Human T-cell leukemia virus type 1 pX-I and pX-II open reading frames are dispensable for the immortalization of primary lymphocytes

Human T-cell leukemia virus type 1 (HTLV-1) infects and transforms CD4+ T-lymphocytes both in vivo and in vitro. Although the Tax protein of HTLV-1 has been strongly implicated as a transforming agent, other virally encoded proteins may also play a role in the transformation process. In addition to the rex and tax genes, the pX region of the HTLV-1 genome contains two open reading frames (pX-I and pX-II) which encode the putative viral accessory proteins known as p12I, p30II, and p13II. Mutations in the ACH molecular clone of HTLV-1 that are predicted to abrogate the expression of p12I, p13II and p30II were constructed. These mutations had no effect on viral replication or the immortalization of primary lymphocytes. Although these proteins are dispensable for viral replication and immortalization in vitro, it remains possible that they alter infection in vivo.

Characterization of a human foamy virus 170-kilodalton Env-Bet fusion protein generated by alternative splicing

Primate foamy viruses (FVs) express, in addition to the 130-kDa envelope protein, a 170-kDa glycoprotein, which reacts with antisera specific for the envelope and Bel proteins. We determined the exact nature of this 170-kDa glycoprotein by using the molecularly cloned human FV (HFV). Radioimmunoprecipitation analysis of 293T cells transfected with appropriate expression constructs by using antisera specific for the HFV Env, Bel1, and Bel2 proteins, as well as reverse transcription-PCR analysis of HFV-infected cells, demonstrated that this protein is an Env-Bet fusion protein that is secreted into the supernatant. However, it is only loosely associated, or not associated, with viral particles. gp170 is generated by an alternatively spliced Env mRNA using a splice donor and splice acceptor pair localized within the env open reading frame (ORF), which is normally used to generate Bell and Bet transcripts derived from the internal promoter within the env ORF. gp170 is expressed at a level 30 to 50% of the Env precursor gp130. However, it alone does not confer infectivity to HFV particles, because capsids derived from proviruses expressing only the gp170 were not released into the supernatant. In contrast, viruses derived from proviral clones deficient in gp170 expression showed similar in vitro infectivity and replication kinetics to wild-type virus. Furthermore, both types of viruses were inactivated to a similar extent by neutralizing sera, indicating that shedding of gp170 probably does not affect the humoral immune response in the infected host.

HIV-1 Vpr interacts with a human 34-kDa mov34 homologue, a cellular factor linked to the G2/M phase transition of the mammalian cell cycle

Several important and possibly interrelated functions have been identified for the HIV-1 accessory gene product Vpr. These include import of the HIV reverse transcription complex into the nucleus of nondividing cells, cellular differentiation including cell cycle arrest at the G2/M phase border, immune suppression, and enhancement of virus replication. We have cloned a candidate Vpr ligand, termed human Vpr interacting protein (hVIP/MOV34), by using a yeast two-hybrid assay. This gene is homologous to a simultaneously identified 34-kDa human mov34 homologue. The MOV34 family includes proteins that function as transcriptional and proteolytic regulators of cell growth and differentiation. We demonstrate direct interactions between the putative ligand hVIP/MOV34 and Vpr in vitro and in vivo. hVIP/MOV34 localizes to the nucleus and appears to function as a component of the cell cycle cascade. We observe an association between the induction of cell cycle arrest at the G2/M phase border by Vpr and a change in the subcellular localization of hVIP/MOV34 from a nuclear to a perinuclear localization. This was further associated with the inhibition of maturation promoting factor-associated histone H1 kinase activity. We conclude that hVIP/MOV34 is involved in the regulation of the cell cycle and a likely cellular cofactor for HIV-1 Vpr.

Identification and functional characterization of a high-affinity Bel-1 DNA binding site located in the human foamy virus internal promoter

The transcription of genes carried by primate foamy viruses is dependent on two distinct promoter elements. These are the long terminal repeat (LTR) promoter, which regulates expression of the viral structural proteins, and a second internal promoter, located towards the 3' end of the env gene, that directs expression of the viral auxiliary proteins. One of these auxiliary proteins is a potent transcriptional transactivator, termed Bel-1 in human foamy virus (HFV) and Tas or Taf in the related simian foamy viruses, that is critical for foamy virus replication. Previously, it has been demonstrated that the LTR promoter element of HFV contains a DNA binding site for Bel-1 that is critical for transcriptional activation (F. He, W. S. Blair, J. Fukushima, and B. R. Cullen, J. Virol. 70:3902-3908, 1996). Here, we extended this earlier work by using methylation interference analysis to identify and characterize the Bel-1 DNA binding sites located in the HFV LTR and internal promoter elements. Based on these data, we propose a minimal, 25-bp DNA binding site for Bel-1, derived from the HFV internal promoter element, and show that this short DNA sequence mediates efficient Bel-1 binding both in vitro and in vivo. We further demonstrate that, as determined by both in vitro and in vivo assays, the Bel-1 target site located within the HFV internal promoter binds Bel-1 with a significantly higher affinity than the cap-proximal Bel-1 target site located in the LTR promoter. This result may provide a mechanistic explanation for the observation that the internal promoter is activated significantly earlier than the LTR promoter during the foamy virus life cycle.

Intracisternal A-type particles express their proteinase in a separate reading frame by translational frameshifting, similar to D-type retroviruses

Intracisternal A-type particles (IAP) are defective endogenous retroviruses that accumulate in the endoplasmic reticulum of rodent cells. IAP genomes share extensive sequence homologies with D-type retroviruses, but were presumed to express the viral proteinase (PR) as part of the gag open reading frame (ORF) while D-type retroviruses express PR in a separate ORF. Here we show that expression of the murine IAP element MIA14 yields three major translation products, corresponding to the Gag, Gag-PR, and Gag-PR-Pol polyproteins. Sequence analysis revealed that MIA14 PR is encoded in its own reading frame, separate from gag and pol. Frameshifting occurred with an efficiency of approximately 25% between the gag and pro ORFs and 35% between pro and pol. The region containing the putative gag-pro frameshift signal consists of a heptanucleotide slippery sequence (A6C) and a stem-loop structure probably forming a pseudoknot. Deletion of this structure element almost completely abolished frameshifting. Insertion of an additional base next to the frameshift signal placed gag and pro in the same ORF and resulted in predominant formation of Gag-PR and Gag-PR-Pol polyproteins which were not processed following in vitro translation. Expression of a similar construct in tissue culture cells, on the other hand, led to efficient intracellular processing of the mutant polyproteins.

