Structure and functional implications of the polymerase active site region in a complex of HIV-1 RT with a double-stranded DNA template-primer and an antibody Fab fragment at 2.8 A resolution

Revisiting plus-strand DNA synthesis in retroviruses and long terminal repeat retrotransposons: dynamics of enzyme: substrate interactions

Although polypurine tract (PPT)-primed initiation of plus-strand DNA synthesis in retroviruses and LTR-containing retrotransposons can be accurately duplicated, the molecular details underlying this concerted series of events remain largely unknown. Importantly, the PPT 3' terminus must be accommodated by ribonuclease H (RNase H) and DNA polymerase catalytic centers situated at either terminus of the cognate reverse transcriptase (RT), and in the case of the HIV-1 enzyme, ∼70Å apart. Communication between RT and the RNA/DNA hybrid therefore appears necessary to promote these events. The crystal structure of the HIV-1 RT/PPT complex, while informative, positions the RNase H active site several bases pairs from the PPT/U3 junction, and thus provides limited information on cleavage specificity. To fill the gap between biochemical and crystallographic approaches, we review a multidisciplinary approach combining chemical probing, mass spectrometry, NMR spectroscopy and single molecule spectroscopy. Our studies also indicate that nonnucleoside RT inhibitors affect enzyme orientation, suggesting initiation of plus-strand DNA synthesis as a potential therapeutic target.

Conformational landscape of the human immunodeficiency virus type 1 reverse transcriptase non-nucleoside inhibitor binding pocket: lessons for inhibitor design from a cluster analysis of many crystal structures

Clustering of 99 available X-ray crystal structures of HIV-1 reverse transcriptase (RT) at the flexible non-nucleoside inhibitor binding pocket (NNIBP) provides information about features of the conformational landscape for binding non-nucleoside inhibitors (NNRTIs), including effects of mutation and crystal forms. The ensemble of NNIBP conformations is separated into eight discrete clusters based primarily on the position of the functionally important primer grip, the displacement of which is believed to be one of the mechanisms of inhibition of RT. Two of these clusters are populated by structures in which the primer grip exhibits novel conformations that differ from the predominant cluster by over 4 A and are induced by the unique inhibitors capravirine and rilpivirine/TMC278. This work identifies a new conformation of the NNIBP that may be used to design NNRTIs. It can also be used to guide more complete exploration of the NNIBP free energy landscape using advanced sampling techniques.

Structural insight into processive human mitochondrial DNA synthesis and disease-related polymerase mutations

Human mitochondrial DNA polymerase (Pol gamma) is the sole replicase in mitochondria. Pol gamma is vulnerable to nonselective antiretroviral drugs and is increasingly associated with mutations found in patients with mitochondriopathies. We determined crystal structures of the human heterotrimeric Pol gamma holoenzyme and, separately, a variant of its processivity factor, Pol gammaB. The holoenzyme structure reveals an unexpected assembly of the mitochondrial DNA replicase where the catalytic subunit Pol gammaA interacts with its processivity factor primarily via a domain that is absent in all other DNA polymerases. This domain provides a structural module for supporting both the intrinsic processivity of the catalytic subunit alone and the enhanced processivity of holoenzyme. The Pol gamma structure also provides a context for interpreting the phenotypes of disease-related mutations in the polymerase and establishes a foundation for understanding the molecular basis of toxicity of anti-retroviral drugs targeting HIV reverse transcriptase.

