Characterization of a replication-defective human immunodeficiency virus type 1 att site mutant that is blocked after the 3' processing step of retroviral integration

Two activities of retroviral integrase, 3' processing and DNA strand transfer, are required to integrate viral cDNA into a host cell chromosome. Integrase activity has been analyzed in vitro using purified protein and recombinant DNA substrates that model the U3 and U5 ends of viral cDNA or by using viral preintegration complexes (PICs) that form during virus infection. Numerous studies have investigated changes in integrase or viral DNA for effects on both 3' processing and DNA strand transfer activities using purified protein, but similar analyses have not been carried out using PICs. Here, we analyzed PICs from human immunodeficiency virus type 1 (HIV-1) strain 604del, an integration-defective mutant lacking 26 bp of U5, and revE1, a revertant of 604del containing an additional 19-bp deletion, for levels of 3' processing activity that occurred in infected cells and for levels of in vitro DNA strand transfer activity. Whereas revE1 supported one-third to one-half of the level of wild-type DNA strand transfer activity, the level of 604del DNA strand transfer activity was undetectable. Surprisingly, integrase similarly processed the 3' ends of 604del and revE1 in vivo. We therefore conclude that 604del is blocked in its ability to replicate in cells after the 3' processing step of retroviral integration. Whereas Western blotting showed that wild-type, revE1, and 604del PICs contained similar levels of integrase protein, Mu-mediated PCR footprinting revealed only minimal protein-DNA complex formation at the ends of 604del cDNA. We propose that 604del is replication defective because proteins important for DNA strand transfer activity do not stably associate with this cDNA after in vivo 3' processing by integrase.     

3'-end processing and kinetics of 5'-end joining during retroviral integration in vivo.

Retroviral replication depends on integration of viral DNA into a host cell chromosome. Integration proceeds in three steps: 3'-end processing, the endonucleolytic removal of the two terminal nucleotides from each 3' end of the viral DNA; strand transfer, the joining of the 3' ends of viral DNA to host DNA; and 5'-end joining (or gap repair), the joining of the 5' ends of viral DNA to host DNA. The 5'-end joining step has never been investigated, either for retroviral integration or for any other transposition process. We have developed an assay for 5'-end joining in vivo and have examined the kinetics of 5'-end joining for Moloney murine leukemia virus (MLV). The interval between 3'-end and 5'-end joining is estimated to be less than 1 h. This assay will be a useful tool for examining whether viral or host components mediate 5'-end joining. MLV integrates its DNA only after its host cell has completed mitosis. We show that the extent of 3'-end processing is the same in unsynchronized and aphidicolin-arrested cells. 3'-end processing therefore does not depend on mitosis.     

Changes to the HIV long terminal repeat and to HIV integrase differentially impact HIV integrase assembly, activity, and the binding of strand transfer inhibitors

Human immunodeficiency virus (HIV) integrase enzyme is required for the integration of viral DNA into the host cell chromosome. Integrase complex assembly and subsequent strand transfer catalysis are mediated by specific interactions between integrase and bases at the end of the viral long terminal repeat (LTR). The strand transfer reaction can be blocked by the action of small molecule inhibitors, thought to bind in the vicinity of the viral LTR termini. This study examines the contributions of the terminal four bases of the nonprocessed strand (G(2)T(1)C(-1)A(-2)) of the HIV LTR on complex assembly, specific strand transfer activity, and inhibitor binding. Base substitutions and abasic replacements at the LTR terminus provided a means to probe the importance of each nucleotide on the different functions. An approach is described wherein the specific strand transfer activity for each integrase/LTR variant is derived by normalizing strand transfer activity to the concentration of active sites. The key findings of this study are as follows. 1) The G(2):C(2) base pair is necessary for efficient assembly of the complex and for maintenance of an active site architecture, which has high affinity for strand transfer inhibitors. 2) Inhibitor-resistant enzymes exhibit greatly increased sensitivity to LTR changes. 3) The strand transfer and inhibitor binding defects of a Q148R mutant are due to a decreased affinity of the complex for magnesium. 4) Gln(148) interacts with G(2), T(1), and C(-1) at the 5' end of the viral LTR, with these four determinants playing important and overlapping roles in assembly, strand transfer catalysis and high affinity inhibitor binding.     