Functional RT and IN incorporated into HIV-1 particles independently of the Gag/Pol precursor protein

The expression and incorporation of retroviral enzymes into virions in the form of Gag/Pol precursor polyproteins is believed to be important for the assembly of infectious viral particles. HIV-1 encodes a 160 kDa Gag/Pol precursor that includes Gag, protease (PR), reverse transcriptase (RT) and integrase (IN). We have developed the use of HIV accessory proteins (Vpr and Vpx) as vehicles to incorporate protein of both viral and non-viral origin into virions by expression in trans as heterologous fusion proteins (Wu et al., 1995, 1996a). To analyze the role of Gag/Pol in the formation of infectious virions, we incorporated RT and IN into HIV-1 particles in trans, as fusion partners of viral protein R (Vpr). Virions derived from an RT and IN minus proviral clone were infectious and replicated through a complete cycle of infection when RT and IN proteins were provided in trans. These results demonstrate that functional RT and IN proteins can be provided in trans, and that their expression and incorporation into virions as components of Gag/Pol are not required for the formation of infectious virions. Thus, for the first time, we have demonstrated for a human pathogenic retrovirus that processes of assembly and the function of critical viral enzymes can be unlinked. This finding will provide unique opportunities to explore retroviral RT/IN function and the role of Gag/Pol in the formation of infectious virions in the context of a replicating virus (in vivo).

Complementation of integrase function in HIV-1 virions

Proviral integration is essential for HIV-1 replication and represents an important potential target for antiviral drug design. Although much is known about the integration process from studies of purified integrase (IN) protein and synthetic target DNA, provirus formation in virally infected cells remains incompletely understood since reconstituted in vitro assays do not fully reproduce in vivo integration events. We have developed a novel experimental system in which IN-mutant HIV-1 molecular clones are complemented in trans by Vpr-IN fusion proteins, thereby enabling the study of IN function in replicating viruses. Using this approach we found that (i) Vpr-linked IN is efficiently packaged into virions independent of the Gag-Pol polyprotein, (ii) fusion proteins containing a natural RT/IN processing site are cleaved by the viral protease and (iii) only the cleaved IN protein complements IN-defective HIV-1 efficiently. Vpr-mediated packaging restored IN function to a wide variety of IN-deficient HIV-1 strains including zinc finger, catalytic core and C-terminal domain mutants as well as viruses from which IN was completely deleted. Furthermore, trans complemented IN protein mediated a bona fide integration reaction, as demonstrated by the precise processing of proviral ends (5'-TG...CA-3') and the generation of an HIV-1-specific (5 bp) duplication of adjoining host sequences. Intragenic complementation between IN mutants defective in different protein domains was also observed, thereby providing the first evidence for IN multimerization in vivo.

Infection of the germ line by retroviral particles produced in the follicle cells: a possible mechanism for the mobilization of the gypsy retroelement of Drosophila

The gypsy retroelement of Drosophila moves at high frequency in the germ line of the progeny of females carrying a mutation in the flamenco (flam) gene. This high rate of de novo insertion correlates with elevated accumulation of full-length gypsy RNA in the ovaries of these females, as well as the presence of an env-specific RNA. We have prepared monoclonal antibodies against the gypsy Pol and Env products and found that these proteins are expressed in the ovaries of flam females and processed in the manner characteristic of vertebrate retroviruses. The Pol proteins are expressed in both follicle and nurse cells, but they do not accumulate at detectable levels in the oocyte. The Env proteins are expressed exclusively in the follicle cells starting at stage 9 of oogenesis, where they accumulate in the secretory apparatus of the endoplasmic reticulum. They then migrate to the inner side of the cytoplasmic membrane where they assemble into viral particles. These particles can be observed in the perivitelline space starting at stage 10 by immunoelectron microscopy using anti-Env antibodies. We propose a model to explain flamenco-mediated induction of gypsy mobilization that involves the synthesis of gypsy viral particles in the follicle cells, from where they leave and infect the oocyte, thus explaining gypsy insertion into the germ line of the subsequent generation.

Plasma membrane targeting of chimeric intracisternal A-type particle polyproteins leads to particle release and specific activation of the viral proteinase

Retrovirus morphogenesis involves assembly of structural Gag polyproteins with subsequent budding from the plasma membrane, followed by proteolytic cleavage by the viral proteinase (PR) and extracellular maturation to the infectious virion. Intracisternal A-type particles (IAPs) are defective retroviruses that assemble and bud at the membranes of the endoplasmic reticulum (ER), where they remain as immature particles consisting exclusively of uncleaved polyproteins. To analyze requirements for intracellular polyprotein transport and PR activation, we constructed deletion and substitution mutations in the IAP gag gene, including the putative ER-targeting signal. Mutant polyproteins were transported to various intracellular locations, including the nucleus, the cytoplasm, the ER, and the plasma membrane. Interestingly, assembly of capsid-like particle structures occurred at almost all sites. However, only those polyproteins transported to the plasma membrane were efficiently and specifically cleaved by viral PR, with cleavage occurring predominantly within the virus particle. Thus, at least in the experimental system presented here, retroviral particle assembly can occur at almost any location within the cell, while polyprotein processing and, consequently, virion maturation are confined to a specific cellular site. These results suggest that a factor restricted to the plasma membrane is required to trigger PR activation and maturation of infectious retroviruses.

A human B-lymphoblastoid cell line constitutively producing Epstein-Barr herpesvirus and JHK retrovirus

The human B-lymphoblastoid cell line, designated JHK-3, with pre-B-cell characteristics, chronically produces two viruses, Epstein-Barr virus (EBV) and JHK virus, an apparently novel retrovirus. The JHK-3 cells are much more productive of extracellular EBV than the high-producer marmoset line B95-8. The extracellular virus of the JHK-3 EBV strain is relatively fragile, more broadly dispersed in an ultracentrifuged sucrose gradient than the B95-8 EBV and more susceptible to disruption by combined treatment with urea and dithiothreitol. By restriction fragment length polymorphism analysis, the JHK-3 EBV strain resembles the EBV strain FF-41. The JHK-3 cells also produce an incompletely characterized, relatively fragile, enveloped, icosahedral RNA virus that contains Mn(++)-dependent reverse transcriptase. JHK virions measure 85 nm in ultrathin sections, much smaller than other Retroviridae. The JHK virus exhibits a distinctive morphogenesis, most nearly resembling C-type retroviruses. The JHK-3 cell line provides a human cell model for investigating virus/virus interactions and their pathogenetic affects on host cells which chronically and simultaneously produce DNA and RNA viruses.

