Adaptation of chimeric retroviruses in vitro and in vivo: isolation of avian retroviral vectors with extended host range

A B cell screen against endogenous retroviruses identifies glycan-reactive IgM that recognizes a broad array of enveloped viruses

Endogenous retroviruses (ERVs), comprising a substantial portion of the vertebrate genome, are remnants of ancient genetic invaders. ERVs with near-intact coding potential reactivate in B cell-deficient mice. To study how B cells contribute to host anti-ERV immunity, we used an antigen-baiting strategy to enrich B cells reactive to ERV surface antigens. We identified ERV-reactive B-1 cells expressing germline-encoded natural IgM antibodies in naïve mice, the level of which further increases upon innate immune sensor stimulation. B cell receptor repertoire profiling of ERV-reactive B-1 cells revealed increased usage of the Igh VH gene that gives rise to glycan-specific antibodies targeting terminal N-acetylglucosamine moieties on ERV glycoproteins, which further engage the complement pathway to mediate anti-ERV responses. These same antibodies also recognize glycoproteins of other enveloped viruses but not self-proteins. These results reveal an innate antiviral mechanism of germline-encoded antibodies with broad reactivity to enveloped viruses, which constitutes a natural antibody repertoire capable of preventing the emergence of infectious ERVs.

Lipidomimetic Compounds Act as HIV-1 Entry Inhibitors by Altering Viral Membrane Structure

The envelope of Human Immunodeficiency Virus type 1 (HIV-1) consists of a liquid-ordered membrane enriched in raft lipids and containing the viral glycoproteins. Previous studies demonstrated that changes in viral membrane lipid composition affecting membrane structure or curvature can impair infectivity. Here, we describe novel antiviral compounds that were identified by screening compound libraries based on raft lipid-like scaffolds. Three distinct molecular structures were chosen for mode-of-action studies, a sterol derivative (J391B), a sphingosine derivative (J582C) and a long aliphatic chain derivative (IBS70). All three target the viral membrane and inhibit virus infectivity at the stage of fusion without perturbing virus stability or affecting virion-associated envelope glycoproteins. Their effect did not depend on the expressed envelope glycoproteins or a specific entry route, being equally strong in HIV pseudotypes carrying VSV-G or MLV-Env glycoproteins. Labeling with laurdan, a reporter of membrane order, revealed different membrane structure alterations upon compound treatment of HIV-1, which correlated with loss of infectivity. J582C and IBS70 decreased membrane order in distinctive ways, whereas J391B increased membrane order. The compounds' effects on membrane order were reproduced in liposomes generated from extracted HIV lipids and thus independent both of virion proteins and of membrane leaflet asymmetry. Remarkably, increase of membrane order by J391B required phosphatidylserine, a lipid enriched in the HIV envelope. Counterintuitively, mixtures of two compounds with opposite effects on membrane order, J582C and J391B, did not neutralize each other but synergistically inhibited HIV infection. Thus, altering membrane order, which can occur by different mechanisms, constitutes a novel antiviral mode of action that may be of general relevance for enveloped viruses and difficult to overcome by resistance development.

Efficient method to optimize antibodies using avian leukosis virus display and eukaryotic cells

Antibody-based therapeutics have now had success in the clinic. The affinity and specificity of the antibody for the target ligand determines the specificity of therapeutic delivery and off-target side effects. The discovery and optimization of high-affinity antibodies to important therapeutic targets could be significantly improved by the availability of a robust, eukaryotic display technology comparable to phage display that would overcome the protein translation limitations of microorganisms. The use of eukaryotic cells would improve the diversity of the displayed antibodies that can be screened and optimized as well as more seamlessly transition into a large-scale mammalian expression system for clinical production. In this study, we demonstrate that the replication and polypeptide display characteristics of a eukaryotic retrovirus, avian leukosis virus (ALV), offers a robust, eukaryotic version of bacteriophage display. The binding affinity of a model single-chain Fv antibody was optimized by using ALV display, improving affinity >2,000-fold, from micromolar to picomolar levels. We believe ALV display provides an extension to antibody display on microorganisms and offers virus and cell display platforms in a eukaryotic expression system. ALV display should enable an improvement in the diversity of properly processed and functional antibody variants that can be screened and affinity-optimized to improve promising antibody candidates.

