An adeno-associated virus (AAV) initiator protein, Rep78, catalyzes the cleavage and ligation of single-stranded AAV ori DNA

HUH Endonuclease: A Sequence-specific Fusion Protein Tag for Precise DNA-Protein Conjugation

Synthetic DNA-protein conjugates have found widespread applications in diagnostics and therapeutics, prompting a growing interest in developing chemical biology methodologies for the precise and site-specific preparation of covalent DNA-protein conjugates. In this review article, we concentrate on techniques to achieve precise control over the structural and site-specific aspects of DNA-protein conjugates. We summarize conventional methods involving unnatural amino acids and self-labeling proteins, accompanied by a discussion of their potential limitations. Our primary focus is on introducing HUH endonuclease as a novel generation of fusion protein tags for DNA-protein conjugate preparation. The detailed conjugation mechanisms and structures of representative endonucleases are surveyed, showcasing their advantages as fusion protein tag in sequence selectivity, biological orthogonality, and no requirement for DNA modification. Additionally, we present the burgeoning applications of HUH-tag-based DNA-protein conjugates in protein assembly, biosensing, and gene editing. Furthermore, we delve into the future research directions of the HUH-tag, highlighting its significant potential for applications in the biomedical and DNA nanotechnology fields.

Inducible HEK293 AAV packaging cell lines expressing Rep proteins

Packaging or producer cell lines for scalable recombinant adeno-associated virus (rAAV) production have been notoriously difficult to create due in part to the cytostatic nature of the Rep proteins required for AAV production. The most difficult challenge being creating AAV packaging cell lines using HEK293 parental cells, currently the best mammalian platform for rAAV production due to the constitutive expression of E1A in HEK293 cells, a key REP transcription activator. Using suspension and serum-free media adapted HEK293SF carrying a gene expression regulation system induced by addition of cumate and coumermycin, we were able to create REP-expressing AAV packaging cells. This was achieved by carefully choosing two of the AAV Rep proteins (Rep 40 and 68), using two inducible promoters with different expression levels and integrating into the cells through lentiviral vector transduction. Three of our best clones produced rAAV titers comparable to titers obtained by standard triple plasmid transfection of their parental cells. These clones were stable for up to 7 weeks under continuous cultures condition. rAAV production from one clone was also validated at scale of 1 L in a wave bioreactor using serum-free suspension culture.

Mechanistic modeling explains the production dynamics of recombinant adeno-associated virus with the baculovirus expression vector system

Current manufacturing processes for recombinant adeno-associated viruses (rAAVs) have less-than-desired yields and produce significant amounts of empty capsids. The increasing demand and the high cost of goods for rAAV-based gene therapies motivate development of more efficient manufacturing processes. Recently, the US Food and Drug Administration (FDA) approved the first rAAV-based gene therapy product manufactured in the baculovirus expression vector system (BEVS), a technology that demonstrated production of high titers of full capsids. This work presents a first mechanistic model describing the key extracellular and intracellular phenomena occurring during baculovirus infection and rAAV maturation in the BEVS. The model predictions are successfully validated for in-house and literature experimental measurements of the vector genome and of structural and non-structural proteins collected during rAAV manufacturing in the BEVS with the TwoBac and ThreeBac constructs. A model-based analysis of the process is carried out to identify the bottlenecks that limit full capsid formation. Vector genome amplification is found to be the limiting step for rAAV production in Sf9 cells using either the TwoBac or ThreeBac system. In turn, vector genome amplification is hindered by limiting Rep78 levels. Transgene and non-essential baculovirus protein expression in the insect cell during rAAV manufacturing also negatively influences the rAAV production yields.

Ewing sarcoma treatment: a gene therapy approach

Ewing sarcoma (ES) is an aggressive malignant tumor, characterized by non-random chromosomal translocations that produce fusion genes. Fusion genes and fusion protein products are promising targets for gene therapy. Therapeutic approaches and strategies vary based on target molecules (nucleotides, proteins) of interest. We present an extensive literature review of active molecules for gene therapy and methods of gene therapy delivery, both of which are necessary for successful treatment.

The adeno-associated virus 2 (AAV2) genome and rep 68/78 proteins interact with cellular sites of DNA damage

Nuclear DNA viruses simultaneously access cellular factors that aid their life cycle while evading inhibitory factors by localizing to distinct nuclear sites. Adeno-Associated Viruses (AAVs), which are Dependoviruses in the family Parvovirinae, are non-enveloped icosahedral viruses, that have been developed as recombinant AAV vectors (rAAV) to express transgenes. AAV2 expression and replication occur in nuclear viral replication centers (VRCs), which relies on cellular replication machinery as well as coinfection by helper viruses such as adenoviruses or herpesviruses, or exogenous DNA damage to host cells. AAV2 infection induces a complex cellular DNA damage response (DDR), either in response to viral DNA or viral proteins expressed in the host nucleus during infection, where VRCs colocalize with DDR proteins. We have previously developed a modified iteration of a viral chromosome conformation capture (V3C-seq) assay to show that the autonomous parvovirus Minute Virus of Mice (MVM) localizes to cellular sites of DNA damage to establish and amplify its replication. Similar V3C-seq assays to map AAV2 show that the AAV2 genome colocalized with cellular sites of DNA damage under both non-replicating and replicating conditions. The AAV2 non-structural protein Rep 68/78, also localized to cellular DDR sites during both non-replicating and replicating infections, and also when ectopically expressed. Ectopically expressed Rep could be efficiently re-localized to DDR sites induced by micro-irradiation. Recombinant AAV2 gene therapy vector genomes derived from AAV2 localized to sites of cellular DNA damage to a lesser degree, suggesting that the Inverted Terminal Repeat (ITR) origins of replication were insufficient for targeting.

Molecular detection and characterization of Carnivore chaphamaparvovirus 1 in dogs

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Canine chaphamaparvovirus (CaChPV) is a novel parvovirus recently discovered in dogs;

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Herein, stool samples from dogs with or without enteric signs were screened for CaChPV;

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CaChPV DNA was found either in diarrhoeic (1.9 %) or asymptomatic (1.6 %) dogs;

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The nearly complete genome sequences were determined for two strains;

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The Italian CaChPV strains tightly clustered with the American reference viruses.

Canine chaphamaparvovirus (CaChPV) is a newly recognised parvovirus discovered by metagenomic analysis during an outbreak of diarrhoea in dogs in Colorado, USA, in 2017 and more recently detected in diarrhoeic dogs in China. Whether the virus plays a role as canine pathogen and whether it is distributed elsewhere, in other geographical areas, is not known. We performed a case-control study to investigate the possible association of CaChPV with enteritis in dogs. CaChPV DNA was detected both in the stools of diarrhoeic dogs (1.9 %, 3/155) and of healthy animals (1.6 %, 2/120). All the CaChPV-infected dogs with diarrhea were mixed infected with other enteric viruses such as canine parvovirus (formerly CPV-2), canine bufavirus (CBuV) and canine coronavirus (CCoV), whilst none of the asymptomatic CaChPV positive animals resulted co-infected. The nearly full-length genome and the partial capsid protein (VP) gene of three canine strains, Te/36OVUD/19/ITA, Te/37OVUD/19/ITA and Te/70OVUD/19/ITA, were reconstructed. Upon phylogenetic analyses based on the NS1 and VP aa sequences, the Italian CaChPV strains tightly clustered with the American reference viruses. Distinctive residues could be mapped to the deduced variable regions of the VP of canine and feline chaphamaparvoviruses, considered as important markers of host range and pathogenicity for parvoviruses.