High degree of sensitivity of the simian immunodeficiency virus (SIVmac) envelope glycoprotein subunit association to amino acid changes in the glycoprotein 41 ectodomain

The infection of macaques by simian immunodeficiency virus (SIVmac) represents an attractive model to study the pathogenic determinants of primate and human immunodeficiency viruses. The utility of this model would be enhanced if genetic changes in human immunodeficiency virus (HIV-1) associated with interesting in vitro properties would, when introduced into SIVmac, result in similar phenotypes. In this study, we introduced amino acid changes into the SIVmac239 envelope glycoproteins that, in the context of the HIV-1 envelope glycoproteins, disproportionately attenuated in vitro cytopathic effects compared with the viral replication rate. Amino acid changes in the SIVmac239 gp41 ectodomain altered the noncovalent association of the gp120 and gp41 glycoproteins significantly more than did analogous changes in the HIV-1 envelope glycoproteins. Decreases in the affinity of the gp120-gp41 interaction were observed and were associated with a dramatic attenuation of virus replication not seen in the HIV-1 studies. The increased sensitivity of the SIVmac gp120-gp41 interaction to amino acid changes presents an obstacle to the direct extension of results obtained with the HIV-1 envelope glycoproteins to the SIVmacaque model.

Reverse transcriptase activity in chicken embryo fibroblast culture supernatants is associated with particles containing endogenous avian retrovirus EAV-0 RNA

We have recently shown that live attenuated virus vaccines produced on chicken-derived cells contain low levels of particle-associated reverse transcriptase (RT). In both virus and corresponding control harvests produced on chicken embryo fibroblasts, these activities were present at significantly higher concentrations than in the vaccines. In order to identify the putative retrovirus sequence responsible for this activity, a novel method for the selective PCR amplification of particle-associated retrovirus RNA that uses DNA primers complementary to the primer binding sites of the known exogenous retroviruses in combination with an anchor primer was applied. A product of the endogenous avian retrovirus family EAV-0, termed EAV-0(B1), was reproducibly generated with a tRNA(Trp)-derived primer from the RT peak fraction of a sucrose density gradient run with a harvest of a live attenuated measles vaccine. In contrast, no products were detected with primers derived from tRNA(Pro), tRNA(Lys)1,2 or tRNA(Lys)3. In the same fraction, genomic RNA of EAV-0(B1) was demonstrated by long PCR. Analysis of several sucrose density gradients from different harvests of various manufacturers demonstrated accumulation of, and colocalization with, RT activity for the EAV-0(B1) RNA but not for a chicken cellular mRNA. Synthesis of cDNA from EAV-0(B1) RNA was shown by endogenous RT reaction. Furthermore, complexes of naturally primed EAV-0(B1) RNA with RT were demonstrated. Taken together, these data strongly suggest that EAV-0 is able to produce virus-like particles with an active RT.

Analysis of cleavage site mutations between the NC and PR Gag domains of Rous sarcoma virus

In retroviruses, the viral protease (PR) is released as a mature protein by cleavage of Gag, Gag-Pro, or Gag-Pro-Pol precursor polypeptides. In avian sarcoma and leukemia viruses (ASLV), PR forms the C-terminal domain of Gag. Based on the properties of a mutation (cs22) in the cleavage site between the upstream NC domain and the PR domain, the proteolytic liberation of PR previously was inferred to be essential for processing of Gag and Pol proteins. To study this process in more detail, we have analyzed the effects that several mutations at the NC-PR cleavage site have on proteolytic processing in virus-like particles expressed in COS and quail cells. Mutant Gag proteins carrying the same mutations also were synthesized in vitro and tested for processing with purified PR. In both types of studies, N-terminal sequencing of the liberated PR domain was carried out to exactly identify the site of cleavage. Finally, synthetic peptides corresponding to the mutant proteins were assessed for the ability to act as substrates for PR. The results were all consistent and led to the following conclusions. (i) In vivo, if normal processing between NC and PR is prevented by mutations, limited cleavage occurs at a previously unrecognized alternative site three amino acids downstream, i.e., in PR. This N-terminally truncated PR is inactive as an enzyme, as inferred from the global processing defect in cs22 and a similar mutant. (ii) In Gag proteins translated in vitro, purified PR cleaves this alternative site as rapidly as it does the wild-type site. (iii) Contrary to previously accepted rules describing retroviral cleavage sites, an isoleucine residue placed at the P1 position of the NC-PR cleavage site does not hinder normal processing. (iv) A proline residue placed at the P2 position in this cleavage site blocks normal processing.

The human T-cell lymphotropic virus type 1 Tof protein contains a bipartite nuclear localization signal that is able to functionally replace the amino-terminal domain of Rex

The X region of human T-cell lymphotropic virus type 1 (HTLV-1) encodes two nucleolar/nuclear proteins, the posttranscriptional regulator of mRNA expression Rex and a protein of unknown function named Tof. To gain insight into the possible biological role of Tof, we investigated the mechanism governing its intracellular trafficking and identified its nucleolar/nuclear localization signal (NLS). Mutational analysis of Tof revealed that its NLS was located between amino acids 71 and 98 and contained two arginine-rich domains that functioned in an interdependent manner. Studies of Tof-Rex hybrid proteins showed that the Tof NLS could functionally replace the NLS of Rex at the level of nuclear targeting. As the NLS of Rex is known to mediate its interaction with its RNA target, the Rex-responsive element (RXRE), we tested whether the NLS of Tof could replace that of Rex in mediating activation of a RXRE-containing mRNA. Results showed that the NLS of Tof was indeed able to mediate activation of RXRE-containing mRNAs, suggesting that Tof itself may function as a regulator of RNA expression and utilization. A comparison of their compartmentalization in response to actinomycin D treatment indicated that Tof did not share Rex's shuttling pathway. Expression of Tof from its natural multiply spliced mRNA required the presence of Rex, suggesting that Tof may regulate viral or cellular mRNA expression during the later stages of viral replication.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

RNase H domain of Moloney murine leukemia virus reverse transcriptase retains activity but requires the polymerase domain for specificity