Gα13 mediates human cytomegalovirus-encoded chemokine receptor US28-induced cell death in melanoma

US28, a constitutively active G-protein-coupled receptor encoded by the human cytomegalovirus, leads to mechanistically unknown programmed cell death. Here we show that expression of wild-type US28 in human melanoma cells leads to apoptotic cell death via caspase 3 activation along with reduced cell proliferation. Reduced tumor growth upon US28 expression was observed in a xenograft mouse model. The signaling mute US28R129A showed a reduced antiproliferative effect. On evaluating different G-proteins coupled to US28 for signal transduction, Gα13 was identified as the main G-protein executing the apoptotic effect. Silencing of Gα13 but not Gαq resulted in a substantial increase in cell survival. Overexpression of Gα13 but not Gαq and their GTPase deficient forms Gα13Q226L and GαqQ209L, respectively, confirmed the requirement of Gα13 for US28 mediated cell death. Increasing expression of Gα13 alone induced cell death underscoring its relay function for US28 mediated decreased cell viability. Further reduced expression of Gα13 in melanoma cell lines isolated from advanced lesions and melanoma tissue was observed. These findings identified Gα13 as crucial for US28-induced cell death, substantiating that the effect of US28 on cell fate depends on preferred G-protein binding.

Functional characterization of the putative hepatitis B virus core protein late domain using retrovirus chimeras

The hepatitis B virus (HBV) Core protein encodes a late (L)-domain like motif (₁₂₉PPAYRPPNAP¹³⁸) that has been purported to serve as a docking site for recruitment of host factors such as Nedd4 that can mediate viral particle release from infected cells. However, mutation of this region of Core typically disrupts nucleocapsid formation in the cytoplasm, making it difficult to ascertain if the Core PPAY motif constitutes a functional L-domain that mediates HBV release in the context of replicating virus. Since many viral L-domains are functionally interchangeable between different virus families, and such swapping experiments have been used as a tool to identify other viral sequences with L-domain activity, we generated chimeric constructs between murine leukemia virus (MLV) Gag and HBV Core to determine if the potential HBV L-domain motif is sufficient to stimulate virus release. We found that the HBV Core PPAY motif, but not the PNAP motif, demonstrates L-domain activity in the context of MLV replication to direct virus release and infectious virion production. Additionally, we found that overexpression of the cellular Nedd4 or WWP1 ubiquitin ligases stimulates release of a partially defective PPAY domain mutant, providing further evidence supporting a role for the Nedd4 ubiquitin ligase in promoting HBV release. These studies lend further insight into the mechanisms used by HBV to mediate its release from infected cells.

Use of lentiviral vectors to deliver and express bicistronic transgenes in developing chicken embryos

The abilities of lentiviral vectors to carry large transgenes (∼8kb) and to efficiently infect and integrate these genes into the genomes of both dividing and non-dividing cells make them ideal candidates for transport of genetic material into cells and tissues. Given the properties of these vectors, it is somewhat surprising that they have seen only limited use in studies of developing tissues and in particular of the developing nervous system. Over the past several years, we have taken advantage of the large capacity of these vectors to explore the expression characteristics of several dual promoter and 2A peptide bicistronic transgenes in developing chick neural retina, with the goal of identifying transgene designs that reliably express multiple proteins in infected cells. Here we summarize the activities of several of these transgenes in neural retina and provide detailed methodologies for packaging lentivirus and delivering the virus into the developing neural tubes of chicken embryos in ovo, procedures that have been optimized over the course of several years of use in our laboratory. Conditions to hatch injected embryos are also discussed. The chicken-specific techniques will be of highest interest to investigators using avian embryos, development and packaging of lentiviral vectors that reliably express multiple proteins in infected cells should be of interest to all investigators whose experiments demand manipulation and expression of multiple proteins in developing cells and tissues.

