viruses, which depend on host cell nuclear transcriptional and replication machinery. After endosomal exposure of nuclear localization sequence and a phospholipase A₂ domain on the capsid surface, and escape into the cytosol, parvovirus capsids enter the nucleus. Due to the small capsid diameter of 18-26 nm, intact capsids can potentially pass into the nucleus through nuclear pore complexes (NPCs). This might be facilitated by active nuclear import, but capsids may also follow an alternative entry pathway that includes activation of mitotic factors and local transient disruption of the nuclear envelope. The nuclear entry is followed by currently undefined events of viral genome uncoating. After genome release, viral replication compartments are initiated and infection proceeds. Parvoviral genomes replicate during cellular S phase followed by nuclear capsid assembly during virus-induced S/G2 cell cycle arrest. Nuclear egress of capsids occurs upon nuclear envelope degradation during apoptosis and cell lysis. An alternative pathway for nuclear export has been described using active transport through the NPC mediated by the chromosome region maintenance 1 protein, CRM1, which is enhanced by phosphorylation of the N-terminal domain of VP2. However, other alternative but not yet uncharacterized nuclear export pathways cannot be excluded.

Particle‐based analysis elucidates the real retention capacities of virus filters and enables optimal virus clearance study design with evaluation systems of diverse virological characteristics

In virus clearance study (VCS) design, the amount of virus loaded onto the virus filters (VF) must be carefully controlled. A large amount of virus is required to demonstrate sufficient virus removal capability; however, too high a viral load causes virus breakthrough and reduces log reduction values. We have seen marked variation in the virus removal performance for VFs even with identical VCS design. Understanding how identical virus infectivity, materials and operating conditions can yield such different results is key to optimizing VCS design. The present study developed a particle number‐based method for VCS and investigated the effects on VF performance of discrepancies between apparent virus amount and total particle number of minute virus of mice. Co‐spiking of empty and genome‐containing particles resulted in a decrease in the virus removal performance proportional to the co‐spike ratio. This suggests that empty particles are captured in the same way as genome‐containing particles, competing for retention capacity. In addition, between virus titration methods with about 2.0 Log₁₀ difference in particle‐to‐infectivity ratios, there was a 20‐fold decrease in virus retention capacity limiting the throughput that maintains the required LRV (e.g., 4.0), calculated using infectivity titers. These findings suggest that ignoring virus particle number in VCS design can cause virus overloading and accelerate filter breakthrough. This article asserts the importance of focusing on virus particle number and discusses optimization of VCS design that is unaffected by virological characteristics of evaluation systems and adequately reflect the VF retention capacity.

Viral targeting of glioblastoma stem cells with patient-specific genetic and post-translational p53 deregulations

Cancer therapy urges targeting of malignant subsets within self-renewing heterogeneous stem cell populations. We dissect the genetic and functional heterogeneity of human glioblastoma stem cells (GSCs) within patients by their innate responses to non-pathogenic mouse parvoviruses that are tightly restrained by cellular physiology. GSC neurospheres accumulate assembled capsids but restrict viral NS1 cytotoxic protein expression by an innate PKR/eIF2α-P response counteractable by electric pulses. NS1 triggers a comprehensive DNA damage response involving cell-cycle arrest, neurosphere disorganization, and bystander disruption of GSC-derived brain tumor architecture in rodent models. GSCs and cancer cell lines permissive to parvovirus genome replication require p53-Ser15 phosphorylation (Pp53S15). NS1 expression is enhanced by exogeneous Pp53S15 induction but repressed by wtp53. Consistently, patient-specific GSC subpopulations harboring p53 gain-of-function mutants and/or Pp53S15 are selective viral targets. This study provides a molecular foundation for personalized biosafe viral therapies against devastating glioblastoma and other cancers with deregulated p53 signaling.