The reverse transcriptase-associated RNase H activity of Moloney murine leukemia virus specifically cleaves within the polypurine tract region of the viral genome to generate the primer for plus-strand DNA synthesis and removes the tRNA primer after minus-strand initiation by preferentially cleaving the RNA one nucleotide before the RNA-DNA junction. Moreover, the enzyme is unable to cleave the extended tRNA substrate at the RNA-DNA junction even at high enzyme concentrations. The RNase H domain of the reverse transcriptase was expressed as a glutathione S-transferase fusion protein and purified from Escherichia coli extracts. Following removal of the glutathione S-transferase portion of the protein, the specificity of the isolated RNase H domain was determined in the plus-strand primer reaction and in the tRNA primer removal reaction. Although the isolated domain lacked specificity in both cases, it was still unable to cleave the tRNA substrate precisely at the RNA-DNA junction. Specificity in both cases could be restored by adding back a truncated form of Moloney murine leukemia virus reverse transcriptase lacking the RNase H domain. These results implicate the polymerase domain as a specificity determinant for the RNase H activity of reverse transcriptase. The isolated RNase H domain had higher activity in the presence of Mn2+ than in the presence of Mg2+, but neither the RNase H domain alone nor the RNase H domain coupled to the polymerase domain in wild-type protein exhibited the normal cleavage specificities in the presence of the nonphysiological divalent cation.

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.

The human T-cell leukemia/lymphotropic virus type 1 p12I proteins bind the interleukin-2 receptor beta and gammac chains and affects their expression on the cell surface

p12I is a small hydrophobic protein encoded by the human T-cell leukemia/lymphotropic virus type 1 (HTLV-1) that interacts with the 16-kDa component of the H+ vacuolar ATPase and cooperates with bovine papillomavirus 1 E5 oncoprotein in cell transformation. Just as an important step in E5 action appears to be its binding to the platelet-derived growth factor receptor, it was found that p12I binds specifically to both the beta and gamma(c) chains of the interleukin-2 receptor (IL-2R). The IL-2R beta and gamma(c) chains associated with p12I are endoglycosidase-H sensitive, suggesting that their interaction occurs in a pre-Golgi compartment. p12I stabilizes the immature forms of the IL-2R beta and gamma(c) chains and decreases their cell surface expression. The interactions of p12I with IL-2R beta and gamma(c) may have important implications in the immunosuppressive effect of HTLV-1 in vivo as well as in the ligand-independent HTLV-1-mediated T-cell proliferation.

The human foamy virus Bel-1 transcription factor is a sequence-specific DNA binding protein

The Bel-1 transcriptional transactivator encoded by human foamy virus (HFV) can efficiently activate gene expression directed by both the HFV long terminal repeat (LTR) and internal (Int) promoter elements. By DNA footprinting and gel retardation analysis, we demonstrate that Bel-1 can specifically bind to discrete sites in both the LTR and Int promoter elements in vitro. However, transactivation of the HFV LTR by Bel-1 was observed to require not only the promoter-proximal Bel-1 binding site identified in vitro but also additional promoter-distal sequences. These data suggest that Bel-1 binding is necessary but not sufficient for efficient transactivation of Bel-1-responsive promoters in mammalian cells and therefore raise the possibility that Bel-1 function may require the action of a cellular DNA binding protein(s). Importantly, these data demonstrate that Bel-1 is unique among retroviral regulatory proteins in being a sequence-specific DNA binding protein.

Role of polymorphic residues of human leucocyte antigen-DR molecules on the binding of human immunodeficiency virus peptides

A study was made of the binding properties of 96 human immunodeficiency virus peptides to human leucocyte antigen (HLA)-DR1 and HLA-DR103 molecules, which differ by three amino acids at positions 67, 70 and 71 in the beta chains. The affinity of the peptides was characterized by their inhibitory concentrations in competitive binding assays which displace half of the labelled influenza haemagglutinin peptide HA306-318 (IC50). Among the high-affinity peptides (IC50 < or = 1 microM), seven bound to DR1, three to DR103 and five equally well to both alleles (promiscuous peptides). Thirty-two other peptides showed medium or low affinity for DR molecules. The role of polymorphic residues was analysed using six mutated DR molecules, intermediates between DR1 and DR103 and differing by one or two substitutions at positions 67, 70 or 71. We reached the same conclusions when using DR1-specific or DR103-specific peptides: modification of residue 70 had no effect on peptide affinity, but single substitution at positions 67 or 71 decreased the allele specificity of the peptides while double substitution at 67 and 71 completely reversed the peptide specificity. In functional assays, DR-binding peptides are able to outcompete specific T-cell proliferation. Furthermore, modification at position 67 or 70 significantly affects the T-cell response and mutation at position 71 abolishes completely the T-cell proliferation. Thus, the polymorphic positions 67 and 71 contributed to the peptide binding with direct effects on T-cell receptor (TCR) recognition while position 70 seems to be mostly engaged in TCR interactions. Furthermore, our results suggest that polymorphic residues may select allele-specific peptides and also influence the conformation of promiscuous peptides.

Analysis of the determinants of neurotropism and neurotoxicity of HFV in transgenic mice

The Bel-1 protein of human foamy virus (HFV) is a transactivator acting on the U3 region of the long terminal repeat and on an internal promoter (IP) immediately upstream of the bel genes. An HFV transgene called delta gpe, containing both promoters and all bel genes, is expressed in the central nervous system and induces neurodegeneration in mice. To dissect the role of individual promoters and bel genes on transgene expression and neurotoxicity we generated transgenic mice with a construct termed pL-bel1, which lacks the IP and the ancillary genes except bel-1. L-bel1 mice transcribed the HFV transgene in more tissues than delta gpe mice, suggesting that CNS specificity is dictated by cis-acting elements not present in the pLbel-1 construct. Unlike delta gpe mice, L-bel1 mice did not develop neurodegenerative changes and did not show induction of nitric oxide synthase expression, although both strains expressed Bel-1 in the brain. Therefore, Bel-1 expression is not sufficient for neurotoxicity. Our results suggest that Bet, a fusion protein between bel-1 and bel-2 which is highly expressed in delta gpe but not in L-bel1 mice, is a candidate for neurotoxicity.

Retroviral integrases and their cousins

The recently determined structures of the catalytic domains of HIV integrase, avian sarcoma virus integrase and the Mu transposase are strikingly similar to each other and also exhibit significant similarity to several nucleases. All these enzymes of cut polynucleotides, leaving 3'OH and 5'PO4 groups. The integrase and transposase also possess a strand-transfer activity that splices DNA. The structural similarities among members of this superfamily of polynucleotidyl transferases suggest that they share a similar mechanism of catalysis.

