Molecular analysis of the envelope gene and long terminal repeat of Friend mink cell focus-inducing virus: implications for the functions of these sequences

Six host range variants of the xenotropic/polytropic gammaretroviruses define determinants for entry in the XPR1 cell surface receptor

Background

The evolutionary interactions between retroviruses and their receptors result in adaptive selection of restriction variants that can allow natural populations to evade retrovirus infection. The mouse xenotropic/polytropic (X/PMV) gammaretroviruses rely on the XPR1 cell surface receptor for entry into host cells, and polymorphic variants of this receptor have been identified in different rodent species.

Results

We screened a panel of X/PMVs for infectivity on rodent cells carrying 6 different XPR1 receptor variants. The X/PMVs included 5 well-characterized laboratory and wild mouse virus isolates as well as a novel cytopathic XMV-related virus, termed Cz524, isolated from an Eastern European wild mouse-derived strain, and XMRV, a xenotropic-like virus isolated from human prostate cancer. The 7 viruses define 6 distinct tropisms. Cz524 and another wild mouse isolate, CasE#1, have unique species tropisms. Among the PMVs, one Friend isolate is restricted by rat cells. Among the XMVs, two isolates, XMRV and AKR6, differ from other XMVs in their PMV-like restriction in hamster cells. We generated a set of Xpr1 mutants and chimeras, and identified critical amino acids in two extracellular loops (ECLs) that mediate entry of these different viruses, including 3 residues in ECL3 that are involved in PMV entry (E500, T507, and V508) and can also influence infectivity by AKR6 and Cz524.

Conclusion

We used a set of natural variants and mutants of Xpr1 to define 6 distinct host range variants among naturally occurring X/PMVs (2 XMV variants, 2 PMVs, 2 different wild mouse variants). We identified critical amino acids in XPR1 that mediate entry of these viruses. These gammaretroviruses and their XPR1 receptor are thus highly functionally polymorphic, a consequence of the evolutionary pressures that favor both host resistance and virus escape mutants. This variation accounts for multiple naturally occurring virus resistance phenotypes and perhaps contributes to the widespread distribution of these viruses in rodent and non-rodent species.

Control of pathogenicity and disease specificity of a T-lymphomagenic gammaretrovirus by E-box motifs but not by an overlapping glucocorticoid response element

Although transcription factors of the basic helix-loop-helix family have been shown to regulate enhancers of lymphomagenic gammaretroviruses through E-box motifs, the overlap of an E-box motif (Egre) with the glucocorticoid response element (GRE) has obscured their function in vivo. We report here that Egre, but not the GRE, affects disease induction by the murine T-lymphomagenic SL3-3 virus. Mutating all three copies of Egre prolonged the tumor latency period from 60 to 109 days. Further mutating an E-box motif (Ea/s) outside the enhancer prolonged the latency period to 180 days, suggesting that Ea/s works as a backup site for Egre. While wild-type SL3-3 and GRE and Ea/s mutants exclusively induced T-cell lymphomas with wild-type latencies mainly of the CD4(+) CD8(-) phenotype, Egre as well as the Egre and Ea/s mutants induced B-cell lymphomas and myeloid leukemia in addition to T-cell lymphomas. T-cell lymphomas induced by the two Egre mutants had the same phenotype as those induced by wild-type SL3-3, indicating the incomplete disruption of T-cell lymphomagenesis, which is in contrast to previous findings for a Runx site mutant of SL3-3. Mutating the Egre site or Egre and Ea/s triggered several tumor phenotype-associated secondary enhancer changes encompassing neighboring sites, none of which led to the regeneration of an E-box motif. Taken together, our results demonstrate a role for the E-box but not the GRE in T lymphomagenesis by SL3-3, unveil an inherent broader disease specificity of the virus, and strengthen the notion of selection for more potent enhancer variants of mutated viruses during tumor development.

In vivo interactions of ecotropic and polytropic murine leukemia viruses in mixed retrovirus infections

Mixed retrovirus infections are the rule rather than the exception in mice and other species, including humans. Interactions of retroviruses in mixed infections and their effects on disease induction are poorly understood. Upon infection of mice, ecotropic retroviruses recombine with endogenous proviruses to generate polytropic viruses that utilize different cellular receptors. Interactions among the retroviruses of this mixed infection facilitate disease induction. Using mice infected with defined mixtures of the ecotropic Friend murine leukemia virus (F-MuLV) and different polytropic viruses, we demonstrate several dramatic effects of mixed infections. Remarkably, inoculation of F-MuLV with polytropic MuLVs completely suppressed the generation of new recombinant viruses and dramatically altered disease induction. Co-inoculation of F-MuLV with one polytropic virus significantly lengthened survival times, while inoculation with another polytropic MuLV induced a rapid and severe neurological disease. In both instances, the level of the polytropic MuLV was increased 100- to 1,000-fold, whereas the ecotropic MuLV level remained unchanged. Surprisingly, nearly all of the polytropic MuLV genomes were packaged within F-MuLV virions (pseudotyped) very soon after infection. At this time, only a fractional percentage of cells in the mouse were infected by either virus, indicating that the co-inoculated viruses had infected the same small subpopulation of susceptible cells. The profound amplification of polytropic MuLVs in coinfected mice may be facilitated by pseudotyping or, alternatively, by transactivation of the polytropic virus in the coinfected cells. This study illustrates the complexity of the interactions between components of mixed retrovirus infections and the dramatic effects of these interactions on disease processes.

Precise identification of endogenous proviruses of NFS/N mice participating in recombination with moloney ecotropic murine leukemia virus (MuLV) to generate polytropic MuLVs

Polytropic murine leukemia viruses (MuLVs) are generated by recombination of ecotropic MuLVs with env genes of a family of endogenous proviruses in mice, resulting in viruses with an expanded host range and greater virulence. Inbred mouse strains contain numerous endogenous proviruses that are potential donors of the env gene sequences of polytropic MuLVs; however, the precise identification of those proviruses that participate in recombination has been elusive. Three different structural groups of proviruses in NFS/N mice have been described and different ecotropic MuLVs preferentially recombine with different groups of proviruses. In contrast to other ecotropic MuLVs such as Friend MuLV or Akv that recombine predominantly with a single group of proviruses, Moloney MuLV (M-MuLV) recombines with at least two distinct groups. In this study, we determined that only three endogenous proviruses, two of one group and one of another group, are major participants in recombination with M-MuLV. Furthermore, the distinction between the polytropic MuLVs generated by M-MuLV and other ecotropic MuLVs is the result of recombination with a single endogenous provirus. This provirus exhibits a frameshift mutation in the 3' region of the surface glycoprotein-encoding sequences that is excluded in recombinants with M-MuLV. The sites of recombination between the env genes of M-MuLV and endogenous proviruses were confined to a short region exhibiting maximum homology between the ecotropic and polytropic env sequences and maximum stability of predicted RNA secondary structure. These observations suggest a possible mechanism for the specificity of recombination observed for different ecotropic MuLVs.

Novel host range and cytopathic variant of ecotropic Friend murine leukemia virus

A variant ecotropic Friend murine leukemia virus, F-S MLV, is capable of inducing the formation of large multinucleated syncytia in Mus dunni cells. This cytopathicity resembles that of Spl574 MLV, a novel variant recently isolated from the spleen of a Mus spicilegus mouse neonatally inoculated with Moloney MLV. F-S MLV is an N-tropic Friend MLV that also has the unusual ability to infect hamster cells, which are normally resistant to mouse ecotropic MLVs. Syncytium induction by both F-S MLV and Spl574 is accompanied by the accumulation of large amounts of unintegrated viral DNA, a hallmark of pathogenic retroviruses, but not previously reported for mouse ecotropic gammaretroviruses. Sequencing and site-specific mutagenesis determined that the syncytium-inducing phenotype of F-S MLV can be attributed to a single amino acid substitution (S84A) in the VRA region of the viral env gene. This site corresponds to that of the single substitution previously shown to be responsible for the cytopathicity of Spl574, S82F. The S84A substitution in F-S MLV also contributes to the ability of this virus to infect hamster cells, but Spl574 MLV is unable to infect hamster cells. Because this serine residue is one of the critical amino acids that form the CAT-1 receptor binding site, and because M. dunni and hamster cells have variant CAT-1 receptors, these results suggest that syncytium formation as well as altered host range may be a consequence of altered interaction between virus and receptor.

