Regulation of human immunodeficiency virus env expression by the rev gene product

A dynamic in vivo view of the HIV-I Rev-RRE interaction

The export of pre-mRNAs coding for the structural genes of the human immunodeficiency virus type I depends on the interaction of the Rev protein with a highly structured viral RNA sequence, the Rev-responsive element (RRE). To gain information about the structure of the RRE and the determinants of the in vivo RRE-Rev interaction, we have analyzed the structure of the 351 nt RRE RNA within living yeast (Saccharomyces cerevisiae) by dimethyl sulfate probing with or without Rev. The in vivo structure in the absence of Rev is generally similar to the previously established solution structure. In addition, we observe a single hypermethylated guanine residue (G128), located within the Rev high-affinity binding site, in vitro as well as in vivo. The important homopurine interaction between residues 129 and 106 is required for the hyperreactivity, confirming its biological relevance. Expression of wild-type Rev leads to a protection of this region and to modifications of the RRE structure: the high-affinity site becomes further structured, and Stem IIA is destabilized. High-level expression of the oligomerization-defective mutant M4 protein leads to the same protections without destabilization of Stem IIA. Taken together with other observations, the data suggest that Rev captures the unusual conformation of the high-affinity site, followed by additional changes in the structure of the RRE.

Posttranscriptional regulation by Rev protein of human immunodeficiency virus type 1 results in nonrandom nuclear localization of gag mRNA

The expression of the human immunodeficiency virus type 1 mRNAs containing the Rev-responsive element is regulated at the posttranscriptional level by the viral Rev protein. Rev increases the nucleocytoplasmic export of these mRNAs, leading to high expression. Using in situ hybridization and electron microscopy, we investigated the localization of a subgenomic gag mRNA in the absence and presence of Rev. In addition to the previously shown cytoplasmic accumulation of the Rev-dependent mRNA, we observed that in the presence of Rev the nuclear gag mRNA accumulates nonrandomly and forms specific localization patterns at the nuclear membrane and in the nucleoplasm. Cellular mRNAs for beta-actin and glyceraldehyde-3-phosphate dehydrogenase were not found to form such patterns. These data suggest that Rev leads the gag mRNA to specific subnuclear locations, which further supports the transport function of Rev.

Characterization of Rev function using subgenomic and genomic constructs in T and COS cells

The effect of the human immunodeficiency type 1 (HIV-1) Rev protein on the splicing and cytoplasmic accumulation of HIV-1 RNAs was investigated in COS and T cells. Subgenomic and genomic constructs were used which expressed varying levels of complexity in their potential RNA constituents. Using all constructs, in both cell types, an inhibitory effect of Rev on the level of fully spliced HIV-1 RNAs could be demonstrated. An increase in the nuclear level of unspliced pre-mRNA was seen in the presence of Rev with genomic constructs. Thus, the inhibitory effect on splicing was not merely due to enhancement of nuclear export of the pre-mRNA with these constructs. In both cell types, a positive effect of Rev on the cytoplasmic accumulation of HIV-1 RNAs could also be seen. However, in T cells, the Rev-dependent RNAs were still capable of accumulating at a reduced level in the cytoplasmic fraction in the absence of Rev. The identity of the cell type, construct, and RNA species impacted on the phenotypic manifestation of Rev function.

Leptomycin B is an inhibitor of nuclear export: inhibition of nucleo-cytoplasmic translocation of the human immunodeficiency virus type 1 (HIV-1) Rev protein and Rev-dependent mRNA

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) regulatory protein Rev is required for unspliced and incompletely spliced viral mRNAs to appear in the cytoplasm and thus for viral replication. Translocation of Rev from the nucleus to the cytoplasm is essential if Rev is to function. We wanted to identify inhibitors of this transport process because they would be potential antiviral agents.

RESULTS

The Streptomyces metabolite, leptomycin B, and other antibiotics of the leptomycin/kazusamycin family were identified as inhibitors of the nucleo-cytoplasmic translocation of Rev at nanomolar concentrations. Rev-dependent export of mRNA into the cytoplasm is also blocked by leptomycin B, which inhibits Rev-dependent, but not Rev-independent gene expression in a short-term transfection assay. In primary human monocytes, leptomycin B suppresses HIV-1 replication.

CONCLUSIONS

Leptomycin B is the first low molecular weight inhibitor of nuclear export to be identified. Although it cannot be used therapeutically, it should serve as a valuable tool for dissecting nuclear export pathways.

Preferential binding of poly(A)-binding protein 1 to an inhibitory RNA element in the human immunodeficiency virus type 1 gag mRNA

Human immunodeficiency virus type 1 (HIV-1) mRNAs encoding structural proteins contain multiple inhibitory/instability elements (INS), which decrease the efficiency of viral protein expression. We have previously identified a strong INS element (INS-1) within the p17(gag) coding region. Here we show that poly(A)-binding protein 1 (PABP1) binds preferentially to INS-1 within the p17(gag) mRNA, but not to a mutated mRNA in which INS-1 function is eliminated. Competition experiments performed in the presence of different nucleic acids and homoribopolymers demonstrated preferential binding of PABP1 to the INS-1-containing RNA. In contrast to HeLa cells and several lymphoid cell lines, certain human glioma cell lines exhibit high levels of gag expression in the absence of Rev upon transient transfection with wild type gag expression vectors. We analyzed extracts of different cell lines and found that the binding of PABP1 to INS-1 RNA is significantly diminished in glial cell extracts. The expression levels of gag correlate with the absence of binding of PABP1 to the INS-1 RNA in cellular extracts. These results suggest a role for PABP1 in the inhibition of gag expression mediated through INS-1.

Identification of an RNA sequence within an intracisternal-A particle element able to replace Rev-mediated posttranscriptional regulation of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) replication depends on the posttranscriptional regulation by the viral Rev protein and can be replaced with the posttranscriptional RNA control element (CTE) of the type D simian retroviruses. We have identified a sequence which shares only nucleotide sequences of the internal loops and secondary structure with the CTE and which is part of a novel murine intracisternal-A particle (IAP) retroelement, inserted within the transcribed mouse osteocalcin-related gene. This sequence, named CTE(IAP), can replace the Rev-mediated regulation of HIV-1, hence it is a posttranscriptional regulatory element. Related elements have been identified in other IAPs. These results suggest that insertional mutagenesis can affect gene expression by providing a functional posttranscriptional control element. The CTE(IAP) and CTEs of the type D simian retroviruses represent a novel class of RNA elements characterized by unique sequences within the internal loops which are predicted to represent the interaction site with cellular factor(s). These findings suggest that such elements may be involved in posttranscriptional regulation of cellular mRNAs.

A structured retroviral RNA element that mediates nucleocytoplasmic export of intron-containing RNA

A common feature of gene expression in all retroviruses is that unspliced, intron-containing RNA is exported to the cytoplasm despite the fact that cellular RNAs which contain introns are usually restricted to the nucleus. In complex retroviruses, the export of intron-containing RNA is mediated by specific viral regulatory proteins (e.g., human immunodeficiency virus type 1 [HIV-1] Rev) that bind to elements in the viral RNA. However, simpler retroviruses do not encode such regulatory proteins. Here we show that the genome of the simpler retrovirus Mason-Pfizer monkey virus (MPMV) contains an element that serves as an autonomous nuclear export signal for intron-containing RNA. This element is essential for MPMV replication; however, its function can be complemented by HIV-1 Rev and the Rev-responsive element. The element can also facilitate the export of cellular intron-containing RNA. These results suggest that the MPMV element mimics cellular RNA transport signals and mediates RNA export through interaction with endogenous cellular factors.