The transcriptional transactivator of simian foamy virus 1 binds to a DNA target element in the viral internal promoter

The transcriptional transactivator (Tas) of simian foamy virus type 1 strongly augments gene expression directed by both the promoter in the viral long terminal repeat and the newly discovered internal promoter located within the env gene. A region of 121 bp, located immediately 5' to the TATA box in the internal promoter, is required for transactivation by Tas. The present study aimed to identify the precise Tas-responsive target(s) in this region and to determine the role of Tas in transcriptional regulation. By analysis of both clustered-site mutations and hybrid promoters in transient expression assays in murine and simian cells, two separate sequence elements within this 121-bp region were shown to be Tas-dependent transcriptional enhancers. These targets, each < 30 bp in length and displaying no apparent sequence homology one to the other, are designated the promoter-proximal and promoter-distal elements. By means of the gel electrophoresis mobility-shift assays, using purified glutathione S-transferase-Tas fusion protein expressed in Escherichia coli, the target proximal to the TATA box exhibited strong binding to glutathione S-transferase-Tas, whereas the distal element appears not to bind. In addition, footprint analysis revealed that 26 bp in the promoter proximal element was protected by glutathione S-transferase-Tas from DNase I. We propose a model for transactivation of the simian foamy virus type 1 internal promoter in which Tas interacts directly with the proximal target element positioned immediately 5' to the TATA box. In this model, Tas attached to this element is presumed to interact with a component(s) of the cellular RNA polymerase II initiation complex and thereby enhance transcription directed by the viral internal promoter.

Isolation and characterization of infectious full-length DNA clones of chimpanzee foamy viruses SFV6 and SFV7: evidence for a Taf-dependent internal promoter

We have cloned complete viral genomes directly from Hirt supernatant DNAs of simian foamy virus types 6 and 7 (SFV6 and SFV7) -infected cells. These clones were shown to be infectious by transfection into cells and subsequent infection of susceptible cells either by cocultivation or by passage of cell-free supernatants. The presence of virus particles, suggested by a typical cytopathic effect, was confirmed by electron microscopy. These viruses were characterized at different levels of the replication cycle. The proviral genomes revealed a taf deletion comparable to that previously described in the human foamy virus (HFV) bel1 gene. Analysis of viral RNAs revealed similar patterns of transcripts for SFV6- and SFV7-infected cells, with predominant expression of accessory genes. Characteristic major viral polypeptides were identified by radioimmunoprecipitation for both isolates. Sequences homologous to the gene encoding Taf and to a potential internal promoter were identified in the infectious clones and subcloned into expression vectors. Their functional properties were tested by transfection assays, which provided evidence for the presence of a Taf-dependent internal promoter in both SFV6 and SFV7 isolates.

Infectious proviral clones of chimpanzee foamy virus (SFVcpz) generated by long PCR reveal close functional relatedness to human foamy virus

Infectious proviral clones of simian foamy virus isolated from chimpanzee (SFVcpz) were generated by long PCR. Two overlapping fragments representing the complete provirus were amplified from genomic DNA of infected cells. Four 8.8-kbp amplimers extending from base 1 of the provirus into the env gene and five 4.45-kbp amplimers reaching from env to the end of the 3'-LTR were cloned into pCR II. Subsequently, the proviral fragments were combined in a chessboard manner to generate 20 plasmids containing full-length proviral DNA. Four plasmids produced infectious virus after transfection of susceptible cells. A distinct proviral form bearing a deletion in the transactivator gene joining both exons of a second regulatory gene present in wild-type foamy virus-infected cells started to emerge 48 hr after transfection of BHK cells with infectious SFVcpz DNA. This observation supports a novel hypothesis to explain establishment of foamy virus latency. The transactivator protein Taf of SFVcpz transcomplemented for the homologous protein Bel-1 of the unique human foamy virus isolate (HFV) and Bel-1 exhibited the reciprocal activity, suggesting that HFV could represent a variant of chimpanzee foamy virus.

Characterization of the calmodulin binding domain of SIV transmembrane glycoprotein by NMR and CD spectroscopy

Recent experimental evidence has shown that the C-terminal peptide of the HIV/SIV transmembrane glycoprotein 41 (gp41) can bind very tightly to calmodulin (CaM). These findings imply a potential mechanism for HIV/SIV cytopathogenesis, which involves the uncoupling of some critical cellular signal transduction pathways that are normally mediated by CaM. Here, we present circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy studies of a 28-residue synthetic peptide, SIV-L, corresponding to the C-terminal portion of the SIV transmembrane glycoprotein gp41. CD studies recorded in aqueous solution show a dramatic increase in the amount of alpha-helical structure of the SIV-L peptide upon binding to calcium-CaM. Two-dimensional NMR experiments were performed to determine the secondary structure of the peptide in 25% aqueous trifluoroethanol solution. In this alpha-helix inducing solvent, the observed nuclear Overhauser effects, as well as the alpha 1H and alpha 13C chemical shift changes, demonstrate that a continuous alpha-helix is formed from W3 to L28, although there is some distortion around P17. This result is in accordance with those obtained for many other CaM-binding peptides. Subsequent one-dimensional NMR titration experiments of calcium-CaM and the SIV-L peptide suggest that the peptide can bind to CaM with a 1:1 stoichiometry and that the peptide binding involves both the N- and C-lobe of CaM. However, gel mobility shift assays suggest that the peptide CaM interaction may be more complicated, as oligomeric forms of CaM and the SIV-L peptide were found. These studies provide a potential molecular basis for HIV/SIV cytopathogenesis.