Antigenic subclasses of polytropic murine leukemia virus (MLV) isolates reflect three distinct groups of endogenous polytropic MLV-related sequences in NFS/N mice

Polytropic murine leukemia viruses (MLVs) are generated by recombination of ecotropic MLVs with members of a family of endogenous proviruses in mice. Previous studies have indicated that polytropic MLV isolates comprise two mutually exclusive antigenic subclasses, each of which is reactive with one of two monoclonal antibodies termed MAb 516 and Hy 7. A major determinant of the epitopes distinguishing the subclasses mapped to a single amino acid difference in the SU protein. Furthermore, distinctly different populations of the polytropic MLV subclasses are generated upon inoculation of different ecotropic MLVs. Here we have characterized the majority of endogenous polytropic MLV-related proviruses of NFS/N mice. Most of the proviruses contain intact sequences encoding the receptor-binding region of the SU protein and could be distinguished by sequence heterogeneity within that region. We found that the endogenous proviruses comprise two major groups that encode the major determinant for Hy 7 or MAb 516 reactivity. The Hy 7-reactive proviruses correspond to previously identified polytropic proviruses, while the 516-reactive proviruses comprise the modified polytropic proviruses as well as a third group of polytropic MLV-related proviruses that exhibit distinct structural features. Phylogenetic analyses indicate that the latter proviruses reflect features of phylogenetic intermediates linking xenotropic MLVs to the polytropic and modified polytropic proviruses. These studies elucidate the relationships of the antigenic subclasses of polytropic MLVs to their endogenous counterparts, identify a new group of endogenous proviruses, and identify distinguishing characteristics of the proviruses that should facilitate a more precise description of their expression in mice and their participation in recombination to generate recombinant viruses.

Characterization of a polytropic murine leukemia virus proviral sequence associated with the virus resistance gene Rmcf of DBA/2 mice

The DBA/2 mouse Rmcf gene is responsible for in vivo and in vitro resistance to infection by the polytropic mink cell focus-forming (MCF) virus subgroup of murine leukemia viruses (MLVs). Previous studies suggested that Rmcf resistance is mediated by expression of an interfering MCF MLV envelope (Env) gene. To characterize this env gene, we examined resistance in crosses between Rmcf(r) DBA/2 mice and Mus castaneus, a species that lacks endogenous MCF env sequences. In backcross progeny, inheritance of Rmcf resistance correlated with inheritance of a specific endogenous MCF virus env-containing 4.6-kb EcoRI fragment. This fragment was present in the DBA/2N substrain with Rmcf-mediated resistance but not in virus-susceptible DBA/2J substrain mice. This fragment contains a provirus with a 5' long terminal repeat and the 5' half of env; the gag and pol genes have been partially deleted. The Env sequence is identical to that of a highly immunogenic viral glycoprotein expressed in the DBA/2 cell line L5178Y and closely resembles the env genes of modified polytropic proviruses. The coding sequence for the full-length Rmcf Env surface subunit was amplified from DNAs from virus-resistant backcross mice and was cloned into an expression vector. NIH 3T3 and BALB 3T3 cells stably transfected with this construct showed significant resistance to infection by MCF MLV but not by amphotropic MLV. This study identifies an Rmcf-linked MCF provirus and indicates that, like the ecotropic virus resistance gene Fv4, Rmcf may mediate resistance through an interference mechanism.

Polymorphisms of the cell surface receptor control mouse susceptibilities to xenotropic and polytropic leukemia viruses

The differential susceptibilities of mouse strains to xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs, respectively) are poorly understood but may involve multiple mechanisms. Recent evidence has demonstrated that these viruses use a common cell surface receptor (the X-receptor) for infection of human cells. We describe the properties of X-receptor cDNAs with distinct sequences cloned from five laboratory and wild strains of mice and from hamsters and minks. Expression of these cDNAs in resistant cells conferred susceptibilities to the same viruses that naturally infect the animals from which the cDNAs were derived. Thus, a laboratory mouse (NIH Swiss) X-receptor conferred susceptibility to P-MLVs but not to X-MLVs, whereas those from humans, minks, and several wild mice (Mus dunni, SC-1 cells, and Mus spretus) mediated infections by both X-MLVs and P-MLVs. In contrast, X-receptors from the resistant mouse strain Mus castaneus and from hamsters were inactive as viral receptors. These results suggest that X-receptor polymorphisms are a primary cause of resistances of mice to members of the X-MLV/P-MLV family of retroviruses and are responsible for the xenotropism of X-MLVs in laboratory mice. By site-directed mutagenesis, we substituted sequences between the X-receptors of M. dunni and NIH Swiss mice. The NIH Swiss protein contains two key differences (K500E in presumptive extracellular loop 3 [ECL 3] and a T582 deletion in ECL 4) that are both required to block X-MLV infections. Accordingly, a single inverse mutation in the NIH Swiss protein conferred X-MLV susceptibility. Furthermore, expression of an X-MLV envelope glycoprotein in Chinese hamster ovary cells interfered efficiently with X-MLV and P-MLV infections mediated by X-receptors that contained K500 and/or T582 but had no effect on P-MLV infections mediated by X-receptors that lacked these amino acids. In contrast, moderate expression of a P-MLV (MCF247) envelope glycoprotein did not cause substantial interference, suggesting that X-MLV and P-MLV glycoproteins interfere nonreciprocally with X-receptor-mediated infections. We conclude that P-MLVs have become adapted to utilize X-receptors that lack K500 and T582. A penalty for this adaptation is a reduced ability to interfere with superinfection. Because failure of interference is a hallmark of several exceptionally pathogenic retroviruses, we propose that it contributes to P-MLV-induced diseases.

Cloning of E6 and E7 genes of human papilloma virus type 18 and transformation potential of E7 gene and its mutants

E6 and E7 genes of human papilloma virus type 18 have been subcloned from plasmid pC7, carrying an insert of DNA from squamous cell carcinoma of cervix. Both genes in comparison to prototype variant contain one mutation that changes asparagine to leucine. In the case of E6 gene this mutation is mapped in codon 129, in the case of E7 the same change AAC to AAA mapped in codon 92. In addition both genes contain few point mutations that do not change the aminoacid sequences of the protein. Two mutants of E7 gene have been constructed by site directed mutagenesis based on PCR technology-one in codon 10 (change Asp to Asn) and one in codon 24 (change Asp to Gly). The first type of mutation did not influence the transformation potential of the E7 gene in comparison to the parental one with mutation in codon 92. The mutation in codon 24 (region responsible for the interaction with Rb protein) eliminate the transformation potential of the gene. The cells transformed with E7 mutants in codons 10 and 92 were tumorigenic for syngenic rats.

Molecular and phylogenetic analysis of SRS 19-6 murine leukemia virus

The complete nucleotide sequence of the genome of Solid-type Reticulum cell Sarcoma 19-6 murine leukemia virus (SRS 19-6 MuLV) was determined. This virus was isolated in mainland China from laboratory mice that had been separated from western mice since the 1930s. The genome is 8,256 nucleotides in length and exhibits a genetic organization characteristic of replication competent MuLVs. Phylogenies constructed from reverse transcriptase (RT) domains showed that SRS 19-6 MuLV is closely related to other MuLV-related retroviruses; however, it has clearly diverged from previously isolated MuLVs. Comparative sequence analysis of the env sequences indicated that SRS 19-6 MuLV encodes a surface (SU) glycoprotein that is related to other ecotropic MuLVs in the VR-A and VR-B variable regions. However, SRS 19-6 MuLV env glycoprotein was distinct from all other MuLVs (ecotropic and non-ecotropic) in the proline-rich hypervariable region. No evidence for recombination with endogenous MuLV env sequences in generation of SRS 19-6 MuLV was observed. Comparisons of long terminal repeat (LTR) sequences revealed that the GV 1.4 molecular clone of Graffi MuLV contained 96% sequence identity to SRS 19-6 MuLV's LTR with 99% identity when comparisons were restricted to the U3 regions of the two viruses. The consensus enhancer binding motifs contained in the U3 regions of the two viruses were nearly identical. Nevertheless the two viruses have previously been shown to induce distinct patterns of disease. Comparisons between 196 and Graffi GV1.4 MuLVs may provide insights into the mechanisms of disease specificity induced by MuLVs.

