Identification of lentivirus tat functional domains through generation of equine infectious anemia virus/human immunodeficiency virus type 1 tat gene chimeras

Equine infectious anemia virus Tat is a predominantly helical protein

Nuclear magnetic resonance (NMR) spectroscopy revealed features of the secondary structure of the equine infectious anemia virus (EIAV) Tat protein in solution. We could show that this protein, which is required in the replication cycle of lentiviruses, forms a predominantly helical structure in trifluoroethanol/water (40% by vol.) solution. In particular, the basic RNA-binding region and the adjacent core domain, which are highly conserved among lentiviral Tat proteins, show helix-type secondary structure under these conditions. Our observations, in concert with recent biochemical data from other laboratories, suggest that the core sequence region and the basic sequence region form interdependent structural domains, both possibly necessary for correct RNA binding.

Identification of the activation domain of equine infectious anemia virus rev

Several members of the lentivirus family of complex retroviruses have been shown to encode proteins that are functionally equivalent to the Rev posttranscriptional regulatory protein of human immunodeficiency virus type 1 (HIV-1). Furthermore, the domain organization of HIV-1 Rev, featuring a highly basic N-terminal RNA binding domain and a leucin-rich C-terminal effector domain, has also been shown to be highly conserved among Rev proteins derived from not only the primate but also the ovine and caprine lentiviruses. Although it has therefore appeared highly probable that the lentivirus equine infectious anemia virus (EIAV) also encodes a Rev, the predicted amino acid sequence of this putative EIAV regulatory protein does not display any evident homology to the basic and leucine-rich motifs characteristic of other known Rev proteins. By fusion of different segments of the proposed EIAV Rev protein to the well-defined RNA binding domain of either HIV-1 or visna virus Rev, we have identified a segment of this EIAV protein that can efficiently substitute in cis for the otherwise essential activation motif. Interestingly, the minimal EIAV Rev activation motif identified in this study comprises approximately 18 amino acids located toward the protein N terminus that lack any evident similarity to the leucine-rich activation domains found in these other lentivirus Rev proteins. It therefore appears that the Rev protein of EIAV, while analogous in function to Rev proteins defined in lentiviruses of primate, ovine, and caprine origin, is nevertheless distinguished by an entirely novel domain organization.

Oncogene activation of HIV-LTR-driven expression via the NF-kappa B binding sites

The Raf-1 proto-oncogene product is a highly regulated serine/threonine kinase that functions in signal transduction downstream from growth factor receptors and upstream from nuclear proto-oncogene products. Using a transient cotransfection assay we have found that activated Raf-1 activates expression from the HIV-LTR. Analysis of a series of 5' deletion and point mutations revealed the NF-kappa B motifs as the Raf-responsive element in the HIV-LTR. Moreover, Raf-BXB activated expression from heterologous promoters driven by the HIV NF-kappa B binding sites. In addition to Raf, we show that v-Src, v-H-Ras and v-Mos activate HIV-LTR expression through the NF-kappa B binding sites and v-H-Ras-induced HIV-LTR expression is mediated by Raf-1. These findings may have implications for the involvement of the cellular homologues of these oncogenes in the switch from latent to productive infection by HIV in response to T-cell activation.

Genetic evidence that the Tat proteins of human immunodeficiency virus types 1 and 2 can multimerize in the eukaryotic cell nucleus

The formation of dimers or higher-order multimers is critical to the biological activity of many eukaryotic regulatory proteins. However, biochemical analyses of the multimerization capacity of the Tat trans activator of human immunodeficiency virus types 1 (HIV-1) and 2 (HIV-2) have yielded contradictory results. We used the two-hybrid genetic assay for protein-protein interactions in the eukaryote Saccharomyces cerevisiae (S. Fields and O.-K. Song, Nature [London] 340:245-246, 1989) to examine the multimerization of Tat in vivo. Both HIV-1 and HIV-2 Tat are shown to form specific homo- but not heteromultimers in the yeast cell nucleus. Mutational analysis indicates a critical role for the essential core motif of Tat in mediating this interaction but demonstrates that efficient Tat multimerization does not require an intact cysteine motif. These data raise the possibility that the multimerization of Tat may be important for Tat function in higher eukaryotes.

Genetic analysis of the cofactor requirement for human immunodeficiency virus type 1 Tat function

The Tat protein of human immunodeficiency virus type 1 is a potent transcriptional trans activator of the viral long terminal repeat promoter element. Tat function requires the direct interaction of Tat with a cis-acting viral RNA target sequence termed the trans-activation response (TAR) element and has also been proposed to require at least one cellular cofactor. We have used a genetic approach to attempt to experimentally define the role of the cellular cofactor in Tat function and TAR binding. Our data suggest that neither Tat nor the cellular cofactor binds to TAR alone in vivo and indicate, instead, that the interaction of Tat with its cellular cofactor is a prerequisite for TAR binding. The known species tropism of lentivirus Tat proteins appears to arise from the fact that not only Tat but also the cellular cofactor can markedly influence the RNA sequence specificity of the resultant protein complex. These data also suggest that the Tat cofactor is likely a cellular transcription factor that has been highly conserved during vertebrate evolution. We hypothesize that the primary function of Tat is to redirect this cellular factor to a novel viral RNA target site and to thereby induce activation of viral gene expression.

