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

Requirement for HIV-1 TAR sequences for Tat activation in rodent cells

HIV-1 gene expression is activated via an interaction between the virally encoded Tat protein and a target RNA, TAR. TAR is located at the immediate 5' end of all viral mRNAs and comprises a partially base-paired stem with a tripyrimidine bulge in the upper stem and a hexanucleotide loop. In vitro, Tat binds specifically to the bulge and upper stem region with no requirement for the loop. In contrast, when Tat activation is analyzed in primate cells, mutations in the loop abolish activation, suggesting a critical role for loop binding cellular factors. However, in rodent cells the reverse is true. Messages with a mutation in the TAR loop are activated whereas messages harboring a wild-type TAR sequence are not activated. By testing the effect of mutations in the bulge and stem in the context of mutation in the loop we now show that this loop-independent activation by Tat in rodent cells requires the critical bulge-stem sequences needed for Tat binding in vitro. These data suggest that in rodent cells, as in vitro, Tat does not require a loop binding cofactor. In rodent cells containing human chromosome 12 (CHO12), however, Tat activation is both bulge and loop dependent. It appears that rodent cells, but not CHO12 cells, are refractory to the normal Tat/TAR activation pathway not by virtue of lacking a loop binding cofactor, but rather by the presence of a loop binding inhibitor of either Tat binding or the activation process.

HIV-1 Tat protein is able to efficiently transactivate the HIV-2 LTR through a TAR RNA element lacking both dinucleotide bulge binding sites

Each of the two stem-loop structures in the HIV-2 TAR (TAR-2) RNA element contains a dinucleotide bulge that specifies a binding site in vitro for the HIV-2 Tat transactivator protein. A TAR-2 RNA with both bulges deleted is very weakly transactivated in vivo by the HIV-2 Tat protein. To gain insight into general features of Tat protein:TAR RNA interactions, we have analyzed the significance of the dinucleotide bulges in TAR-2 RNA for in vitro binding and in vivo transactivation by the related HIV-1 Tat protein. The HIV-1 Tat protein has been shown previously to bind efficiently to wild-type TAR-2 RNA and fully transactivates the HIV-2 LTR. We found that the 5' proximal bulge and the 3' distal bulge appear to specify a high and low affinity binding site in vitro, respectively, for the HIV-1 Tat protein. Wild-type TAR-2 RNA was found to be able to bind HIV-1 Tat proteins simultaneously at each bulge binding site in vitro. A TAR-2 RNA with both bulges deleted was greatly defective for in vitro binding by the HIV-1 Tat protein. Surprisingly, the TAR-2 RNA with both bulges deleted was efficiently transactivated in vivo by the HIV-1 Tat protein, indicating that the HIV-1 Tat protein (but not HIV-2 Tat protein) is able to strongly activate transcription of a TAR RNA with no apparent bulge binding site.

Interactions of human immunodeficiency virus type 1 transactivator of transcription protein with signal transduction pathways

The current state of knowledge investigating Tat interactions with signal transduction pathways is still in its infancy but has made significant progress toward understanding HIV pathology. This area is of great interest because Tat is among a small group of newly discovered RNA-based regulators of transcription. What is more important, however, are the implications of understanding these interactions concerning HIV-infected individuals. With the failure to develop effective HIV vaccines after years of development, it is becoming more feasible to conjecture therapies that target Tat as a means to keep HIV in its quiescent state rather than to eliminate the virus. In either case, the intense study of Tat and signal transduction pathways promises to provide a wealth of information about transcriptional control as well as the regulation of immune cell activation.

Biological activity and intracellular location of the Tat protein of equine infectious anemia virus

The Tat protein of equine infectious anemia virus (EIAV) was synthesized in Escherichia coli using the inducible expression plasmid, pET16b, which contains a His.Tag leader, thus allowing for rapid and efficient enrichment of the histidine-tagged protein by metal affinity chromatography. Yields of up to 20 mg of Tat were obtained from 10(11) bacterial cells. The recombinant Tat protein was shown to potently trans-activate the EIAV long terminal repeat (LTR) following its introduction into canine cells by 'scrape loading'. The EIAV Tat protein was found to localize predominantly within the cytoplasm, in contrast to HIV-1 Tat. The availability of large amounts of purified functional EIAV Tat protein should greatly facilitate detailed structure-function analyses.

Transcriptional activation in vitro by the human immunodeficiency virus type 1 Tat protein: evidence for specific interaction with a coactivator(s)

The Tat protein encoded by human immunodeficiency virus type 1 is a strong transcriptional activator of gene expression from the viral long terminal repeat and is essential for virus replication. We have investigated the molecular mechanism of Tat trans-activation by using a cell-free transcription system. We find that the trans-activation domain of Tat, amino acid residues 1-48 [Tat-(1-48)], can inhibit specifically--i.e., "squelch," transcriptional activation by full-length Tat [Tat-(1-86)]. Squelching depends upon the functional integrity of the Tat trans-activation domain because the mutant [Ala41]Tat-(1-48), which is defective in Tat trans-activation in vivo and in vitro, does not squelch in vitro Tat trans-activation. Inhibition is selective because Tat-activated transcription, but not Tat-independent transcription, is squelched. Preincubation experiments with Tat or Tat-(1-48) and nuclear extracts show that the trans-activation region of Tat can interact with cellular coactivator(s) required for Tat trans-activation and that this interaction can occur in the absence of the human immunodeficiency virus long terminal repeat promoter. Furthermore, the putative coactivator(s) mediating trans-activation by Tat differ from those mediating trans-activation by the acidic activator VP16, as shown by reciprocal squelching experiments in vitro. Our results suggest that specific cellular coactivator(s) are required for mediating activated transcription by human immunodeficiency virus type 1 Tat.