Evolution of dependoparvoviruses across geological timescales-implications for design of AAV-based gene therapy vectors

Endogenous viral elements (EVEs) are genetic remnants of viruses that have integrated into host genomes millions of years ago and retained as heritable elements passed on to offspring until present-day. As a result, EVEs provide an opportunity to analyse the genomes of extinct viruses utilizing these genomic viral fossils to study evolution of viruses over large timescales. Analysis of sequences from near full-length EVEs of dependoparvoviral origin identified within three mammalian taxa, Whippomorpha (whales and hippos), Vespertilionidae (smooth-nosed bats), and Lagomorpha (rabbits, hares, and pikas), indicates that distinct ancestral dependoparvovirus species integrated into these host genomes approximately 77 to 23 million years ago. These ancestral viruses are unique relative to modern adeno-associated viruses (AAVs), and distinct from extant species of genus Dependoparvovirus. These EVE sequences show characteristics previously unseen in modern, mammalian AAVs, but instead appear more similar to the more primitive, autonomously replicating and pathogenic waterfowl dependoparvoviruses. Phylogeny reconstruction suggests that the whippomorph EVE orthologue derives from exogenous ancestors of autonomous and highly pathogenic dependoparvovirus lineages, believed to have uniquely co-evolved with waterfowl birds to present date. In contrast, ancestors of the two other mammalian orthologues (Lagomorpha and Vespertilionidae) likely shared the same lineage as all other known mammalian exogenous AAVs. Comparative in silico analysis of the EVE genomes revealed remarkable overall conservation of AAV rep and cap genes, despite millions of years of integration within the host germline. Modelling these proteins identified unexpected variety, even between orthologues, in previously defined capsid viral protein (VP) variable regions, especially in those related to the three- and fivefold symmetry axes of the capsid. Moreover, the normally well-conserved phospholipase A2 domain of the predicted minor VP1 also exhibited a high degree of sequence variance. These findings may indicate unique biological properties for these virus ‘fossils’ relative to extant dependoparvoviruses and suggest key regions to explore within capsid sequences that may confer novel properties for engineered gene therapy vectors based on paleovirology data.

Genomic and transcriptional analyses of novel parvoviruses identified from dead peafowl

To identify potential pathogens responsible for a disease outbreak of cultured peafowls in China in 2013, metagenomic sequencing was conducted. The genomes of two closely related parvoviruses, namely peafowl parvovirus 1 (PePV1) and PePV2, were identified with size of 4428 bp and 4348 bp, respectively. Phylogenetic analysis revealed that both viruses are novel parvoviruses, belonging to the proposed genus Chapparvovirus of Parvoviridae. The transcriptional profile of PePV1 was analyzed by transfecting a nearly complete PePV1 genome into HEK-293T cells. Results revealed that PePV1 employs one promoter and two polyadenylation sites to start and terminate its transcriptions, with one donor site and two acceptor sites for pre-mRNA splicing. PePV1 DNA and structural protein were detected in several tissues of a dead peafowl, which appeared to have suffered enteritis, pneumonia and viremia. These results provide novel information of chapparvoviruses, and call for attention to the potential pathogens.

Exchange of functional domains between a bacterial conjugative relaxase and the integrase of the human adeno-associated virus

Endonucleases of the HUH family are specialized in processing single-stranded DNA in a variety of evolutionarily highly conserved biological processes related to mobile genetic elements. They share a structurally defined catalytic domain for site-specific nicking and strand-transfer reactions, which is often linked to the activities of additional functional domains, contributing to their overall versatility. To assess if these HUH domains could be interchanged, we created a chimeric protein from two distantly related HUH endonucleases, containing the N-terminal HUH domain of the bacterial conjugative relaxase TrwC and the C-terminal DNA helicase domain of the human adeno-associated virus (AAV) replicase and site-specific integrase. The purified chimeric protein retained oligomerization properties and DNA helicase activities similar to Rep68, while its DNA binding specificity and cleaving-joining activity at oriT was similar to TrwC. Interestingly, the chimeric protein could catalyse site-specific integration in bacteria with an efficiency comparable to that of TrwC, while the HUH domain of TrwC alone was unable to catalyze this reaction, implying that the Rep68 C-terminal helicase domain is complementing the TrwC HUH domain to achieve site-specific integration into TrwC targets in bacteria. Our results illustrate how HUH domains could have acquired through evolution other domains in order to attain new roles, contributing to the functional flexibility observed in this protein superfamily.

Novel Mutant AAV2 Rep Proteins Support AAV2 Replication without Blocking HSV-1 Helpervirus Replication

As their names imply, parvoviruses of the genus Dependovirus rely for their efficient replication on the concurrent presence of a helpervirus, such as herpesvirus, adenovirus, or papilloma virus. Adeno-associated virus 2 (AAV2) is such an example, which in turn can efficiently inhibit the replication of each helpervirus by distinct mechanisms. In a previous study we have shown that expression of the AAV2 rep gene is not compatible with efficient replication of herpes simplex virus 1 (HSV-1). In particular, the combined DNA-binding and ATPase/helicase activities of the Rep68/78 proteins have been shown to exert opposite effects on the replication of AAV2 and HSV-1. While essential for AAV2 DNA replication these protein activities account for the Rep-mediated inhibition of HSV-1 replication. Here, we describe a novel Rep mutant (Rep-D371Y), which displayed an unexpected phenotype. Rep-D371Y did not block HSV-1 replication, but still supported efficient AAV2 replication, at least when a double-stranded AAV2 genome template was used. We also found that the capacity of Rep-D371Y to induce apoptosis and a Rep-specific DNA damage response was significantly reduced compared to wild-type Rep. These findings suggest that AAV2 Rep-helicase subdomains exert diverging activities, which contribute to distinct steps of the AAV2 life cycle. More important, the novel AAV2 mutant Rep-D371Y may allow deciphering yet unsolved activities of the AAV2 Rep proteins such as DNA second-strand synthesis, genomic integration or packaging, which all involve the Rep-helicase activity.