Integrase of Mason-Pfizer monkey virus

The gene encoding an integrase of Mason-Pfizer monkey virus (M-PMV) is located at the 3'-end of the pol open reading frame. The M-PMV integrase has not been previously isolated and characterized. We have now cloned, expressed, isolated, and characterized M-PMV integrase and compared its activities and primary structure with those of HIV-1 and other retroviral integrases. M-PMV integrase prefers untranslated 3'-region-derived long-terminal repeat sequences in both the 3'-processing and the strand transfer activity assays. While the 3'-processing reaction catalyzed by M-PMV integrase was significantly increased in the presence of Mn(2+) and Co(2+) and was readily detectable in the presence of Mg(2+) and Ni(2+) cations, the strand transfer activity was strictly dependent only on Mn(2+). M-PMV integrase displays more relaxed substrate specificity than HIV-1 integrase, catalyzing the cleavage and the strand transfer of M-PMV and HIV-1 long-terminal repeat-derived substrates with similar efficiency. The structure-based sequence alignment of M-PMV, HIV-1, SIV, and ASV integrases predicted critical amino acids and motifs of M-PMV integrase for metal binding, interaction with nucleic acids, dimerization, protein structure maintenance and function, as well as for binding of human immunodeficiency virus type 1 and Rous avian sarcoma virus integrase inhibitors 5-CI-TEP, DHPTPB and Y-3.     

Probing of HIV-1 integrase/DNA interactions using novel analogs of viral DNA

The specific activity of the human immunodeficiency virus, type 1 (HIV-1), integrase on the viral long terminal repeat requires the binding of the enzyme to certain sequences located in the U3 and U5 regions at the ends of viral DNA, but the determinants of this specific DNA-protein recognition are not yet completely understood. We synthesized DNA duplexes mimicking the U5 region and containing either 2'-modified nucleosides or 1,3-propanediol insertions and studied their interactions with HIV-1 integrase, using Mn2+ or Mg2+ ions as integrase cofactors. These DNA modifications had no strong effect on integrase binding to the substrate analogs but significantly affected 3'-end processing rate. The effects of nucleoside modifications at positions 5, 6, and especially 3 strongly depended on the cationic cofactor used. These effects were much more pronounced in the presence of Mg2+ than in the presence of Mn2+. Modifications of base pairs 7-9 affected 3'-end processing equally in the presence of both ions. Adenine from the 3rd bp is thought to form at least two hydrogen bonds with integrase that are crucial for specific DNA recognition. The complementary base, thymine, is not important for integrase activity. For other positions, our results suggest that integrase recognizes a fine structure of the sugar-phosphate backbone rather than heterocyclic bases. Integrase interactions with the unprocessed strand at positions 5-8 are more important than interactions with the processed strand for specific substrate recognition. Based on our results, we suggest a model for integrase interaction with the U5 substrate.     

Structural determinants for HIV-1 integrase inhibition by beta-diketo acids.

Among all the HIV-1 integrase inhibitors, the beta-diketo acids (DKAs) represent a major lead in anti-HIV-1 integrase drug design. These derivatives inhibit the integration reaction in vitro with a strong specificity for the 3'-end joining step. They are also antiviral and inhibit integration in vivo. The aim of the present study has been to investigate the molecular interactions between DKAs and HIV-1 integrase. We have compared 5CITEP with one of the most potent DKAs reported by the Merck group (L-708,906) and found that 5CITEP inhibits 3'-processing at concentrations where L-708,906 is only active on strand transfer. We also report a novel bifunctional DKA derivative that inhibits 3'-processing even more effectively than 5CITEP. The interactions of these inhibitors with the viral DNA donor ends have been studied by performing experiments with oligonucleotides containing defined modifications. We propose that the bifunctional DKA derivative binds to both the acceptor and donor sites of HIV-1 integrase, whereas the monofunctional L-708,906 derivative binds selectively to the acceptor site.     

Branched oligonucleotide-intercalator conjugate forming a parallel stranded structure inhibits HIV-1 integrase.