Structural change in Rev responsive element RNA of HIV-1 on binding Rev peptide

The HIV-1 Rev responsive element (RRE) high-affinity binding site was studied by homonuclear and heteronuclear NMR. Two Rev binding element (RBE) RNA oligonucleotides were used as model systems in this study: RBE3, which contains the wild-type Rev high-affinity binding site, and RBE3-A which is identical except for the deletion of a bulged A. The temperature dependence of the two-dimensional spectra of the free RNAs indicates that at lower temperatures more than one conformation is present. However, at higher temperatures a single conformation predominates. Model structures of RBE3 and RBE3-A as well as the RBE3-A complexed with a peptide derived from the RNA binding domain of HIV-1 Rev, were calculated using NMR-derived restraints. The Rev high-affinity binding site of the HIV-1 RRE contains a structured internal loop with two purine-purine base-pairs and an extrahelical U. Comparison of the free and bound RNA structures reveals that upon peptide binding there is a distinct change in the backbone at G24, which is involved in a G-G base-pair. In the free RNA, G24 is in the syn conformation, and the backbone is in a relatively normal configuration, antiparallel to the other strand. In the bound RNA, the backbone at G24 has flipped over so that it is parallel to the other strand. G24 in the bound RNA still forms a base-pair with G6, but is now in the anti conformation.

Human immunodeficiency virus type 1 Rev function requires continued synthesis of its target mRNA

Synthesis of human immunodeficiency virus structural proteins is dependent on expression of the virus-encoded Rev protein due to the constitutive nuclear sequestration of mRNAs coding for the structural proteins. The pathway by which Rev, through interaction with the Rev-responsive element (RRE) within the mRNA, achieves export of the mRNA remains unclear. To probe the mechanism by which Rev induces nuclear export of its target mRNAs, the effect of inhibiting mRNA synthesis on the function of Rev was examined. Two approaches to address this issue were pursued: (i) the use of general transcription inhibitors such as 5,6-dichlorobenzimidazole riboside (DRB) and actinomycin D, and (ii) the more selective modulation of target gene transcription permitted by the use of a tetracycline-regulated promoter. Addition of either DRB or actinomycin D inhibited Rev action despite the presence of significant quantities of the target mRNA throughout the course of drug treatment. Furthermore, prolonged DRB treatment was found to improve rather than diminish the induction observed. Subsequent analysis using the tetracycline-modulated promoter demonstrated that Rev function was dependent on the transcription rate of the target mRNA and independent of target mRNA concentration. These data strongly indicate that Rev functions through interaction with newly synthesized target mRNA, facilitating its export by preventing its interaction with the host factors that effect nuclear sequestration.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.

Cooperation between herpes simplex virus type 1-encoded ICP0 and Tat to support transcription of human immunodeficiency virus type 1 long terminal repeat in vivo can occur in the absence of the TAR binding site

Expression of human immunodeficiency virus type 1 (HIV-1) provirus can be stimulated by herpes simplex virus type 1 (HSV-1) infection; the stimulation occurs at the level of transcriptional activation of the HIV long terminal repeat (LTR) and is mediated by both cellular and HSV-1-encoded transactivators. We have shown in this study that HSV-1 immediate-early gene ICP0 cooperates effectively with the HIV-1-encoded transactivator, Tat, in the stimulation of HIV-1 LTR-directed transcription. The cooperation between ICP0 and Tat is specific for the HIV-1 LTR and was not observed with other promoters (e.g., ICP0) that can be transactivated by ICP0 but not by Tat. Analyses of HIV-1 LTR deletion mutants have shown that ICP0 not only transactivates an HIV-1 LTR mutant that is unresponsive to NF-kappaB and Tat-mediated transactivation, such as the HIV-1 LTR with the enhancer deleted (-83 LTR) and TAR deleted (+20 to +81), but also restores responsiveness to Tat. ICP0 also showed cooperation with Gal4-Tat fusion protein-mediated transactivation of Gal4-HIV-1 LTR with TAR deleted. Enhancement of the transcriptional activation of ICP0 by Tat requires both the cysteine-rich and core domains of Tat and is inhibited by RO5-3335. ICP0 stimulates transcription of not only the HIV-1 LTR but also the TAR-defective HIV-1 provirus. We suggest that ICP0 can (i) recruit Tat to the vicinity of the HIV-1 promoter, thereby providing an alternative binding site for Tat, and (ii) substitute for the enhancer-binding proteins that are required for efficient Tat transactivation in T cells.

Localization of HIV-1 RNA in mammalian nuclei

The Rev protein of human immunodeficiency virus type 1 (HIV-1) facilitates the nuclear export of unspliced and partially spliced viral RNAs. In the absence of Rev, these intron-containing HIV-1 RNAs are retained in the nucleus. The basis for nuclear retention is unclear and is an important aspect of Rev regulation. Here we use in situ hybridization and digital imaging microscopy to examine the intranuclear distributions of intron-containing HIV RNAs and to determine their spatial relationships to intranuclear structures. HeLa cells were transfected with an HIV-1 expression vector, and viral transcripts were localized using oligonucleotide probes specific for the unspliced or spliced forms of a particular viral RNA. In the absence of Rev, the unspliced viral RNAs were predominantly nuclear and had two distinct distributions. First, a population of viral transcripts was distributed as approximately 10-20 intranuclear punctate signals. Actinomycin D chase experiments indicate that these signals represent nascent transcripts. A second, stable population of viral transcripts was dispersed throughout the nucleoplasm excluding nucleoli. Rev promoted the export of this stable population of viral RNAs to the cytoplasm in a time-dependent fashion. Significantly, the distributions of neither the nascent nor the stable populations of viral RNAs coincided with intranuclear speckles in which splicing factors are enriched. Using splice-junction-specific probes, splicing of human beta-globin pre-mRNA occurred cotranscriptionally, whereas splicing of HIV-1 pre-mRNA did not. Taken together, our results indicate that the nucleolus and intranuclear speckles are not involved in Rev regulation, and provide further evidence that efficient splicing signals are critical for cotranscriptional splicing.

Full-length sequence and mosaic structure of a human immunodeficiency virus type 1 isolate from Thailand

Human immunodeficiency virus type 1 isolates of envelope genotype E are contributing substantially to the global pandemic. These strains appear to be mosaics, with the gag gene from clade A and the envelope from clade E; the parental clade E strain has not been found. Here we report the first full genomic sequence of one such mosaic virus, isolate CM240 from Thailand. Multiple breakpoints between the two parental genotypes have been found in a CM240 virus. The entire gag-pol region and most, if not all, of the accessory genes vif, vpr, tat, rev, and vpu appear to derive from clade A. The genotype switches to E shortly after the signal peptide of the envelope and back to clade A near the middle of gp41; thus, the portion of the envelope that lies on the cytoplasmic side of the membrane appears to be principally derived not from clade E, as previously thought, but from clade A. Another small segment not belonging to any recognized clade and presumably also contributed by the parental E strain has been found in the long terminal repeat. It may be significant that the implied virion structure resembles a pseudotype virus with the matrix and core from one clade and the outer envelope from another. In the long terminal repeat, differences were observed between CM240 and other clades in the number of NF-kappa B binding sites, the sequence of the TATA box, and the putative secondary structure of the transactivation response region stem-loop. The mosaic structure of a CM240 virion is suggestive of phenotypic differences which might have contributed to the emergence of this variant.