Germline viral "fossils" guide in silico reconstruction of a mid-Cenozoic era marsupial adeno-associated virus

Germline endogenous viral elements (EVEs) genetically preserve viral nucleotide sequences useful to the study of viral evolution, gene mutation, and the phylogenetic relationships among host organisms. Here, we describe a lineage-specific, adeno-associated virus (AAV)-derived endogenous viral element (mAAV-EVE1) found within the germline of numerous closely related marsupial species. Molecular screening of a marsupial DNA panel indicated that mAAV-EVE1 occurs specifically within the marsupial suborder Macropodiformes (present-day kangaroos, wallabies, and related macropodoids), to the exclusion of other Diprotodontian lineages. Orthologous mAAV-EVE1 locus sequences from sixteen macropodoid species, representing a speciation history spanning an estimated 30 million years, facilitated compilation of an inferred ancestral sequence that recapitulates the genome of an ancient marsupial AAV that circulated among Australian metatherian fauna sometime during the late Eocene to early Oligocene. In silico gene reconstruction and molecular modelling indicate remarkable conservation of viral structure over a geologic timescale. Characterisation of AAV-EVE loci among disparate species affords insight into AAV evolution and, in the case of macropodoid species, may offer an additional genetic basis for assignment of phylogenetic relationships among the Macropodoidea. From an applied perspective, the identified AAV “fossils” provide novel capsid sequences for use in translational research and clinical applications.

AAV2 X increases AAV6 rep/cap-driven rAAV production

We have recently identified a new gene, involved in DNA replication, at the far 3' end of the adeno-associated virus type 2 (AAV2) genome. The AAV type 6 (AAV6) genome has a disrupted X open reading frame (ORF) whose two halves, when combined, have full-length homology and comparable size to AAV2 X. Hypothesizing that AAV6 X is inactive, we assessed if AAV2 X augments recombinant (r)AAV2 DNA replication and virion production, but with rep and cap trans-functions of AAV6. Using AAV2 X expressing HEK293 cell lines we show AAV2 X significantly boosts rAAV DNA replication/virion production, driven by AAV6 rep/cap as it does the AAV2 rep/cap system. Protein BLAST search for homology between AAV2 X and various AAV Rep78 proteins suggests that X might be AAV8 Rep78-derived and have some of its activities. These data suggest that AAV2 X, and the corresponding X genes of other AAV types/clades, warrant further study.

Adeno-associated virus Rep represses the human integration site promoter by two pathways that are similar to those required for the regulation of the viral p5 promoter

Adeno-associated virus serotype 2 (AAV2) can efficiently replicate in cells that have been infected with helper viruses, such as adenovirus or herpesvirus. However, in the absence of helper virus infection, AAV2 establishes latency by integrating its genome site specifically into PPP1R12C, a gene located on chromosome 19. This integration target site falls into one of the most gene-dense regions of the human genome, thus inviting the question as to whether the virus has evolved mechanisms to control this complex transcriptional environment in order to facilitate integration, maintain an apparently innocuous latency, and/or establish conditions that are conducive to the rescue of the integrated viral genome. The viral replication (Rep) proteins control and direct every known aspect of the viral life cycle and have been shown to tightly control all AAV2 promoters. In addition, a number of heterologous promoters are repressed by the AAV2 Rep proteins. Here, we demonstrate that Rep proteins efficiently repress expression from the target site PPP1R12C promoter. We find evidence that this repression employs mechanisms similar to those described for Rep-mediated AAV2 p5 promoter regulation. Furthermore, we show that the repression of the cellular target site promoter is based on two distinct mechanisms, one relying on the presence of a functional Rep binding motif within the 5′ untranslated region (UTR) of PPP1R12C, whereas the second pathway requires only an intact nucleoside triphosphate (NTP) binding site within the Rep proteins, indicating the possible reliance of this pathway on interactions of the Rep proteins with cellular proteins that mediate or regulate cellular transcription.

IMPORTANCE The observation that repression of transcription from the adeno-associated virus serotype 2 (AAV2) p5 and integration target site promoters is mediated by shared mechanisms highlights the possible coevolution of virus and host and could lead to the identification of host factors that the virus exploits to navigate its life cycle.

Production and characterization of novel recombinant adeno-associated virus replicative-form genomes: a eukaryotic source of DNA for gene transfer

Conventional non-viral gene transfer uses bacterial plasmid DNA containing antibiotic resistance genes, cis-acting bacterial sequence elements, and prokaryotic methylation patterns that may adversely affect transgene expression and vector stability in vivo. Here, we describe novel replicative forms of a eukaryotic vector DNA that consist solely of an expression cassette flanked by adeno-associated virus (AAV) inverted terminal repeats. Extensive structural analyses revealed that this AAV-derived vector DNA consists of linear, duplex molecules with covalently closed ends (termed closed-ended, linear duplex, or "CELiD", DNA). CELiD vectors, produced in Sf9 insect cells, require AAV rep gene expression for amplification. Amounts of CELiD DNA produced from insect cell lines stably transfected with an ITR-flanked transgene exceeded 60 mg per 5 × 10(9) Sf9 cells, and 1-15 mg from a comparable number of parental Sf9 cells in which the transgene was introduced via recombinant baculovirus infection. In mice, systemically delivered CELiD DNA resulted in long-term, stable transgene expression in the liver. CELiD vectors represent a novel eukaryotic alternative to bacterial plasmid DNA.

HUH site-specific recombinases for targeted modification of the human genome

Site-specific recombinases (SSRs) have been crucial in the development of mammalian transgenesis. For gene therapy purposes, this approach remains challenging, because, for example, SSR delivery is largely unresolved and SSR DNA substrates must pre-exist in target cells. In this review, we discuss the potential of His-hydrophobic-His (HUH) recombinases to overcome some of the limitations of conventional SSRs. Members of the HUH protein family cleave single-stranded (ss)DNA, but can mediate site-specific integration with the aid of the host replication machinery. Adeno-associated virus (AAV) Rep remains the only known example to support site-specific integration in human cells, and AAV is an excellent gene delivery vector that can be targeted to specific cells and organelles. Bacterial protein TrwC catalyzes integration into human sequences and can be delivered to human cells covalently linked to DNA, offering attractive new features for targeted genome modification.

Large-scale recombinant adeno-associated virus production

Since recombinant adeno-associated virus (rAAV) was first described as a potential mammalian cell transducing system, frequent reports purportedly solving the problems of scalable production have appeared. Yet few of these processes have enabled the development of robust and economical rAAV production. Two production platforms have emerged that have gained broad support for producing both research and clinical grade vectors. These processes differ fundamentally in several aspects. One approach is based on adherent mammalian cells and uses optimized chemical transient transfection for introducing the essential genetic components into the cells. The other approach utilizes suspension cultures of invertebrate cells. Baculovirus expression vectors are used for introducing the AAV genes into the cells. In addition, the baculovirus provides the helper functions necessary for efficient AAV DNA replication. The use of suspension cell culture provides an intrinsically more scalable platform system than using adherent cells. The upstream processes for suspension cultures are amenable for automation and are easily monitored and regulated to maintain optimum conditions that produce consistent yields of rAAV. Issues relating to developing new and improving existing rAAV production methods are discussed.

Interactions between AAV-2 and HSV-1: implications for hybrid vector design

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have a transgene capacity of up to 150 kbp and can efficiently transduce many different cell types in culture and in vivo without causing cytopathic effects. However, these vectors do not support long-term transgene expression. Adeno-associated virus type 2 (AAV-2) has the capacity to integrate its genome into a specific site on human chromosome 19, but AAV-2-derived gene therapy vectors have a transgene capacity of only 4.5 kb. To combine the large transgene capacity of HSV-1 with the potential for site-specific genomic integration and long-term transgene expression of AAV-2, HSV/AAV hybrid vectors have been developed. This review describes the design, applications and limitations of these hybrid vectors. However, as HSV-1 is a full helper virus for AAV-2 replication, the main focus is the analysis of the molecular mechanisms of interaction between the two viruses. The knowledge of these interactions will have direct implications on the design of novel HSV/AAV hybrid vectors.