Integration of a DNA copy of the HIV-1 genome into chromosomal DNA of infected cells is a key step of viral replication. Integration is carried out by integrase, a viral protein which binds to both ends of viral DNA and catalyses reactions of the 3'-end processing and strand transfer. A 3'-3' branched oligonucleotide functionalised by the intercalator oxazolopyridocarbazole at each 5'-end was found to inhibit integration in vitro. We show that both a specific (G,A) sequence and the OPC intercalating agent contribute to the capability of the branched oligonucleotide to form a parallel stranded structure responsible for the inhibition.     

HIV-1 integrase can process a 3'-end crosslinked substrate.

Integrase of the human immunodeficiency virus type-1 (HIV-1) recognizes specific sequences located in the U3 and U5 regions at the ends of viral DNA. We synthesized DNA duplexes mimicking the U5 region and containing either 2'-aminonucleosides or non-nucleoside 1,3-propanediol insertions at the third and terminal positions and studied their interactions with HIV-1 integrase. Both modifications introduced a local structural distortion in the DNA double helix. Replacement of the terminal nucleosides by corresponding 2'-aminonucleosides had no significant effect on integrase activity. We used an integrase substrate bearing terminal 2'-aminonucleosides in both strands to synthesize a duplex with cross-linked strands. This duplex was then used to determine whether terminal base pair disruption is an obligatory step of retroviral DNA 3'-processing. Processing of the cross-linked analog of the integrase substrate yielded a product of the same length as 3'-processing of the wild-type substrate but the reaction efficiency was lower. Replacement of the third adenosine in the processed strand by a corresponding 2'-aminonucleoside did not affect integrase activity, whereas, its replacement by 1,3-propanediol completely inhibited 3'-processing. Both modifications of the complementary thymidine in the nonprocessed strand increased the initial rate of 3'-processing. The same effect was observed when both nucleosides, at the third position, were replaced by corresponding 2'-aminonucleosides. This indicates that the local duplex distortion facilitated the cleavage of the phosphodiester bond. Thus, a localized destabilization of the third A-T base pair is necessary for efficient 3'-processing, whereas 3'-end-fraying is important but not absolutely required.     

Both substrate and target oligonucleotide sequences affect in vitro integration mediated by human immunodeficiency virus type 1 integrase protein produced in Saccharomyces cerevisiae.

Integration of retroviral DNA into the host cell genome requires the interaction of retroviral integrase (IN) protein with the outer ends of both viral long terminal repeats (LTRs) to remove two nucleotides from the 3' ends (3' processing) and to join the 3' ends to newly created 5' ends in target DNA (strand transfer). We have purified the IN protein of human immunodeficiency virus type 1 (HIV-1) after production in Saccharomyces cerevisiae and found it to have many of the properties described for retroviral IN proteins. The protein performs both 3' processing and strand transfer reactions by using HIV-1 or HIV-2 attachment (att) site oligonucleotides. A highly conserved CA dinucleotide adjacent to the 3' processing site of HIV-1 is important for both the 3' processing and strand transfer reactions; however, it is not sufficient for full IN activity, since alteration of nucleotide sequences internal to the HIV-1 U5 CA also impairs IN function, and Moloney murine leukemia virus att site oligonucleotides are poor substrates for HIV-1 IN. When HIV-1 att sequences are positioned internally in an LTR-LTR circle junction substrate, HIV-1 IN fails to cleave the substrate preferentially at positions coinciding with correct 3' processing, implying a requirement for positioning att sites near DNA ends. The 2 bp normally located beyond the 3' CA in linear DNA are not essential for in vitro integration, since mutant oligonucleotides with single-stranded 3' or 5' extensions or with no residues beyond the CA dinucleotide are efficiently used. Selection of target sites is nonrandom when att site oligonucleotides are joined to each other in vitro. We modified an in vitro assay to distinguish oligonucleotides serving as the substrate for 3' processing and as the target for strand transfer. The modified assay demonstrates that nonrandom usage of target sites is dependent on the target oligonucleotide sequence and independent of the oligonucleotide used as the substrate for 3' processing.     