Human foamy virus Bel1 transactivator contains a bipartite nuclear localization determinant which is sensitive to protein context and triple multimerization domains

The Bel1 protein of human foamy virus is a 300-amino-acid nuclear regulatory protein which transactivates the gene expression directed by the homologous long terminal repeat and the human immunodeficiency virus type 1 long terminal repeat. While previous reports suggested that the single basic domain of Bel1 from residues 211 to 222 and/or 209 to 226 is necessary and sufficient for efficient nuclear localization (L. K. Venkatesh, C. Yang, P. A. Theodorakis, and G. Chinnandurai, J. Virol. 67:161-169, 1993; F. He, J. D. Sun, E. D. Garrett, and B. R. Cullen, J. Virol. 67:1896-1904, 1993), our recent data showed that another basic domain, from amino acid residues 199 to 200, is also required for nuclear localization of Bel1 (C. W. Lee, C. Jun, K. J. Lee, and Y. C. Sung, J. Virol. 68:2708-2719, 1994). To clarify this discrepancy, we constructed various bel1-lacZ chimeric constructs and several linker insertion mutants and determined their subcellular localization. When the region of Bel1 containing basic domains was placed at an internal site of the lacZ gene, the nuclear localization signal (NLS) of Bel1 consisted of two discontinuous basic regions separated by an intervening sequence. Moreover, insertion of specific amino acids between two basic regions disrupted the activity of the Bel1 NLS. On the other hand, Bel1 residues 199 and 200 were not required to direct the Bel1-beta-galactosidase chimeric protein to the nucleus when the Bel1 NLS was appended to the amino terminus of beta-galactosidase. These results indicate that the function of the Bel1 NLS is sensitive to the protein context within which the sequence is present. In addition, we demonstrated that the Bel1 protein forms a multimeric complex in the nuclei of mammalian cells by using a sensitive in vivo protein-protein interaction assay. Mutational analyses revealed that the regions which mediate multimer formation map to three domains of Bel1, i.e., residues 1 to 31, 42 to 82, and 82 to 111. Furthermore, our results show that the region of Bel1 from residues 202 to 226 prevents Bel1 from forming a multimeric complex.

Beta cell-specific expression of retroviral mRNAs and group-specific antigen and the development of beta cell-specific autoimmunity in non-obese diabetic mice

Non-obese diabetic (NOD) mice spontaneously develop autoimmune type 1 diabetes. Earlier studies have shown that retroviruses appear to be associated with the development of the disease in these animals. This investigation was initiated to determine whether any retroviral genes are specifically expressed in pancreatic beta cells from NOD mice, in contrast to their non-diabetic, parental strain, ICR mice. Host chromosomal DNAs from pancreatic islets, kidneys, hearts, and stomachs of NOD and ICR mice contained an equal amount of A-type retroviral genome (DNA); however, A-type retroviral gag, pol, and env mRNAs were detected in only the pancreatic islets from NOD mice. Furthermore, group-specific retroviral antigen (p73 of A-type--gag gene product) was found by immunofluorescent staining using anti-p73 antibody in only pancreatic beta cells from NOD mice. On the basis of these observations, we suggest that tissue and strain differences in transcription of the retroviral genome and beta cell-specific expression of A-type retroviral group-specific antigen p73 in NOD mice may be involved in the initiation of beta cell-specific autoimmunity leading to type 1 diabetes in these animals.

Characterization of the internal promoter of simian foamy viruses

Simian and human foamy viruses (HFV and SFV), genetically related members of the spumavirus genus of retroviruses, have complex genome structures which encode the gag, pol, and env genes for virion proteins as well as additional open reading frames. One of these open reading frames is a viral transactivator, encoded by genes designated taf for SFV and bel-1 for HFV, which augments transcription directed by the long terminal repeat (LTR) through cis-acting targets in the U3 domain of the LTR. Recently, an internal transcriptional promoter has been identified in sequences within the 3' end of the HFV env gene (M. Lochelt, W. Muranyi, and R. M. Flugel, Proc. Natl. Acad. Sci. 90:7317-7321, 1993). We have demonstrated by using transient expression assays in several tissue culture cell lines and by analyzing viral transcripts in infected cells that SFV-1 from a rhesus macaque and SFV-3 from an African green monkey also encode an internal promoter in the env gene. Transcription directed by the internal promoters of SFV-1 and SFV-3 is activated by the taf-1 and taf-3 gene products, respectively, in several cell types. The importance of a TATA box for the SFV-1 internal promoter was established by site-specific mutagenesis, and the 5' ends of transcripts initiating in the internal promoter have been determined. cis-acting sequences in the SFV-1 env gene required for the response to taf-1 are contained within a 121-bp element located 5' to the TATA box in the internal promoter. This taf-1-responsive element in the internal promoter functions in a position- and orientation-independent fashion in a heterologous promoter and thus has the properties of an enhancer which depends on taf-1 activity. Alignments reveal that the SFV-1 internal promoter and the SFV-1 LTR have little sequence relatedness. Cross-transactivation studies show that the transactivators of SFV-1 and HFV function on the internal promoter and LTR of the homologous virus but not on the heterologous virus. In summary, the genomes of simian and human foamy viruses direct viral transcription through both the promoter in the LTR and an internal promoter within the env gene, and each promoter contains unique enhancer-like elements regulated by the viral transactivator.

Differential effect of tumor necrosis factor-alpha and herpes simplex virus type 1 on the Tat-targeted inhibition of human immunodeficiency virus type 1 replication

In this study, we have examined whether the Tat antagonist can inhibit human immunodeficiency virus type 1 (HIV-1) replication in the presence of cofactors that can activate transcription of HIV-1 provirus by an NF-kappa B-mediated mechanism, such as tumor necrosis factor-alpha (TNF-alpha) or herpes simplex virus type 1 (HSV-1) infection. As a prototype, we have chosen a low-molecular-weight Tat inhibitor, Ro5-3335, and analyzed its effect on HIV-1 replication in the presence of TNF-alpha and HSV-1 infection in acutely infected peripheral blood lymphocytes (PBLs) and T cells. Ro5-3335 inhibited HIV-1 replication both in CEM-174 cells and in PBLs, but the magnitude of the inhibition was inversely related to viral inoculum and the inhibition was only temporary; viral replication resumed at later times postinfection in spite of the continuous presence of the drug. In contrast, Ro5-3335 suppressed TNF-alpha-induced activation of HIV-1 replication in chronically infected T cells and monocytes that both expressed only low levels of HIV-1 constitutively, while its effect in high-expressing OM-10.1 cells was negligible in the presence of TNF-alpha. The inhibition of HIV-1 replication by Ro5-3335 was specific for the Tat-mediated effect and this drug was not able to inhibit the TNF-alpha-induced expression of the tat-defective HIV-1 provirus. In contrast to TNF-alpha, HSV-1-stimulated HIV-1 expression in the ACH-2 cells was effectively inhibited in the presence of Ro5-3335. These results demonstrate that Tat plays an essential role in HSV-1-mediated activation of HIV-1 provirus, while the TNF-alpha complementation of Tat shows cell-type specificity. These observations suggest that inhibition of the Tat function alone may not be sufficient for an effective anti-HIV-1 inhibition.