Intracellular localization of adeno-associated viral proteins expressed in insect cells

Production of vectors derived from adeno-associated virus (AAVv) in insect cells represents a feasible option for large-scale applications. However, transducing particles yields obtained in this system are low compared with total capsid yields, suggesting the presence of genome encapsidation bottlenecks. Three components are required for AAVv production: viral capsid proteins (VP), the recombinant AAV genome, and Rep proteins for AAV genome replication and encapsidation. Little is known about the interaction between the three components in insect cells, which have intracellular conditions different to those in mammalian cells. In this work, the localization of AAV proteins in insect cells was assessed for the first time with the purpose of finding potential limiting factors. Unassembled VP were located either in the cytoplasm or in the nucleus. Their transport into the nucleus was dependent on protein concentration. Empty capsids were located in defined subnuclear compartments. Rep proteins expressed individually were efficiently translocated into the nucleus. Their intranuclear distribution was not uniform and differed from VP distribution. While Rep52 distribution and expression levels were not affected by AAV genomes or VP, Rep78 distribution and stability changed during coexpression. Expression of all AAV components modified capsid intranuclear distribution, and assembled VP were found in vesicles located in the nuclear periphery. Such vesicles were related to baculovirus infection, highlighting its role in AAVv production in insect cells. The results obtained in this work suggest that the intracellular distribution of AAV proteins allows their interaction and does not limit vector production in insect cells.

Adeno-associated virus: a key to the human genome?

Adeno-associated viruses (AAV) are widely spread throughout the human population, yet no pathology has been associated with infection. This fact, together with the availability of simple molecular techniques to alter the packaged viral genome, has made AAV a serious contender in the search for an ideal gene therapy delivery vehicle. However, our understanding of the intriguing features of this virus is far from exhausted and it is likely that the mechanisms underlying the viral lifestyle will reveal possible novel strategies that can be employed in future clinical approaches. One such aspect is the unique approach AAV has evolved in order to establish latency. In the absence of a cellular milieu that will support productive viral replication, wild-type AAV can integrate its genome site specifically into a locus on human chromosome 19 (termed AAVS1), where it resides without apparent effects on the host cell until cellular conditions are changed by outside influences, such as adenovirus super-infection, which will lead to the rescue of the viral genome and productive replication. This article will introduce the biology of AAV, the unique viral strategy of targeted genome integration and address relevant questions within the context of attempts to establish therapeutic approaches that will utilize targeted gene addition to the human genome.

Inhibition of herpes simplex virus type 1 replication by adeno-associated virus rep proteins depends on their combined DNA-binding and ATPase/helicase activities

Adeno-associated virus (AAV) has previously been shown to inhibit the replication of its helper virus herpes simplex virus type 1 (HSV-1), and the inhibitory activity has been attributed to the expression of the AAV Rep proteins. In the present study, we assessed the Rep activities required for inhibition of HSV-1 replication using a panel of wild-type and mutant Rep proteins lacking defined domains and activities. We found that the inhibition of HSV-1 replication required Rep DNA-binding and ATPase/helicase activities but not endonuclease activity. The Rep activities required for inhibition of HSV-1 replication precisely coincided with the activities that were responsible for induction of cellular DNA damage and apoptosis, suggesting that these three processes are closely linked. Notably, the presence of Rep induced the hyperphosphorylation of a DNA damage marker, replication protein A (RPA), which has been reported not to be normally hyperphosphorylated during HSV-1 infection and to be sequestered away from HSV-1 replication compartments during infection. Finally, we demonstrate that the execution of apoptosis is not required for inhibition of HSV-1 replication and that the hyperphosphorylation of RPA per se is not inhibitory for HSV-1 replication, suggesting that these two processes are not directly responsible for the inhibition of HSV-1 replication by Rep.

Stimulation of homology-directed gene targeting at an endogenous human locus by a nicking endonuclease

Homologous recombination (HR) is a highly accurate mechanism of DNA repair that can be exploited for homology-directed gene targeting. Since in most cell types HR occurs very infrequently (approximately 10(-6) to 10(-8)), its practical application has been largely restricted to specific experimental systems that allow selection of the few cells that become genetically modified. HR-mediated gene targeting has nonetheless revolutionized genetics by greatly facilitating the analysis of mammalian gene function. Recent studies showed that generation of double-strand DNA breaks at specific loci by designed endonucleases greatly increases the rate of homology-directed gene repair. These findings opened new perspectives for HR-based genome editing in higher eukaryotes. Here, we demonstrate by using donor DNA templates together with the adeno-associated virus (AAV) Rep78 and Rep68 proteins that sequence- and strand-specific cleavage at a native, predefined, human locus can also greatly enhance homology-directed gene targeting. Our findings argue for the development of other strategies besides direct induction of double-strand chromosomal breaks to achieve efficient and heritable targeted genetic modification of cells and organisms. Finally, harnessing the cellular HR pathway through Rep-mediated nicking expands the range of strategies that make use of AAV elements to bring about stable genetic modification of human cells.

Site-specific integration of adeno-associated virus involves partial duplication of the target locus

A variety of viruses establish latency by integrating their genome into the host genome. The integration event generally occurs in a nonspecific manner, precluding the prediction of functional consequences from resulting disruptions of affected host genes. The nonpathogenic adeno-associated virus (AAV) is unique in its ability to stably integrate in a site-specific manner into the human MBS85 gene. To gain a better understanding of the integration mechanism and the consequences of MBS85 disruption, we analyzed the molecular structure of AAV integrants in various latently infected human cell lines. Our study led to the observation that AAV integration causes an extensive but partial duplication of the target gene. Intriguingly, the molecular organization of the integrant leaves the possibility that a functional copy of the disrupted target gene could potentially be preserved despite the resulting rearrangements. A latently infected, Mbs85-targeted mouse ES cell line was generated to study the functional consequences of the observed duplication-based integration mechanism. AAV-modified ES cell lines continued to self-renew, maintained their multilineage differentiation potential and contributed successfully to mouse development when injected into blastocysts. Thus, our study reveals a viral strategy for targeted genome addition with the apparent absence of functional consequences.

Toward exascale production of recombinant adeno-associated virus for gene transfer applications

To gain acceptance as a medical treatment, adeno-associated virus (AAV) vectors require a scalable and economical production method. Recent developments indicate that recombinant AAV (rAAV) production in insect cells is compatible with current good manufacturing practice production on an industrial scale. This platform can fully support development of rAAV therapeutics from tissue culture to small animal models, to large animal models, to toxicology studies, to Phase I clinical trials and beyond. Efforts to characterize, optimize and develop insect cell-based rAAV production have culminated in successful bioreactor-scale production of rAAV, with total yields potentially capable of approaching the exa-(10(18)) scale. These advances in large-scale AAV production will allow us to address specific catastrophic, intractable human diseases such as Duchenne muscular dystrophy, for which large amounts of recombinant vector are essential for successful outcome.