The rev-responsive element negatively regulates human immunodeficiency virus type 1 env mRNA expression in primate cells

The human immunodeficiency virus type 1 (HIV-1) Rev protein mediates the accumulation of unspliced and singly spliced viral transcripts within the cytoplasm of infected cells, late in the infection cycle, leading to the expression of the viral structural proteins, Gag, Pol, and Env. Rev binds to a complex RNA structure, the Rev-responsive element (RRE), present in all Rev-responsive viral transcripts, relieving their nuclear sequestration. The precise mechanism by which RRE-containing transcripts are retained within the nucleus in the absence of Rev protein is not well understood. We previously demonstrated that the RRE alone plays a crucial role in the nuclear retention of RRE-containing env transcripts in stably transfected Drosophila cells. Here we extend our previous observations and demonstrate that the RRE is a principal determinant of nuclear retention for envelope transcripts in primate cells and, in particular, human CD4+ T cells.

The posttranscriptional control element of the simian retrovirus type 1 forms an extensive RNA secondary structure necessary for its function

It was previously shown that a 240-nucleotide (nt) RNA element (cis-acting transactivation element [CTE]) located between the env gene and the 3' long terminal repeat of simian retrovirus type 1 (SRV-1) can functionally replace posttranscriptional activation directed by Rev and the Rev-responsive element (RRE) when inserted into a Rev- and RRE-deficient molecular clone of human immunodeficiency virus type 1, resulting in efficient virus replication. Here, we analyze the molecular and structural requirements for function of this RNA element. Deletion mutagenesis demonstrated that the core element spans 173 nt. SRV-2 and Mason-Pfizer monkey virus have highly homologous elements, which function similarly when inserted into the Rev/RRE-deficient human immunodeficiency virus type 1. Computer prediction indicated that the core CTEs of all three viruses have similar extensive secondary structures. Mutagenesis of the SRV-1 CTE revealed that both sequence and secondary structure are essential for function. Nuclease probing of the SRV-1 CTE further supported the genetic analysis and confirmed the predicted structural features of the RNA element. Sequence analysis of the 240-nt SRV-1 CTE, after continuous long-term propagation of the Rev-independent viruses, revealed that the genetically defined core element remained unchanged, while regions outside the core CTE underwent deletions or duplications. These data further support our in vitro mutagenesis data and demonstrate the importance of the sequence and structure of the SRV-1 CTE for appropriate function.

Location of the HIV-1 Rev protein during mitosis: inactivation of the nuclear export signal alters the pathway for postmitotic reentry into nucleoli

The HIV-1 Rev protein localizes predominantly to the nucleolus of HIV-1-infected or Rev-expressing cells. The subcellular location of Rev during mitotic nucleolar disintegration was examined at various stages of mitosis in synchronized Rev-expressing CMT3 cells. During early prophase Rev was predominantly located in disintegrating nucleoli and began to accumulate at the peripheral regions of chromosomes in late prophase, eventually distributing uniformly on all chromosomes in prometaphase. In anaphase Rev remained associated with the perichromosomal regions, but significant amounts of Rev were also seen in numerous nucleolus-derived foci. The movement of Rev from disintegrating nucleoli to perichromosomal regions and foci was similar to that of nonribosomal nucleolar proteins, including fibrillarin, nucleolin, protein B23 and p52 of the granular component. During telophase Rev remained associated with perichromosomal regions and mitotic foci until the nuclear envelope started to reform. When nuclear envelope formation was complete in late telophase, nonribosomal nucleolar proteins were present in prenucleolar bodies (PNBs) which were eventually incorporated into nucleoli; at the same time, Rev was excluded from nuclei. In contrast, a trans-dominant negative Rev protein containing an inactive nuclear export signal reentered nuclei by the nonribosomal nucleolar protein pathway in late telophase, associating with PNBs and reformed nucleoli. Rev protein reentry into postmitotic nuclei was delayed until early G1 phase, but before the arrival of ribosomal protein S6. Thus, Rev behaves like a nonribosomal nucleolar protein through mitosis until early telophase; however, its nuclear reentry seems to require reestablishment of both a nuclear import system and active nucleoli.

The human T-cell leukemia virus type 1 posttranscriptional trans-activator Rex contains a nuclear export signal

The Rex protein of human T-cell leukemia virus type 1 is required for the nuclear export of unspliced viral mRNA and, therefore, for virus replication. In this manuscript, we demonstrate that Rex shuttles between the nucleus and the cytoplasm and that its activation domain constitutes a nuclear export signal that specifies efficient transport to the cytoplasm. These findings are consistent with a model for Rex-mediated trans-activation in which Rex-viral mRNA complexes are targeted for nuclear export by the direct action of the activation domain.

Protein sequence requirements for function of the human T-cell leukemia virus type 1 Rex nuclear export signal delineated by a novel in vivo randomization-selection assay

The Rex protein of human T-cell leukemia virus type 1, like the functionally equivalent Rev protein of human immunodeficiency virus type 1, contains a leucine-rich activation domain that specifically interacts with the human nucleoporin-like Rab/hRIP cofactor. Here, this Rex sequence is shown to function also as a protein nuclear export signal (NES). Rex sequence libraries containing randomized forms of the activation domain/NES were screened for retention of the ability to bind Rab/hRIP by using the yeast two-hybrid assay. While the selected sequences differed widely in primary sequence, all were functional as Rex activation domains. In contrast, randomized sequences that failed to bind Rab/hRIP lacked Rex activity. The selected sequences included one with homology to the Rev activation domain/NES and a second that was similar to the NES found in the cellular protein kinase inhibitor alpha. A highly variant, yet fully active, activation domain sequence selected on the basis of Rab/hRIP binding retained full NES function even though this sequence preserved only a single leucine residue. In contrast, nonfunctional activation domain mutants that were unable to bind Rab/hRIP had also lost NES function. These data demonstrate that NES activity is a defining characteristic of the activation domains found in the Rev/Rex class of retroviral regulatory proteins and strongly support the hypothesis that the Rab/hRIP cofactor plays a critical role in mediating the biological activity of these NESs. In addition, these data suggest a consensus sequence for NESs of the Rev/Rex class.

cis-acting elements in human immunodeficiency virus type 1 RNAs direct viral transcripts to distinct intranuclear locations

Two distinct intranuclear locations were identified for alternatively spliced RNA transcripts expressed from the pNL4-3 infectious molecular clone of human immunodeficiency virus (HIV) type 1. Multiply spliced HIV RNA encoding tat was detected within the nucleus in large clusters; immunostaining and colocalization studies using laser-scanning confocal microscopy revealed that these structures contained the non-small nuclear ribonucleoprotein RNA processing factor, SC35. In contrast, unspliced gag RNA was detected in much smaller granules distributed throughout the nucleus, with little or no association with SC35-containing granules. Analyses of nuclear RNA expressed from recombinant plasmids encoding gag (pCMVgag-2) alone or tat (pCMVtat-2) alone revealed distributions corresponding to those obtained with pNL4-3, indicating that expression within the context of the HIV provirus was not required for the distinct RNA locations detected for these transcripts. The presence of unspliced gag RNA in small granules was confirmed in infections of H9 T-lymphocytic cells, indicating that gag localization was not restricted to transient expression systems. The intranuclear distribution of gag RNA was dependent on specific RNA sequences. Deletion of a portion of the gag gene of pCMVgag-2, containing a cis-repressing inhibitory region, resulted in redirection of unspliced gag RNA from small granules into large SC35-containing clusters. The addition of the Rev-responsive element, RRE, to the deleted pCMVgag-2 construct resulted in RNA transcripts which were no longer associated with SC35. We also identified a cellular intron, rabbit beta-globin-intervening sequence 2 (IVS-2) which, when introduced into pCMVgag-2, redirected unspliced gag RNA into SC35-containing granules and permitted rev-independent Gag expression. These findings suggest that redirecting intranuclear RNA localization may influence gene expression. Color micrographs from this article are available for view at http//128.231.216.2/lmmhome.htm.