Equine Parvovirus-Hepatitis in China: Characterization of Its Genetic Diversity and Evidence for Natural Recombination Events Between the Chinese and American Strains

Equine parvovirus-hepatitis (EqPV-H) was first reported in a horse that died of equine serum hepatitis in the USA in 2018, and was determined having a strong association with equine serum hepatitis in the following studies. As a newly discovered virus, the genomic sequences of only seven EqPV-H strains have been reported. Considering this, an epidemiological study was performed to investigate the prevalence of EqPV-H in equines in Guangdong Province in China, and obtain genomic sequences of the field prevalent EqPV-H strains. The detection rate of EqPV-H was finally determined to be 8.33% (95% CI: 2.8-18.4%), and EqPV-H's coinfection with equine hepacivirus and equine pegivirus was also determined. Then, the genomes of the Chinese field EqPV-H strains were obtained by PCR, sequencing, and assembly. Through bootscanning analysis, Simplot analysis, and phylogenetic analysis, strong evidence for natural recombination events were found in two Chinese field EqPV-H strains. The natural recombination events occurred between the Chinese and American strains, and were determined within VP protein. Finally, the genetic distance of EqPV-H strains was investigated. Nucleotide identities of 97.1-99.9% and 95.2-100% were found for NS and VP between EqPV-H strains, respectively. Together with other molecular evidence obtained in the present study, the genetic diversity of EqPV-H was determined. Taken together, the results of the present study expand our knowledge on the epidemiological characteristics, genetic variability, and evolution of EqPV-H.

Antiangiogenic Vascular Endothelial Growth Factor-Blocking Peptides Displayed on the Capsid of an Infectious Oncolytic Parvovirus: Assembly and Immune Interactions

As many tumor cells synthetize vascular endothelial growth factors (VEGF) that promote neo-vascularization and metastasis, frontline cancer therapies often administer anti-VEGF (α-VEGF) antibodies. To target the oncolytic parvovirus minute virus of mice (MVM) to the tumor vasculature, we studied the functional tolerance, evasion of neutralization, and induction of α-VEGF antibodies of chimeric viruses in which the footprint of a neutralizing monoclonal antibody within the 3-fold capsid spike was replaced by VEGF-blocking peptides: P6L (PQPRPL) and A7R (ATWLPPR). Both peptides allowed viral genome replication and nuclear translocation of chimeric capsid subunits. MVM-P6L efficiently propagated in culture, exposing the heterologous peptide on the capsid surface, and evaded neutralization by the anti-spike monoclonal antibody. In contrast, MVM-A7R yielded low infectious titers and was poorly recognized by an α-A7R monoclonal antibody. MVM-A7R showed a deficient assembly pattern, suggesting that A7R impaired a transitional configuration that the subunits must undergo in the 3-fold axis to close up the capsid shell. The MVM-A7R chimeric virus consistently evolved in culture into a mutant carrying the P6Q amino acid substitution within the A7R sequence, which restored normal capsid assembly and infectivity. Consistent with this finding, anti-native VEGF antibodies were induced in mice by a single injection of MVM-A7R empty capsids, but not by MVM-A7R virions. This fundamental study provides insights to endow an infectious parvovirus with immune antineovascularization and evasion capacities by replacing an antibody footprint in the capsid 3-fold axis with VEGF-blocking peptides, and it also illustrates the evolutionary capacity of single-stranded DNA (ssDNA) viruses to overcome engineered capsid structural restrictions.IMPORTANCE Targeting the VEGF signaling required for neovascularization by vaccination with chimeric capsids of oncolytic viruses may boost therapy for solid tumors. VEGF-blocking peptides (VEbp) engineered in the capsid 3-fold axis endowed the infectious parvovirus MVM with the ability to induce α-VEGF antibodies without adjuvant and to evade neutralization by MVM-specific antibodies. However, these properties may be compromised by structural restraints that the capsid imposes on the peptide configuration and by misassembly caused by the heterologous peptides. Significantly, chimeric MVM-VEbp resolved the structural restrictions by selecting mutations within the engineered peptides that restored efficient capsid assembly. These data show the promise of antineovascularization vaccines using chimeric VEbp-icosahedral capsids of oncolytic viruses but also raise safety concerns regarding the genetic stability of manipulated infectious parvoviruses in cancer and gene therapies.

