Minute virus of mice small non-structural protein NS2 localizes within, but is not required for the formation of, Smn-associated autonomous parvovirus-associated replication bodies

Mutations in the Non-Structural Protein-Coding Sequence of Protoparvovirus H-1PV Enhance the Fitness of the Virus and Show Key Benefits Regarding the Transduction Efficiency of Derived Vectors

Single nucleotide changes were introduced into the non-structural (NS) coding sequence of the H-1 parvovirus (PV) infectious molecular clone and the corresponding virus stocks produced, thereby generating H1-PM-I, H1-PM-II, H1-PM-III, and H1-DM. The effects of the mutations on viral fitness were analyzed. Because of the overlapping sequences of NS1 and NS2, the mutations affected either NS2 (H1-PM-II, -III) or both NS1 and NS2 proteins (H1-PM-I, H1-DM). Our results show key benefits of PM-I, PM-II, and DM mutations with regard to the fitness of the virus stocks produced. Indeed, these mutants displayed a higher production of infectious virus in different cell cultures and better spreading capacity than the wild-type virus. This correlated with a decreased particle-to-infectivity (P/I) ratio and stimulation of an early step(s) of the viral cycle prior to viral DNA replication, namely, cell binding and internalization. These mutations also enhance the transduction efficiency of H-1PV-based vectors. In contrast, the PM-III mutation, which affects NS2 at a position downstream of the sequence deleted in Del H-1PV, impaired virus replication and spreading. We hypothesize that the NS2 protein—modified in H1-PM-I, H1-PM-II, and H1-DM—may result in the stimulation of some maturation step(s) of the capsid and facilitate virus entry into subsequently infected cells.

The Minute Virus of Mice NS2 proteins are not essential for productive infection of embryonic murine cells in utero

The P4 promoter of the autonomous parvovirus Minute Virus of Mice (MVM) drives the production of its non-structural proteins, NS1 and NS2. The NS2 isoforms are without enzymatic activity but interact with cellular proteins. While NS2 is crucial to the viral life cycle in cultured murine cells, NS2-null mutant virus productively infects transformed host cells of other species. In the mouse, sensitivity to MVM infection is age dependent, exhibiting limited subclinical infections in adults, but sustained and potentially lethal infection in embryos. We therefore questioned whether the species-dependent requirement for NS2 function in vitro would be retained in utero. We report here that it is not. NS2-null mutant MVMp is capable of mounting a productive, albeit much reduced, infection of normal embryonic mouse cells in vivo. Based on the data, we hypothesize that NS2 may bear an as-yet undescribed immunosuppressive function.

Distribution and dynamics of transcription-associated proteins during parvovirus infection

Canine parvovirus (CPV) infection leads to reorganization of nuclear proteinaceous subcompartments. Our studies showed that virus infection causes a time-dependent increase in the amount of viral nonstructural protein NS1 mRNA. Fluorescence recovery after photobleaching showed that the recovery kinetics of nuclear transcription-associated proteins, TATA binding protein (TBP), transcription factor IIB (TFIIB), and poly(A) binding protein nuclear 1 (PABPN1) were different in infected and noninfected cells, pointing to virus-induced alterations in binding dynamics of these proteins.

An in-frame deletion in the NS protein-coding sequence of parvovirus H-1PV efficiently stimulates export and infectivity of progeny virions

An in-frame, 114-nucleotide-long deletion that affects the NS-coding sequence was created in the infectious molecular clone of the standard parvovirus H-1PV, thereby generating Del H-1PV. The plasmid was transfected and further propagated in permissive human cell lines in order to analyze the effects of the deletion on virus fitness. Our results show key benefits of this deletion, as Del H-1PV proved to exhibit (i) higher infectivity (lower particle-to-infectivity ratio) in vitro and (ii) enhanced tumor growth suppression in vivo compared to wild-type H-1PV. This increased infectivity correlated with an accelerated egress of Del H-1PV progeny virions in producer cells and with an overall stimulation of the viral life cycle in subsequently infected cells. Indeed, virus adsorption and internalization were significantly improved with Del H-1PV, which may account for the earlier appearance of viral DNA replicative forms that was observed with Del H-1PV than wild-type H-1PV. We hypothesize that the internal deletion within the NS2 and/or NS1 protein expressed by Del H-1PV results in the stimulation of some step(s) of the viral life cycle, in particular, a maturation step(s), leading to more efficient nuclear export of infectious viral particles and increased fitness of the virus produced.