Thymidine analogue resistance suppression by V75I of HIV-1 reverse transcriptase: effects of substituting valine 75 on stavudine excision and discrimination

Val(75) of HIV-1 reverse transcriptase (RT) plays a role in positioning the template nucleotide +1 during the formation of the ternary complex. Mutations, such as V75M and V75A, emerge in patients infected with HIV-1 group M subtype B and group O variants, after failing treatment with stavudine (d4T) and other nucleoside RT inhibitors. V75I is an accessory mutation of the Q151M multidrug resistance complex of HIV-1 RT and is rarely associated with thymidine analogue resistance mutations (TAMs). In vitro, it confers resistance to acyclovir. TAMs confer resistance to zidovudine (AZT) and d4T by increasing the rate of ATP-mediated excision of the terminal nucleotide monophosphate (primer unblocking). In a wild-type HIV-1 group O RT sequence context, V75A and V75M conferred increased excision activity on d4T-terminated primers, in the presence of PP(i). In contrast, V75I decreased the PP(i)-mediated unblocking efficiency on AZT and d4T-terminated primers, in different sequence contexts (i.e. wild-type group M subtype B or group O RTs). Interestingly, in the sequence context of an excision-proficient RT (i.e. M41L/A62V/T69SSS/K70R/T215Y), the introduction of V75I led to a significant decrease of its ATP-dependent excision activity on AZT-, d4T-, and acyclovir-terminated primers. The excision rate of d4T-monophosphate in the presence of ATP (3.2 mm) was about 10 times higher for M41L/A62V/T69SSS/K70R/T215Y than for the mutant M41L/A62V/T69SSS/K70R/V75I/T215Y RT. The antagonistic effect of V75I with TAMs was further demonstrated in phenotypic assays. Recombinant HIV-1 containing the M41L/A62V/T69SSS/K70R/V75I/T215Y RT showed 18.3- and 1.5-fold increased susceptibility to AZT and d4T, respectively, in comparison with virus containing the M41L/A62V/T69SSS/K70R/T215Y RT.

Preferred sequences within a defined cleavage window specify DNA 3' end-directed cleavages by retroviral RNases H

The RNase H activity of reverse transcriptase carries out three types of cleavage termed internal, RNA 5' end-directed, and DNA 3' end-directed. Given the strong association between the polymerase domain of reverse transcriptase and a DNA 3' primer terminus, we asked whether the distance from the primer terminus is paramount for positioning DNA 3' end-directed cleavages or whether preferred sequences and/or a cleavage window are important as they are for RNA 5' end-directed cleavages. Using the reverse transcriptases of human immunodeficiency virus, type 1 (HIV-1) and Moloney murine leukemia virus (M-MuLV), we determined the effects of sequence, distance, and substrate end structure on DNA 3' end-directed cleavages. Utilizing sequence-matched substrates, our analyses showed that DNA 3' end-directed cleavages share the same sequence preferences as RNA 5' end-directed cleavages, but the sites must fall in a narrow window between the 15th and 20th nucleotides from the recessed end for HIV-1 reverse transcriptase and between the 17th and 20th nucleotides for M-MuLV. Substrates with an RNA 5' end recessed by 1 (HIV-1) or 2-3 (M-MuLV) bases on a longer DNA could accommodate both types of end-directed cleavage, but further recession of the RNA 5' end excluded DNA 3' end-directed cleavages. For HIV-1 RNase H, the inclusion of the cognate dNTP enhanced DNA 3' end-directed cleavages at the 17th and 18th nucleotides. These data demonstrate that all three modes of retroviral RNase H cleavage share sequence determinants that may be useful in designing assays to identify inhibitors of retroviral RNases H.

Solution characterization of [methyl-(13)C]methionine HIV-1 reverse transcriptase by NMR spectroscopy