The hypervariable domain of the murine leukemia virus surface protein tolerates large insertions and deletions, enabling development of a retroviral particle display system

The surface proteins (SU) of murine type-C retroviruses have a central hypervariable domain devoid of cysteine and rich in proline. This 41-amino-acid region of Friend ecotropic murine leukemia virus SU was shown to be highly tolerant of insertions and deletions. Viruses in which either the N-terminal 30 amino acids or the C-terminal 22 amino acids of this region were replaced by the 7-amino-acid sequence ASAVAGA were fully infectious. Insertions of this 7-amino-acid sequence at the N terminus, center, and the C terminus of the hypervariable domain had little effect on envelope protein (Env) function, while this insertion at a position 10 amino acids following the N terminus partially destabilized the association between the SU and transmembrane subunits of Env. Large, complex domains (either a 252-amino-acid single-chain antibody binding domain [scFv] or a 96-amino-acid V1/V2 domain of HIV-1 SU containing eight N-linked glycosylation sites and two disulfides) did not interfere with Env function when inserted in the center or C-terminal portions of the hypervariable domain. The scFv domain inserted into the C-terminal region of the hypervariable domain was shown to mediate binding of antigen to viral particles, demonstrating that it folded into the active conformation and was displayed on the surface of the virion. Both positive and negative enrichment of virions expressing the V1/V2 sequence were achieved by using a monoclonal antibody specific for a conformational epitope presented by the inserted sequence. These results indicated that the hypervariable domain of Friend ecotropic SU does not contain any specific sequence or structure that is essential for Env function and demonstrated that insertions into this domain can be used to extend particle display methodologies to complex protein domains that require expression in eukaryotic cells for glycosylation and proper folding.

Immunoprotective determinants in friend murine leukemia virus envelope protein

Several immunological epitopes are known to be located within the Friend murine leukemia virus (F-MuLV) envelope protein, but their relative contributions to protection from Friend virus-induced disease are not known. To determine how expression of various immunological determinants affected protection, mice were immunized with recombinant vaccinia viruses expressing different portions of the F-MuLV envelope protein, and they were then challenged with a lethal dose of Friend virus complex. The disease parameters that were followed in the mice were early viremia, early splenomegaly, and late splenomegaly. Both the N-terminal and C-terminal portions of the F-MuLV gp70 were found to protect against late splenomegaly, the primary clinical sign associated with virus-induced erythroleukemia. However, neither region alone protected against early splenomegaly and early viremia, indicating poor immunological control over early virus replication and spread through the spleen and blood. In contrast, mice immunized with a vaccine expressing the entire F-MuLV envelope protein were protected against all three disease parameters. The results indicated that expression of multiple immunological determinants including both T-helper and B cell epitopes was necessary for full protection.

Pathogenicity of a mutant friend spleen focus-forming virus encoding an Env-like membrane glycoprotein (gp55) with substitution by a xenotropic murine leukemia virus Env gp70 sequence

Friend spleen focus-forming virus (F-SFFV) is a replication-defective acutely leukemogenic mouse retrovirus and encodes an envelope protein (Env)-like membrane glycoprotein (gp55) in its defective env gene, which is responsible for the early stage of the viral leukemogenesis. Gp55 is a modified Env protein and contains a polytropic mink cell focus-inducing (MCF) murine leukemia virus (MuLV) Env gp70-derived sequence in its amino-terminal region. To evaluate the possibility that the presumed binding of gp55 to an MCF MuLV receptor protein has some role in leukemogenesis, we examined the biological activities of a mutant gp55 (XE gp55), which has a xenotropic MuLV Env gp70 amino-terminal region. XE gp55 displayed almost the same biological activities as the wild-type gp55, excluding the above possibility.

Origin and rapid evolution of a novel murine erythroleukemia virus of the spleen focus-forming virus family

The Friend spleen focus-forming virus (SFFV) env gene encodes a glycoprotein with apparent Mr of 55,000 that binds to erythropoietin receptors (EpoR) to stimulate erythroblastosis. A retroviral vector that does not encode any Env glycoprotein was packaged into retroviral particles and was coinjected into mice in the presence of a nonpathogenic helper virus. Although most mice remained healthy, one mouse developed splenomegaly and polycythemia at 67 days; the virus from this mouse reproducibly caused the same symptoms in secondary recipients by 2 to 3 weeks postinfection. This disease, which was characterized by extramedullary erythropoietin-independent erythropoiesis in the spleens and livers, was also reproduced in long-term bone marrow cultures. Viruses from the diseased primary mouse and from secondary recipients converted an erythropoietin-dependent cell line (BaF3/EpoR) into factor-independent derivatives but had no effect on the interleukin-3-dependent parental BaF3 cells. Most of these factor-independent cell clones contained a major Env-related glycoprotein with an Mr of 60,000. During further in vivo passaging, a virus that encodes an Mr-55,000 glycoprotein became predominant. Sequence analysis indicated that the ultimate virus is a new SFFV that encodes a glycoprotein of 410 amino acids with the hallmark features of classical gp55s. Our results suggest that SFFV-related viruses can form in mice by recombination of retroviruses with genomic and helper virus sequences and that these novel viruses then evolve to become increasingly pathogenic.

An array of novel murine spleen focus-forming viruses that activate the erythropoietin receptor

The Friend spleen focus-forming virus (SFFV) env gene encodes a 409-amino-acid glycoprotein with an apparent Mr of 55,000 (gp55) that binds to erythropoietin receptors (EpoR) to stimulate erythroblastosis. We reported previously the in vivo selection during serial passages in mice of several evolutionary intermediates that culminated in the formation of a novel SFFV (M. E. Hoatlin, E. Gomez-Lucia, F. Lilly, J. H. Beckstead, and D. Kabat, J. Virol. 72:3602-3609, 1998). A mouse injected with a retroviral vector in the presence of a nonpathogenic helper virus developed long-latency erythroblastosis, and subsequent viral passages resulted in more pathogenic isolates. The viruses taken from these mice converted an erythropoietin-dependent cell line (BaF3/EpoR) into factor-independent derivatives. Western blot analysis of cell extracts with an antiserum that broadly reacts with murine retroviral envelope glycoproteins suggested that the spleen from the initial mouse with mild erythoblastosis contained an array of viral components that were capable of activating EpoR. DNA sequence analysis of the viral genomes cloned from different factor-independent cell clones revealed env genes with open reading frames encoding 644, 449, and 187 amino acids. All three env genes contained 3' regions identical to that of SFFV, including a 6-bp duplication and a single-base insertion that have been shown previously to be critical for pathogenesis. However, the three env gene sequences did not contain any polytropic sequences and were divergent in their 5' regions, suggesting that they had originated by recombination and partial deletions of endogenously inherited MuLV env sequences. These results suggest that the requirements for EpoR activation by SFFV-related viruses are dependent on sequences at the 3' end of the env gene and not on the polytropic regions or on the 585-base deletions that are common among the classical strains of SFFV. Moreover, sequence analysis of the different recombinants and deletion mutants revealed that short direct and indirect repeat sequences frequently flanked the deletions that had occurred, suggesting a reverse transcriptase template jumping mechanism for this rapid retroviral diversification.