Expression of HIV env gene in a human T cell line for a rapid and quantifiable cell fusion assay

Human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins present at the surface of infected cells are known to mediate fusion with CD4-positive target cells. In this study we have developed a novel Env-expressing cell line for investigating the fusion process in a biologically significant system. Cell surface expression of the HIV-1 env gene, isolated from the highly fusogenic strain SF33, was obtained in the CD4-negative T cell line A2.01. To render the system versatile and efficient, HIV-1 regulatory proteins Tat and Rev were supplied in trans. The presence of Env at the cell surface was shown by cytofluorometry and immunofluorescence and precursor processing of gp160 to gp120/gp41 was demonstrated by Western blot. The fusion capacity of A2.01-Env cells was assessed by coculture with CD4-positive T lymphocytes or the fusion indicator cell line, HeLa-CD4-LTR-beta-Gal. By coincubation with CD4-positive T cells such as SupT1, A2.01-Env cells were observed to mediate rapidly numerous well-defined syncytia in a reproducible fashion. By expressing Tat, they also had the capacity to trans-activate the LTR-linked reporter beta-Gal gene following fusion with HeLa-CD4-LTR-beta-Gal cells. The fusion-inhibiting anti-CD4 monoclonal antibodies Q425 and Q428 were used to block specifically Env-mediated fusion with CD4-positive cells and to demonstrate application of this system to the search for potential fusion-blocking agents. Our system thus offers a biologically significant model for studying fusion events with the advantages of being rapid, reproducible and versatile.

Inhibitory activity of the equine infectious anemia virus major 5' splice site in the absence of Rev

The major 5' splice site of equine infectious anemia virus (EIAV) conforms to the consensus 5' splice site in eight consecutive positions and is located immediately upstream of the gag AUG. Our results show that the presence of this 5' splice site on the EIAV gag mRNA decreases Gag production 30- to 60-fold. This is caused by inefficient nuclear mRNA export and inefficient mRNA utilization. Inhibition could be overcome by providing human immunodeficiency virus type 1 Rev/Rev-responsive element, human T-cell leukemia virus type 1 Rex/Rex-responsive element, or simian retrovirus type 1 constitutive transport element. In addition, inhibition could be abolished by introducing single point mutations in the 5' splice site or by moving the 5' splice site away from its natural position immediately upstream of the gag AUG. This demonstrates that both maintenance of a perfect consensus 5' splice site and its proper location on the mRNA are important for inhibitory activity of the EIAV major 5' splice site.

Nuclear export of late HIV-1 mRNAs occurs via a cellular protein export pathway

The Rev protein of HIV-1 is essential for the nuclear export of incompletely spliced viral mRNAs. This action depends on the mutationally defined Rev activation domain, which both binds the nucleoporin-like human cellular cofactor Rab/hRIP and also functions as a nuclear export signal. Protein kinase inhibitor alpha (PKI) also contains a potent nuclear export signal. However, PKI plays no role in nuclear RNA export and instead induces the nuclear export of a specific protein target, the catalytic subunit of cAMP-dependent protein kinase. Here, it is demonstrated that the nuclear export signal of PKI not only binds the Rab/hRIP cofactor specifically but also can effectively substitute for the Rev activation domain in mediating the nuclear export of HIV-1 mRNAs. We conclude that HIV-1 Rev and PKI act through an identical nuclear export pathway and that Rev, rather than using a dedicated RNA export pathway, is instead acting as an adaptor that allows viral mRNAs to access a cellular protein export pathway.

Amphibian transcription factor IIIA proteins contain a sequence element functionally equivalent to the nuclear export signal of human immunodeficiency virus type 1 Rev

The human immunodeficiency virus type 1 (HIV-1) Rev protein is required for nuclear export of late HIV-1 mRNAs. This function is dependent on the mutationally defined Rev activation domain, which also forms a potent nuclear export signal. Transcription factor IIIA (TFIIIA) binds to 5S rRNA transcripts and this interaction has been proposed to play a role in the efficient nuclear export of 5S rRNA in amphibian oocytes. Here it is reported that amphibian TFIIIA proteins contain a sequence element with homology to the Rev activation domain that effectively substitutes for this domain in inducing the nuclear export of late HIV-1 mRNAs. It is further demonstrated that this TFIIIA sequence element functions as a protein nuclear export signal in both human cells and frog oocytes. Thus, this shared protein motif may play an analogous role in mediating the nuclear export of both late HIV-1 RNAs and 5S rRNA transcripts.

A Novel hnRNP Specifically Interacts with HIV-1 RRE RNA

We have identified and obtained the full-length clone of RREBP49, a human nuclear factor which specifically interacts with the Rev-responsive element (RRE) sequence of human immunodeficiency virus type 1. Sequence analysis revealed that RREBP49 is highly homologous to hnRNP F protein and contains three repeated RNA-binding domains. Binding assays demonstrated that Rev and RREBP49 bind to different subregions on the RRE sequence and that binding is mutually nonexclusive. Blocking of endogenous RREBP49 expression by an antisense construct increases Rev activity in CV-1 cells, indicating that RREBP49 and Rev may play antagonistic roles in HIV-1 replication. RREBP49 may function as a splicing factor or a nuclear retention factor for unspliced mRNAs. However, only a slight decrease of Rev activity was observed when exogenous RREBP49 was introduced into CV-1 cells by pSVL-RREBP49 expression vector. This may be explained by a high endogenous level of RREBP49 which is above optimal. Alternatively, additional cellular factors may be required for RREBP49-mediated inhibition of Rev. Copyright 1996 S. Karger AG, Basel

Nuclear transport of human immunodeficiency virus type 1, visna virus, and equine infectious anemia virus Rev proteins: identification of a family of transferable nuclear export signals

The human immunodeficiency virus type 1 Rev trans activator binds directly to unspliced viral mRNA in the nucleus and activates its transport to the cytoplasm. In additon to the sequences that confer RNA binding and nuclear localization, Rev has a carboxy-terminal region, the activation domain, whose integrity is essential for biological activity. Because it has been established that Rev constitutively exits and reenters the nucleus and that the activation domain is required for nuclear exit, it has been proposed that Rev's activation domain is a nuclear export signal (NES). Here, we used microinjection-based assays to demonstrate that the activation domain of human immunodeficiency virus type 1 Rev imparts rapid nuclear export after its transfer to heterologous substrates. NES- mediated export is specific, as it is sensitive both to inactivation by missense mutation and to selective inhibition by an excess of the wild-type, but not mutant, activation domain peptide. Examination of the Rev trans activators of two nonprimate lentiviruses, visna virus and equine infectious anemia virus, revealed that their activation domains are also potent NESs. Taken together, these data demonstrate that nuclear export can be determined by positively acting peptide motifs, namely, NESs, and suggest that Rev proteins activate viral RNA transport by providing export ribonucleoproteins with specific information that targets them to the cytoplasm.