Recruitment of DNA replication and damage response proteins to viral replication centers during infection with NS2 mutants of Minute Virus of Mice (MVM)

MVM NS2 is essential for viral DNA amplification, but its mechanism of action is unknown. A classification scheme for autonomous parvovirus-associated replication (APAR) center development, based on NS1 distribution, was used to characterize abnormal APAR body maturation in NS2null mutant infections, and their organization examined for defects in host protein recruitment. Since acquisition of known replication factors appeared normal, we looked for differences in invoked DNA damage responses. We observed widespread association of H2AX/MDC1 damage response foci with viral replication centers, and sequestration and complex hyperphosphorylation of RPA(32), which occurred in wildtype and mutant infections. Quantifying these responses by western transfer indicated that both wildtype and NS2 mutant MVM elicited ATM activation, while phosphorylation of ATR, already basally activated in asynchronous A9 cells, was downregulated. We conclude that MVM infection invokes multiple damage responses that influence the APAR environment, but that NS2 does not modify the recruitment of cellular proteins.

Parvovirus induced alterations in nuclear architecture and dynamics

The nucleus of interphase eukaryotic cell is a highly compartmentalized structure containing the three-dimensional network of chromatin and numerous proteinaceous subcompartments. DNA viruses induce profound changes in the intranuclear structures of their host cells. We are applying a combination of confocal imaging including photobleaching microscopy and computational methods to analyze the modifications of nuclear architecture and dynamics in parvovirus infected cells. Upon canine parvovirus infection, expansion of the viral replication compartment is accompanied by chromatin marginalization to the vicinity of the nuclear membrane. Dextran microinjection and fluorescence recovery after photobleaching (FRAP) studies revealed the homogeneity of this compartment. Markedly, in spite of increase in viral DNA content of the nucleus, a significant increase in the protein mobility was observed in infected compared to non-infected cells. Moreover, analyzis of the dynamics of photoactivable capsid protein demonstrated rapid intranuclear dynamics of viral capsids. Finally, quantitative FRAP and cellular modelling were used to determine the duration of viral genome replication. Altogether, our findings indicate that parvoviruses modify the nuclear structure and dynamics extensively. Intranuclear crowding of viral components leads to enlargement of the interchromosomal domain and to chromatin marginalization via depletion attraction. In conclusion, parvoviruses provide a useful model system for understanding the mechanisms of virus-induced intranuclear modifications.

Mosquito densonucleosis virus non-structural protein NS2 is necessary for a productive infection

Mosquito densonucleosis viruses synthesize two non-structural proteins, NS1 and NS2. While NS1 has been studied relatively well, little is known about NS2. Antiserum was raised against a peptide near the N-terminus of NS2, and used to conduct Western blot analysis and immuno-fluorescence assays. Western blots revealed a prominent band near the expected size (41 kDa). Immuno-fluorescence studies of mosquito cells transfected with AeDNV indicate that NS2 has a wider distribution pattern than does NS1, and the distribution pattern appears to be a function of time post-infection. Nuclear localization of NS2 requires intact C-terminus but does not require additional viral proteins. Mutations ranging from complete NS2 knock-out to a single missense amino acid substitution in NS2 can significantly reduce viral replication and production of viable progeny.

Replication initiator protein NS1 of the parvovirus minute virus of mice binds to modular divergent sites distributed throughout duplex viral DNA

To initiate DNA synthesis, the NS1 protein of minute virus of mice (MVM) first binds to a simple cognate recognition sequence in the viral origins, comprising two to three tandem copies of the tetranucleotide TGGT. However, this motif is also widely dispersed throughout the viral genome. Using an immunoselection procedure, we show that NS1 specifically binds to many internal sites, so that all viral fragments of more than approximately 170 nucleotides effectively compete for NS1, often binding with higher affinity to these internal sites than to sites in the origins. We explore the diversity of the internal sites using competitive binding and DNase I protection assays and show that they vary between two extreme forms. Simple sites with three somewhat degenerate, tandem TGGT reiterations bind effectively but are minimally responsive to ATP, while complex sites, containing multiple variably spaced TGGT elements arranged as opposing clusters, bind NS1 with an affinity that can be enhanced approximately 10-fold by ATP. Using immuno-selection procedures with randomized sequences embedded within specific regions of the genome, we explore possible binding configurations in these two types of site. We conclude that binding is modular, combinatorial, and highly flexible. NS1 recognizes two to six variably spaced, more-or-less degenerate forms of the 5'-TGGT-3' motif, so that it binds efficiently to a wide variety of sequences. Thus, despite complex coding constraints, binding sites are configured at frequent intervals throughout duplex forms of viral DNA, suggesting that NS1 may serve as a form of chromatin to protect and tailor the environment of replicating genomes.