HIV reverse transcriptase (RT) is a primary target for drug intervention in the treatment of AIDS. We report the first solution NMR studies of [methyl-(13)C]methionine HIV-1 RT, aimed at better understanding the conformational and dynamic characteristics of RT, both in the presence and absence of the non-nucleoside RT inhibitor (NNRTI) nevirapine. The selection of methionine as a structural probe was based both on its favorable NMR characteristics, and on the presence of two important active site methionine residues, M184(66) and M230(66). Observation of the M184 resonance is subunit dependent; in the p66 subunit the solvent-exposed residue produces a readily observed signal with a characteristic resonance shift, while in the globular p51 subunit, the M184(51) resonance is shifted and broadened as M184 becomes buried in the protein interior. In contrast, although structural data indicates that the environment of M230 is also strongly subunit dependent, the M230 resonances from both subunits have very similar shift and relaxation characteristics. A comparison of chemical shift and intensity data with model-based predictions gives reasonable agreement for M184(66), while M230(66), located on the beta-hairpin "primer grip", is more mobile and solvent-exposed than suggested by crystal structures of the apo enzyme which have a "closed" fingers-thumb conformation. This mobility of the primer grip is presumably important for binding of non-nucleoside RT inhibitors (NNRTIs), since the NNRTI binding pocket is not observed in the absence of the inhibitors, requiring instead that the binding pocket be dynamically accessible. In the presence of the nevirapine, both the M184(66) and M230(66) resonances are significantly perturbed, while none of the methionine resonances in the p51 subunit is sensitive to this inhibitor. Site-directed mutagenesis indicates that both M16 and M357 produce two resonances in each subunit, and for both residues, the intensity ratio of the component peaks is strongly subunit dependent. Conformational features that might explain the multiple peaks are discussed.

Molecular basis of human immunodeficiency virus drug resistance: an update

Antiretroviral therapy has led to a significant decrease in human immunodeficiency virus (HIV)-related mortality. Approved antiretroviral drugs target different steps of the viral life cycle including viral entry (coreceptor antagonists and fusion inhibitors), reverse transcription (nucleoside and non-nucleoside inhibitors of the viral reverse transcriptase), integration (integrase inhibitors) and viral maturation (protease inhibitors). Despite the success of combination therapies, the emergence of drug resistance is still a major factor contributing to therapy failure. Viral resistance is caused by mutations in the HIV genome coding for structural changes in the target proteins that can affect the binding or activity of the antiretroviral drugs. This review provides an overview of the molecular mechanisms involved in the acquisition of resistance to currently used and promising investigational drugs, emphasizing the structural role of drug resistance mutations. The optimization of current antiretroviral drug regimens and the development of new drugs are still challenging issues in HIV chemotherapy. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

The search for potent, small molecule NNRTIs: A review

AIDS has become the leading pandemic disease, and is the cause of death worldwide. Presently, HAART treatment, a combination of reverse transcriptase (RT) and protease inhibitors is also unsuccessful due to the virus getting resistant to the drugs because of mutational changes. Two types of RT inhibitors exist namely nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs). The NNRTIs which bind to an allosteric site on RT are an important arsenal of drugs against HIV-1. The specificity of NNRTIs towards HIV-1 has led to extensive structural and molecular modelling studies of enzyme complexes and chemical synthesis of second and third-generation NNRTIs. The major drawbacks of NNRTIs are generation of resistance and pharmacokinetic problems. By mutational studies of non-nucleoside inhibitor binding pocket (NNIBP) some amino acids which were found to play an important role in proper binding resulted less prone to mutation. In this review we present a chronological history of NNRTI development, also highlighting the need for small molecules belonging to the NNRTI class for the management of AIDS.

Antimicrobial strategies: inhibition of viral polymerases by 3'-hydroxyl nucleosides

Over the past 20 years, nucleoside analogues have constituted an arsenal of choice in the fight against HIV, hepatitis B and C viruses, and herpesviruses. Classical antiviral nucleosides such as zidovudine act as obligate chain terminators. Once incorporated as monophosphates into the viral nucleic acid, they immediately block the progression of the polymerase as a result of their lack of a reactive 3'-hydroxyl (3'-OH) group. This review explores beyond the paradigm of obligate chain termination, from a structural and a mechanistic perspective, the strategy of inhibiting viral polymerases (RNA- and DNA-dependant) with nucleoside analogues containing a 3'-OH group. Depending on their mechanism of action, these molecules typically fall into the following three categories: (i) delayed chain terminators; (ii) pseudo-obligate chain terminators; or (iii) mutagenic nucleosides. Delayed chain terminators (i.e. penciclovir, cidofovir and entecavir) block the polymerase at an internal position within the viral nucleic acid, whereas R7128 and the 4'C substituted nucleosides do not permit subsequent incorporation events. Ribavirin, 5-hydroxydeoxycytidine and KP1461 are not chain terminators. Instead, they inhibit viral replication after mispairing with the template base, resulting in random mutations that are often lethal. Finally, brivudine, clevudine and other L-nucleosides have unique or yet to be defined mechanisms of inhibition.