Adeno-associated virus site-specific integration is regulated by TRP-185

Adeno-associated virus (AAV) integrates site specifically into the AAVS1 locus on human chromosome 19. Although recruitment of the AAV nonstructural protein Rep78/68 to the Rep binding site (RBS) on AAVS1 is thought to be an essential step, the mechanism of the site-specific integration, particularly, how the site of integration is determined, remains largely unknown. Here we describe the identification and characterization of a new cellular regulator of AAV site-specific integration. TAR RNA loop binding protein 185 (TRP-185), previously reported to associate with human immunodeficiency virus type 1 TAR RNA, binds to AAVS1 DNA. Our data suggest that TRP-185 suppresses AAV integration at the AAVS1 RBS and enhances AAV integration into a region downstream of the RBS. TRP-185 bound to Rep68 directly, changing the Rep68 DNA binding property and stimulating Rep68 helicase activity. We present a model in which TRP-185 changes the specificity of the AAV integration site from the RBS to a downstream region by acting as a molecular chaperone that promotes Rep68 complex formation competent for 3'-->5' DNA helicase activity.

Adeno-associated virus interactions with B23/Nucleophosmin: identification of sub-nucleolar virion regions

Adeno-associated virus (AAV) is a human parvovirus that normally requires a helper virus such as adenovirus (Ad) for replication. The four replication proteins (Rep78, 68, 52 and 40) encoded by AAV are pleiotropic effectors of virus integration, replication, transcription and virion assembly. Using Rep68 column chromatography and mass spectrometry, we have identified the nucleolar, B23/Nucleophosmin (NPM) protein as an Rep-interacting partner. Rep-NPM interactions were verified by co-immunofluorescence and chemical cross-linking studies. We have found that there is demonstrable, but limited co-localization between Rep and NPM in co-infected cells. In contrast, there was significant co-localization between NPM and AAV Cap proteins. In vitro experiments using purified MBPRep78 and NPM show that NPM stimulates MBPRep78 interactions with the AAV ITR as well as endonuclease activity. These studies suggest that NPM plays a role in AAV amplification affecting Rep function and virion assembly.

Site-directed genome modification: derivatives of DNA-modifying enzymes as targeting tools

The modification of mammalian genomes is an important goal in gene therapy and animal transgenesis. To generate stable genetic and biochemical changes, the therapeutic genes or transgenes need to be incorporated into the host genome. Ideally, the integration of the foreign gene should occur at sites that ensure their continual expression in the absence of any unwanted side effects on cellular metabolism. In this article, we discuss the opportunities provided by natural DNA-modifying enzymes, such as transposases, recombinases and integrases, to mediate the stable integration of foreign genes into host genomes. In addition, we discuss the approaches that have been taken to improve the efficiency and to modify the site-specificity of these enzymes.

Stable secondary structure near the nicking site for adeno-associated virus type 2 Rep proteins on human chromosome 19

Adeno-associated virus serotype 2 (AAV-2) can preferentially integrate its DNA into a 4-kb region of human chromosome 19, designated AAVS1. The nicking activity of AAV-2's Rep68 or Rep78 proteins is essential for preferential integration. These proteins nick at the viral origin of DNA replication and at a similar site within AAVS1. The current nicking model suggests that the strand containing the nicking site is separated from its complementary strand prior to nicking. In AAV serotypes 1 through 6, the nicking site is flanked by a sequence that is predicted to form a stem-loop with standard Watson-Crick base pairing. The region flanking the nicking site in AAVS1 (5'-GGCGGCGGT/TGGGGCTCG-3' [the slash indicates the nicking site]) lacks extensive potential for Watson-Crick base pairing. We therefore performed an empirical search for a stable secondary structure. By comparing the migration of radiolabeled oligonucleotides containing wild-type or mutated sequences from the AAVS1 nicking site to appropriate standards, on native and denaturing polyacrylamide gels, we have found evidence that this region forms a stable secondary structure. Further confirmation was provided by circular dichroism analyses. We identified six bases that appear to be important in forming this putative secondary structure. Mutation of five of these bases, within the context of a double-stranded nicking substrate, reduces the ability of the substrate to be nicked by Rep78 in vitro. Four of these five bases are outside the previously recognized GTTGG nicking site motif and include parts of the CTC motif that has been demonstrated to be important for integration targeting.

Identification of an adeno-associated virus Rep protein binding site in the adenovirus E2a promoter

Adeno-associated virus (AAV) and other parvoviruses inhibit proliferation of nonpermissive cells. The mechanism of this inhibition is not thoroughly understood. To learn how AAV interacts with host cells, we investigated AAV's interaction with adenovirus (Ad), AAV's most efficient helper virus. Coinfection with Ad and AAV results in an AAV-mediated inhibition of Ad5 gene expression and replication. The AAV replication proteins (Rep) activate and repress gene expression from AAV and heterologous transcription promoters. To investigate the role of Rep proteins in the suppression of Ad propagation, we performed chromatin immunoprecipitation analyses that demonstrated in vivo AAV Rep protein interaction with the Ad E2a gene promoter. In vitro binding of purified AAV Rep68 protein to the Ad E2a promoter was characterized by electrophoretic mobility shift assays (Kd= 200 +/- 25 nM). A 38 bp, Rep68-protected region (5'-TAAGAGTCAGCGCGCAGTATTTACTGAAGAGAGCCT-3') was identified by DNase I footprint analysis. The 38-bp protected region contains the weak E2a TATA box, sequence elements that resemble the Rep binding sites identified by random sequence oligonucleotide selection, and the transcription start site. These results suggest that Rep binding to the E2a promoter contributes to the inhibition of E2a gene expression from the Ad E2a promoter and may affect Ad replication.

Adeno-associated virus Rep78/Rep68 promotes localized melting of the rep binding element in the absence of adenosine triphosphate

We have applied fluorescence anisotropy and molecular beacon fluorescence methods to study the interactions between the Adeno-associated virus Rep78/Rep68 protein and the 23-bp Rep binding element (RBE). Rep78/Rep68 stably interacted with both the single- and double-stranded conformations of the RBE, but the interaction mechanisms of single- and double-stranded DNA appeared to be fundamentally different. The stoichiometry of Rep78 association with both the separate top and bottom strands of the RBE was 1:1, and the relative dissociation constant (K(D)) values of these associations were calculated to be 2.3x10(-8) and 3.2x10(-8) M, respectively. In contrast, the stoichiometry of Rep78 association with the double-stranded RBE was 2:1, and the dissociation constant was determined to be 4.2x10(-15) M(2). Moreover, Rep78/Rep68 interaction with the 23-bp duplex RBE appeared to cause localized melting of the double-stranded DNA substrate in the absence of adenosine triphosphate (ATP). This melting activity showed slower kinetics than binding and may contribute to the initiation of ATP-dependent Rep78 helicase activity.