Retroviral replicating vectors in cancer

The use of replication-competent viruses for the treatment of cancer is an emerging technology that shows significant promise. Among the various different types of viruses currently being developed as oncolytic agents, retroviral replicating vectors (RRVs) possess unique characteristics that allow highly efficient, non-lytic, and tumor-selective gene transfer. By retaining all of the elements necessary for viral replication, RRVs are capable of transmitting genes via exponential in situ amplification. Their replication-competence also provides a powerful means whereby novel and useful RRV variants can be generated using natural selection. Their stringent requirement for cell division in order to achieve productive infection, and their preferential replication in cells with defective innate immunity, confer a considerable degree of natural specificity for tumors. Furthermore, their ability to integrate stably into the genome of cancer cells, without immediate cytolysis, contributes to long-lasting therapeutic efficacy. Thus, RRVs show much promise as therapeutic agents for cancer and are currently being tested in the clinic. Here we describe experimental methods for their production and quantitation, for adaptive evolution and natural selection to develop novel or improved RRV, and for in vitro and in vivo assessment of the therapeutic efficacy of RRVs carrying prodrug activator genes for treatment of cancer.

Directed adenovirus evolution using engineered mutator viral polymerases

Adenoviruses (Ads) are the most frequently used viruses for oncolytic and gene therapy purposes. Most Ad-based vectors have been generated through rational design. Although this led to significant vector improvements, it is often hampered by an insufficient understanding of Ad’s intricate functions and interactions. Here, to evade this issue, we adopted a novel, mutator Ad polymerase-based, ‘accelerated-evolution’ approach that can serve as general method to generate or optimize adenoviral vectors. First, we site specifically substituted Ad polymerase residues located in either the nucleotide binding pocket or the exonuclease domain. This yielded several polymerase mutants that, while fully supportive of viral replication, increased Ad’s intrinsic mutation rate. Mutator activities of these mutants were revealed by performing deep sequencing on pools of replicated viruses. The strongest identified mutators carried replacements of residues implicated in ssDNA binding at the exonuclease active site. Next, we exploited these mutators to generate the genetic diversity required for directed Ad evolution. Using this new forward genetics approach, we isolated viral mutants with improved cytolytic activity. These mutants revealed a common mutation in a splice acceptor site preceding the gene for the adenovirus death protein (ADP). Accordingly, the isolated viruses showed high and untimely expression of ADP, correlating with a severe deregulation of E3 transcript splicing.

Retroviruses human immunodeficiency virus and murine leukemia virus are enriched in phosphoinositides

Retroviruses acquire a lipid envelope during budding from the membrane of their hosts. Therefore, the composition of this envelope can provide important information about the budding process and its location. Here, we present mass spectrometry analysis of the lipid content of human immunodeficiency virus type 1 (HIV-1) and murine leukemia virus (MLV). The results of this comprehensive survey found that the overall lipid content of these viruses mostly matched that of the plasma membrane, which was considerably different from the total lipid content of the cells. However, several lipids are enriched in comparison to the composition of the plasma membrane: (i) cholesterol, ceramide, and GM3; and (ii) phosphoinositides, phosphorylated derivatives of phosphatidylinositol. Interestingly, microvesicles, which are similar in size to viruses and are also released from the cell periphery, lack phosphoinositides, suggesting a different budding mechanism/location for these particles than for retroviruses. One phosphoinositide, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], has been implicated in membrane binding by HIV Gag. Consistent with this observation, we found that PI(4,5)P(2) was enriched in HIV-1 and that depleting this molecule in cells reduced HIV-1 budding. Analysis of mutant virions mapped the enrichment of PI(4,5)P(2) to the matrix domain of HIV Gag. Overall, these results suggest that HIV-1 and other retroviruses bud from cholesterol-rich regions of the plasma membrane and exploit matrix/PI(4,5)P(2) interactions for particle release from cells.