Elucidating the inhibition mechanism of HIV-1 non-nucleoside reverse transcriptase inhibitors through multicopy molecular dynamics simulations

Human immunodeficiency virus-1 reverse transcriptase (RT) inhibition is a major focus of current anti-AIDS drug discovery and development programs, comprising 17 of the 31 Food and Drug Administration-approved compounds. The emergence of the non-nucleoside RT inhibitor (NNRTI) class of compounds provides a highly specific and structurally diverse set of drugs, which act noncompetitively to perturb normal RT function. Despite a relatively rich set of crystallographic data of RT in various states, details of the allosteric modulation of RT dynamics by NNRTIs are lacking. Capturing this inhibitory mechanism could fuel the design of more effective inhibitors at the NNRTI site and also drive the identification of novel allosteric sites. To address this, we have performed multicopy molecular dynamics (MD) simulations of RT in the presence and absence of the NNRTI nevirapine (cumulative total simulation time, 360 ns). By comparing the collective motions of the MD and crystallographic structures, we demonstrate that the chief effect of NNRTIs is to constrain a key rigid-body motion between the "fingers" and "thumb" subdomains of the p66 subunit. We show that the NNRTI binding pocket (NNIBP) is proximal to the hinge points for this essential motion, and NNRTIs therefore act as "molecular wedges," sterically blocking the full range of motion. To explain how this impaired movement might result in the experimentally observed loss of polymerase activity, we show that the motion influences the geometry of key catalytic residues on opposite faces of the NNIBP. From a methodological point of view, our results suggest that the multicopy MD simulation approach is very useful when studying proteins that perform such large conformational changes.

Bridge water mediates nevirapine binding to wild type and Y181C HIV-1 reverse transcriptase--evidence from molecular dynamics simulations and MM-PBSA calculations

The important role of the bridge water molecule in the binding of HIV-1 reverse transcriptase (RT) inhibitor complex was elucidated by molecular dynamics (MD) simulations using an MM-PBSA approach. Binding free energies and thermodynamic property differences for nevirapine bound to wild type and Y181C HIV-1 reverse transcriptase were investigated, and the results were compared with available experimental data. MD simulations over 3 ns revealed that the bridge water formed three characteristic hydrogen bonds to nevirapine and two residues, His235 and Leu234, in the binding pocket. The energetic derived model, which was determined from the consecutive addition of a water molecule, confirmed that only the contribution from the bridge water was essential in the binding configuration. Including this bridge water in the MM-PBSA calculations reoriented the binding energies from -32.20 to -37.65 kcal/mol and -28.07 to -29.82 kcal/mol in the wild type and Y181C HIV-1 RT, respectively. From the attractive interactions via the bridge water, His235 and Leu234 became major contributions. We found that the bridge water is the key in stabilizing the bound complex; however, in the Y181C RT complex this bridge water showed weaker hydrogen bond formation, lack of attractive force to nevirapine and lack of binding efficiency, leading to the failure of nevirapine against the Y181C HIV-1 RT. Moreover, the dynamics of Val179, Tyr181Cys, Gly190 and Leu234 in the binding pocket showed additional attractive energetic contributions in helping nevirapine binding. These findings that the presence of a water molecule in the hydrophobic binding site plays an important role are a step towards a quantitative understanding of the character of bridge water in enzyme-inhibitor binding. This can be helpful in developing designs for novel non-nucleoside HIV-1 RT inhibitors active against the mutant enzyme.

When is it time for reverse transcription to start and go?