RNA aptamers selected to bind human immunodeficiency virus type 1 Rev in vitro are Rev responsive in vivo

RNA aptamers (binding sequences) that can interact tightly and specifically with the human immunodeficiency virus type 1 Rev protein have previously been selected from random sequence pools. Although the selected sequences compete with the wild-type Rev-binding element (RBE) in vitro, it was not known whether they would be able to functionally replace the RBE in vivo. Two aptamers that were different from the wild-type RBE in terms of both primary sequence and secondary structure were inserted into the full-length Rev-responsive element (RRE) in place of the RBE. The hybrid RREs were assayed for their ability to mediate Rev function in vivo using a reporter system. The aptamers were found to be functionally equivalent to the wild-type element when the assay system was saturated with Rev and better than the wild-type element when Rev was limiting. These results demonstrate that the affinity of the primary Rev-binding element rather than its particular sequence may be most responsible for conferring Rev responsiveness on viral mRNAs. Moreover, the fact that increased binding ability can lead to increased Rev responsiveness suggests that cellular factors do not directly influence the Rev:RBE interaction. Finally, since sequences distinct from the RBE are found to be Rev responsive, it may be possible for the RBE to readily mutate in response to drugs or gene therapy reagents that target the Rev:RBE interaction.

Specific binding of polypyrimidine tract binding protein and hnRNP A1 to HIV-1 CRS elements

The human immunodeficiency virus (HIV) Rev and human T-cell leukemia virus (HTLV) Rex proteins regulate viral RNA processing. Both proteins act to overcome the block to viral structural gene expression, at least in part, by reversing the inhibitory effect of intronic RNA sequences, termed cis-acting repressive (CRS) sequences. Using HTLV type II (HTLV-II) as a model, we recently showed that the function of a 5' long terminal repeat (LTR) CRS correlates with in vitro binding by both polypyrimidine tract binding (PTB) protein (also known as hnRNP I) and hnRNP A1 to CRS RNA (1,2). Using radioimmunoprecipitation of proteins ultraviolet (UV) crosslinked to each HIV CRS RNA with monoclonal anti-hnRNP antibodies, we now demonstrate that hnRNP I and hnRNP A1 bind to two different HIV-1 CRS RNAs. In addition, we show that hnRNP I and hnRNP A1 binding to HIV-1 CRS RNAs can be specifically competed by HTLV-II CRS RNAs using electrophoretic mobility shift assay (EMSA)/UV crosslinking assays. Binding by both hnRNP I and hnRNP A1 to HIV-1 and HTLV-II CRS RNAs suggests a role for these proteins in CRS function that may be influenced by the Rev and Rex proteins, respectively.

Production of avian leukosis virus particles in mammalian cells can be mediated by the interaction of the human immunodeficiency virus protein Rev and the Rev-responsive element

In human immunodeficiency virus type 1-infected cells, the efficient expression of viral proteins from unspliced and singly spliced RNAs is dependent on two factors: the presence in the cell of the viral protein Rev and the presence in the viral RNA of the Rev-responsive element (RRE). We show here that the HIV-1 Rev/RRE system can increase the expression of avian leukosis virus (ALV) structural proteins in mammalian cells (D-17 canine osteosarcoma) and promote the release of mature ALV virions from these cells. In this system, the Rev/RRE interaction appears to facilitate the export of full-length unspliced ALV RNA from the nucleus to the cytoplasm, allowing increased production of the ALV structural proteins. Gag protein is produced in the cytoplasm of the ALV-transfected cells even in the absence of a Rev/RRE interaction. However, a functional Rev/RRE interaction increases the amount of Gag present intracellularly and, more strikingly, results in the release of mature ALV particles into the supernatant. RCAS virus containing an RRE is replication-competent in chicken embryo fibroblasts; however, we have been unable to determine whether the particles produced in D-17 cells are as infectious as the particles produced in chicken embryo fibroblasts.

Polypyrimidine tract-binding protein and heterogeneous nuclear ribonucleoprotein A1 bind to human T-cell leukemia virus type 2 RNA regulatory elements

Efficient expression of human T-cell leukemia virus (HTLV) and human immunodeficiency virus structural proteins requires Rx and Rev proteins, respectively. Decreased expression of Gag and Env appears to be due, in part, to intragenic RNA sequences, termed cis-acting repressive sequences (CRS), and may be mediated by binding of specific cellular factors. We demonstrated previously that two cellular proteins, p60CRS and p40CRS, interact with HTLV type 2.5' long terminal repeat CRS RNA and that the interaction of both proteins with CRS RNA correlates with function (A. C. Black, C. T. Ruland, J. Luo, A. Bakker, J. K. Fraser, and J. D. Rosenblatt, Virology 200:29-41, 1994). By radioimmunoprecipitation of HeLa nuclear proteins UV cross-linked to CRS RNAs with murine monoclonal antibodies, we now show that p40CRS is heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and p60CRS is polypyrimidine tract-binding protein or hnRNP I. These immunoprecipitation results were confirmed by an immunobinding assay with hnRNP I and hnRNP AI antibodies and by cross-competition electrophoretic mobility shift experiments. In addition, we mapped a putative hnRNP A1 binding site in U5 RNA and demonstrated that p40CRS (hnRNP A1) binding to that site correlates with CRS function. Since both hnRNP I and hnRNP A1 have been shown to influence splicing and potentially other steps in RNA processing, the binding of both hnRNP I and hnRNP A1 to HTLV RNA regulatory elements may alter retrovirus RNA processing and may be involved in regulation by Rex.

Efficient expression of the human papillomavirus type 16 L1 protein in epithelial cells by using Rev and the Rev-responsive element of human immunodeficiency virus or the cis-acting transactivation element of simian retrovirus type 1

Production of the human papillomavirus (HPV) late gene products L1 and L2 is limited to terminally differentiated keratinocytes. Here, we demonstrate that mRNA encoding the HPV-16 L1 capsid protein contains cis-acting RNA elements that inhibit expression at the posttranscriptional level. While cytoplasmic L1 mRNA is detectable in transfected HeLa cells, L1 protein is not produced. We have identified at least one major inhibitory element that is located within the L1 open reading frame, whereas another negative element had been reported to lie in the 3'-untranslated region of L1. The presence of these elements may explain the lack of HPV late gene expression in undifferentiated epithelial cells. Efficient production of HPV-16 L1 could be achieved with posttranscriptional regulatory elements of human immunodeficiency virus type 1 or simian retrovirus type 1. L1 protein was expressed in the presence of human immunodeficiency virus type 1 Rev from hybrid mRNAs containing the RNA binding site for Rev (Rev-responsive element). In addition, we have achieved efficient expression of L1 from hybrid mRNAs containing a cis-acting transactivation element from simian retrovirus type 1. Our data show that HPV-16 L1 protein production is regulated posttranscriptionally. This regulated expression may allow virus production in terminally differentiated epithelial cells and is probably a conserved and important mechanism for HPV expression.

Intracellular trafficking of the human immunodeficiency virus type 1 Rev protein: involvement of continued rRNA synthesis in nuclear retention

We have explored the mechanism directing the intracellular trafficking and nucleolar accumulation of the human immunodeficiency virus type 1 (HIV-1) Rev protein. Treatment of Rev-expressing cells with mycophenolic acid, an inhibitor of inosine monophosphate dehydrogenase, resulted in a redistribution of Rev from the nucleoli to the nucleoplasm and cytoplasm. In contrast, a Rev effector domain mutant was retained in the nucleus, indicating the involvement of this domain in the protein's nuclear retention/nucleocytoplasmic transport. Identical results were obtained by inhibiting transcription using actinomycin D or 5,6-dichlorobenzimidazole riboside. All three drugs were found to inhibit biosynthetic labeling of ribosomal RNA and to disrupt nucleolar morphology, suggesting a correlation between nucleolar/nuclear retention of Rev, continued ribosomal RNA synthesis, and intact nucleolar architecture. Results of binding/immunofluorescence assays using isolated, permeabilized nuclei and extracts of cells expressing Rev demonstrated that the protein is able to bind to nucleoli in vitro, in the absence of active cellular processes or eukaryotic posttranslational modifications. Rev derived from actinomycin D-treated cells showed equivalent binding, indicating that the inhibitor did not directly interfere with the ability of the protein to interact with nucleolar structures. Rev's interaction with nucleoli was directed by the protein's arginine-rich RNA-binding/nucleolar localization domain, and was abrogated by pretreatment of the nuclei with RNaseA, indicating a requirement for RNA, probably ribosomal RNA.