HIV-2 viral protein X association with the GAG p27 capsid protein

VPX is a 16 kDa accessory protein expressed in cells infected with HIV-2 and most SIV strains and is packaged into virus particles. In order to define the requirements for incorporation of VPX into virions, VPX and HIV-2 GAG-POL were expressed independently from a vaccinia virus-based transient expression system. Under these conditions, VPX was exported from transfected cells only when coexpressed with the HIV-2 GAG-POL plasmid. A 27 kDa protein coprecipitating with VPX was found to have an identical electrophoretic mobility as the GAG capsid protein, and reacted with an anti-GAG antiserum. Coexpression of VPX and GAG-POL resulted in virus-like particles containing both proteins, as determined by sucrose gradient analyses. Expression of VPX and HIV-2 GAG without POL gave similar results. VPX association with HIV-2 GAG p27 capsid protein was specific, since no association was found with the HIV-1 GAG p25/p24 capsid protein.

Function of the cytoplasmic domain of a retroviral transmembrane protein: p15E-p2E cleavage activates the membrane fusion capability of the murine leukemia virus Env protein

In the murine leukemia viruses (MuLVs), the Env complex is initially cleaved by a cellular protease into gp70SU and pre15ETM. After the virus particle is released from the cell, the C-terminal 16 residues are removed from the cytoplasmic domain of pre15E by the viral protease, yielding the mature p15ETM and p2E. We have investigated the function of this cleavage by generating a Moloney MuLV mutant, termed p2E-, in which the Env coding region terminates at the cleavage site. This mutant synthesizes only the truncated, mature form of TM rather than its extended precursor. When cells expressing this truncated Env protein are cocultivated with NIH 3T3 cells, they induce rapid cell-cell fusion. Thus, the truncated form, which is normally found in virions but not in virus-producing cells, is capable of causing membrane fusion. We conclude that the 16-residue p2E tail inhibits this activity of Env until the virus has left the cell. p2E- virions were found to be infectious, though with a lower specific infectivity than that of the wild type, showing that p2E does not play an essential role in the process of infection. Fusion was also observed with a chimeric p2E- virus in which gp70SU and nearly all of p15ETM are derived from amphotropic, rather than Moloney, MuLV. In a second mutant, an amino acid at the cleavage site was changed. The pre15E protein in this mutant is not cleaved. While the mutant Env complex is incorporated into virions, these particles have a very low specific infectivity. This result suggests that the cleavage event is essential for infectivity, in agreement with the idea that removal of p2E activates the membrane fusion capability of the Env complex.

Chemoprevention of retroviral infection: success is determined by virus inoculum strength and cellular immunity

We demonstrated earlier that post-exposure prophylaxis with 3'-azido-3'-deoxythymidine (AZT, zidovudine) or with AZT + interferon-alpha (IFN-alpha) prevented viremia and disease in BALB/c mice inoculated with Rauscher murine leukemia virus (RLV). After the 20-day treatment course, most animals were resistant to rechallenge with live virus. Adoptive transfer of T cells from such resistant but not from normal mice into naive recipients provided full protection against virus challenge. From these experiments, we concluded that post-exposure chemoprophylaxis restricted virus replication and allowed the animals to form protective, long-lasting cellular immune responses. Here, the role for cellular immunity during antiviral chemoprophylaxis was tested by comparing treatment success in normal BALB/c mice and in their nude, athymic counterparts. Both were inoculated with equal doses of RLV (10(4) plaque-forming units, pfu). Single-agent AZT or combination therapy with AZT + IFN-alpha, started before or after RLV inoculation, prevented viremia in all normal but not in most nude mice. A significant number of nude mice were completely protected by chemoprevention only when given a 10 times lower virus dose. When normal mice were injected with a 10 times higher virus dose (10(5) pfu), complete protection by chemoprevention was lost. These results demonstrate that the success of chemoprevention depends critically on the virus inoculum. The differential success of chemoprevention in normal and T-cell-deficient mice implies that effective cellular immunity plays an important role in protecting virus-exposed animals against viremia and disease.

Synthetic approaches to continuous assays of retroviral proteases

This chapter has described a number of approaches for continuous assay of retroviral proteases using either chromogenic or fluorogenic synthetic substrates. The significant progress in this area has been catalyzed by the intense interest in HIV protease as a therapeutic target, but these versatile methods will be used widely in future for studies of many other proteases.

Assay methods for retroviral proteases

A variety of assay methods for retroviral proteases have been developed in response to different experimental requirements, such as initial identification of a protease, subsequent enzymatic characterization, and high-capacity automated screening of potential inhibitors. This chapter has reviewed a number of these methods above; most have been closely tailored to match specific experimental requirements, and some of them are described in greater detail in other chapters in this volume. They include analysis of polyprotein cleavage using SDS-PAGE, analysis of the determinants of substrate cleavage using either chromogenic peptides or reversed-phase HPLC for product separation after cleavage of unmodified peptides, and the design and utilization of quenched fluoregenic peptides for use in continuous assay.

Expression of biologically active envelope glycoprotein from the acutely pathogenic simian immunodeficiency virus SIVsmmPBj

The full-length envelope (env) gene from the most acutely pathogenic primate lentivirus described so far, the simian immunodeficiency virus SIVsmmPBj14 was expressed by a recombinant vaccinia virus vector (vv-env4) and was completely characterized as a previous step for its use as an immunogen in vaccination trials. Radioimmunoprecipitation and Western blot experiments indicated that SIVsmmPBj gp160 precursor was processed into gp120 and gp41 subunits, and that gp120 was released into the medium. Flow cytometry analysis showed that recombinant SIVsmmPBj was transported to and expressed on the surface of vvenv4-infected cells. Biochemical analysis of virus-like particles produced by coinfection of cells with recombinant vaccinia viruses expressing SIVsmmPBj Env (vv-env4) and Gag (vv-wtgag) proteins revealed that the Env glycoprotein was incorporated into core-like particles. Furthermore, cells expressing SIVsmmPBj env gene products were found to undergo fusion with the same CD4+ cell lines in which the whole provirus has been shown to form syncytia.