Upon cell infection by a retrovirus, the viral DNA polymerase, called reverse transcriptase (RT), copies the genomic RNA to generate the proviral DNA flanked by two long terminal repeats (LTR). A discovery twenty years ago demonstrated that the structural viral nucleocapsid protein (NC) encoded by Gag is an essential cofactor of reverse transcription, chaperoning RT during viral DNA synthesis. However, it is only recently that NC was found to exert a control on the timing of reverse transcription, in a spatio-temporal manner. This brief review summarizes findings on the timing of reverse transcription in wild type HIV-1 and in nucleopcapsid (NC) mutants where virions contain a large amount of newly made viral DNA. This brief review also proposes some explanations of how NC may control late reverse transcription during Gag assembly in virus producer cells.

Mutations in the thumb–connection and RNase H domain of HIV type-1 reverse transcriptase of antiretroviral treatment-experienced patients

Background

Antiretroviral therapy that targets HIV type-1 (HIV-1) reverse transcriptase (RT) can be linked to mutations in the thumb–connection (amino acids [AA] 241–426) and RNase H (AA 427–560) domains, which could affect drug resistance.

Methods

Genotypical and statistical analyses were performed on HIV-1 RT from 100 antiretroviral treatment-naive and 248 antiretroviral treatment-experienced patients, the majority of whom were infected with HIV-1 subtype B. The RT region was analysed in three parts: the polymerase (AA 1–240), thumb–connection (AA 241–426) and RNase H (AA 427–560) domains.

Results

The polymerase domain had statistically significant changes between the two groups at 24 AA positions that are known resistance sites. Within the thumb–connection domain, R284 and N348 had statistically significant changes between the groups ( P=0.007 and P≤0.001, respectively). In treatment-experienced patients, 17.3% had R284K, whereas 24.5% had N348I substitutions. Both R284 and N348 were 100% conserved in treatment-naive patients. Within the RNase H domain, only K451 showed a statistically significant change ( P≤0.001), with K451R present in 11% of treatment-experienced patients but remaining 100% conserved among treatment-naive patients.

Conclusions

RT mutations at three positions outside of the polymerase region were associated with antiretroviral therapy: R284K, N348I and K451R. Both R284K and K451R interact with the phosphate backbone of the template or primer in HIV-1 RT crystal structures and could potentially influence the positioning of the primer strand, thus affecting polymerization, the efficiency of nucleoside reverse transcriptase inhibitor excision and/or RNase H activity.

Identifying and characterizing a functional HIV-1 reverse transcriptase-binding site on integrase

Integrase (IN) from human immunodeficiency virus, type 1 (HIV-1) exerts pleiotropic effects in the viral replication cycle. Besides integration, IN mutations can impact nuclear import, viral maturation, and reverse transcription. IN and reverse transcriptase (RT) interact in vitro, and the IN C-terminal domain (CTD) is both necessary and sufficient for binding RT. We used nuclear magnetic resonance spectroscopy to identify a putative RT-binding surface on the IN CTD, and surface plasmon resonance to obtain kinetic parameters and the binding affinity for the IN-RT interaction. An IN K258A substitution that disrupts reverse transcription in infected cells is located at the putative RT-binding surface, and we found that this substitution substantially weakens IN CTD-RT interactions. We also identified two additional IN amino acid substitutions located at the putative RT-binding surface (W243E and V250E) that significantly impair viral replication in tissue culture. These results strengthen the notion that IN-RT interactions are biologically relevant during HIV-1 replication and also provide insights into this interaction at the molecular level.

HIV ribonuclease H: continuing the search for small molecule antagonists

Members of the ribonuclease H (RNase H) family of enzymes (EC 3.1.26.4), which are found in nearly all organisms, are endoribonucleases that specifically hydrolyze the phosphodiester bond of RNA in a RNA-DNA hybrid. In retroviruses such as HIV-1, the RNase H activity is part of reverse transcriptase, the enzyme that converts the viral ssRNA into dsDNA suitable for integration into the host cell genome. In HIV-1, RNase H plays an essential role in various stages of reverse transcription, and it has been known for 20 years that inhibiting RNase H activity renders HIV noninfectious. However, the development of potent and selective antagonists of HIV RNase H has made surprisingly slow progress, and so far no RNase H inhibitor is in clinical trial, rendering this enzyme an important, but as yet underexplored, drug target. The recently described crystal structure of human RNase H in complex with a RNA-DNA hybrid provides new insight into the mechanism of HIV RNase H activity, with the potential to unveil new niches for therapeutic intervention. The current status of RNase H screening efforts is reviewed here.