Characterization of the mouse adeno-associated virus AAVS1 ortholog

The nonpathogenic human adeno-associated virus (AAV) has developed a mechanism to integrate its genome into human chromosome 19 at 19q13.4 (termed AAVS1), thereby establishing latency. Here, we provide evidence that the chromosomal signals required for site-specific integration are conserved in the mouse genome proximal to the recently identified Mbs85 gene. These sequence motifs can be specifically nicked by the viral Rep protein required for the initiation of site-specific AAV DNA integration. Furthermore, these signals can serve as a minimal origin for Rep-dependent DNA replication. In addition, we isolated the mouse Mbs85 proximal promoter and show transcriptional activity in three mouse cell lines.

Adeno-associated virus site-specific integration and AAVS1 disruption

Adeno-associated virus (AAV) is a single-stranded DNA virus with a unique biphasic lifestyle consisting of both a productive and a latent phase. Typically, the productive phase requires coinfection with a helper virus, for instance adenovirus, while the latent phase dominates in healthy cells. In the latent state, AAV is found integrated site specifically into the host genome at chromosome 19q13.4 qtr (AAVS1), the only animal virus known to integrate in a defined location. In this study we investigated the latent phase of serotype 2 AAV, focusing on three areas: AAV infection, rescue, and integration efficiency as a function of viral multiplicity of infection (MOI); efficiency of site-specific integration; and disruption of the AAVS1 locus. As expected, increasing the AAV MOI resulted in an increase in the percentage of cells infected, with 80% of cells infected at an MOI of 10. Additional MOI only marginally effected a further increase in percentage of infected cells. In contrast to infection, we found very low levels of integration at MOIs of less than 10. At an MOI of 10, at which 80% of cells are infected, less than 5% of clonal cell lines contained integrated AAV DNA. At an MOI of 100 or greater, however, 35 to 40% of clonal cell lines contained integrated AAV DNA. Integration and the ability to rescue viral genomes were highly correlated. Analysis of integrated AAV indicated that essentially all integrants were AAVS1 site specific. Although maximal integration efficiency approached 40% of clonal cell lines (essentially 50% of infected cells), over 80% of cell lines contained a genomic disruption at the AAVS1 integration locus on chromosome 19 ( approximately 100% of infected cells). Rep expression by itself and in the presence of a plasmid integration substrate was able to mediate this disruption of the AAVS1 site. We further characterized the disruption event and demonstrated that it resulted in amplification of the AAVS1 locus. The data are consistent with a revised model of AAV integration that includes preliminary expansion of a defined region in AAVS1.

The nuclease domain of adeno-associated virus rep coordinates replication initiation using two distinct DNA recognition interfaces

Integration into a particular location in human chromosomes is a unique property of the adeno-associated virus (AAV). This reaction requires the viral Rep protein and AAV origin sequences. To understand how Rep recognizes DNA, we have determined the structures of the Rep endonuclease domain separately complexed with two DNA substrates: the Rep binding site within the viral inverted terminal repeat and one of the terminal hairpin arms. At the Rep binding site, five Rep monomers bind five tetranucleotide direct repeats; each repeat is recognized by two Rep monomers from opposing faces of the DNA. Stem-loop binding involves a protein interface on the opposite side of the molecule from the active site where ssDNA is cleaved. Rep therefore has three distinct binding sites within its endonuclease domain for its different DNA substrates. Use of these different interfaces generates the structural asymmetry necessary to regulate later events in viral replication and integration.

Characterization of the adenoassociated virus Rep protein complex formed on the viral origin of DNA replication

Interaction between the adenoassociated virus (AAV) replication proteins, Rep68 and 78, and the viral terminal repeats (TRs) is mediated by a DNA sequence termed the Rep-binding element (RBE). This element is necessary for Rep-mediated unwinding of duplex DNA substrates, directs Rep catalyzed cleavage of the AAV origin of DNA replication, and is required for viral transcription and proviral integration. Six discrete Rep complexes with the AAV TR substrates have been observed in vitro, and cross-linking studies suggest these complexes contain one to six molecules of Rep. However, the functional relationship between Rep oligomerization and biochemical activity is unclear. Here we have characterized Rep complexes that form on the AAV TR. Both Rep68 and Rep78 appear to form the same six complexes with the AAV TR, and ATP seems to stimulate formation of specific, higher order complexes. When the sizes of these Rep complexes were estimated on native polyacrylamide gels, the four slower migrating complexes were larger than predicted by an amount equivalent to one or two TRs. To resolve this discrepancy, the molar ratio of protein and DNA was calculated for the three largest complexes. Data from these experiments indicated that the larger complexes included multiple TRs in addition to multiple Rep molecules and that the Rep-to-TR ratio was approximately 2. The two largest complexes were also associated with increased Rep-mediated, origin cleavage activity. Finally, we characterized a second, Rep-mediated cleavage event that occurs adjacent to the normal nicking site, but on the opposite strand. This second site nicking event effectively results in double-stranded DNA cleavage at the normal nicking site.

Adeno-associated virus type 2 Rep40 modulates the proliferation rate of Rep52-expressing HeLa cells

OBJECTIVE

To investigate the growth-inhibitory effect of Rep proteins on cells.

METHODS

We generated Rep-expressing plasmids that containing wild-type rep genes or mutant rep genes with point mutations in their ATP-binding sites. We obtained several HeLa cell clones expressing Rep proteins constitutively by the regulator plasmid. The expression of each Rep protein was detected by Western blotting with an anti-Rep monoclonal antibody.

RESULTS

Clones that expressed Rep52 and Rep40 grew more slowly than those which exhibited no detectable expression of Rep or mutant Rep protein. In contrast, clone w-33, which proliferated as quickly as control HeLa cells, expressed only Rep40. By comparing the expression levels of Rep proteins in these clones, we found that the ratio of Rep40 to Rep52 gradually increased in the faster growing clones.

CONCLUSION

These results suggest that while Rep52 expression retards cell proliferation, Rep40 promotes recovery from Rep52-induced HeLa cell growth inhibition.

Cloning of an avian adeno-associated virus (AAAV) and generation of recombinant AAAV particles

Recent studies have proposed that adeno-associated viruses (AAVs) are not evolutionarily linked to other mammalian autonomous parvoviruses but are more closely linked to the autonomous parvoviruses of birds. To better understand the relationship between primate and avian AAVs (AAAVs), we cloned and sequenced the genome of an AAAV (ATCC VR-865) and generated recombinant AAAV particles. The genome of AAAV is 4,694 nucleotides in length and has organization similar to that of other AAVs. The entire genome of AAAV displays 56 to 65% identity at the nucleotide level with the other known AAVs. The AAAV genome has inverted terminal repeats of 142 nucleotides, with the first 122 forming the characteristic T-shaped palindromic structure. The putative Rep-binding element consists of a tandem (GAGY)(4) repeat, and the putative terminal resolution site (trs), CCGGT/CG, contains a single nucleotide substitution relative to the AAV(2) trs. The Rep open reading frame of AAAV displays 50 to 54% identity at the amino acid level with the other AAVs, with most of the diversity clustered at the carboxyl and amino termini. Comparison of the capsid proteins of AAAV and the primate dependoviruses indicate that divergent regions are localized to surface-exposed loops. Despite these sequence differences, we were able to produce recombinant AAAV particles carrying a lacZ reporter gene by cotransfection in 293T cells and were able to examine transduction efficiency in both chicken primary cells and several cell lines. Our findings indicate that AAAV is the most divergent AAV described to date but maintains all the characteristics unique to the genera of dependovirus.