Construction and characterization of the infectious clone of porcine parvovirus carrying genetic marker

Porcine parvovirus (PPV) is one of the major pathogens that bring about reproductive failure of pregnant sows. However, the study of the pathogenesis mechanism is circumscribed due to the lack of efficient genetic manipulation method. Infectious clone is a powerful tool for further studying the genetic mechanisms of PPV. In the present study, the gene fragment (157-4812) of PPV was amplified by PPV China isolate strain as a template, and PPV DNA fragments (1-182) forming Y-structure within in 5' end and (4788-5074) forming U-structure in 3' end were synthesized. And then, the above three fragments were inserted into plasmid pKQLL to congregate a PPV full-length recombinant plasmid by means of In-Fusion cloning technology. After the successful sequencing identification of the recombinant plasmid, the EcoR I restriction site was brought out as a genetic marker by nonsense mutation (A3058 T) to produce plasmid Y-PPV, which was transfected into PK-15 cells for rescue of virus. The rescued viral particles were observed under transmission electron microscopy, and the sequencing analysis showed that Y-PPV could stably carry the genetic marker. It could be seen that Y-PPV has similar replicate capability and pathogenicity as the wild-type parental PPV strain by cellular and animal experiments. These results confirmed that Y-PPV maintain similar biological characteristics with wild-type parental PPV strain. Infectious clone could be a valuable tool for studying the individual genes of PPV and applications in gene deletion or live vector vaccines.

Systematic analysis of biological roles of charged amino acid residues located throughout the structured inner wall of a virus capsid

Structure-based mutational analysis of viruses is providing many insights into the relationship between structure and biological function of macromolecular complexes. We have systematically investigated the individual biological roles of charged residues located throughout the structured capsid inner wall (outside disordered peptide segments) of a model spherical virus, the minute virus of mice (MVM). The functional effects of point mutations that altered the electrical charge at 16 different positions at the capsid inner wall were analyzed. The results revealed that MVM capsid self-assembly is rather tolerant to point mutations that alter the number and distribution of charged residues at the capsid inner wall. However, mutations that either increased or decreased the number of positive charges around capsid-bound DNA segments reduced the thermal resistance of the virion. Moreover, mutations that either removed or changed the positions of negatively charged carboxylates in rings of acidic residues around capsid pores were deleterious by precluding a capsid conformational transition associated to through-pore translocation events. The results suggest that number, distribution and specific position of electrically charged residues across the inner wall of a spherical virus may have been selected through evolution as a compromise between several different biological requirements.

Differential phosphorylation and n-terminal configuration of capsid subunits in parvovirus assembly and viral trafficking

The T1 parvovirus Minute Virus of Mice (MVM) was used to study the roles that phosphorylation and N-terminal domains (Nt) configuration of capsid subunits may play in icosahedral nuclear viruses assembly. In synchronous MVM infection, capsid subunits newly assembled as two types of cytoplasmic trimeric intermediates (3VP2, and 1VP1:2VP2) harbored a VP1 phosphorylation level fivefold higher than that of VP2, and hidden Nt. Upon nuclear translocation at S phase, VP1-Nt became exposed in the heterotrimer and subsequent subviral assembly intermediates. Empty capsid subunits showed a phosphorylation level restored to VP1:VP2 stoichiometry, and the Nt concealed in their interior. However ssDNA-filled virus maturing at S/G2 lacked VP1 phosphorylation and one major VP2 phosphopeptide, and exposed VP2-Nt. Endosomal VP2-Nt cleavage resulted in VP3 subunits devoid of any phospholabel, implying that incoming viral particles specifically harbor a low phosphorylation status. Phosphorylation provides a mechanistic coupling of parvovirus nuclear assembly to the cell cycle.

Protoparvovirus Cell Entry

The Protoparvovirus (PtPV) genus of the Parvoviridae family of viruses includes important animal pathogens and reference molecular models for the entire family. Some virus members of the PtPV genus have arisen as promising tools to treat tumoral processes, as they exhibit marked oncotropism and oncolytic activities while being nonpathogenic for humans. The PtPVs invade and replicate within the nucleus making extensive use of the transport, transcription and replication machineries of the host cells. In order to reach the nucleus, PtPVs need to cross over several intracellular barriers and traffic through different cell compartments, which limit their infection efficiency. In this review we summarize molecular interactions, capsid structural transitions and hijacking of cellular processes, by which the PtPVs enter and deliver their single-stranded DNA genome into the host cell nucleus. Understanding mechanisms that govern the complex PtPV entry will be instrumental in developing approaches to boost their anticancer therapeutic potential and improving their safety profile.