Sequence flexibility in the polytropic env gp70-derived region of the membrane glycoprotein (gp55) of Friend spleen focus-forming virus affects its biological activity

We previously reported (N. Watanabe, M. Nishi, Y. Ikawa, and H. Amanuma, J. Virol. 65:132-137, 1991) that the mutant Friend spleen focus-forming virus (F-SFFV(MS)), which encodes a mutant gp55 membrane glycoprotein with an ecotropic env gp70 sequence, was nonpathogenic. Here we injected the F-SFFV(MS)-Friend murine leukemia virus (F-MuLV) clone 57 complex into newborn DBA/2 mice. We obtained four groups of pathogenic variant F-SFFV complexes, each showing a different degree of pathogenicity in adult mice and a different gp55 profile. Of these, group 1 variant F-SFFV was particularly interesting, because it was the most frequently obtained and because it produced doublet bands of gp55 (59 and 57 kDa), neither of which reacted with the nonecotropic gp70-specific monoclonal antibody, and because its DNA intermediate did not hybridize with the nonecotropic env-specific probe. Cloning and DNA sequence analysis of the env region of one isolate of the group 1 variant F-SFFV revealed that this virus consisted of two distinct F-SFFV genomes; one (clone 117) differed from the other (clone 118) due to the presence of a 39-bp in-frame deletion. Reconstitution to full-length F-SFFV genomes and a pathogenicity assay showed that each reconstituted F-SFFV was pathogenic, with clone 117 showing a higher degree of pathogenicity than clone 118. Both reconstituted F-SFFVs caused activation of the mouse erythropoietin receptor in the factor-independent cell proliferation assay, although much less efficiently than the wild-type polycythemia-inducing isolate F-SFFVp. Clone 118 produced a gp55 of 59 kDa, while clone 117 produced one of 57 kDa. Clone 118 had a substitution by the F-MuLV clone 57 gp70 sequence, indicating that it was derived from the F-SFFV(MS) env gene by a homologous recombination with the F-MuLV clone 57 env gene. The site of the 39-bp deletion in clone 117 corresponded to the portion of the clone 118 sequence which was unique to the ecotropic env genes. These results indicated the importance for the biological activity of gp55 of the sequences in the gp70 differential region, which are contained in both polytropic and ecotropic env genes.

A retroviral peptide encoded by mutated env p15E gene is recognized by specific CD8+ T lymphocytes on drug-treated murine mastocytoma P815

Highly immunogenic ("xenogenized") tumour variants appear after treatment of murine mastocytoma P815 with the triazene derivative DTIC, a phenomenon associated with the appearance of structurally abnormal p15E env proteins in the variant cells. In the present study, we have isolated and sequenced several p15E cDNA gene fragments amplified by means of polymerase chain reaction (PCR) from parental (P815) and xenogenized (P815/DTIC) tumour cells. Compared to known p15E sequences in parental cells, one p15E sequence from xenogenized cells presented three distinct nucleotide changes, one of which was apparently unique to P815/DTIC DNA and cDNA upon single-nucleotide primer extension assay. One major histocompatibility complex (MHC) class I-binding peptide, corresponding to a putative mutation in the p15E sequence, was tested in parallel with the parental peptide for recognition by P815/DTIC-specific cytotoxic T cells in vitro. The results suggested that the amino acid substitution at the relevant position of the p15E protein may produce an antigenic T cell epitope. By skin test assay of mice primed with either the synthetic peptide or P815/DTIC cells, evidence was obtained that the mutated peptide is immunogenic in vivo, and that the neoepitope is expressed by P815/DTIC cells. In accordance with previous data in the L5178Y/DTIC tumour model system, these findings reinforce the notion that xenogenization of tumour cells may result in the expression of class I-binding mutated peptides of retroviral origin.

Two separate envelope regions influence induction of brain disease by a polytropic murine retrovirus (FMCF98)

The major determinants involved in neurological disease induction by polytropic murine leukemia virus FMCF98 are encoded by the envelope gene. To map these determinants further, we produced four chimeras which contained neurovirulent FMCF98 envelope sequences combined with envelope sequences from the closely related nonneurovirulent polytropic virus FMCF54. Surprisingly, two chimeric viruses containing completely separate envelope regions from FMCF98 could both induce neurological disease. Clinical signs caused by both neurovirulent chimeras appeared to be indistinguishable from those caused by FMCF98, although the incubation periods were longer. One neurovirulence determinant mapped to the N-terminal portion of gp7O, which contains the VRA and VRB receptor-binding regions, while the other determinant mapped downstream of both of the variable regions. Western blot (immunoblot) analyses and immunohistochemical staining of tissue sections indicated that the variations in neurovirulence of these viruses could not be explained by differences in either the quantitative level or the location of virus expression in the brain.

Novel integration sites at the distal 3' end of the c-myb locus in retrovirus-induced promonocytic leukemias

In BALB/c nu/nu and sublethally irradiated DBA/2 mice, promonocytic leukemia was induced by intravenous inoculation of Friend murine leukemia virus (F-MuLV) strain C57 in conjunction with intraperitoneal injection of pristane. These tumors appear to be identical morphologically to previously reported ones induced by other MuLVs, such as Moloney, amphotropic 4070A, and F-MuLV FB29, which most commonly have provirus integrations in the 5' end of the c-myb locus. Interestingly, 2 of the 16 F-MuLV-induced tumors had viruses integrated in the distal 3' end of c-myb. To determine the precise locations of these integrations, it was necessary to clone sequences encoding the 3' c-myb exons and to prepare a physical map of this region. Exons 10 to 15 were positioned on the map, and it was found that the proviruses in the aforementioned tumors were located within narrow region in the beginning of the large (greater than 11 kb) intron 14. The predicted protein product encoded by the affected alleles is truncated by 38 amino acids. This represents a novel virus integration site which is most likely associated with oncogenic activation of the c-myb gene during leukemogenesis.

Chimeric envelope glycoproteins constructed between amphotropic and xenotropic murine leukemia retroviruses

A set of chimeric envelope proteins between amphotropic and xenotropic murine leukemia retroviruses (MuLV), two closely related members in the MuLV family, were constructed. The purpose was to examine the regions that could be successfully exchanged between these two similar viral envelope proteins. The data indicate that fully active chimeras can be built when the junction is either at the EcoRI site (amino acid 169) 42 amino acids N-terminal to the polyproline hinge of gp70 (named CH4) or at the ScaI site (aa 593) in the membrane spanning portion of p15E (CH1). However, a chimera at the AflII site (aa 125, CH5) and two in the C-terminal end of gp70 (aa 418, CH2; aa 326, CH3) were inactive. These results, taken together with other data from our laboratory and others, suggest that the entire gp70/p15E structure is sensitive to alterations and that even envelope proteins that are very similar have only a limited ability to exchange sequences.

Characterization of nuclear protein binding to a site in the long terminal repeat of a murine leukemia virus: comparison with the NFAT complex

We previously identified a protein-binding site (MLPal) that is located downstream of the enhancer element in the long terminal repeat (LTR) of a mink cell focusing-forming (MCF) murine leukemia virus (F. K. Yoshimura, K. Diem, H. Chen, and J. Tupper, J. Virol. 67:2298-2304, 1993). We determined that the MLPal site regulates transcription specifically in T cells and affects the lymphomagenicity of the MCF isolate 13 murine leukemia virus with a single enhancer repeat in its LTR. In this report, we present evidence that two different proteins, a T-cell-specific protein and a ubiquitous protein, bind the MLPal site in a sequence-specific manner. By mutational analysis, we determined that the T-cell-specific and the ubiquitous proteins require different nucleotides in the MLPal sequence for DNA binding. By competitive electrophoretic mobility shift assays, we demonstrated that the T-cell-specific protein that binds MLPal is identical or similar to a protein from nonactivable T cells that interacts with the binding site of the nuclear factor of activated T cells (NFAT). Unlike the NFAT-binding site, however, the MLPal site does not bind proteins that are inducible by T-cell activation. We observed that the MLPal sequence is conserved in the LTRs of other mammalian retroviruses that cause T-cell diseases. Furthermore, the MLPal sequence is present in the transcriptional regulatory regions of cellular genes that either are expressed specifically in T cells or are commonly rearranged by provirus integration in thymic lymphomas. Thus, the MLPal-binding proteins may play a role in the transcriptional regulation not only of the MCF virus LTR but also of cellular genes involved in T-cell development.