Packaging of human chromosome 19-specific adeno-associated virus (AAV) integration sites in AAV virions during AAV wild-type and recombinant AAV vector production

Adeno-associated virus type 2 (AAV-2) establishes latency by site-specific integration into a unique locus on human chromosome 19, called AAVS1. During the development of a sensitive real-time PCR assay for site-specific integration, AAV-AAVS1 junctions were reproducibly detected in highly purified AAV wild-type and recombinant AAV vector stocks. A series of controls documented that the junctions were packaged in AAV capsids and were newly generated during a single round of AAV production. Cloned junctions displayed variable AAV sequences fused to AAVS1. These data suggest that packaged junctions represent footprints of AAV integration during productive infection. Apparently, AAV latency established by site-specific integration and the helper virus-dependent, productive AAV cycle are more closely related than previously thought.

Identification of active site residues of the adeno-associated virus type 2 Rep endonuclease

Adeno-associated virus type 2 Rep endonuclease activity is necessary for both viral DNA replication and site-specific integration of the viral genome into human chromosome 19. The biochemical activities required for site-specific endonuclease activity (namely specific DNA binding and transesterification activity) have been mapped to the amino-terminal domain of the AAV2 Rep protein. The amino-terminal 208 amino acids are alone sufficient for site-specific endonuclease activity, and nicking by this domain is metal-dependent. To identify this metal-binding site, we have employed a cysteine mutagenesis approach that targets conserved acidic amino acids. By using this technique, we provide functional biochemical data supporting a role for glutamate 83 in the coordination of metal ions in the context of Rep endonuclease activity. In addition, our biochemical data suggest that glutamate 164, although not involved in the coordination of metal ions, is closely associated with the active site. Thus, in lieu of a crystal structure for the AAV type 2 amino-terminal domain, our data corroborate the recently published structural studies of the AAV type 5 endonuclease and suggest that although the two enzymes are not highly conserved with respect to the AAV family, their active sites are highly conserved.

Analysis of site-specific transgene integration following cotransduction with recombinant adeno-associated virus and a rep encodingplasmid

BACKGROUND

Recombinant adeno-associated virus (rAAV) has many advantages for gene therapeutic applications in comparison with other vector systems. One of the most promising features is the ability of wild-type (wt) AAV to integrate site-specifically into human chromosome 19. However, this feature is lost in rAAV vectors due to the removal of the rep-coding sequences.

METHODS

HeLa cells were transfected with a rep expression plasmid, infected by rAAV and grown with or without selection pressure. Single cell clones were generated and genomic DNA was analyzed for site-specific integration by Southern blotting analysis and fluorescence in situ hybridization (FISH).

RESULTS

Transfection of HeLa cells with a rep expression plasmid followed by transduction with a rAAV vector resulted in site-specific integration of the transgene at AAVS1 on human chromosome 19 in 7 of 10 cell clones analyzed. In marked contrast, transduction of cells with rAAV alone did not result in any site-specific integration of the transgene.

CONCLUSIONS

The high frequency with which the site-specific integration took place in the presence of Rep protein is comparable with the results observed with wtAAV. These results offer opportunities for the development of specifically integrating rAAV vectors.

Addition of six-His-tagged peptide to the C terminus of adeno-associated virus VP3 does not affect viral tropism or production

Production of large quantities of recombinant adeno-associated virus (AAV) is difficult and not cost-effective. To overcome this problem, we have explored the feasibility of creating a recombinant AAV encoding a 6xHis tag on the VP3 capsid protein. We generated a plasmid vector containing a six-His (6xHis)-tagged AAV VP3. A second plasmid vector was generated that contained the full-length AAV capsid capable of producing VP1 and VP2, but not VP3 due to a mutation at position 2809 that encodes the start codon for VP3. These plasmids, necessary for production of AAV, were transfected into 293 cells to generate a 6xHis-tagged VP3mutant recombinant AAV. The 6xHis-tagged VP3 did not affect the formation of AAV virus, and the physical properties of the 6xHis-modified AAV were equivalent to those of wild-type particles. The 6xHis-tagged AAV did not affect the production titer of recombinant AAV and could be used to purify the recombinant AAV using an Ni-nitrilotriacetic acid column. Addition of the 6xHis tag did not alter the viral tropism compared to wild-type AAV. These observations demonstrate the feasibility of producing high-titer AAV containing a 6xHis-tagged AAV VP3 capsid protein and to utilize the 6xHis-tagged VP3 capsid to achieve high-affinity purification of this recombinant AAV.

A cis-acting element that directs circular adeno-associated virus replication and packaging

A novel pathway of adeno-associated virus (AAV) replication marked by the assembly of circular monomer duplex intermediates (cAAV) has been recently discovered. In the present report we identify a single AD domain of the inverted terminal repeat as a minimal origin of cAAV replication. A small internal palindrome (BB'), necessary for optimal Rep-inverted terminal repeat interaction, does not contribute to the efficiency of cAAV replication, while the terminal resolution site is an essential cis-acting element. Furthermore, recombinant cAAV vectors that encompass only the AD domain replicate exclusively in a circular form and no detectable linear duplex replicative intermediates are generated, suggesting that both pathways of AAV replication are independent and can be separated. In addition, we show that cAAVs are efficient templates for encapsidation of single-stranded DNA genomes, an observation that assigns a biological role for these novel replication species. Together, these findings shed new light on the current model of AAV replication and packaging.

Genome engineering using site-specific recombinases

The targeted modification of the mammalian genome has a variety of applications in research, medicine, and biotechnology. Site-specific recombinases have become significant tools in all of these areas. Conditional gene targeting using site-specific recombinases has enabled the functional analysis of genes, which cannot be inactivated in the germline. The site-specific integration of adeno-associated virus, a major gene therapy vehicle, relies on the recombinase activity of the viral rep proteins. Site-specific recombinases also allow the precise integration of open reading frames encoding pharmaceutically relevant proteins into highly active gene loci in cell lines and transgenic animals. These goals have been accomplished by using a variety of genetic strategies but only a few recombinase proteins. However, the vast repertoire of recombinases, which has recently become available as a result of large-scale sequencing projects, may provide a rich source for the development of novel strategies to precisely alter mammalian genomes.

Structural unity among viral origin binding proteins: crystal structure of the nuclease domain of adeno-associated virus Rep

Adeno-associated virus (AAV), unique among animal viruses in its ability to integrate into a specific chromosomal location, is a promising vector for human gene therapy. AAV Replication (Rep) protein is essential for viral replication and integration, and its amino terminal domain possesses site-specific DNA binding and endonuclease activities required for replication initiation and integration. This domain displays a novel endonuclease fold and demonstrates an unexpected structural relationship to other viral origin binding proteins such as the papillomavirus E1 protein and the SV40 T antigen. The active site, located at the bottom of a positively charged cleft, is formed by the spatial convergence of a divalent metal ion and two conserved sequence motifs that define the rolling circle replication superfamily.