Protoparvovirus Knocking at the Nuclear Door

Protoparvoviruses target the nucleus due to their dependence on the cellular reproduction machinery during the replication and expression of their single-stranded DNA genome. In recent years, our understanding of the multistep process of the capsid nuclear import has improved, and led to the discovery of unique viral nuclear entry strategies. Preceded by endosomal transport, endosomal escape and microtubule-mediated movement to the vicinity of the nuclear envelope, the protoparvoviruses interact with the nuclear pore complexes. The capsids are transported actively across the nuclear pore complexes using nuclear import receptors. The nuclear import is sometimes accompanied by structural changes in the nuclear envelope, and is completed by intranuclear disassembly of capsids and chromatinization of the viral genome. This review discusses the nuclear import strategies of protoparvoviruses and describes its dynamics comprising active and passive movement, and directed and diffusive motion of capsids in the molecularly crowded environment of the cell.

Structural basis for biologically relevant mechanical stiffening of a virus capsid by cavity-creating or spacefilling mutations

Recent studies reveal that the mechanical properties of virus particles may have been shaped by evolution to facilitate virus survival. Manipulation of the mechanical behavior of virus capsids is leading to a better understanding of viral infection, and to the development of virus-based nanoparticles with improved mechanical properties for nanotechnological applications. In the minute virus of mice (MVM), deleterious mutations around capsid pores involved in infection-related translocation events invariably increased local mechanical stiffness and interfered with pore-associated dynamics. To provide atomic-resolution insights into biologically relevant changes in virus capsid mechanics, we have determined by X-ray crystallography the structural effects of deleterious, mechanically stiffening mutations around the capsid pores. Data show that the cavity-creating N170A mutation at the pore wall does not induce any dramatic structural change around the pores, but instead generates subtle rearrangements that propagate throughout the capsid, resulting in a more compact, less flexible structure. Analysis of the spacefilling L172W mutation revealed the same relationship between increased stiffness and compacted capsid structure. Implications for understanding connections between virus mechanics, structure, dynamics and infectivity, and for engineering modified virus-based nanoparticles, are discussed.

Structural Analysis of a Temperature-Induced Transition in a Viral Capsid Probed by HDX-MS

Icosahedral viral capsids are made of a large number of symmetrically organized protein subunits whose local movements can be essential for infection. In the capsid of the minute virus of mice, events required for infection that involve translocation of peptides through capsid pores are associated with a subtle conformational change. In vitro, this change can be reversibly induced by overcoming the energy barrier through mild heating of the capsid, but little is known about the capsid regions involved in the process. Here, we use hydrogen-deuterium exchange coupled to mass spectrometry to analyze the dynamics of the minute virus of mice capsid at increasing temperatures. Our results indicate that the transition associated with peptide translocation involves the structural rearrangement of regions distant from the capsid pores. These alterations are reflected in an increased dynamics of some secondary-structure elements in the capsid shell from which spikes protrude, and a decreased dynamics in the long intertwined loops that form the large capsid spikes. Thus, the translocation events through capsid pores involve a global conformational rearrangement of the capsid and a complex alteration of its equilibrium dynamics. This study additionally demonstrates the potential of hydrogen-deuterium exchange coupled to mass spectrometry to explore in detail temperature-dependent structural dynamics in large macromolecular protein assemblies. Most importantly, it paves the way for undertaking novel studies of the relationship between structure, dynamics, and biological function in virus particles and other large protein cages.