Identification and mapping of inhibitory sequences in the human immunodeficiency virus type 2 vif gene

Human immunodeficiency virus (HIV) regulates the expression of its genes temporally at the mRNA processing step. A subset of the mRNA species which encode the structural and some accessory genes contains inhibitory sequences (INS or CRS elements) which prevent nuclear export of the RNA or its utilization in the cytoplasm. Such inhibition is overridden by the interaction of a viral protein, Rev, with its RNA target sequence, RRE. The vif gene product, which is essential for virus replication in vivo, is encoded by a singly spliced mRNA, and its expression is dependent on rev in infected cells. However, INS elements have not been found in the HIV-1 vif gene itself, although such elements have been observed in Gag, Pol, and Env coding sequences. We have now identified an INS within the 5' half of HIV-2 vif which does not show any homology with cellular mRNAs or other previously identified INS and CRS elements of HIV. These results suggest that retroviral mRNAs have novel labile sequences different from those of cellular mRNAs.

Identification of positive and negative splicing regulatory elements within the terminal tat-rev exon of human immunodeficiency virus type 1

The requirement of human immunodeficiency virus type 1 to generate numerous proteins from a single primary transcript is met largely by the use of suboptimal splicing to generate over 30 mRNAs. To ensure that appropriate quantities of each protein are produced, there must be a signal(s) that controls the efficiency with which any particular splice site in the RNA is used. To identify this control element(s) and to understand how it operates to generate the splicing pattern observed, we have initially focused on the control of splicing of the tat-rev intron, which spans the majority of the env open reading frame. Previous analysis indicated that a suboptimal branchpoint and polypyridimine tract in this intron contribute to its suboptimal splicing (A. Staffa and A. Cochrane, J. Virol. 68:3071-3079, 1994). In this report, we identify two additional elements within the 3'-terminal exon, an exon-splicing enhancer (ESE) and an exon splicing silencer (ESS), that modulate the overall efficiency with which the 3' tat-rev splice site is utilized. Both elements are capable of functioning independently of one another. Furthermore, while both the ESE and ESS can function in a heterologous context, the function of the ESS is extremely sensitive to the sequence context into which it is placed. In conclusion, it would appear that the presence of a suboptimal branchpoint and a polypyrimidine tract as well as the ESE and ESS operate together to yield the balanced splicing of the tat-rev intron observed in vivo.

Expression and purification of the HIV type 1 Rev protein produced in Escherichia coli and its use in the generation of monoclonal antibodies

We have developed a simple and rapid procedure for the purification of large amounts of Rev protein overexpressed in E. coli. The purification method, which does not require denaturation of the protein, takes advantage of the positively charged nature of Rev and the ability of Rev to interact with nucleic acids. The purified protein was used to develop three novel murine monoclonal antibodies against Rev. Using fusion proteins between glutathione S-transferase (GST) and various fragments of the Rev protein, we mapped the specificity of these antibodies to different regions of the Rev protein. One antibody, 3H6, is directed against the nucleolar localization/RRE-binding domain of Rev between amino acids 38 and 44. Another antibody, 3G4, recognizes an epitope between amino acids 90 and 116 of Rev. A third antibody, 2G2, does not recognize any of the fusion proteins, and may be directed against a conformational epitope. All three antibodies are able to detect Rev on Western blots and to immunoprecipitate Rev under native conditions. However, only 3H6 and 3G4 immunoprecipitate Rev under denaturing conditions and are able to detect Rev expressed in transfected cells by indirect immunofluorescence. These antibodies should prove useful in further studies of Rev function.

The roles of nucleolar structure and function in the subcellular location of the HIV-1 Rev protein

The human immunodeficiency virus 1 (HIV-1) Rev transactivator protein plays a critical role in the regulation of expression of structural proteins by controlling the pathway of mRNA transport. The Rev protein is located predominantly in the nucleoli of HIV-1 infected or Rev-expressing cells. Previous studies demonstrated that the Rev protein forms a specific complex in vitro with protein B23 which is suggested to be a nucleolar receptor and/or carrier for the Rev protein. To study the role of the nucleolus and nucleolar proteins in Rev function, transfected COS-7 or transformed CMT3 cells expressing the Rev protein were examined for subcellular locations of Rev and other proteins using indirect immunofluorescence and immunoelectron microscopy. One day after transfection the Rev protein was found in most cells only in the nucleolar dense fibrillar and granular components where it colocalized with protein B23. These were designated class 1 cells. In a second class of cells Rev and B23 accumulated in the nucleoplasm as well as in nucleoli. Treatment of class 1 cells with actinomycin D (AMD) under conditions that blocked only RNA polymerase I transcription caused Rev to completely redistribute from nucleoli to the cytoplasm. Simultaneously, protein B23 was partially released from nucleoli, mostly into the nucleoplasm, with detectable amounts in the cytoplasm. In cells recovering from AMD treatment in the presence of cycloheximide Rev and B23 showed coincident relocation to nucleoli. Class 2 cells were resistant to AMD-induced Rev redistribution. Selective inhibition of RNA polymerase II transcription by alpha-amanitin or by DRB did not cause Rev to be released into the cytoplasm suggesting that active preribosomal RNA transcription is required for the nucleolar location of Rev. However, treatment with either of the latter two drugs at higher doses and for longer times caused partial disruption of nucleoli accompanied by translocation of the Rev protein to the cytoplasm. These results suggest that the nucleolar location of Rev depends on continuous preribosomal RNA transcription and a substantially intact nucleolar structure.

Presence of exon splicing silencers within human immunodeficiency virus type 1 tat exon 2 and tat-rev exon 3: evidence for inhibition mediated by cellular factors

Human immunodeficiency virus type 1 (HIV-1) pre-mRNA splicing is regulated in order to maintain pools of unspliced and partially spliced viral RNAs as well as the appropriate levels of multiply spliced mRNAs during virus infection. We have previously described an element in tat exon 2 that negatively regulates splicing at the upstream tat 3' splice site 3 (B. A. Amendt, D. Hesslein, L.-J. Chang, and C. M. Stoltzfus, Mol. Cell. Biol. 14:3960-3970, 1994). In this study, we further defined the element to a 20-nucleotide (nt) region which spans the C-terminal vpr and N-terminal tat coding sequences. By analogy with exon splicing enhancer (ESE) elements, we have termed this element an exon splicing silencer (ESS). We show evidence for another negative cis-acting region within tat-rev exon 3 of HIV-1 RNA that has sequence motifs in common with a 20-nt ESS element in tat exon 2. This sequence is juxtaposed to a purine-rich ESE element to form a bipartite element regulating splicing at the upstream tat-rev 3' splice site. Inhibition of the splicing of substrates containing the ESS element in tat exon 2 occurs at an early stage of spliceosome assembly. The inhibition of splicing mediated by the ESS can be specifically abrogated by the addition of competitor RNA. Our results suggest that HIV-1 RNA splicing is regulated by cellular factors that bind to positive and negative cis elements in tat exon 2 and tat-rev exon 3.