Structure and function of HIV-1 reverse transcriptase: molecular mechanisms of polymerization and inhibition

The rapid replication of HIV-1 and the errors made during viral replication cause the virus to evolve rapidly in patients, making the problems of vaccine development and drug therapy particularly challenging. In the absence of an effective vaccine, drugs are the only useful treatment. Anti-HIV drugs work; so far drug therapy has saved more than three million years of life. Unfortunately, HIV-1 develops resistance to all of the available drugs. Although a number of useful anti-HIV drugs have been approved for use in patients, the problems associated with drug toxicity and the development of resistance means that the search for new drugs is an ongoing process. The three viral enzymes, reverse transcriptase (RT), integrase (IN), and protease (PR) are all good drug targets. Two distinct types of RT inhibitors, both of which block the polymerase activity of RT, have been approved to treat HIV-1 infections, nucleoside analogs (NRTIs) and nonnucleosides (NNRTIs), and there are promising leads for compounds that either block the RNase H activity or block the polymerase in other ways. A better understanding of the structure and function(s) of RT and of the mechanism(s) of inhibition can be used to generate better drugs; in particular, drugs that are effective against the current drug-resistant strains of HIV-1.

Interactions between HIV-1 reverse transcriptase and the downstream template strand in stable complexes with primer-template

Background

Human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) forms stable ternary complexes in which RT is bound tightly at fixed positions on the primer-template (P/T). We have probed downstream interactions between RT and the template strand in the complex containing the incoming dNTP (+1 dNTP•RT•P/T complex) and in the complex containing the pyrophosphate analog, foscarnet (foscarnet•RT•P/T complex).

Methods and Results

UV-induced cross-linking between RT and the DNA template strand was most efficient when a bromodeoxyuridine residue was placed in the +2 position (the first template position downstream from the incoming dNTP). Furthermore, formation of the +1 dNTP•RT•P/T complex on a biotin-containing template inhibited binding of streptavidin when biotin was in the +2 position on the template but not when the biotin was in the +3 position. Streptavidin pre-bound to a biotin residue in the template caused RT to stall two to three nucleotides upstream from the biotin residue. The downstream border of the complex formed by the stalled RT was mapped by digestion with exonuclease RecJ_F. UV-induced cross-linking of the complex formed by the pyrophosphate analog, foscarnet, with RT and P/T occurred preferentially with bromodeoxyuridine in the +1 position on the template in keeping with the location of RT one base upstream in the foscarnet•RT•P/T complex (i.e., in the pre-translocation position).

Conclusions

For +1 dNTP•RT•P/T and foscarnet•RT•P/T stable complexes, tight interactions were observed between RT and the first unpaired template nucleotide following the bound dNTP or the primer terminus, respectively.

Delayed chain termination protects the anti-hepatitis B virus drug entecavir from excision by HIV-1 reverse transcriptase