The strand transfer oligonucleotide inhibitors of HIV-integrase

Retroviral integrase participates in two catalytic reactions, which require interactions with the two ends of the viral DNA in the 3'processing reaction, and with a targeted host DNA in the strand transfer reaction. The 3'-hydroxyl group of 2'-deoxyadenosine resulting from the specific removing of GT dinucleotide from the viral DNA in the processing reaction provides the attachment site for the host DNA in a transesterification reaction. We synthesized oligonucleotides (ONs) of various lengths that mimic the processed HIV-1 U5 terminus of the proviral long terminal repeat (LTR) and are ended by 2'-deoxyadenosine containing a 3'-O-phosphonomethyl group. The duplex stability of phosphonomethyl ONs was increased by covalent linkage of the modified strand with its complementary strand by a triethylene glycol loop (TEG). Modified ONs containing up to 10 bases inhibited in vitro the strand transfer reaction catalyzed by HIV-1 integrase at nanomolar concentrations.     

Synthesis, biological evaluation and 3D-QSAR studies of 3-keto salicylic acid chalcones and related amides as novel HIV-1 integrase inhibitors

HIV-1 integrase is one of the three most important enzymes required for viral replication and is therefore an attractive target for anti retroviral therapy. We herein report the design and synthesis of 3-keto salicylic acid chalcone derivatives as novel HIV-1 integrase inhibitors. The most active compound, 5-bromo-2-hydroxy-3-[3-(2,3,6-trichlorophenyl)acryloyl]benzoic acid (25) was selectively active against integrase strand transfer, with an IC(50) of 3.7 μM. While most of the compounds exhibited strand transfer selectivity, a few were nonselective, such as 5-bromo-3-[3-(4-bromophenyl)acryloyl]-2-hydroxybenzoic acid (15), which was active against both 3'-processing and strand transfer with IC(50) values of 11±4 and 5±2 μM, respectively. The compounds also inhibited HIV replication with potencies comparable with their integrase inhibitory potencies. Thus, 5-bromo-2-hydroxy-3-[3-(2,3,6-trichlorophenyl)acryloyl]benzoic acid (25) and 5-bromo-3-[3-(4-bromophenyl)acryloyl]-2-hydroxybenzoic acid (15) inhibited HIV-1 replication with EC(50) values of 7.3 and 8.7 μM, respectively. A PHASE pharmacophore hypothesis was developed and validated by 3D-QSAR, which gave a predictive r(2) of 0.57 for an external test set of ten compounds. Phamacophore derived molecular alignments were used for CoMFA and CoMSIA 3D-QSAR modeling. CoMSIA afforded the best model with q(2) and r(2) values of 0.54 and 0.94, respectively. This model predicted all the ten compounds of the test set within 0.56 log units of the actual pIC(50) values; and can be used to guide the rational design of more potent novel 3-keto salicylic acid integrase inhibitors.     

The strand transfer oligonucleotide inhibitors of HIV-integrase

Retroviral integrase participates in two catalytic reactions, which require interactions with the two ends of the viral DNA in the 3′processing reaction, and with a targeted host DNA in the strand transfer reaction. The 3′-hydroxyl group of 2′-deoxyadenosine resulting from the specific removing of GT dinucleotide from the viral DNA in the processing reaction provides the attachment site for the host DNA in a transesterification reaction. We synthesized oligonucleotides (ONs) of various lengths that mimic the processed HIV-1 U5 terminus of the proviral long terminal repeat (LTR) and are ended by 2′-deoxyadenosine containing a 3′-O-phosphonomethyl group. The duplex stability of phosphonomethyl ONs was increased by covalent linkage of the modified strand with its complementary strand by a triethylene glycol loop (TEG). Modified ONs containing up to 10 bases inhibited in vitro the strand transfer reaction catalyzed by HIV-1 integrase at nanomolar concentrations.     