HnRNP L binds a cis-acting RNA sequence element that enables intron-dependent gene expression

Most pre-mRNAs require an intron for efficient processing in higher eukaryotes. To test the hypothesis that intron-independent gene expression involves positive, cis-acting RNA sequence elements, we constructed chimeric genes in which various regions of the naturally intronless HSV-TK gene were inserted into an intronless variant of the highly intron-dependent human beta-globin gene. Using a transient transfection assay, we identified a 119-nucleotide sequence element contained within the transcribed region of the HSV-TK gene that enables efficient cytoplasmic accumulation of globin RNA in the absence of splicing. RNA UV-cross-linking assays indicated that a 68-kD protein present in nuclear extracts of HeLa and COS cells specifically binds to this HSV-TK sequence element. This 68-kD protein was found to cross-react with an antiserum specific to hnRNP L. Recombinant hnRNP L was shown to bind with high sequence specificity to this RNA sequence element. Analysis of substitution mutants in this element indicated that binding of hnRNP L correlates with accumulation of the RNA in the cytoplasm. Thus, we conclude that (1) hnRNP L binds in a sequence-specific manner to this RNA sequence element that enables intron-independent gene expression, and (2) intron-independent pre-mRNA processing and transport involves sequence-specific RNA-protein interactions between cis-acting RNA sequence elements and proteins such as hnRNP L. This sequence element may be of general use for the efficient expression of cDNA versions of intron-dependent genes.

Efficiency of reinitiation of translation on human immunodeficiency virus type 1 mRNAs is determined by the length of the upstream open reading frame and by intercistronic distance

In this study, we examined the mechanism of translation of the human immunodeficiency virus type 1 tat mRNA in eucaryotic cells. This mRNA contains the tat open reading frame (ORF), followed by rev and nef ORFs, but only the first ORF, encoding tat, is efficiently translated. Introduction of premature stop codons in the tat ORF resulted in efficient translation of the downstream rev ORF. We show that the degree of inhibition of translation of rev is proportional to the length of the upstream tat ORF. An upstream ORF spanning 84 nucleotides was predicted to inhibit 50% of the ribosomes from initiating translation at downstream AUGs. Interestingly, the distance between the upstream ORF and the start codon of the second ORF also played a role in efficiency of downstream translation initiation. It remains to be investigated if these conclusions relate to translation of mRNAs other than human immunodeficiency virus type 1 mRNAs. The strong inhibition of rev translation exerted by the presence of the tat ORF may reflect the different roles of Tat and Rev in the viral life cycle. Tat acts early to induce high production of all viral mRNAs. Rev induces a switch from the early to the late phase of the viral life cycle, resulting in production of viral structural proteins and virions. Premature Rev production may result in entrance into the late phase in the presence of suboptimal levels of viral mRNAs coding for structural proteins, resulting in inefficient virus production.

Localization of human immunodeficiency virus Rev in transfected and virus-infected cells

The rev gene product of human immunodeficiency virus (HIV) is obligatory for viral replication. Rev interacts specifically with a structured RNA sequence within the viral genome termed the REV response element (RRE). Although the importance of Rev for the expression of viral proteins is well documented, its functional mechanism remains unresolved. Previous studies identified Rev in the absence of RRE to be a nuclear protein localized primarily within the nucleoli. To extend our understanding of the role of Rev in viral replication, immunolocalization studies of Rev and other nuclear components were carried out in transfected cells expressing both the Rev protein and RRE-containing mRNA and in cells infected with HIV. In both types of cells, Rev-like immunoreactivity was distributed both in the nucleoplasm and cytoplasm. Within the nucleus, Rev immunoreactivity was not evenly distributed but was present within focal concentrations. In transfected cells that were double labeled for Rev and SC-35, which labels a known component of spliceosomes, the foci of Rev labeling were distinct from the "speckles" labeled by SC-35, although Rev foci and speckles were often juxtaposed. In addition, morphological changes in the three-dimensional network of speckles were observed in both transfected cells expressing both the Rev protein and RRE-containing mRNA and in cells infected with HIV-1 and HIV-2. Our observations are consistent with the proposed dual role of Rev in mRNA transport and splicing.

Nuclear export of the human immunodeficiency virus type 1 nucleocytoplasmic shuttle protein Rev is mediated by its activation domain and is blocked by transdominant negative mutants

The human immunodeficiency virus type 1 nucleocytoplasmic shuttle protein Rev moves repeatedly between the cytoplasm, a perinuclear zone, the nucleoli, and nucleoplasmic speckles. In this study, we demonstrated by both indirect immunofluorescence and Western immunoblot analysis that nuclear exit of Rev transdominant negative mutants was defective compared with that of wild-type Rev. The basic and activation domains of Rev signal import and export, respectively, of Rev across the nuclear membrane. In cotransfection experiments, mutants containing mutations of Rev inhibited the nuclear egress of wild-type Rev, thus revealing a novel transdominant negative phenotype.

Requirement of N-terminal amino acid residues of gp41 for human immunodeficiency virus type 1-mediated cell fusion

An expression vector was designed to test the structural requirements of the gp41 N terminus for human immunodeficiency virus type 1-induced membrane fusion. Mutations in the region coding for the N terminus of gp41 were found to disrupt glycoprotein expression because of deleterious effects on the Rev-responsive element (RRE). Insertion of an additional RRE in the 3'-noncoding sequence of env made possible efficient glycoprotein expression, irrespective of the mutations introduced into the RRE in the natural location. This permitted the insertion of the unique restriction site SpeI within the N-terminal sequences of gp41, allowing convenient and efficient mutation of the gp41 N terminus by using double-stranded synthetic oligonucleotides. Mutants with deletions of 1 to 7 amino acids of the N terminus were constructed. Expression and cleavage of all mutants were confirmed by Western immunoblot analysis with anti-gp41 antibodies. The capability of mutants to induce membrane fusion was monitored following transfection of HeLa-T4+ cell lines with wild-type and mutant expression vectors by electroporation and microinjection. The efficiency of cell-fusing activity decreased drastically with deletion of 3 and 4 amino acids and was completely lost with deletion of 5 amino acids. Cotransfection of the parent and mutant expression vectors resulted in reduced cell-fusing activity. The extent of this dominant interference by mutant glycoprotein paralleled the decrease in cell-fusing activity of the mutants alone. This suggests the existence of a specific N-terminal structure required for fusing activity. However, there does not appear to be a stringent requirement for the precise length of the N terminus. This finding is supported by the length variation of this region among natural human immunodeficiency virus type 1 isolates and is in contrast to the apparent stringency in the length of analogous N-terminal structures of influenza A virus and paramyxovirus fusion glycoproteins.

The human immunodeficiency virus type 1 Rev protein and the Rev-responsive element counteract the effect of an inhibitory 5' splice site in a 3' untranslated region

A 5' splice site located in a 3' untranslated region (3'UTR) has been shown previously to inhibit gene expression. Natural examples of inhibitory 5' splice sites have been identified in the late 3'UTRs of papillomaviruses and are thought to inhibit viral late gene expression at early stages of the viral life cycle. In this study, we demonstrate that the interaction of the human immunodeficiency virus type 1 Rev protein with the Rev-responsive element (RRE) overcomes the inhibitory effects of a 5' splice site located within a 3'UTR. This was studied by using both a bovine papillomavirus type 1 L1 cDNA expression vector and a chloramphenicol acetyltransferase expression vector containing a 5' splice site in the 3'UTR. In both systems, coexpression of Rev enhanced cytoplasmic expression from vectors containing the RRE even when the RRE and the inhibitory 5' splice site were separated by up to 1,000 nucleotides. In addition, multiple copies of a 5' splice site in a 3'UTR were shown to act synergistically, and this effect could also be moderated by the interaction of Rev and the RRE. These studies provide additional evidence that at least one mechanism of Rev action is through interactions with the splicing machinery. We have previously shown that base pairing between the U1 small nuclear RNA and a 3'UTR 5' splice site is required for inhibition of gene expression. However, experiments by J. Kjems and P. A. Sharp (J. Virol. 67:4769-4776, 1993) have suggested that Rev acts on spliceosome assembly at a stage after binding of the U1 small nuclear ribonucleoprotein to the 5' splice site. This finding suggests that binding of additional small nuclear ribonucleoproteins, as well as other splicing factors, may be necessary for the inhibitory action of a 3'UTR 5' splice site. These data also suggest that expression of the papillomavirus late genes in terminally differentiated keratinocytes can be regulated by a viral or cellular Rev-like activity.