Reflections on scrapie and related disorders, with consideration of the possibility of a viral aetiology

The transmissible spongiform encephalopathies of domesticated animals, scrapie in sheep and bovine spongiform encephalopathy (BSE), and transmissible mink encephalopathy are more than a scientific curiosity; under certain circumstances their impact on commercial activities can be calamitous. Knowledge of their causation and pathogenesis is still rudimentary, but many consider than an unconventional agent, the prion (a brain protein, PrP), that is not associated with nucleic acid is involved in both. Others believe that conventional viruses, which replicate by virtue of their nucleic acid-defined genes, are involved in the causation and progression of the encephalopathies but that technical problems have prevented their identification. Others postulate even more exotic causative agents. While this paper will particularly address the possibility of a viral aetiology for these diseases, it is also emphasized that our knowledge of the state of the immune system in animals with encephalopathy needs broadening. There are remarkable gaps in our knowledge of the histopathology of these diseases, particularly the nature of the characteristic vacuoles. Much further work is needed on the biochemical changes in the brain and the serum, particularly of the latter as it could lead to an additional means of recognizing clinical cases without waiting for the animal to die with subsequent examination of the brain for characteristic lesions and the presence of protease-K-resistant PrP.

Basic helix-loop-helix proteins in murine type C retrovirus transcriptional regulation

E boxes, recognition sequences for basic helix-loop-helix (bHLH) transcription factors, are detected in the enhancer and promoter regions of several murine type C retroviruses. Here we show that ALF1, a member of bHLH protein family of transcription factors, in vitro binds with differing affinities to distinct E-box sequences found in the U3 regulatory regions of Friend, Moloney, SL3-3, and Akv murine leukemia viruses (MLVs) as well as Friend spleen focus-forming virus (SFFV). In NIH 3T3 fibroblasts, ALF1 overexpression elevated transcription from the U3 region of Moloney MLV and the complete long terminal repeat regions of Friend SFFV, Akv MLV, and SL3-3 MLV but neither from the U3 region nor from the complete long terminal repeat of Friend MLV. Introduction of mutations in the Akv MLV E boxes showed the E-box cis elements to be required for the function of ALF1 as a transcription factor. ALF1 and the glucocorticoid receptor, with overlapping DNA binding sequences, did not act synergistically with respect to transcriptional trans activation of expression from the Akv MLV promoter-enhancer region. We conclude that ALF1 in vivo may be an important transcription regulator for Akv, SL3-3, and Moloney MLVs as well as for Friend SFFV.

Characterization of epitopes defining two major subclasses of polytropic murine leukemia viruses (MuLVs) which are differentially expressed in mice infected with different ecotropic MuLVs

Polytropic murine leukemia viruses (MuLVs) arise in mice by recombination of ecotropic MuLVs with endogenous retroviral envelope genes and have been implicated in the induction of hematopoietic proliferative diseases. Inbred mouse strains contain many endogenous sequences which are homologous to the polytropic env genes; however, the extent to which particular sequences participate in the generation of the recombinants is unknown. Previous studies have established antigenic heterogeneity among the env genes of polytropic MuLVs, which may reflect recombination with distinct endogenous genes. In the present study, we have examined many polytropic MuLVs and found that nearly all isolates fall into two mutually exclusive antigenic subclasses on the basis of the ability of their SU proteins to react with one of two monoclonal antibodies, termed Hy 7 and MAb 516. Epitope-mapping studies revealed that reactivity to the two antibodies is dependent on the identity of a single amino acid residue encoded in a variable region of the receptor-binding domain of the env gene. This indicated that the two antigenic subclasses of MuLVs arose by recombination with distinct sets of endogenous genes. Evaluation of polytropic MuLVs in mice revealed distinctly different ratios of the two subclasses after inoculation of different ecotropic MuLVs, suggesting that individual ecotropic MuLVs preferentially recombine with distinct sets of endogenous polytropic env genes.

Structural elements in glycoprotein 70 from polytropic Friend mink cell focus-inducing virus and glycoprotein 71 from ecotropic Friend murine leukemia virus, as defined by disulfide-bonding pattern and limited proteolysis

The disulfide-bonding pattern of glycoprotein 70 (gp70), the surface glycoprotein (SU) encoded by the envelope gene of polytropic Friend milk cell focus-inducing virus, was elucidated and compared with that of glycoprotein 71 (gp71), the corresponding glycoprotein of the ecotropic Friend murine leukemia virus, which had previously been determined (M. Linder, D. Linder, J. Hahnen, H.-H. Schott, and Stirm, Eur. J. Biochem. 203:65-73, 1992). In the carboxy-terminal constant domain, in which these glycoproteins have about 97% sequence homology, the location of the four disulfide bonds was found to be analogous. In the amino-terminal differential domain, with about 37% sequence homology, 8 of the 12 cysteine residues of the ecotropic SU are conserved in the polytropic SU. In this domain, a similar clustering of disulfide bonds was detected, which led to the identification of three distinct disulfide-bonded regions in both glycoproteins. However, because of deletions and sequence deviations, the glycoproteins must have significantly different three-dimensional structures in these regions. Since the receptor-binding functions of both glycoproteins have been attributed to their amino-terminal domains and since each binds to a different receptor, these disulfide-bonded structures are likely candidates for receptor-binding functions. Limited proteolysis of both glycoproteins with various endoproteinases led to the identification of preferential proteolytic sites between disulfide-bonded regions, at the beginning of the hypervariable proline-rich region, and between differential and constant domains, further confirming the structural organization of the folded glycoproteins.

Characterization of replication-competent retroviruses from nonhuman primates with virus-induced T-cell lymphomas and observations regarding the mechanism of oncogenesis

Rapidly progressive T-cell lymphomas were observed in 3 of 10 rhesus monkeys several months after autologous transplantation of enriched bone marrow stem cells that had been transduced with a retroviral vector preparation containing replication-competent virus (R. E. Donahue, S. W. Kessler, D. Bodice, K. McDonagh, C. Dunbar, S. Goodman, B. Agricola, E. Byrne, M. Raffeld, R. Moen, J. Bacher, K. M. Zsebo, and A. W. Nienhuis, J. Exp. Med. 176:1124-1135, 1992). The animals with lymphoma appeared to be tolerant to retroviral antigens in that their sera lacked antibodies reactive with viral proteins and contained 10(4) to 10(5) infectious virus particles per ml. By molecular cloning and DNA sequencing, we have now demonstrated that the serum from one of the monkeys contained a replication-competent retrovirus that arose by recombination between vector and packaging encoding sequences (vector/helper [V/H] recombinant) in the producer clone used for transduction of bone marrow stem cells. Southern blot analysis demonstrated 14 or 25 copies of this genome per cell where present in two animals. The genome of a second replication-competent virus was also recovered by molecular cloning; it arose by recombination involving the genome of the V/H recombinant and endogenous murine retroviral genomes in the producer clone. Twelve copies of this amphotropic virus/mink cell focus-forming virus genome were present in tumor DNA of one animal, but it was not found in tumor DNA of the other two animals with lymphoma. Southern blot analysis of DNA from various tissues demonstrated common insertion site bands in several samples of tumor DNA from one animal, suggesting clonal origin of the lymphoma. Our data are most consistent with a pathogenic mechanism in which chronic productive retroviral infection allowed insertional mutagenesis of critical growth control genes, leading to cell transformation and clonal tumor evolution.

Presentation of native epitopes in the V1/V2 and V3 regions of human immunodeficiency virus type 1 gp120 by fusion glycoproteins containing isolated gp120 domains

The immune response to viral glycoproteins is often directed against conformation- and/or glycosylation-dependent structures; synthetic peptides and bacterially expressed proteins are inadequate probes for the mapping of such epitopes. This report describes a retroviral vector system that presents such native epitopes on chimeric glycoproteins in which protein fragments of interest are fused to the C terminus of the N-terminal domain of the murine leukemia virus surface protein, gp70. The system was used to express two disulfide-bonded domains from gp120, the surface protein of human immunodeficiency virus type 1 (HIV-1), that include potent neutralization epitopes. The resulting fusion glycoproteins were synthesized at high levels and were efficiently transported and secreted. A fusion protein containing the HXB2 V1/V2 domain was recognized by an HIVIIIB-infected patient serum as well as by 17 of 36 HIV-1 seropositive hemophiliac, homosexual male and intravenous drug user patient sera. Many of these HIV+ human sera reacted with V1/V2 domains from several HIV-1 clones expressed in fusion glycoproteins, indicating the presence of cross-reactive antibodies against epitopes in the V1/V2 domain. Recognition of gp(1-263):V1/V2HXB2 by the HIVIIIB-infected human patient serum was largely blocked by synthetic peptides matching V1 but not V2 sequences, while recognition of this construct by a broadly cross-reactive hemophiliac patient serum was not blocked by individual V1 or V2 peptides or by mixtures of these peptides. A construct containing the V3 domain of the IIIB strain of HIV-1, gp(1-263):V3HXB2, was recognized by sera from a human and a chimpanzee that had been infected by HIVIIIB but not by sera from hemophiliac patients who had been infected with HIV-1 of MN-like V3 serotype. The reactive sera had significantly higher titers when assayed against gp(1-263):V3HXB2 than when assayed against matching V3 peptides. Immunoprecipitation of this fusion glycoprotein by the human serum was only partially blocked by V3 peptide, indicating that this infected individual produced antibodies against epitopes in V3 that were expressed on the fusion glycoprotein but not by synthetic peptides. These data demonstrated that the chimeric glycoproteins described here effectively present native epitopes present in the V1/V2 and V3 domains of gp120 and provide efficient methods for detection of antibodies directed against native epitopes in these regions and for characterization of such epitopes.