Juxtaposition between activation and basic domains of human immunodeficiency virus type 1 Tat is required for optimal interactions between Tat and TAR

trans activation of the human immunodeficiency virus type 1 long terminal repeat requires that the viral trans activator Tat interact with the trans-acting responsive region (TAR) RNA. Although the N-terminal 47 amino acids represent an independent activation domain that functions via heterologous nucleic acid-binding proteins, sequences of Tat that are required for interactions between Tat and TAR in cells have not been defined. Although in vitro binding studies suggested that the nine basic amino acids from positions 48 to 57 in Tat bind efficiently to the 5' bulge in the TAR RNA stem-loop, by creating several mutants of Tat and new hybrid proteins between Tat and the coat protein of bacteriophage R17, we determined that this arginine-rich domain is not sufficient for interactions between Tat and TAR in vivo. Rather, the activation domain is also required and must be juxtaposed to the basic domain. Thus, in vitro TAR RNA binding does not translate to function in vivo, which suggests that other proteins are important for specific and productive interactions between Tat and TAR.

Translation of equine infectious anemia virus bicistronic tat-rev mRNA requires leaky ribosome scanning of the tat CTG initiation codon

We have examined the translational regulation of the equine infectious anemia virus (EIAV) bicistronic tat-rev mRNA. Site-directed mutagenesis of the tat leader region followed by expression of the tat-rev cDNA both in vitro and in transiently transfected cells established that tat translation is initiated exclusively at a CTG codon. Increasing the efficiency of tat translation by altering the CTG initiator to ATG resulted in a dramatic decrease in translation of the downstream (rev) cistron, indicating that leaky scanning of the tat CTG initiation codon permitted translation of the downstream rev cistron. Since the tat leader sequences precede the major EIAV splice donor and are therefore present at the 5' termini of both spliced and unspliced viral mRNAs, the expression of all EIAV structural and regulatory proteins is dependent on leaky scanning of the tat initiator.

The VP16 transcription activation domain is functional when targeted to a promoter-proximal RNA sequence

Among eukaryotic transcription trans-activators, the human immunodeficiency virus type 1 (HIV-1) Tat protein is exceptional in that its target site TAR is an RNA rather than a DNA sequence. Here, we confirm that fusion of Tat to the RNA-binding domain of the HIV-1 Rev protein permits the efficient activation of an HIV-1 long terminal repeat (LTR) promoter in which critical TAR sequences have been replaced by RNA sequences derived from the HIV-1 Rev response element (RRE). An RRE target sequence as small as 13 nucleotides is shown to form an effective in vivo target for Rev binding. More important, a fusion protein consisting of Rev attached to the VP16 transcription activation domain was also observed to efficiently activate the HIV-1 LTR from this nascent RNA target. These data demonstrate that trans-activation of transcription by acidic activation domains does not require a stable interaction with the promoter DNA and suggest that VP16, like Tat, can act on steps subsequent to the formation of the HIV-1 LTR preinitiation complex. The finding that the activation domains of VP16 and Tat are functionally interchangeable raises the possibility that these apparently disparate viral trans-activators may nevertheless act via similar mechanisms.

Mechanism of action of regulatory proteins encoded by complex retroviruses

Complex retroviruses are distinguished by their ability to control the expression of their gene products through the action of virally encoded regulatory proteins. These viral gene products modulate both the quantity and the quality of viral gene expression through regulation at both the transcriptional and posttranscriptional levels. The most intensely studied retroviral regulatory proteins, termed Tat and Rev, are encoded by the prototypic complex retrovirus human immunodeficiency virus type 1. However, considerable information also exists on regulatory proteins encoded by human T-cell leukemia virus type I, as well as several other human and animal complex retroviruses. In general, these data demonstrate that retrovirally encoded transcriptional trans-activators can exert a similar effect by several very different mechanisms. In contrast, posttranscriptional regulation of retroviral gene expression appears to occur via a single pathway that is probably dependent on the recruitment of a highly conserved cellular cofactor. These two shared regulatory pathways are proposed to be critical to the ability of complex retroviruses to establish chronic infections in the face of an ongoing host immune response.