Adeno-associated virus type 2 Rep78 inhibition of PKA and PRKX: fine mapping and analysis of mechanism

Hormones and neurotransmitters utilize cyclic AMP (cAMP) as a second messenger in signal transduction pathways to regulate cell growth and division, differentiation, gene expression, and metabolism. Adeno-associated virus type 2 (AAV-2) nonstructural protein Rep78 inhibits members of the cAMP signal transduction pathway, the protein kinases PKA and PRKX. We mapped the kinase binding and inhibition domain of Rep78 for PRKX to amino acids (aa) 526 to 561 and that for PKA to aa 526 to 621. These polypeptides were as potent as full-length Rep78 in kinase inhibition, which suggests that the kinase-inhibitory domain is entirely contained in these Rep peptides. Steady-state kinetic analysis of Rep78-mediated inhibition of PKA and PRKX showed that Rep78 appears to increase the K(m) value of the peptide kinase substrate, while the maximal velocity of the reaction was unaffected. This indicates that Rep78 acts as a competitive inhibitor with respect to the peptide kinase substrate. We detected homology between a cellular pseudosubstrate inhibitor of PKA, the protein kinase inhibitor PKI, and the PRKX and PKA inhibition domains of Rep78. Due to this homology and the competitive inhibition mechanism of Rep78, we propose that Rep78 inhibits PKA and PRKX kinase activity by pseudosubstrate inhibition.

Biochemical characterization of Junonia coenia densovirus nonstructural protein NS-1

Junonia coenia densovirus (JcDNV) is an autonomous parvovirus that infects the larvae of the common buckeye butterfly, Junonia coenia. Unlike vertebrate parvoviruses, the genes encoding the structural protein and nonstructural (NS) proteins of JcDNV are in opposite orientations; thus, each strand contains a sense and antisense open reading frame (ORF). The promoter at map position 93 controls expression of NS ORFs 2, 3, and 4, which encode three NS proteins, NS-1, NS-2, and NS-3. These proteins are likely to be involved in viral DNA replication, among other functions. In contrast to the nonstructural proteins of the vertebrate parvoviruses, the NS proteins of the Densovirinae have not been characterized. Here, we describe biochemical properties of the NS-1 protein of JcDNV. The NS-1 ORF was cloned in frame with the Escherichia coli malE gene, which encodes the bacterial maltose binding protein (MBP). Using electrophoretic mobility shift and DNase I protection assays, we identified the region of the JcDNV terminal sequence that is recognized specifically by the MBP-NS-1 fusion protein. The site consists of (GAC)4 and is located on the A-A' region of the terminal palindrome. In addition, the MBP-NS-1 fusion protein catalyzes the cleavage of single-stranded DNA (ssDNA) substrates derived from the JcDNV putative origin of replication, primarily at two sites in the motif 5'-G*TAT*TG-3'. One cleavage site is between the thymidine dinucleotide at positions 92 and 93 and the other site corresponds to thymidine at nucleotide 95; both sites are on the complementary strand of the sequence assigned GenBank accession number A12984. Cleavage of ssDNA is dependent on the presence of a divalent metal cofactor but does not require nucleoside triphosphate hydrolysis. Parvovirus NS proteins contain the phylogenically conserved Walker A- and B-site ATPase motifs. These sites in JcDNV NS-1 diverge from the consensus, yet despite these atypical motifs our analyses support that MBP-NS-1 has ATP-dependent helicase activity. These results indicate that JcDNV NS-1 possesses activities common to the superfamily of rolling-circle replication initiator proteins in general and the parvovirus replication proteins in particular, and they provide a basis for comparative analyses of the structure and function relationships among the parvovirus NS-1 equivalents.

The left-end and right-end origins of minute virus of mice DNA differ in their capacity to direct episomal amplification and integration in vivo

Previously it was shown that a 53-nucleotide viral replication origin, derived from the left-end (3') telomere of minute virus of mice (MVM) DNA, directed integration of infecting MVM genomes into an Epstein-Barr virus (EBV)-based episome in cell culture. Integration depended upon the presence, in the episome, of a functional origin sequence which could be nicked by NS1, the viral initiator protein. Here we extend our studies to the genomic right-end (5') origin and report that three 131- to 135-nucleotide right-end origin sequences failed to target MVM episomal integration even though the same sequences were functional in NS1-driven DNA replication assays in vitro. Additionally, we observed amplification of episomal DNA in response to MVM infection in cell lines harboring episomes which directed integration, but not in cell lines containing episomes which did not direct integration, including those with inserts of the MVM right-end origin.

A Rep recognition sequence is necessary but not sufficient for nicking of DNA by adeno-associated virus type-2 Rep proteins

The strand-specific, site-specific endonuclease (nicking) activity of the Rep68 and Rep78 (Rep68/78) proteins of adeno-associated virus type 2 (AAV) is involved in AAV replication, and appears to be involved in AAV site-specific integration. Rep68/78 cuts within the inverted terminal repeats (ITRs) of the AAV genome and in the AAV preferred integration locus on human chromosome 19 (AAVS1). The known endonuclease cut sites are 11-16 bases away from the primary binding sites, known as Rep recognition sequences (RRSs). A linear, double-stranded segment of DNA, containing an RRS and a cut site, has previously been shown to function as a substrate for the Rep68/78 endonuclease activity. We show here that mutation of the Rep recognition sequence, within such a DNA segment derived from the AAV ITRs, eliminates the ability of this substrate to be cleaved detectably by Rep78. Rep78 nicks the RRS-containing site from AAVS1 about half as well as the linear ITR sequence. Eighteen other RRS-containing sequences found in the human genome, but outside AAVS1, are not cleaved by Rep78. These results may help to explain the specificity of AAV integration.

Amino-terminal domain exchange redirects origin-specific interactions of adeno-associated virus rep78 in vitro

The unique ability of adeno-associated virus type 2 (AAV) to site-specifically integrate its genome into a defined sequence on human chromosome 19 (AAVS1) makes it of particular interest for use in targeted gene delivery. The objective underlying this study is to provide evidence for the feasibility of retargeting site-specific integration into selected loci within the human genome. Current models postulate that AAV DNA integration is initiated through the interactions of the products of a single viral open reading frame, REP, with sequences present in AAVS1 that resemble the minimal origin for AAV DNA replication. Here, we present a cell-free system designed to dissect the Rep functions required to target site-specific integration using functional chimeric Rep proteins derived from AAV Rep78 and Rep1 of the closely related goose parvovirus. We show that amino-terminal domain exchange efficiently redirects the specificity of Rep to the minimal origin of DNA replication. Furthermore, we establish that the amino-terminal 208 amino acids of Rep78/68 constitute a catalytic domain of Rep sufficient to mediate site-specific endonuclease activity.