Structural insights into the retroviral DNA integration apparatus

Retroviral replication depends on successful integration of the viral genetic material into a host cell chromosome. Virally encoded integrase, an enzyme from the DDE(D) nucleotidyltransferase superfamily, is responsible for the key DNA cutting and joining steps associated with this process. Insights into the structural and mechanistic aspects of integration are directly relevant for the development of antiretroviral drugs. Recent breakthroughs have led to biochemical and structural characterization of the principal integration intermediates revealing the tetramer of integrase that catalyzes insertion of both 3' viral DNA ends into a sharply bent target DNA. This review discusses the mechanism of retroviral DNA integration and the mode of action of HIV-1 integrase strand transfer inhibitors in light of the recent visualization of the prototype foamy virus intasome, target DNA capture and strand transfer complexes.     

Concerted integration of viral DNA termini by purified avian myeloblastosis virus integrase.

Concerted integration of retroviral DNA termini, which produces a characteristic duplication of sequences at the integration site and formation of the proviral state, is a necessary step of the retroviral life cycle. We investigated the pairwise integration reaction catalyzed by purified avian retrovirus integrase by measuring the response to solution parameters and how the sequences of the viral termini, which comprise the avian imperfect inverted repeat, affect the reaction. When we optimized the reaction, an efficiency was achieved which approached that measured in systems using cytoplasmic extracts from virus-infected cells. The response of purified avian integrase to solution parameters was similar to that of the integration activity derived from cellular extracts. For strand transfer, the U3 viral terminal sequences were preferred to those of the U5 termini, a result we previously showed for the trimming reaction. That the sequence preference was the same for trimming and strand transfer may be further evidence that only one catalytic site is used for both reactions. A significant number of integration sites were sequenced. Interesting trends were found for the fidelity of the host duplications to the avian 6-bp duplication size, the clustering of the integration sites in the nonessential region of the lambda host DNA, and the sequence characteristics of the duplication sites.     

Four novel bis-(naphtho-gamma-pyrones) isolated from Fusarium species as inhibitors of HIV-1 integrase

Integration of viral DNA into host cell DNA is an essential step in retroviral (HIV-1) replication and is catalyzed by HIV-1 integrase. HIV-1 integrase is a novel therapeutic target and is the focus of efforts to identify effective inhibitors that will prevent/or cure HIV infections. Four novel naphtho-gamma-pyrones, belonging to the chaetochromin and ustilaginoidin family, were discovered as inhibitors of HIV-1 integrase from the screening of fungal extracts using a recombinant in vitro assay. These compounds inhibit both the coupled and strand transfer activity of HIV-1 integrase with IC(50) values of 1-3 and 4-12 microM, respectively. The discovery, structure elucidation, chemical modification and the structure-activity relationship of these compounds are described.     

Activity of recombinant HIV-1 integrase on mini-HIV DNA

Integration of the human immunodeficiency virus type 1 (HIV-1) cDNA into the genome of a human cell is an essential step in the viral replication cycle. Understanding of the integration process has been facilitated by the development of in vitro assays using specific oligonucleotides and recombinant integrase. However, understanding of the biology of retroviral integration will require in vitro and in vivo model systems using long DNA substrates that mimic the HIV cDNA. We have now studied the activity of recombinant HIV-1 integrase on a linear 4.7 kb double-stranded DNA, containing flanking regions of approximately 200 bp that represent the intact ends of the HIV-1 long terminal repeat (LTR) sequences (mini-HIV). The strand transfer products of the integration reaction can be directly visualized after separation in agarose gels by ethidium bromide staining. The most prominent reaction product resulted from integration of one LTR end into another LTR end (U5 into U5 and U5 into U3). Sequence analysis of the reaction products showed them to be products of legitimate integration preceded by correct processing of the viral LTR ends. Hotspots for integration were detected. Electron microscopy revealed the presence of a range of reaction products resulting from single or multiple integration events. The binding of HIV-1 integrase to mini-HIV DNA was visualized. Oligomers of integrase seem to induce DNA looping whereby the enzyme often appears to be bound to the DNA substrate that adopts the structure of a three-site synapsis that is reminiscent of the Mu phage transposase complex.     