Mutational analysis of the envelope gene of Moloney murine leukemia virus

The env gene products of Moloney murine leukemia virus are required for binding and entry of the virus into the target cell. Thirty-three linker insertion mutations were constructed throughout the env gene of Moloney murine leukemia virus. Twenty of the mutations were located in the surface protein (SU), and the remaining thirteen were located in the transmembrane protein (TM). The viability of the viruses containing these env gene mutations was determined by performing transient transfections and screening for the release of reverse transcriptase. Eleven viable mutants were isolated, nine in SU and two in TM. Three of the viable mutants were temperature sensitive. Four of the viable mutants were clustered in the carboxy terminus of SU. The env gene products of transfected cell lines which produced viable virus were analyzed. Our results indicated two regions of SU important for the stability of the SU/TM heteropolymer and one region important for the interaction of the env gene products with the viral core.

A Friend virus mutant that overcomes Fv-2rr host resistance encodes a small glycoprotein that dimerizes, is processed to cell surfaces, and specifically activates erythropoietin receptors

The env gene of Friend spleen focus-forming virus (SFFV) encodes a membrane glycoprotein (gp55) that is inefficiently (3 to 5%) processed from the rough endoplasmic reticulum to form a larger dimeric plasma membrane derivative (gp55p). Moreover, the SFFV env glycoprotein associates with erythropoietin receptors (EpoR) to cause proliferation of infected erythroblasts [J.-P. Li, A. D. D'Andrea, H. F. Lodish, and D. Baltimore, Nature (London) 343:762-764, 1990]. Interestingly, the mitogenic effect of SFFV is blocked in mice homozygous for the Fv-2r resistance gene, but mutant SFFVs can overcome this resistance. Recent evidence suggested that these mutants contain partial env deletions that truncate the membrane-proximal extracellular domain of the encoded glycoproteins (M. H. Majumdar, C.-L. Cho, M. T. Fox, K. L. Eckner, S. Kozak, D. Kabat, and R. W. Geib, J. Virol. 66:3652-3660, 1992). Mutant BB6, which encodes a gp42 glycoprotein that has a large deletion in this domain, causes erythroblastosis in DBA/2 (Fv-2s) as well as in congenic D2.R (Fv-2r) mice. Analogous to gp55, gp42 is processed inefficiently as a disulfide-bonded dimer to form cell surface gp42p. Retroviral vectors with SFFV and BB6 env genes have no effect on interleukin 3-dependent BaF3 hematopoietic cells, but they cause growth factor independency of BaF3/EpoR cells, a derivative that contains recombinant EpoR. After binding 125I-Epo to surface EpoR on these factor-independent cells and adding the covalent cross-linking reagent disuccinimidyl suberate, complexes that had immunological properties and sizes demonstrating that they consisted of 125I-Epo-gp55p and 125I-Epo-gp42p were isolated from cell lysates. Contrary to a previous report, SFFV or BB6 env glycoproteins did not promiscuously activate other members of the EpoR superfamily. Although the related env glycoproteins encoded by dualtropic murine leukemia viruses formed detectable complexes with EpoR, strong mitogenic signalling did not ensue. Our results indicate that the SFFV and BB6 env glycoproteins specifically activate EpoR; they help to define the glycoprotein properties important for its functions; and they strongly suggest that the Fv-2 leukemia control gene encodes an EpoR-associated regulatory factor.

The erythropoietin receptor transmembrane region is necessary for activation by the Friend spleen focus-forming virus gp55 glycoprotein

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated by binding either erythropoietin (EPO) or gp55, the Friend spleen focus-forming virus glycoprotein. The highly specific interaction between gp55 and EPO-R triggers cell proliferation and thereby causes the first stage of Friend virus-induced erythroleukemia. We have generated functional chimeric receptors containing regions of the EPO-R and the interleukin-3 receptor (AIC2A polypeptide), a related cytokine receptor which does not interact with gp55. All chimeric receptors were expressed at similar levels, had similar binding affinities for EPO, and conferred EPO-dependent cell growth. Only those chimeric receptors which contained the EPO-R transmembrane region were activated by gp55. These results demonstrate that the transmembrane region of the EPO-R is critical for activation by gp55. In addition, analysis of a soluble, secreted EPO-R and cysteine point mutants of the EPO-R show that the extracytoplasmic region of the EPO-R specifically interacts with gp55.

Basis for receptor specificity of nonecotropic murine leukemia virus surface glycoprotein gp70SU

Murine leukemia viruses (MuLVs) initiate infection of NIH 3T3 cells by binding of the viral envelope (Env) protein to a cell surface receptor. Interference assays have shown that MuLVs can be divided into four groups, each using a distinct receptor: ecotropic, polytropic, amphotropic, and 10A1. In this study, we have attempted to map the determinants within viral Env proteins by constructing chimeric env genes. Chimeras were made in all six pairwise combinations between Moloney MCF (a polytropic MuLV), amphotropic MuLV, and 10A1, using a conserved EcoRI site in the middle of the Env coding region. The receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity seems to map to the N-terminal portion of surface glycoprotein gp70SU. The difference between amphotropic and 10A1 receptor specificity can be attributed to one or more of only six amino acid differences in this region. Nearly all other cases showed evidence of interaction between Env domains in the generation of receptor specificity. Thus, a chimera composed exclusively of MCF and amphotropic sequences was found to exhibit 10A1 receptor specificity. None of the chimeras were able to infect cells by using the MCF receptor; however, two chimeras containing the C-terminal portion of MCF gp70SU could bind to this receptor, while they were able to infect cells via the amphotropic receptor. This result raises the possibility that receptor binding maps to the C-terminal portion of MCF gp70SU but requires MCF N-terminal sequences for a functional interaction with the MCF receptor.

The erythropoietin receptor transmembrane region is necessary for activation by the Friend spleen focus-forming virus gp55 glycoprotein

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated by binding either erythropoietin (EPO) or gp55, the Friend spleen focus-forming virus glycoprotein. The highly specific interaction between gp55 and EPO-R triggers cell proliferation and thereby causes the first stage of Friend virus-induced erythroleukemia. We have generated functional chimeric receptors containing regions of the EPO-R and the interleukin-3 receptor (AIC2A polypeptide), a related cytokine receptor which does not interact with gp55. All chimeric receptors were expressed at similar levels, had similar binding affinities for EPO, and conferred EPO-dependent cell growth. Only those chimeric receptors which contained the EPO-R transmembrane region were activated by gp55. These results demonstrate that the transmembrane region of the EPO-R is critical for activation by gp55. In addition, analysis of a soluble, secreted EPO-R and cysteine point mutants of the EPO-R show that the extracytoplasmic region of the EPO-R specifically interacts with gp55.