Identification and characterization of the bovine immunodeficiency-like virus tat gene

A cDNA clone of the bovine immunodeficiency-like virus (BIV) trans-activator gene (tat) was identified and characterized. The tat cDNA clone was generated by splicing, and on the basis of sequence analysis, the Tat protein was found to be encoded entirely by the first exon. It is 103 amino acids in size and shares sequence homology with the human immunodeficiency virus (HIV) Tat. The BIV tat clone can trans activate the BIV promoter effectively, as measured by the expression of the bacterial chloramphenicol acetyltransferase gene, when transfected into bovine cells. Besides activating the BIV promoter, the BIV Tat can also trans activate the HIV promoter effectively. It is possible that BIV Tat and HIV Tat employ similar mechanisms in trans activation of the viral long terminal repeat-directed gene expression.

Human chromosome 12 is required for optimal interactions between Tat and TAR of human immunodeficiency virus type 1 in rodent cells

Levels of trans activation of the human immunodeficiency virus type 1 long terminal repeat (HIV-1 LTR) by the virally encoded transactivator Tat show marked species-specific differences. For example, levels of transactivation observed in Chinese hamster ovary (CHO) rodent cells are 10-fold lower than those in human cells or in CHO cells that contain the human chromosome 12. Thus, the human chromosome 12 codes for a protein or proteins that are required for optimal Tat activity. Here, the function of these cellular proteins was analyzed by using a number of modified HIV-1 LTRs and Tats. Neither DNA-binding proteins that bind to the HIV-1 LTR nor proteins that interact with the activation domain of Tat could be implicated in this defect. However, since species-specific differences were no longer observed with hybrid proteins that contain the activation domain of Tat fused to heterologous RNA-binding proteins, optimal interactions between Tat and the trans-acting responsive RNA (TAR) must depend on this factor(s).

Functional analysis of biologically distinct genetic variants of simian immunodeficiency virus isolated from a mandrill

We examined the biological properties of two infectious clones of a simian immunodeficiency virus, SIVMND, which were designated as pMD121 and pMD122. Upon transfection into CD4-negative cells, pMD122 generated virions much less efficiently than pMD121. Likewise, the growth kinetics in CD4-positive cells of virus derived from pMD122 were remarkably delayed relative to those of virus from pMD121. The cytocidal activity of the MD122 virus was also low. A series of recombinant clones were constructed from pMD121 and pMD122 to determine the sequence responsible for the low virulence of the MD122 virus. The genetic determinant in pMD122 responsible for its properties was mapped to within a region (316 base pairs) encompassing the tat, rev, and env coding sequences. Sequence analysis revealed that the two clones differed by only one nucleotide in this region. A nucleotide substitution G (pMD121) to T (pMD122) altered an arginine codon to a serine codon in the first tat coding exon. Transient transfection experiments showed that the tat activity of pMD122 was about twofold less than that of pMD121. These findings indicate that small differences in tat activity can have a dramatic effect on the biological behavior of SIVMND.

Functional classification of simian immunodeficiency virus isolated from a chimpanzee by transactivators

In reporter-based transient expression systems, we characterized simian immunodeficiency virus from a chimpanzee (SIVCPZ), with special reference to the human immunodeficiency virus type 1 (HIV-1). SIVCPZ was not equally activated by tat and rev transactivators derived from representative primate lentiviruses. HIV-1 alone activated SIVCPZ to the full extent in both tat and rev assays. The tat and rev gene products of SIVCPZ, as well as those of HIV-1, efficiently transactivated the other viruses. These results indicate that SIVCPZ is identical to HIV-1 with regard to the compatibility of tat and rev gene activities among four subgroups of primate lentiviruses.

Functional comparison of the basic domains of the Tat proteins of human immunodeficiency virus types 1 and 2 in trans activation

The trans-activator Tat proteins coded by human immunodeficiency virus type 1 (HIV-1) and HIV-2 appear to be similar in structure and function. However, the Tat protein of HIV-2 (Tat2) activates the HIV-1 long terminal repeat (LTR) less efficiently than Tat1 (M. Emerman, M. Guyader, L. Montagnier, D. Baltimore, and M. A. Muesing, EMBO J. 6:3755-3760, 1987). To determine the functional domain of Tat2 which contributes to this incomplete reciprocity, we have carried out domain substitution between Tat1 and Tat2 by exchanging the basic domains involved in Tat interaction with its target trans-activation-response (TAR) RNA structure. Our results indicate that Tat1 proteins containing substitutions of either 8 or 14 amino acids of the basic domain of Tat2 exhibited reduced trans activation of the HIV-1 LTR by about 1/20 or one-fourth the level induced by wt Tat1. In contrast, Tat2 containing a substitution of the 9-amino-acid basic domain of Tat1 trans activated HIV-1 LTR like native Tat1. A substitution of the highly conserved core domain of Tat2 with that of Tat1 did not have any significant effect on trans activation of the HIV-1 LTR. These results indicate that the basic domain of Tat2 contributes to its inefficient trans activation of the HIV-1 LTR. Mutation of an acidic residue (Glu) located between the core domain and the Arg-rich basic domain of Tat2 at position 77 to a Gly residue increased the activity of Tat2 substantially. These results further suggest that the presence of an acidic residue (Glu) adjacent to Arg-rich sequences may at least partially contribute to the reduced activity of the Tat2 basic domain.