Inhibition of Moloney murine leukemia virus integration using polyamides targeting the long-terminal repeat sequences

The retroviral integrase (IN) carries out the integration of the viral DNA into the host genome. Both IN and the DNA sequences at the viral long-terminal repeat (LTR) are required for the integration function. In this report, a series of minor groove binding hairpin polyamides targeting sequences within terminal inverted repeats of the Moloney murine leukemia virus (M-MuLV) LTR were synthesized, and their effects on integration were analyzed. Using cell-free in vitro integration assays, polyamides targeting the conserved CA dinucleotide with cognate sites closest to the terminal base pairs were effective at blocking 3' processing but not strand transfer. Polyamides which efficiently inhibited 3' processing and strand transfer targeted the LTR sequences through position 9. Polyamides that inhibited integration were effective at nanomolar concentrations and showed subnanomolar affinity for their cognate LTR sites. These studies highlight the role of minor groove interactions within the LTR termini for retroviral integration.     

Biochemical analysis of HIV-1 integrase variants resistant to strand transfer inhibitors

In this study, eight different HIV-1 integrase proteins containing mutations observed in strand transfer inhibitor-resistant viruses were expressed, purified, and used for detailed enzymatic analyses. All the variants examined were impaired for strand transfer activity compared with the wild type enzyme, with relative catalytic efficiencies (k(p)/K(m)) ranging from 0.6 to 50% of wild type. The origin of the reduced strand transfer efficiencies of the variant enzymes was predominantly because of poorer catalytic turnover (k(p)) values. However, smaller second-order effects were caused by up to 4-fold increases in K(m) values for target DNA utilization in some of the variants. All the variants were less efficient than the wild type enzyme in assembling on the viral long terminal repeat, as each variant required more protein than wild type to attain maximal activity. In addition, the variant integrases displayed up to 8-fold reductions in their catalytic efficiencies for 3'-processing. The Q148R variant was the most defective enzyme. The molecular basis for resistance of these enzymes was shown to be due to lower affinity binding of the strand transfer inhibitor to the integrase complex, a consequence of faster dissociation rates. In the case of the Q148R variant, the origin of reduced compound affinity lies in alterations to the active site that reduce the binding of a catalytically essential magnesium ion. Finally, except for T66I, variant viruses harboring the resistance-inducing substitutions were defective for viral integration.     

Assembly of prototype foamy virus strand transfer complexes on product DNA bypassing catalysis of integration

Integrase is the key enzyme that mediates integration of retroviral DNA into cellular DNA which is essential for viral replication. Inhibitors of HIV‐1 that target integrase recognize the nucleoprotein complexes formed by integrase and viral DNA substrate (intasomes) rather than the free enzyme. Atomic resolution structures of HIV‐1 intasomes are therefore required to understand the mechanisms of inhibition and drug resistance. To date, prototype foamy virus (PFV) is the only retrovirus for which such structures have been determined. We show that PFV strand transfer complexes (STC) can be assembled on product DNA without going through the normal forward reaction pathway. The finding that a retroviral STC can be assembled in this way may provide a powerful tool to alleviate the obstacles that impede structural studies of nucleoprotein intermediates in HIV‐1 DNA integration.     

Functional nucleotides of U5 LTR determining substrate specificity of prototype foamy virus integrase

In order to study functional nucleotides in prototype foamy virus (PFV) DNA on specific recognition by PFV integrase (IN), we designed chimeric U5 long terminal repeat (LTR) DNA substrates by exchanging comparative sequences between human immunodeficiency virus type-1 (HIV-1) and PFV U5 LTRs, and investigated the 3'-end processing reactivity using HIV-1 and PFV INs, respectively. HIV-1 IN recognized the nucleotides present in the fifth and sixth positions at the 3'-end of the substrates more specifically than any other nucleotides in the viral DNA. However, PFV IN recognized the eighth and ninth nucleotides as distinctively as the fifth and sixth nucleotides in the reactions. In addition, none of the nucleotides present in the twelfth, sixteenth, seventeenth, eighteenth, nineteenth, and twentieth positions were not differentially recognized by HIV-1 and PFV INs, respectively. Therefore, our results suggest that the functional nucleotides that are specifically recognized by its own IN in the PFV U5 LTR are different from those in the HIV-1 U5 LTR in aspects of the positions and nucleotide sequences. Furthermore, it is proposed that the functional nucleotides related to the specific recognition by retroviral INs are present inside ten nucleotides from the 3'-end of the U5 LTR.