Structure of the equine infectious anemia virus Tat protein

Trans-activator (Tat) proteins regulate the transcription of lentiviral DNA in the host cell genome. These RNA binding proteins participate in the life cycle of all known lentiviruses, such as the human immunodeficiency viruses (HIV) or the equine infectious anemia virus (EIAV). The consensus RNA binding motifs [the trans-activation responsive element (TAR)] of HIV-1 as well as EIAV Tat proteins are well characterized. The structure of the 75-amino acid EIAV Tat protein in solution was determined by two- and three-dimensional nuclear magnetic resonance methods and molecular dynamics calculations. The protein structure exhibits a well-defined hydrophobic core of 15 amino acids that serves as a scaffold for two flexible domains corresponding to the NH2- and COOH-terminal regions. The core region is a strictly conserved sequence region among the known Tat proteins. The structural data can be used to explain several of the observed features of Tat proteins.

Protein interactions with DNA elements in variant equine infectious anemia virus enhancers and their impact on transcriptional activity

The long terminal repeats (LTRs) from various cloned equine infectious anemia virus (EIAV) proviruses differ significantly, but all contain cis-acting DNA elements identical to MDBP-, PEA2-, AP-1-, and PU.1 (ets)-binding sites. A prototype EIAV LTR would contain one of each of these conserved elements. The LTR variations originate from the insertion of novel sequences between the PEA2 and AP-1 elements in the transcriptional enhancer unit. Viewed in this way, the LTR from provirus clone lambda 12 has an 11-bp insertion containing a PEA2 site and the LTR of the lambda 6 provirus has a 31-bp insertion/duplication containing PEA2, AP-1, and PU.1 sites. Two other LTRs were cloned by amplification of cDNAs from the persistently infected cell line, EIAV-FEA. A third LTR was generated by site-directed mutagenesis of one of the LTRs from EIAV-FEA cells. The latter three had a single base change in the element next to the TATA box that abolished PU.1 binding; however, the variable regions of these LTRs were shown by gel mobility shift assays to contain one or two PU.1 sites. One variable region was shown to have an octamer site overlapping its tandem PU.1 elements. Basal, PMA-activated, and Tat trans-activated transcriptional activities of the LTRs were compared in several different cell lines by transient transfection. The various promoters displayed different relative levels of activity depending on the cell line used and the condition of activation. This natural set of variant promoters may help define how changes in the components of the transcription complex influence transactivation by Tat. The diverse LTRs could endow their respective proviruses with a unique pattern of expression and activation in vivo.

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.

Tat and the HIV-1 promoter

The HIV-1 Tat protein enhances the formation of productive RNA polymerase II elongation complexes, potentially acting through a positive-acting, DRB-sensitive elongation factor. Tat is usually recruited to the HIV-1 promoter through the Tat trans-activation response element RNA stem-loop structure; however, recent data suggest that in certain cell types it can be directed instead through upstream enhancer elements. New studies also reveal that the response element overlaps a novel motif that promotes the assembly of abortive elongation complexes in the absence of Tat.

Electrostatic interactions modulate the RNA-binding and transactivation specificities of the human immunodeficiency virus and simian immunodeficiency virus Tat proteins

The transcriptional activating (Tat) proteins from human immunodeficiency virus and simian immunodeficiency virus are sequence-specific RNA-binding proteins. In human immunodeficiency virus Tat, a single arginine residue, flanked on each side by three to four basic amino acids, mediates specific binding to a bulge region in trans-acting responsive element (TAR) RNA. We have systematically mutated the flanking charged residues and found that, in addition to the position of the sequence-specific arginine, the particular arrangement of nonspecific electrostatic interactions is an important determinant of RNA-binding specificity and transactivation activity. These additional electrostatic contacts may help stabilize the structure of TAR RNA when bound to arginine. One critical electrostatic interaction, located two residues N-terminal to the arginine, is absent in the simian immunodeficiency virus Tat protein and accounts for the difference in promoter specificities of the human and simian immunodeficiency viral proteins.

TAR RNA binding properties and relative transactivation activities of human immunodeficiency virus type 1 and 2 Tat proteins

Using gel shift assays, we found that the human immunodeficiency virus type 1 (HIV-1) Tat protein (Tat-1) bound both HIV-1 and HIV-2 TAR RNAs with similar high affinities. In contrast, the HIV-2 Tat protein (Tat-2) bound only TAR-2 RNA with high affinity. We conclude that the weak in vivo activity of Tat-2 on the HIV-1 long terminal repeat that has been observed previously is likely the result of low affinity for TAR-1 RNA. Additionally, TAR-2 RNA was found to contain multiple specific binding sites for Tat proteins. GAL4-Tat fusion proteins were analyzed to compare the relative transactivation activities of Tat-1 and Tat-2 in the absence of requirements for binding to TAR RNAs. The GAL4-Tat-2 protein was found to transactivate synthetic promoters containing GAL4 binding sites at levels severalfold higher than did the GAL4-Tat-1 protein.

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.

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).