Amino Acid Side Chains Buried along Intersubunit Interfaces in a Viral Capsid Preserve Low Mechanical Stiffness Associated with Virus Infectivity

Single-molecule experimental techniques and theoretical approaches reveal that important aspects of virus biology can be understood in biomechanical terms at the nanoscale. A detailed knowledge of the relationship in virus capsids between small structural changes caused by single-point mutations and changes in mechanical properties may provide further physics-based insights into virus function; it may also facilitate the engineering of viral nanoparticles with improved mechanical behavior. Here, we used the minute virus of mice to undertake a systematic experimental study on the contribution to capsid stiffness of amino acid side chains at interprotein interfaces and the specific noncovalent interactions they establish. Selected side chains were individually truncated by introducing point mutations to alanine, and the effects on local and global capsid stiffness were determined using atomic force microscopy. The results revealed that, in the natural virus capsid, multiple, mostly hydrophobic, side chains buried along the interfaces between subunits preserve a comparatively low stiffness of most (S2 and S3) regions. Virtually no point mutation tested substantially reduced stiffness, whereas most mutations increased stiffness of the S2/S3 regions. This stiffening was invariably associated with reduced virus yields during cell infection. The experimental evidence suggests that a comparatively low stiffness at S3/S2 capsid regions may have been biologically selected because it facilitates capsid assembly, increasing infectious virus yields. This study demonstrated also that knowledge of individual amino acid side chains and biological pressures that determine the physical behavior of a protein nanoparticle may be used for engineering its mechanical properties.

Role of capsid proteins in parvoviruses infection

The parvoviruses are widely spread in many species and are among the smallest DNA animal viruses. The parvovirus is composed of a single strand molecule of DNA wrapped into an icosahedral capsid. In a viral infection, the massy capsid participates in the entire viral infection process, which is summarized in this review. The capsid protein VP1 is primarily responsible for the infectivity of the virus, and the nuclear localization signal (NLS) of the VP1 serves as a guide to assist the viral genome in locating the nucleus. The dominant protein VP2 provides an “anti-receptor”, which interacts with the cellular receptor and leads to the further internalization of virus, and, the N-terminal of VP2 also cooperates with the VP1 to prompt the process of nucleus translocation. Additionally, a cleavage protein VP3 is a part of the capsid, which exists only in several members of the parvovirus family; however, the function of this cleavage protein remains to be fully determined. Parvoviruses can suffer from the extreme environmental conditions such as low pH, or even escape from the recognition of pattern recognition receptors (PRRs), due to the protection of the stable capsid, which is thought to be an immune escape mechanism. The applications of the capsid proteins to the screening and the treatment of diseases are also discussed. The processes of viral infection should be noted, because understanding the virus-host interactions will contribute to the development of therapeutic vaccines.

The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.

Cellular and viral life cycles are connected through multiple, though poorly understood, mechanisms. Parvoviruses infect humans and a broad spectrum of animals, causing a variety of diseases, but they are also used in experimental cancer therapy and serve as vectors for gene therapy. Parvoviruses can only multiply in proliferating cells providing essential replicative and transcriptional functions. However, it is unknown whether the cell cycle regulatory machinery may also control parvovirus assembly. We found that the nuclear translocation of parvovirus MVM capsid subunits (VPs) was highly dependent on physiological cell cycle regulations in mammalian fibroblasts, including: quiescence, progression through G1/S boundary, DNA synthesis, and cell to cell contacts. VPs nuclear translocation was significantly more sensitive to cell cycle controls than viral genome replication and gene expression. The results support nuclear capsid assembly as the major driving process of parvoviruses biological hallmarks, such as pathogenesis in proliferative tissues and tropism for cancer cells. In addition, disturbing the tight coupling of parvovirus assembly with the cell cycle may determine viral persistence in quiescent and post-mitotic host tissues. These findings may contribute to understand cellular regulations on the assembly of other nuclear eukaryotic viruses, and to develop cell cycle-based avenues for antiviral therapy.

Quantitatively probing propensity for structural transitions in engineered virus nanoparticles by single-molecule mechanical analysis