The effects of alternate polypurine tracts (PPTs) and mutations of sequences adjacent to the PPT on viral replication and cleavage specificity of the Rous sarcoma virus reverse transcriptase

We previously reported that a mutant Rous sarcoma virus (RSV) with an alternate polypurine tract (PPT), DuckHepBFlipPPT, had unexpectedly high titers and that the PPT was miscleaved primarily at one position following a GA dinucleotide by the RNase H of reverse transcriptase (RT). This miscleavage resulted in a portion of the 3' end of the PPT (5'-ATGTA) being added to the end of U3 of the linear viral DNA. To better understand the RNase H cleavage by RSV RT, we made a number of mutations within the DuckHepBFlipPPT and in the sequences adjacent to the PPT. Deleting the entire ATGTA sequence from the DuckHepBFlipPPT increased the relative titer to wild-type levels, while point mutations within the ATGTA sequence reduced the relative titer but had minimal effects on the cleavage specificity. However, mutating a sequence 5' of ATGTA affected the relative titer of the virus and caused the RNase H of RSV RT to lose the ability to cleave the PPT specifically. In addition, although mutations in the conserved stretch of thymidine residues upstream of the PPT did not affect the relative titer or cleavage specificity, the mutation of some of the nucleotides immediately upstream of the PPT did affect the titer and cleavage specificity. Taken together, our studies show that the structure of the PPT in the context of the cognate RT, rather than a specific sequence, is important for the proper cleavage by RSV RT.

The core element of a CpG island protects avian sarcoma and leukosis virus-derived vectors from transcriptional silencing

Unmethylated CpG islands are known to keep adjacent promoters transcriptionally active. In the CpG island adjacent to the adenosine phosphoribosyltransferase gene, the protection against transcriptional silencing can be attributed to the short CpG-rich core element containing Sp1 binding sites. We report here the insertion of this CpG island core element, IE, into the long terminal repeat of a retroviral vector derived from Rous sarcoma virus, which normally suffers from progressive transcriptional silencing in mammalian cells. IE insertion into a specific position between enhancer and promoter sequences led to efficient protection of the integrated vector from silencing and gradual CpG methylation in rodent and human cells. Individual cell clones with IE-modified reporter vectors display high levels of reporter expression for a sustained period and without substantial variegation in the cell culture. The presence of Sp1 binding sites is important for the protective effect of IE, but at least some part of the entire antisilencing capacity is maintained in IE with mutated Sp1 sites. We suggest that this strategy of antisilencing protection by the CpG island core element may prove generally useful in retroviral vectors.

Transgene expression facilitated by the v-src splice acceptor can impair replication kinetics and lead to genomic instability of Rous sarcoma virus-based vectors

Rous sarcoma virus (RSV) can be used for the simple generation of high-titer replication-competent retroviral (RCR) vectors. Retroviruses undergo frequent genomic recombination, however, and vectors with reduced replication kinetics are rapidly overgrown by mutant forms. Vector design is hence critical to vector efficacy. In this study, two different designs of RSV-based RCR vectors were evaluated. Vectors in which transgene expression was facilitated by the v-src splice acceptor were revealed to have greatly reduced replication kinetics and genomic stability in comparison to vectors in which transgene expression was mediated by an internal ribosome entry site in the 3' untranslated region.

Transduction of rhesus macaque hematopoietic stem and progenitor cells with avian sarcoma and leukosis virus vectors