Residual expression of reporter genes in constructs mimicking HIV genome organization

Plasmids were constructed whereby the expression of a reporter gene, either the cDNA corresponding to the secreted form of human alkaline phosphatase (SEAP) or the herpes simplex virus type 1 (HSV1) thymidine kinase (tk) gene, was rendered dependent upon the expression of the human immunodeficiency virus type 1 (HIV1) tat and rev proteins. The SEAP or tk genes were placed between HIV1 splice donor and acceptor sites. One SEAP construct carried a series of alternating splice donor and acceptor sites. In all cases, the rev response element mapped within an intron. Despite such mimicry of the HIV1 genome, residual expression of the reporter gene in the absence of tat and rev was observed. These results, as well as non-specific T-cell recruitment, suggest limits to the specificity of using HIV-activated toxic gene expression to kill HIV-infected cells.

The Rev protein of human immunodeficiency virus type 1 counteracts the effect of an AU-rich negative element in the human papillomavirus type 1 late 3' untranslated region

We have identified a sequence in the late 3' untranslated region of human papillomavirus type 1 mRNAs that acts posttranscriptionally to repress gene expression. Deletion analysis localized the inhibitory element to an AU-rich sequence between nucleotides 6958 and 6984 on the human papillomavirus type 1 genome. This sequence inhibits gene expression in an orientation-dependent manner. Upon transfection of eucaryotic cells with plasmids containing this sequence, approximately 4-fold-lower cytoplasmic mRNA levels and 64- to 128-fold-lower protein levels were produced compared with those produced by plasmids lacking the inhibitory sequence. Interestingly, providing the constitutive transport element of simian retrovirus type 1 in sense orientation counteracted inhibition exerted by the human papillomavirus type 1 sequence. Inhibition could also be overcome by the presence of human immunodeficiency virus type 1 Rev protein in trans and its target sequence, the Rev-responsive element, in cis. Rev is a nuclear protein and acts by promoting nuclear export of human immunodeficiency virus type 1 mRNAs encoding structural proteins. Our results are consistent with a model for human papillomavirus type 1 late-gene expression in which mRNAs containing human papillomavirus type 1 inhibitory sequences enter a nonproductive route in the nucleus, resulting in inefficient mRNA utilization. Rev directs mRNA containing inhibitory sequences to a productive route by interacting with the Rev-responsive element.

Restriction of human immunodeficiency virus type 1 production in a human astrocytoma cell line is associated with a cellular block in Rev function

Chronically human immunodeficiency virus type 1 (HIV-1) strain IIIB-infected human TH4-7-5 astrocytoma cells show low-level virus production. Cocultivation of TH4-7-5 cells with myelomonocytic cells led to active virus production in these target cells after a lag period, indicating cell-determined restriction of virus replication in the glial cells. HIV-1 transcript patterns of TH4-7-5 cells contained only a small proportion of Rev-dependent mRNA species, mimicking a Rev-negative phenotype despite the presence of rev mRNAs and protein. Sequencing of the single provirus integrated in TH4-7-5 cells demonstrated that the rev gene and the Rev-responsive element are intact. These results suggested inhibited function of the Rev-regulatory unit in these astrocytoma cells. Transfection of TH4-7-5 cells with a Rev expression plasmid resulted in weak or no induction of proviral p24gag antigen levels compared with the dramatic increase observed in Rev-permissive HeLa cells. Immunofluorescence analysis of TH4-7-5 cells transfected with a rev-expressing plasmid revealed prominent cytoplasmic and nuclear-nucleolar localization of Rev, in contrast to the predominant nuclear-nucleolar localization pattern of Rev in HeLa cells. We conclude that restriction of virus production in TH4-7-5 cells is at least partially due to a block in Rev-dependent posttranscriptional regulation of HIV expression.

Sequence-specific inhibition of gene expression by a novel antisense oligodeoxynucleotide phosphorothioate directed against a nonregulatory region of the human immunodeficiency virus type 1 genome

Previous studies have demonstrated that oligodeoxynucleotide phosphorothioates complementary to human immunodeficiency virus type 1 (HIV-1) RNA are more nuclease resistant and are effective inhibitors of HIV-1 replication than their unmodified counterpart. In this study, antisense oligodeoxynucleotide sequences were evaluated for therapeutic potential in the treatment of HIV infections. The use of HIV-infected lymphocytes to test the efficacy of a drug is very complex, and therefore it is difficult to draw conclusions about the mechanism. We used a COS-like Monkey kidney cell line (CMT3) stably transfected with plasmids pCMVgagpol-rre-r (containing gag and pol genes) and pCMVrev (containing the rev gene of HIV-1), derived from cDNA clone BH10, as a model. A biologically active provirus that transcribes and translates their nucleotide sequences into viral proteins p24, p39/41, p55, and p160 was generated. Sequence-specific and dose-dependent inhibition of HIV-1 viral protein synthesis and significant inhibition at the mRNA level were demonstrated by antisense construct GPI2A, directed against a nonregulatory region of the HIV-1 genome. Also, our studies demonstrated enhancement of the antisense effect through encapsulation in a cationic lipid preparation. The observed attenuation of HIV-1 mRNA levels suggests that, at least in part, the mechanism of action of GPI2A was at the transcript level. Further studies have also shown antiviral activity of this construct as determined by the reverse transcriptase assay using acutely and chronically infected cells of lymphoid origin (H9 cells). Toxicological studies involving cell growth characteristics, colony-forming ability, effects on cellular proteins, specific activities of labeled proteins, and DNA synthesis in cell culture showed no cytotoxic effects of GPI2A.

Human immunodeficiency virus type 1 cellular RNA load and splicing patterns predict disease progression in a longitudinally studied cohort

We report the results of a longitudinal study of RNA splicing patterns in 31 early-stage human immunodeficiency virus disease patients with an average follow-up time of 3 years. Eighteen patients showed no evidence for disease progression, whereas 13 patients either showed a > or = 50% reduction in baseline CD4 count or developed opportunistic infections. Levels of unspliced, tat, rev, and nef mRNAs in peripheral blood mononuclear cells were measured by a reverse transcriptase-quantitative, competitive PCR assay. Viral RNA was detected in all patients at all time points. All 13 rapid progressors had viral RNA loads that were > or = 1 log unit greater than those of the slow progressors. In addition, seven of the rapid progressors showed a reduction of more than threefold in the ratio of spliced to unspliced RNA over the 3 years of follow-up. Conversely, two slow progressors with intermediate levels of viral RNA showed no splicing shift. These results confirm earlier observations that viral RNA is uniformly expressed in early-stage patients. We further show that cellular RNA viral load is predictive of disease progression. Importantly, the shift from a predominately spliced or regulatory viral mRNA pattern to a predominately unspliced pattern both is associated with disease progression and adds predictive utility to measurement of either RNA class alone.