Viruses are increasingly being studied from the perspective of fundamental physics at the nanoscale as biologically evolved nanodevices with many technological applications. In viral particles of the minute virus of mice (MVM), folded segments of the single-stranded DNA genome are bound to the capsid inner wall and act as molecular buttresses that increase locally the mechanical stiffness of the particle. We have explored whether a quantitative linkage exists in MVM particles between their DNA-mediated stiffening and impairment of a heat-induced, virus-inactivating structural change. A series of structurally modified virus particles with disrupted capsid-DNA interactions and/or distorted capsid cavities close to the DNA-binding sites were engineered and characterized, both in classic kinetics assays and by single-molecule mechanical analysis using atomic force microscopy. The rate constant of the virus inactivation reaction was found to decrease exponentially with the increase in elastic constant (stiffness) of the regions closer to DNA-binding sites. The application of transition state theory suggests that the height of the free energy barrier of the virus-inactivating structural transition increases linearly with local mechanical stiffness. From a virological perspective, the results indicate that infectious MVM particles may have acquired the biological advantage of increased survival under thermal stress by evolving architectural elements that rigidify the particle and impair non-productive structural changes. From a nanotechnological perspective, this study provides proof of principle that determination of mechanical stiffness and its manipulation by protein engineering may be applied for quantitatively probing and tuning the conformational dynamics of virus-based and other protein-based nanoassemblies.

Classic nuclear localization signals and a novel nuclear localization motif are required for nuclear transport of porcine parvovirus capsid proteins

Nuclear targeting of capsid proteins (VPs) is important for genome delivery and precedes assembly in the replication cycle of porcine parvovirus (PPV). Clusters of basic amino acids, corresponding to potential nuclear localization signals (NLS), were found only in the unique region of VP1 (VP1up, for VP1 unique part). Of the five identified basic regions (BR), three were important for nuclear localization of VP1up: BR1 was a classic Pat7 NLS, and the combination of BR4 and BR5 was a classic bipartite NLS. These NLS were essential for viral replication. VP2, the major capsid protein, lacked these NLS and contained no region with more than two basic amino acids in proximity. However, three regions of basic clusters were identified in the folded protein, assembled into a trimeric structure. Mutagenesis experiments showed that only one of these three regions was involved in VP2 transport to the nucleus. This structural NLS, termed the nuclear localization motif (NLM), is located inside the assembled capsid and thus can be used to transport trimers to the nucleus in late steps of infection but not for virions in initial infection steps. The two NLS of VP1up are located in the N-terminal part of the protein, externalized from the capsid during endosomal transit, exposing them for nuclear targeting during early steps of infection. Globally, the determinants of nuclear transport of structural proteins of PPV were different from those of closely related parvoviruses. Importance: Most DNA viruses use the nucleus for their replication cycle. Thus, structural proteins need to be targeted to this cellular compartment at two distinct steps of the infection: in early steps to deliver viral genomes to the nucleus and in late steps to assemble new viruses. Nuclear targeting of proteins depends on the recognition of a stretch of basic amino acids by cellular transport proteins. This study reports the identification of two classic nuclear localization signals in the minor capsid protein (VP1) of porcine parvovirus. The major protein (VP2) nuclear localization was shown to depend on a complex structural motif. This motif can be used as a strategy by the virus to avoid transport of incorrectly folded proteins and to selectively import assembled trimers into the nucleus. Structural nuclear localization motifs can also be important for nuclear proteins without a classic basic amino acid stretch, including multimeric cellular proteins.

A slender tract of glycine residues is required for translocation of the VP2 protein N-terminal domain through the parvovirus MVM capsid channel to initiate infection

Viruses constitute paradigms to study conformational dynamics in biomacromolecular assemblies. Infection by the parvovirus MVM (minute virus of mice) requires a conformational rearrangement that involves the intracellular externalization through capsid channels of the 2Nt (N-terminal region of VP2). We have investigated the role in this process of conserved glycine residues in an extended glycine-rich tract located immediately after 2Nt. Based on the virus structure, residues with hydrophobic side chains of increasing volume were substituted for glycine residues 31 or 33. Mutations had no effect on capsid assembly or stability, but inhibited virus infectivity. All mutations, except those to alanine residues which had minor effects, impaired 2Nt externalization in nuclear maturing virions and in purified virions, to an extent that correlated with the side chain size. Different biochemical and biophysical analyses were consistent with this result. Importantly, all of the tested glycine residue replacements impaired the capacity of the virion to initiate infection, at ratios correlating with their restrictive effects on 2Nt externalization. Thus small residues within the evolutionarily conserved glycine-rich tract facilitate 2Nt externalization through the capsid channel, as required by this virus to initiate cell entry. The results demonstrate the exquisite dependence on geometric constraints of a biologically relevant translocation event in a biomolecular complex.