Genome-wide integration site analyses showed that Moloney murine leukemia virus (MoMLV)- and lentivirus-derived vectors integrate preferentially into the coding regions of genes, posing a risk of insertional mutagenesis. Avian sarcoma and leukosis viruses (ASLVs) were previously reported to have a weak preference for gene-coding regions in a cell line study as compared with human immunodeficiency virus and MoMLV; however, thus far these vectors have not been studied for their potential efficacy in transduction of hematopoietic progenitor and stem cells. In this study we investigated for the first time the ability of ASLV-derived RCAS (replication-competent ALV LTR [avian leukosis virus long terminal repeat] with a splice acceptor) vectors to transduce rhesus macaque hematopoietic progenitors and long-term repopulating cells, in an autologous transplantation model. RCAS vectors can efficiently and stably transduce rhesus macaque CD34+ hematopoietic progenitor cells with an efficiency of transduction of up to 34% ex vivo. In two animals transplanted with RCAS vector-transduced autologous CD34+ cells, highly polyclonal hematopoietic reconstitution with sustained gene-marking levels in myeloid and lymphoid lineages was observed up to 18 months post-transplantation. These findings are encouraging and suggest that this vector system should be explored and further optimized for gene therapy applications targeting hematopoietic stem and progenitor cells.

Adaptive evolution of a tagged chimeric gammaretrovirus: identification of novel cis-acting elements that modulate splicing

Retroviruses are well known for their ability to incorporate envelope (Env) proteins from other retroviral strains and genera, and even from other virus families. This characteristic has been widely exploited for the generation of replication-defective retroviral vectors, including those derived from murine leukemia virus (MLV), bearing heterologous Env proteins. We investigated the possibility of "genetically pseudotyping" replication-competent MLV by replacing the native env gene in a full-length viral genome with that of another gammaretrovirus. Earlier, we developed replication-competent versions of MLV that stably transmit and express transgenes inserted into the 3' untranslated region of the viral genome. In one such tagged MLV expressing green fluorescent protein, we replaced the native env sequence with that of gibbon ape leukemia virus (GALV). Although the GALV Env protein is commonly used to make high-titer pseudotypes of MLV vectors, we found that the env replacement greatly attenuated viral replication. However, extended cultivation of cells exposed to the chimeric virus resulted in selection of mutants exhibiting rapid replication kinetics and different variants arose in different infections. Two of these variants had acquired mutations at or adjacent to the splice acceptor site, and three others had acquired dual mutations within the long terminal repeat. Analysis of the levels of unspliced and spliced viral RNA produced by the parental and adapted viruses showed that the mutations gained by each of these variants functioned to reverse an imbalance in splicing caused by the env gene substitution. Our results reveal the presence of previously unknown cis-acting sequences in MLV that modulate splicing of the viral transcript and demonstrate that tagging of the retroviral genome with an easily assayed transgene can be combined with in vitro evolution as an approach to efficiently generating and screening for replicating mutants of replication-impaired recombinant viruses.

Alternate polypurine tracts (PPTs) affect the rous sarcoma virus RNase H cleavage specificity and reveal a preferential cleavage following a GA dinucleotide sequence at the PPT-U3 junction

Retroviral polypurine tracts (PPTs) serve as primers for plus-strand DNA synthesis during reverse transcription. The generation and removal of the PPT primer requires specific cleavages by the RNase H activity of reverse transcriptases; removal of the PPT primer defines the left end of the linear viral DNA. We replaced the endogenous PPT from RSVP(A)Z, a replication-competent shuttle vector based on Rous sarcoma virus (RSV), with alternate retroviral PPTs and the duck hepatitis B virus "PPT." Viruses in which the endogenous RSV PPT was replaced with alternate PPTs had lower relative titers than the wild-type virus. 2-LTR circle junction analysis showed that the alternate PPTs caused significant decreases in the fraction of viral DNAs with complete (consensus) ends and significant increases in the insertion of part or all of the PPT at the 2-LTR circle junctions. The last two nucleotides in the 3' end of the RSV PPT are GA. Examination of the (mis)cleavages of the alternate PPTs revealed preferential cleavages after GA dinucleotide sequences. Replacement of the terminal 3' A of the RSV PPT with G caused a preferential miscleavage at a GA sequence spanning the PPT-U3 boundary, resulting in the deletion of the terminal adenine normally present at the 5' end of the U3. A reciprocal G-to-A substitution at the 3' end of the murine leukemia virus PPT increased the relative titer of the chimeric RSV-based virus and the fraction of consensus 2-LTR circle junctions.