A 585-bp deletion found in the spleen focus-forming virus (SFFV) env gene is responsible for the defective intracellular transport of SFFV gp52

The Friend spleen focus-forming virus (F-SFFV) codes for a transport defective, leukemogenic envelope glycoprotein designated as gp52. Gp52 closely resembles the envelope glycoproteins (gp70-p15E) encoded by the mink cell focus-forming viruses (MCFV). The major differences between SFFV and MCFV include a 585-bp deletion and a frame-shift mutation near the 3' end of the SFFV env gene. We have constructed a mutant MCFV env gene, which contains a 585-bp deletion like that found in the SFFV env gene, and expressed this gene using recombinant vaccinia vectors or retroviral vectors. The mutant MCFV env gene expressed a truncated, transport defective glycoprotein (gp57). Only a small proportion of gp57 underwent further oligosaccharide processing. Intracellular gp57 remained predominantly monomeric and only a small proportion of gp57 (and its processed forms) formed disulfide-linked dimers and trimers which resembled those formed by SFFV gp52. Processed forms of gp57 were found on the cell surfaces and in culture fluids. The extracellular forms had a faster electrophoretic mobility than the intracellular-processed forms of gp57. These results indicate that the 585-bp deletion found in SFFV env gene is responsible for the folding, transport, and secretion of gp52. Retroviral vectors carrying the mutant MCFV env gene were nonpathogenic (or weakly pathogenic) in adult mice. The results indicate that the 585-bp deletion, although essential, is not the sole determinant of SFFV-induced disease in adult mice.

Receptor choice determinants in the envelope glycoproteins of amphotropic, xenotropic, and polytropic murine leukemia viruses

The envelope glycoproteins (SU) of mammalian type C retroviruses possess an amino-terminal domain of about 200 residues, which is involved in binding a cell surface receptor. In this domain, highly conserved amino acid sequences are interrupted by two segments of variable length and sequence, VRA and VRB. We have studied the role of these variable regions in receptor recognition and binding by constructing chimeric molecules in which portions of the amino-terminal domains from amphotropic (4070A), xenotropic (NZB), and polytropic (MCF 247) murine leukemia virus SU proteins were permuted. These chimeras, which exchanged either one or two variable regions, were expressed at the surface of replication-defective viral particles by a pseudotyping assay. Wild-type or recombinant env genes were transfected into a cell line producing Moloney murine leukemia virus particles devoid of envelope glycoproteins in which a retrovirus vector genome carrying an Escherichia coli lacZ gene was packaged. The host range and sensitivity to interference of pseudotyped virions were assayed, and we observed which permutations resulted in receptor switch or loss of function. Our results indicate that the determinants of receptor choice are found within the just 120 amino acids of SU proteins. Downstream sequences contribute to the stabilization of the receptor-specific structure.

Localization of the intrachain disulfide bonds of the envelope glycoprotein 71 from Friend murine leukemia virus

Envelope glycoprotein 71 from Friend murine leukemia virus was purified to homogeneity by reversed-phase HPLC. It could be shown that all 20 cysteine residues of the molecule are linked by disulfide bonds. After complete tryptic digestion, peptides containing cystine were identified by comparison of the reversed-phase HPLC profile of the digest with that of a reduced aliquot which had been subjected to affinity chromatography on thiol-Sepharose. The locations of the 10 disulfide bonds were determined by isolation, further digestion and analysis of peptides containing cystine. The first cysteine residue of the sequence (Cys46) was shown to be coupled to the sixth (Cys98), leading to a large loop containing four additional cysteine residues. Computer model building and energy calculations led to the assignment of Cys72 to Cys87 and Cys73 to Cys83. The following four cysteine residues of the sequence also constitute a structural unit, with Cys121 bonded to Cys141 and Cys133 to Cys146, and the last two cysteine residues in the amino-terminal domain of glycoprotein 71 form a small loop (Cys178 to Cys184). The first two cysteine residues of the carboxy-terminal domain produce a very small hydrophobic loop (Cys312-Cys315). Cys361 is bound to Cys373, Cys342 to Cys396 and Cys403 to Cys416. A model for the folding pattern of the viral glycoprotein is proposed.

Mutational analysis of N-linked glycosylation sites of Friend murine leukemia virus envelope protein

The roles played by the N-linked glycans of the Friend murine leukemia virus envelope proteins were investigated by site-specific mutagenesis. The surface protein gp70 has eight potential attachment sites for N-linked glycan; each signal asparagine was converted to aspartate, and mutant viruses were tested for the ability to grow in NIH 3T3 fibroblasts. Seven of the mutations did not affect virus infectivity, whereas mutation of the fourth glycosylation signal from the amino terminus (gs4) resulted in a noninfectious phenotype. Characterization of mutant gene products by radioimmunoprecipitation confirmed that glycosylation occurs at all eight consensus signals in gp70 and that gs2 carries an endoglycosidase H-sensitive glycan. Elimination of gs2 did not cause retention of an endoglycosidase H-sensitive glycan at a different site, demonstrating that this structure does not play an essential role in envelope protein function. The gs3- mutation affected a second posttranslational modification of unknown type, which was manifested as production of gp70 that remained smaller than wild-type gp70 after removal of all N-linked glycans by peptide N-glycosidase F. The gs4- mutation decreased processing of gPr80 to gPr90, completely inhibited proteolytic processing of gPr90 to gp70 and Pr15(E), and prevented incorporation of envelope products into virus particles. Brefeldin A-induced mixing of the endoplasmic reticulum and parts of the Golgi apparatus allowed proteolytic processing of wild-type gPr90 to occur in the absence of protein transport, but it did not overcome the cleavage defect of the gs4- precursor, indicating that gs4- gPr90 is resistant to the processing protease. The work reported here demonstrates that the gs4 region is important for env precursor processing and suggests that gs4 may be a critical target in the disruption of murine leukemia virus env product processing by inhibitors of N-linked glycosylation.

An amino-terminal fragment of the Friend murine leukemia virus envelope glycoprotein binds the ecotropic receptor

Retrovirus entry into cells is mediated by specific binding of the envelope glycoprotein to a cell membrane receptor. Constitutive envelope gene expression prevents infection by interfering with the binding of viruses which recognize the same receptor. We have used this property to investigate the receptor binding capacities of deleted or truncated murine leukemia virus ecotropic envelope glycoproteins. Friend murine leukemia virus envelope glycoproteins bearing internal amino-terminal deletions, or a soluble 245-amino-acid gp70 amino-terminal fragment, were expressed in NIH 3T3 cells. The susceptibility of these cells to ecotropic and amphotropic virus infection was determined. We observed that both membrane-bound and soluble forms of the gp70 245-amino-acid amino-terminal domain induced resistance to ecotropic virus, indicating that this fragment binds the ecotropic receptor. Binding occurs both at the cell surface and in the endoplasmic reticulum, as shown by the use of soluble envelope fragments either secreted in the culture supernatants or retained in the endoplasmic reticulum lumen by a KDEL sequence. These results suggest that the gp70 amino-terminal domain folds into a structure which recognizes the ecotropic receptor regardless of the carboxy-terminal part of the molecule.

Construction and properties of an "artificial" spleen focus-forming virus

The replication-defective Friend spleen focus-forming virus (F-SFFV) induces acute erythroblastosis in adult mice. The envelope-related (env) gene and LTR are the only functional elements of the viral genome. The env-coded glycoprotein gp55 has been shown to be responsible for target cell specificity and for the short latency of the disease caused by SFFV. This molecule closely resembles the env coded proteins gp70 + p15E of mink cell focus inducing viruses (MCFV). The only substantial differences between these two env genes are a large deletion spanning 585 nucleotides in the middle of the F-SFFV gene and a frameshift mutation near the 3' end leading to a modified and shortened membrane anchor in the mature protein. To determine if the large deletion and/or the frameshift mutation are capable of changing the properties of a nonpathogenic MCFV into those of an acutely pathogenic SFFV we introduced these changes into the env gene of an MCFV. The results show that the mutated MCFV is as acutely pathogenic as F-SFFV. We therefore conclude that the modified membrane anchor of gp55 and the change caused by the large deletion are the essential determinants of the high pathogenicity of SFFV.