Dynamics and interactions of parvoviral NS1 protein in the nucleus

Nuclear positioning and dynamic interactions of viral proteins with nuclear substructures play essential roles during infection with DNA viruses. Visualization of the intranuclear interactions and motility of the parvovirus replication protein (NS1) in living cells gives insight into specific parvovirus protein-cellular structure interactions. Confocal analysis of highly synchronized infected Norden Laboratory Feline Kidney cells showed accumulation of nuclear NS1 in discrete interchromosomal foci. NS1 fused with enhanced yellow fluorescence protein (NS1-EYFP) provided a marker in live cells for dynamics of NS1 traced by photobleaching techniques. Fluorescence Recovery after Photobleaching suggested that the NS1 protein is not freely diffusing but undergoes transient interactions with nuclear compartments. Fluorescence Loss in Photobleaching demonstrated for the first time the shuttling of a parvoviral protein between the nucleus and the cytoplasm as assayed with NS1-EYFP. Finally, time-lapse imaging of infected cells revealed that the intranuclear distribution of NS1-EYFP evolves dramatically starting from the formation of NS1 foci and proceeding to a homogenous distribution extending throughout the nucleus.

Differential roles for the C-terminal hexapeptide domains of NS2 splice variants during MVM infection of murine cells

The MVM NS2 proteins are required for viral replication in cells of its normal murine host, but are dispensable in transformed human 324K cells. Alternate splicing at the minor intron controls synthesis of three forms of this protein, which differ in their C-terminal hexapeptides and in their relative abundance, with NS2P and NS2Y, the predominant isoforms, being expressed at a 5:1 ratio. Mutant genomes were constructed with premature termination codons in the C-terminal exons of either NS2P or NS2Y, which resulted in their failure to accumulate in vivo. To modulate their expression levels, we also introduced a mutation at the putative splice branch point of the large intron, dubbed NS2(lo), that reduced total NS2 expression in murine A9 cells such that NS2P accumulated to approximately half the level normally seen for NS2Y. All mutants replicated productively in human 324K cells. In A9 cells, NS2Y(-) mutants replicated like wildtype, and the NS2(lo) mutants expressed NS1 and replicated duplex viral DNA like wildtype, although their progeny single-strand DNA synthesis was reduced. However, while NS2P(-) and NS2-null viruses initiated infection efficiently in A9 cells, they gave diminished NS1 levels, and viral macromolecular synthesis appeared to become paralyzed shortly after the onset of viral duplex DNA amplification, such that no progeny single-strand DNA could be detected. Thus, the NS2P isoform, even when expressed at a level lower than that of NS2Y, performs a critical role in infection of A9 cells that cannot be accomplished by the NS2Y isoform alone.

Identification of functional elements in the murine Gabp alpha/ATP synthase coupling factor 6 bi-directional promoter

The GA-repeat binding protein (GABP) is a ubiquitous transcription factor involved in transcriptional regulation of genes encoding proteins involved in a variety of cellular processes including adipocyte differentiation, mitochondrial respiration, and neuromuscular signaling. GABP is composed of two subunits; the GABP alpha subunit is a member of the Ets-family of transcription factors, and the unrelated ankyrin repeat containing GABP beta subunit. We previously identified a bidirectional promoter directing the expression of Gabpa (GAA) gene in one direction and ATP Synthase Coupling Factor 6 (Atp5j) (CF6) gene in the other [Chinenov, Y., Coombs, C. and Martin, M. E., 2000a. "Isolation of a bi-directional promoter directing the expression of the mouse GABP alpha and ATP Synthase Coupling Factor 6 genes. Gene 261:311-320.]. In this study we characterize sequence elements and regulatory factors contributing to the promoter activities of the GAA/CF6 bidirectional promoter. The core of the GAA/CF6 bidirectional promoter is retained within a 400 bp sequence and contains four GABP binding sites, a Sp1/3 binding site and an YY1 binding site. Site-directed mutagenesis demonstrated that while no single factor binding site was essential for promoter activity in either direction, the GA1 site located proximal to the previously mapped transcription start sites functioned cooperatively with the other GABP binding sites and with the Sp1/3 and YY1 sites to provide transcriptional activation of the GAA and CF6 promoters. The other GABP sites and the Sp1/3 and YY1 binding sites were functionally redundant for basal promoter activities in both directions. Electrophoretic mobility shift assays identified multiple DNA-protein complexes containing GABP alpha, GABP beta, Sp1, Sp3 or YY1 proteins, including one ternary complex containing GABP alpha, GABP beta and Sp1 proteins. Binding of GABP to the GAA/CF6 bi-directional promoter provides the potential for autoregulation of GABP alpha expression and confirms the importance of GABP in the coordinate expression of respiratory chain components.