Mutations of a residue within the polyproline-rich region of Env alter the replication rate and level of cytopathic effects in chimeric avian retroviral vectors

Previous attempts to extend the host range of the avian sarcoma/leukosis virus (ASLV)-based RCASBP vectors produced two viral vectors, RCASBP M2C (4070A) and RCASBP M2C (797-8), which replicate using the amphotropic murine leukemia virus 4070A Env protein (2). Both viruses were adapted to replicate efficiently in the avian cell line DF-1, but RCASBP M2C (4070A) caused extensive cytopathic effects (CPE) in DF-1 cells whereas RCASBP M2C (797-8) induced low levels of CPE. The two viruses differed only at amino acid 242 of the polyproline-rich region in the surface (SU) subunit of the Env protein. In RCASBP M2C (4070A), an isoleucine replaced the wild-type proline residue, whereas a threonine residue was found in RCASBP M2C (797-8). In the present study, we show that other amino acid substitutions at position 242 strongly influence the CPE and replication rate of the chimeric viruses. There was a correlation between the amount of unintegrated linear retroviral DNA present in infected DF-1 cells and the level of CPE. This suggests that there may be a role for superinfection in the CPE. The treatment of RCASBP M2C (4070A)-infected cells with dantrolene, which inhibits the release of calcium from the endoplasmic reticulum (ER), reduced the amount of CPE seen during infection with the highly cytotoxic virus. Dantrolene treatment did not appear to affect virus production, suggesting that Ca2+ release from the ER had a role in the CPE caused by these viruses.

Evidence that stable retroviral transduction and cell survival following DNA integration depend on components of the nonhomologous end joining repair pathway

We have previously reported several lines of evidence that support a role for cellular DNA repair systems in completion of the retroviral DNA integration process. Failure to repair an intermediate in the process of integrating viral DNA into host DNA appears to trigger growth arrest or death of a large percentage of infected cells. Cellular proteins involved in the nonhomologous end joining (NHEJ) pathway (DNA-PK(CS)) and the damage-signaling kinases (ATM and ATR) have been implicated in this process. However, some studies have suggested that NHEJ proteins may not be required for the completion of lentiviral DNA integration. Here we provide additional evidence that NHEJ proteins are required for stable transduction by human immunodeficiency type 1 (HIV-1)-based vectors. Our analyses with two different reporters show that the number of stably transduced DNA-PK(CS)-deficient scid fibroblasts was reduced by 80 to 90% compared to the number of control cells. Furthermore, transduction efficiency can be restored to wild-type levels in scid cells that are complemented with a functional DNA-PK(CS) gene. The efficiency of stable transduction by an HIV-1-based vector is also reduced upon infection of Xrcc4 and ligase IV-deficient cells, implying a role for these components of the NHEJ repair pathway. Finally, we show that cells deficient in ligase IV are killed by infection with an integrase-competent but not an integrase-deficient HIV-1 vector. Results presented in this study lend further support to a general role for the NHEJ DNA repair pathway in completion of the retroviral DNA integration process.

Visualization of retroviral replication in living cells reveals budding into multivesicular bodies

Retroviral assembly and budding is driven by the Gag polyprotein and requires the host-derived vacuolar protein sorting (vps) machinery. With the exception of human immunodeficiency virus (HIV)-infected macrophages, current models predict that the vps machinery is recruited by Gag to viral budding sites at the cell surface. However, here we demonstrate that HIV Gag and murine leukemia virus (MLV) Gag also drive assembly intracellularly in cell types including 293 and HeLa cells, previously believed to exclusively support budding from the plasma membrane. Using live confocal microscopy in conjunction with electron microscopy of cells generating fluorescently labeled virions or virus-like particles, we observed that these retroviruses utilize late endosomal membranes/multivesicular bodies as assembly sites, implying an endosome-based pathway for viral egress. These data suggest that retroviruses can interact with the vps sorting machinery in a more traditional sense, directly linked to the mechanism by which cellular proteins are sorted into multivesicular endosomes.