Entecavir (ETV) is a potent antiviral nucleoside analogue that is used to treat hepatitis B virus (HBV) infection. Recent clinical studies have demonstrated that ETV is also active against the human immunodeficiency virus type 1 (HIV-1). Unlike all approved nucleoside analogue reverse transcriptase RT) inhibitors (NRTIs), ETV contains a 3'-hydroxyl group that allows further nucleotide incorporation events to occur. Thus, the mechanism of inhibition probably differs from classic chain termination. Here, we show that the incorporated ETV-monophosphate (MP) can interfere with three distinct stages of DNA synthesis. First, incorporation of the next nucleotide at position n + 1 following ETV-MP is compromised, although DNA synthesis eventually continues. Second, strong pausing at position n + 3 suggests a long range effect, referred to as "delayed chain-termination." Third, the incorporated ETV-MP can also act as a "base pair confounder" during synthesis of the second DNA strand, when the RT enzyme needs to pass the inhibitor in the template. Enzyme kinetics revealed that delayed chain termination is the dominant mechanism of action. High resolution foot-printing experiments suggest that the incorporated ETV-MP "repels" the 3'-end of the primer from the active site of HIV-1 RT, which, in turn, diminishes incorporation of the natural nucleotide substrate at position n + 4. Most importantly, delayed chain termination protects ETV-MP from phosphorolytic excision, which represents a major resistance mechanism for approved NRTIs. Collectively, these findings provide a rationale and important tools for the development of novel, more potent delayed chain terminators as anti-HIV agents.

Probing DNA polymerase activity with stereoisomeric 2′-fluoro-β-D-arabinose (2′F-araNTPs) and 2′-fluoro-β-D-ribose (2′F-rNTPs) nucleoside 5′-triphosphates

2′-Deoxy-2′-fluoro-β-D-ribonucleosides (2′F-rN) and 2′-deoxy-2′-fluoro-β-D-arabinonucleosides (2′F-araN) differ solely in the stereochemistry at the 2′-carbon of the furanose sugar ring. 2′F-rN 5′-triphosphates (2′F-rNTPs) are among the most commonly used sugar-modified nucleoside 5′-triphosphates (NTPs) for in vitro selection; however, the epimeric 2′F-araN 5′-triphosphates (2′F-araNTPs) have only recently been applied to polymerase-directed biosynthesis [C.G. Peng and M.J. Damha. J. Am. Chem. Soc. 129, 5310 (2007)]. The present study describes primer extension assays that compare, for the first time, the incorporation efficiency of the two isomeric NTPs, namely, 2′F-araNTPs or 2′F-rNTPs, by four DNA polymerases [Deep Vent (exo-), 9°Nm, HIV-1 RT, and MMLV-RT]. Under the conditions used, incorporation of 2′F-araTTP proceeded more efficiently relative to 2′F-rUTP, while the incorporation of 2′F-araCTP is comparable or slightly less efficient than that observed with 2′F-rCTP. Interestingly, these preferences were observed for all four of the DNA polymerases tested. Unexpected differences in NTP incorporation were observed for 2′F-rCTP vs. rCTP. Despite their seemingly similar conformation, they behaved striking differently in the in vitro polymerization assays. 2′F-rCTP is a much better substrate than the native counterpart (rCTP), an observation first made with human DNA polymerases [F.C. Richardson, R.D. Kuchta, A. Mazurkiewicz, K.A. Richardson. Biochem. Pharmacol. 59, 1045 (2000)]. In contrast, 2′F-rUTP behaved like rUTP, providing poor yield of full-length products. Taken together, this indicates that 2′F-rCTP is very unusual with regard to enzyme/substrate recognition; an observation that can be exploited for the production of DNA oligomers enriched with both ribose and arabinose modifications. These findings are timely given the significant interest and growing need to develop chemically modified oligonucleotides for therapeutic and diagnostic research. By examining the structure-activity relationship (SAR) of the ribose and arabinose sugar, this study furthers our understanding of how the nature of the 2′ substituent (e.g., α vs. β; F vs. OH) and the heterocyclic base affect NTP selection (specificity) by DNA polymerases.Key words: 2′F-rNTPs, 2′F-araNTPs, DNA polymerases, biosynthesis, modified nucleoside triphosphates.