Retroviral RNA packaging: a review

In retroviruses, the "Gag" or core polyprotein is capable of assembling into virus particles and packaging the genomic RNA of the virus. How this protein recognizes viral RNA is not understood. Gag polyproteins contain a zinc-finger domain; mutants with changes in this domain assemble into virions, but a large fraction of these particles lack viral RNA. Thus, one crucial element in the RNA packaging mechanism is the zinc-finger domain. RNA sequences required for packaging ("packing signals") have been studied both by deletion analysis and by measuring encapsidation of nonviral mRNAs containing limited insertions of viral sequence. These experiments show that all or part of the packaging signal in viral RNA is located near the 5 end of the genome. These signals appear to be quite large, i.e., hundreds of nucleotides. Each virus particle actually contains a dimer of two identical, + strand genomic RNA molecules. The nature of the dimeric linkage is not understood. In some experimental situations (including zinc-finger mutants), only a small fraction of the particles in a virus preparation contain genomic RNA. It is striking that the genomic RNA packaged in these situations is dimeric. Because of this important observation, it is speculated that only dimers are packaged, and that the dimeric structure is an element of the packaging signal. It is also suggested that the dimers undergo a conformational change ("RNA maturation") after the virus is released from the cell, and that this change may depend upon the cleavage of the Gag polyprotein, a post-assembly event catalyzed by the virus-coded protease.

In vitro translation of fractionated virus-specific RNA isolated from plasma of chicken infected by avian myeloblastosis virus. Unprocessed and processed myeloblastosis-associated virus env-polypeptide precursors

The 60 S viral RNA complex isolated from leukaemic plasma of chicken infected by avian myeloblastosis virus (AMV) was denatured, the poly(A)-RNA selected and centrifuged in a linear sucrose density gradient. RNA from each fraction was translated in vitro and the products were analyzed by slab polyacrylamide gel electrophoresis (PAGE). Unprocessed primary translation product (p64env) of MAV env gene from 21 S RNA fraction was immunoprecipitated by anti-gp85 serum. If, however, this RNA was translated in the presence of dog pancreas microsomal membranes (DPM), the processed 92 K MAV glycoprotein precursor (p92env) was immunoprecipitated by anti-gp85 serum. This precursor, unlike p64env was resistant to exogenous protease.

Reverse transcriptase and protease activities of avian leukosis virus Gag-Pol fusion proteins expressed in insect cells

Protease (PR)-defective avian leukosis virus particles display 300-fold-reduced levels of reverse transcriptase (RT) activity relative to wild-type particles. This observation suggests that during virion assembly RT is activated by proteolytic maturation of the Gag-Pol polyprotein precursor. To study the relationship between proteolytic cleavage and RT activation, we subjected PR-defective virion cores to digestion with purified viral PR and analyzed the structure of the major polypeptides produced as well as RT activity. Under conditions in which Gag precursors were fully matured, the RT domain was only incompletely released from the Gag-Pol precursor, remaining tethered to the upstream Gag domains PR or NC-PR. In the same reaction, RT activity was stimulated only three-fold, or 100-fold less than expected for a fully active RT. The poor activation suggested that the NC or PR domains could repress RT activity. To test this idea, we constructed recombinant baculoviruses expressing 19 different fusion proteins with upstream Gag or downstream Pol sequences attached to RT. Each protein was partially purified and assayed for its inherent RT activity. The results are consistent with the idea that Gag sequences can inhibit RT activity but indicate that the size of the Pol domain as well as the status of the PR domain (wild-type or mutant) also can profoundly influence activity. Several of the constructed Gag-Pol fusion proteins contained a wild-type PR domain. Some of these underwent intracellular PR-mediated processing, while others did not. All proteins in which the PR domain was preceded by upstream Gag sequences showed specific proteolysis. By contrast, all proteins initiated with a methionine placed one residue upstream of the natural N terminus of PR failed to show specific proteolysis. Amino-terminal sequencing of one such protein yielded the correct amino acid sequence and showed that the initiating methionine was not removed. One interpretation of these findings is that activation of PR requires the generation of the precise N terminus of the mature PR.

Functional analysis of human foamy virus accessory reading frames

Foamy viruses belong to the retroviruses which possess a complex genome structure. The human foamy virus (HFV) isolate bears three open reading frames (the so-called bel genes) in the 3' region of the genome which have been reported to give rise to possibly six different proteins via alternative splicing (W. Muranyi and R. M. Flügel, J. Virol. 65:727-735, 1991). In order to analyze the requirements of these proteins for HFV replication in vitro, we constructed a set of single and combinatory bel gene mutants of an infectious molecular clone of HFV. The mutant which lacked the transacting activator, bel-1, was found to be replication incompetent. All other mutants replicated equally well and gave rise to comparable titers of infectious cell-free virus. When HFV proviruses were put under the control of a heterologous promoter (simian virus 40), none of the accessory gene products was found to be required for expression of structural (gag) proteins. There was no evidence for a posttranscriptional regulatory protein that is present in other complex retroviruses.

BEL-1 transactivator responsive sequences in the long terminal repeat of human foamy virus

Cis-regulatory elements in the long terminal repeat (LTR) of human foamy virus (HFV) were identified by using LTR mutants to transiently express the chloramphenicol acetyl-transferase gene after co-transfection with an expression plasmid for the virus bel-1 (transactivator) gene. The R-U5 region and an element in the 5' U3 region were found to negatively influence HFV gene expression. The complete BEL-1 responsive region was mapped to extend from nucleotide position -471 to position -93 relative to the start of transcription. Within this region, three elements were identified that in the homologous or a heterologous (SV40) promoter context can, independently and irrespective of their orientation, act as targets for BEL-1. These elements are located between nucleotide positions -413/-378, -361/-291, and -124/93. The target elements do not share obvious sequence homologies. The mechanism of HFV transactivation appears to be novel among the complex retroviruses and is likely to involve, as yet, undiscovered cellular DNA binding factors.

Identification of discrete functional domains of HIV-1 integrase and their organization within an active multimeric complex

HIV-1 integrase protein possesses the 3' processing and DNA strand transfer activities that are required to integrate HIV DNA into a host chromosome. The N-, C-terminal and core domains of integrase are necessary for both activities in vitro. We find that certain pairs of mutant integrase proteins, which are inactive when each protein is assayed alone, can support near wild type levels of activity when both proteins are present together in the reaction mixture. This complementation implies that HIV-1 integrase functions as a multimer and has enabled us to probe the organization of the functional domains within active mixed multimers. We have identified a minimal set of functional integrase domains that are sufficient for 3' processing and DNA strand transfer and find that some domains are contributed in trans by separate monomers within the functional complex.