Relationship between SU subdomains that regulate the receptor-mediated transition from the native (fusion-inhibited) to the fusion-active conformation of the murine leukemia virus glycoprotein

Envelope glycoproteins (Env) of retroviruses are trimers of SU (surface) and TM (transmembrane) heterodimers and are expressed on virions in fusion-competent forms that are likely to be metastable. Activation of the viral receptor-binding domain (RBD) via its interaction with a cell surface receptor is thought to initiate a cascade of events that lead to refolding of the Env glycoprotein into its stable fusion-active conformation. While the fusion-active conformation of the TM subunit has been described in detail for several retroviruses, little is known about the fusion-competent structure of the retroviral glycoproteins or the molecular events that mediate the transition between the two conformations. By characterizing Env chimeras between the ecotropic and amphotropic murine leukemia virus (MLV) SUs as well as a set of point mutants, we show that alterations of the conformation of the SU glycoprotein strongly elevate Env fusogenicity by disrupting the stability of the Env complex. Compensatory mutations that restored both Env stability and fusion control were also identified, allowing definition of interactions within the Env complex that maintain the stability of the native Env complex. We show that, in the receptor-unbound form, structural interactions between the N terminus of the viral RBD (NTR domain), the proline-rich region (PRR), and the distal part of the C-terminal domain of the SU subunit maintain a conformation of the glycoprotein that is fusion inhibitory. Additionally, we identified mutations that disrupt this fusion-inhibitory conformation and allow fusion activation in the absence of viral receptors, provided that receptor-activated RBD fragments are added in trans during infection. Other mutations were identified that allow fusion activation in the absence of receptors for both the viral glycoprotein and the trans-acting RBD. Finally, we found mutations of the SU that bypass in cis the requirement for the NTR domain in fusion activation. All these different mutations call for a critical role of the PRR in mediating conformational changes of the Env glycoprotein during fusion activation. Our results suggest a model of MLV Env fusion activation in which unlocking of the fusion-inhibitory conformation is initiated by receptor binding of the viral RBD, which, upon disruption of the PRR, allows the NTR domain to promote further events in Env fusion activation. This involves a second type of interaction, in cis or in trans, between the receptor-activated RBD and a median segment of the freed C-terminal domain.

Construction and characterization of a replication-competent retroviral shuttle vector plasmid

We constructed two versions of an RCASBP-based retroviral shuttle vector, RSVP (RCASBP shuttle vector plasmid), containing either the zeocin or blasticidin resistance gene. In this vector, the drug resistance gene is expressed in avian cells from the long terminal repeat (LTR) promoter, whereas in bacteria the resistance gene is expressed from a bacterial promoter. The vector contains a bacterial origin of replication (ColE1) to allow circular viral DNA to replicate as a plasmid in bacteria. The vector also contains the lac operator sequence, which binds to the lac repressor protein, providing a simple and rapid way to purify the vector DNA. The RSVP plasmid contains the following sequence starting with the 5" end: LTR, gag, pol, env, drug resistance gene, lac operator, ColE1, LTR. After this plasmid was transfected into DF-1 cells, we were able to rescue the circularized unintegrated viral DNA from RSVP simply by transforming the Hirt DNA into Escherichia coli. Furthermore, we were able to rescue the integrated provirus. DNA from infected cells was digested with an appropriate restriction enzyme (ClaI) and the vector-containing segments were enriched using lac repressor protein and then self-ligated. These enriched fractions were used to transform E. coli. The transformation was successful and we did recover integration sites, but higher-efficiency rescue was obtained with electroporation. The vector is relatively stable upon passage in avian cells. Southern blot analyses of genomic DNAs derived from successive viral passages under nonselective conditions showed that the cassette (drug resistance gene-lac operator-ColE1) insert was present in the vector up to the third viral passage for both resistance genes, which suggests that the RSVP vectors are stable for approximately three viral passages. Together, these results showed that RSVP vectors are useful tools for cloning unintegrated or integrated viral DNAs.