Substitution of leucine for isoleucine in a sequence highly conserved among retroviral envelope surface glycoproteins attenuates the lytic effect of the Friend murine leukemia virus

Friend murine leukemia virus is a replication-competent retrovirus that contains no oncogene and that exerts lytic and leukemogenic properties. Thus, newborn mice inoculated with Friend murine leukemia virus develop severe early hemolytic anemia before appearance of erythroleukemia. To identify the retroviral determinants regulating these effects, we used chimeric infectious constructions and site-directed point mutations between a virulent Friend murine leukemia virus strain and a naturally occurring variant attenuated in lytic and leukemogenic effects. We found that severe hemolytic anemia was always associated with higher numbers of blood reticulocytes with budding retroviral particles. Furthermore, a remarkably conservative leucine to isoleucine change in the extracellular SU component of the retroviral envelope was sufficient to attenuate this lytic effect. Also, this leucine at position 348 of the envelope precursor protein was located within the only stretch of five amino acids that is conserved in the extracellular SU component of all murine, feline, and primate type C and type D retroviral envelopes. This observation suggested an important structural function for this yet undescribed conserved sequence of the envelope. Lastly, we observed that lytic and leukemogenic effects were attenuated by a deletion of a second repeat in the transcriptional enhancer region of the viral long terminal repeats of the variant strain.

Mechanism of leukemogenesis induced by mink cell focus-forming murine leukemia viruses

The Friend or Moloney mink cell focus-forming (MCF) virus encodes a recombinant-type envelope glycoprotein, gp70, that is closely related to the membrane glycoprotein, gp55, of Friend spleen focus-forming virus (SFFV). We have shown previously that gp55 has the ability to activate cell growth by binding to the cellular receptor for erythropoietin. Here we show that gp70 encoded by either the Friend or Moloney MCF virus also binds to the erythropoietin receptor and that coexpression of the receptor and gp70 in an interleukin-3 (IL-3)-dependent cell line can activate IL-3-independent growth. Furthermore, when the cDNA for the human IL-2 receptor beta chain, which is related by sequence to the erythropoietin receptor, was introduced into this cell line, it became growth factor independent after infection either with SFFV or with one of the two MCF viruses but not with an ecotropic virus. Based on these observations, we propose a mechanism for the early stage of leukemogenesis induced by the MCF-type murine leukemia viruses.

Identification of an epitope encoded in the env gene of Friend murine leukemia virus recognized by anti-Friend virus cytotoxic T lymphocytes

We have previously shown that strong epitopes recognized by anti-Friend virus (FV) cytotoxic T lymphocytes (CTL) in H-2b mice are encoded in both the env and gag/pol regions of the helper friend leukemia virus genome. Two approaches have been used to identify these epitopes. At the nucleic acid level, we have constructed env genes with either of two in-frame deletions: pKR2, an env gene with a 681-bp deletion in the gp70 region and inserted into the pSV2-gpt-1 expression vector; and pKR1, an env gene with an 81-bp deletion in the p15E region and inserted into pSV2-gpt-1. Cell clones were established by transfecting Fisher rat embryo cells with pDb (the H-2Db restriction element), pNEO (for G418 selection) and either pKR1 or pKR2. Db and env gene expression was monitored by immunoprecipitation with polyclonal antibodies or by detection of viral RNA on Northern blots. Expressor cell clones were tested for susceptibility to lysis by polyclonal anti-FV/Db CTL in 51Cr-release assays. Whereas cells expressing pKR1 were lysed to the same extent as cells expressing the intact env gene, cells expressing pKR2 were resistant to lysis, suggesting that all detectable env epitopes are encoded within the 681-bp deletion. Polypeptides representing the two most likely candidate epitopes encoded in this segment were synthesized and tested for their abilities to sensitize FRE cells expressing Db alone for lysis by the CTL. One 17-mer polypeptide, AGTGDRLLNLVQGAYQA [corrected], functioned as a strong CTL epitope in this assay, but the other 18-mer polypeptide was inactive. Studies of the role of this epitope in the immune response to candidate viral vaccines are in progress.

Glycosylation of the envelope glycoprotein from a polytropic murine retrovirus in two different host cells

A polytropic recombinant retrovirus containing the envelope gene of Friend mink cell focus-inducing virus plus the remainder of the genome of an amphoropic murine leukemia virus was propagated on mouse embryo fibroblasts and mink lung cells. Virus particles, metabolically labeled with [2-3H]mannose, were harvested from the culture supernatants and lysed with detergents. The viral envelope glycoprotein was isolated from the lysates by immunoaffinity chromatography and purified by preparative SDS/PAGE. Oligosaccharides were liberated by sequential treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide-N4-(N-acetyl-beta-glucosaminyl) asparagine amidase F and fractionated by high-performance liquid chromatography. Individual glycans were characterized chromatographically, by methylation analyses and in part, by enzymic microsequencing. The results demonstrated that viral glycoproteins, synthesized in mouse embryo fibroblasts, carried as major constituents partially fucosylated diantennary, 2,4- and 2,6-branched triantennary and tetraantennary complex type N-glycans with 0-4 sialic acid residues and only small amounts of high-mannose type species with 5-9 mannose residues. As a characteristic feature, part of the complex type glycans contained additional Gal(alpha 1-3) substituents. Glycoprotein obtained from virions propagated on mink lung cells, contained partially fucosylated diantennary and 2,4-branched triantennary oligosaccharides with 1-3 sialic acid residues, in addition to trace amounts of high-mannose type species with 8 or 9 mannose residues. Thus, the results reveal that predominantly, the complex type N-glycans of the retroviral envelope glycoprotein display cell-specific variations including differences in oligosaccharide branching, sialylation and substitution by additional Gal(alpha 1-3) residues.

The T----C substitution at -198 of the A gamma-globin gene associated with the British form of HPFH generates overlapping recognition sites for two DNA-binding proteins

Defects in the developmental changes of human hemoglobin production characterized by the continued expression of fetal globin during adult life are classified as hereditary persistence of fetal hemoglobin (HPFH). Among the various molecular lesions associated with this phenotype, the non-deletion forms with point mutations in the promoter region are thought to provide mechanistic clues for gamma-globin gene regulation. The natural occurrence of four different base substitutions mapping within six nucleotides of a homopurine.homopyrimidine motif in the upstream promoter region demarcate a potential control element. To assess its importance for transcriptional activity, we compared the -202 (C----G), -198 (T----C) and -196 (C----T) HPFH mutations with the normal sequence in binding studies with nuclear proteins from erythroid and non-erythroid cells. Wildtype DNA and HPFH mutations at -202 or -196 showed only a weak protein interaction of unclear functional significance. In contrast, -198 (T----C) generated overlapping, high-affinity binding sites for two ubiquitous nuclear proteins. One cognate protein was identified as the transcription factor Sp1. The second one was termed NF-G.C as it interacted strongly with the homopolymer poly(dG).poly(dC). The generation of additional recognition sites for trans-acting factors by the -198 HPFH mutation correlated with a modest increase in promoter activity in vitro specifically with nuclear extracts from erythroid cells. The activation appears to be mediated by binding of Sp1, but it requires interaction with an erythroid-specific factor, most likely GF-1. Templates containing the -196 HPFH mutation showed a transcriptional activity identical to wildtype. This suggests that despite the topological proximity of the mutations, the HPFH phenotype may be established by different mechanisms.

Involvement of nuclear factor I-binding sites in control of Akv virus gene expression

The U3 region of Akv murine leukemia virus carries a 99-base-pair repeat that is associated with transcriptional enhancement in murine NIH 3T3 cells. Deletion analysis points to a critical function of a region within the repeat unit related to the recognition sequences for nuclear factor I proteins but distinct from the sites previously analyzed in related viruses. Nuclear proteins binding to the critical site were detected in NIH 3T3 cells and in mouse livers. A protein fraction binding to this site was purified from mouse livers by ion-exchange and DNA affinity chromatography and shown to have nuclear factor I properties. Mutations that caused a partial or complete reduction of the in vitro binding were introduced into an Akv long terminal repeat with one 99-base-pair repeat copy driving a reporter gene, and the expression activities of the mutants in NIH 3T3 cells were found to correspond to their in vitro binding activities. This correlation strongly supports the role of nuclear factor I proteins in Akv expression. Residual expression activity was, however, detected in mutants devoid of in vitro binding. This residual activity may relate to the presence of additional sequences with homology to nuclear factor I binding sites both within and outside the repeat region. The ability of these sites to bind crude and purified protein fractions with nuclear factor I activity was analyzed, and the role of the sites within and outside the repeat region for control of gene expression of Akv and related viruses is discussed.