Inhibition of human immunodeficiency virus type 1 Tat activity by coexpression of heterologous trans activators

We examined the mechanism of Tat-mediated trans activation through competition experiments employing Tat proteins of human immunodeficiency virus type 1 (HIV-1) and equine infectious anemia virus (EIAV). EIAV Tat, as well as chimeric EIAV/HIV-1 Tat proteins, inhibited HIV-1 Tat-mediated trans activation in a cell-type-dependent fashion. Furthermore, these proteins inhibited trans activation by Tat-bacteriophage R17 coat protein chimeras. Inhibition resulted from competition between activation domains of effectors and competitors for a limiting cellular cofactor. The context in which competitor activation domains were expressed contributed to the extent of inhibition. In transfected cells, EIAV Tat and all chimeric competitors were located primarily in the cytoplasm, whereas HIV-1 Tat was primarily located in the nucleus. These data are consistent with a model for trans activation in which the activation domain of Tat associates with and conveys a cellular factor to the transcription complex via the trans-acting-responsive element (TAR).

Genetic characterization of simian immunodeficiency virus isolated from an African mandrill

We constructed an infectious molecular clone of simian immunodeficiency virus from an African mandrill (SIVMND). Upon transfection, this clone directed the production of progeny virus particles infectious to and cytopathic for CD4+ human leukemia cells. Thirteen frameshift proviral mutants with an alteration in the eight open reading frames of SIVMND were generated by recombinant DNA techniques, and were analyzed biologically and biochemically. While mutations in the structural genes gag, pol, and env abolished viral growth and induction of cytopathology, mutants of the vif, vpr, and nef genes were fully biologically active. Of the tat and rev mutants, only one rev mutant grew in CD4+ cells with delayed kinetics. In reporter-based transient expression systems, transactivation potentials of the tat and rev mutants were evaluated. A mutant lacking 2nd coding exon of tat gene exhibited tat activity similar to that of the wild type clone. The infectious rev mutant was partially defective for rev gene activity.

A minimal lentivirus Tat

Transcriptional regulatory mechanisms found in lentiviruses employ RNA enhancer elements called trans-activation responsive (TAR) elements. These nascent RNA stem-loops are cis-acting targets of virally encoded Tat effectors. Interactions between Tat and TAR increase the processivity of transcription complexes and lead to efficient copying of viral genomes. To study essential elements of this trans activation, peptide motifs from Tats of two distantly related lentiviruses, equine infectious anemia virus (EIAV) and human immunodeficiency virus type 1 (HIV-1), were fused to the coat protein of bacteriophage R17 and tested on the long terminal repeat of EIAV, where TAR was replaced by the R17 operator, the target of the coat protein. This independent RNA-tethering mechanism mapped activation domains of Tats from HIV-1 and EIAV to 47 and 15 amino acids and RNA-binding domains to 10 and 26 amino acids, respectively. Thus, a minimal lentivirus Tat consists of 25 amino acids, of which 15 modify viral transcription and 10 bind to the target RNA stem-loop.

Mutational analysis of the equine infectious anemia virus Tat-responsive element

A hairpinlike structure is predicted to exist at the 5' end of equine infectious anemia virus (EIAV) RNA which is similar in many ways to the human immunodeficiency type 1 (HIV-1) Tat-responsive element (TAR). In EIAV, this structure has a shorter stem than in HIV-1 and lacks the uridine bulge. Primer extension analysis of EIAV RNA was used to identify the transcriptional start site in the viral long terminal repeat. Premature termination of primer elongation at the predicted double-stranded RNA region was frequently observed and suggests that the inferred hairpin structure exists under these conditions. We have functionally characterized EIAV TAR by site-directed mutagenesis and transient gene expression analysis. It is demonstrated here that the secondary structure of this element is essential for Tat action. Mutations that disrupted base pairing abolished TAR function, and compensatory mutations that restored the stem structure resulted in Tat activation. The TAR loop appears to be closed by two U.G base pairs that are likely to provide a unique structural motif recognized by the Tat protein. With one exception, substitutions of nucleotides within the EIAV loop sequence decreased TAR function. All nucleotide substitutions of the cytidine at position +14 increased EIAV Tat responsiveness; however, its deletion abolished trans activation. Our results lead us to propose that the EIAV and HIV-1 Tat systems employ closely related cis- and trans-acting components that probably act by the same mechanism.