Structure of the Tribolium castaneum telomerase catalytic subunit TERT

A common hallmark of human cancers is the overexpression of telomerase, a ribonucleoprotein complex that is responsible for maintaining the length and integrity of chromosome ends. Telomere length deregulation and telomerase activation is an early, and perhaps necessary, step in cancer cell evolution. Here we present the high-resolution structure of the Tribolium castaneum catalytic subunit of telomerase, TERT. The protein consists of three highly conserved domains, organized into a ring-like structure that shares common features with retroviral reverse transcriptases, viral RNA polymerases and B-family DNA polymerases. Domain organization places motifs implicated in substrate binding and catalysis in the interior of the ring, which can accommodate seven to eight bases of double-stranded nucleic acid. Modelling of an RNA-DNA heteroduplex in the interior of this ring demonstrates a perfect fit between the protein and the nucleic acid substrate, and positions the 3'-end of the DNA primer at the active site of the enzyme, providing evidence for the formation of an active telomerase elongation complex.

AKAP149 binds to HIV-1 reverse transcriptase and is involved in the reverse transcription

Like all retroviruses, human immunodeficiency virus type 1 (HIV-1) undergoes reverse transcription during its replication cycle. The cellular cofactors potentially involved in this process still remain to be identified. We show here that A-kinase anchoring protein 149 (AKAP149) interacts with HIV-1 reverse transcriptase (RT) in both the yeast two-hybrid system and human cells. The AKAP149 binding site has been mapped to the RNase H domain of HIV-1 RT. AKAP149 silencing by RNA interference in HIV-1-infected cells inhibited viral replication at the reverse transcription step. We selected single-point mutants of RT defective for AKAP149 binding and demonstrated that mutant G462R, despite retaining significant intrinsic RT activity in vitro, failed to carry out HIV-1 reverse transcription correctly in infected cells. This suggests that the interaction between RT and AKAP149 in infected cells may play an important role in HIV-1 reverse transcription.

Connection domain mutations N348I and A360V in HIV-1 reverse transcriptase enhance resistance to 3'-azido-3'-deoxythymidine through both RNase H-dependent and -independent mechanisms

Thymidine analogue-associated mutations (TAMs) in reverse transcriptase (RT) of the human immunodeficiency virus type 1 (HIV-1) cause resistance to 3'-azido-3'-deoxythymidine (AZT) through excision of the incorporated monophosphate. Mutations in the connection domain of HIV-1 RT can augment AZT resistance. It has been suggested that these mutations compromise RNase H cleavage, providing more time for AZT excision to occur. However, the underlying mechanism remains elusive. Here, we focused on connection mutations N348I and A360V that are frequently observed in clinical samples of treatment-experienced patients. We show that both N348I and A360V, in combination with TAMs, decrease the efficiency of RNase H cleavage and increase excision of AZT in the presence of the pyrophosphate donor ATP. The TAMs/N348I/A360V mutant accumulates transiently formed, shorter hybrids that can rebind to RT before the template is irreversibly degraded. These hybrids dissociate selectively from the RNase H-competent complex, whereas binding in the polymerase-competent mode is either not affected with N348I or modestly improved with A360V. Both connection domain mutations can compensate for TAM-mediated deficits in processive DNA synthesis, and experiments with RNase H negative mutant enzymes confirm an RNase H-independent contribution to increased levels of resistance to AZT. Moreover, the combination of diminished RNase H cleavage and increased processivity renders the use of both PP(i) and ATP advantageous, whereas classic TAMs solely enhance the ATP-dependent reaction. Taken together, our findings demonstrate that distinct, complementary mechanisms can contribute to higher levels of excision of AZT, which in turn can amplify resistance to this drug.

Dynamic binding orientations direct activity of HIV reverse transcriptase

The reverse transcriptase of human immunodeficiency virus (HIV) catalyses a series of reactions to convert the single-stranded RNA genome of HIV into double-stranded DNA for host-cell integration. This task requires the reverse transcriptase to discriminate a variety of nucleic-acid substrates such that active sites of the enzyme are correctly positioned to support one of three catalytic functions: RNA-directed DNA synthesis, DNA-directed DNA synthesis and DNA-directed RNA hydrolysis. However, the mechanism by which substrates regulate reverse transcriptase activities remains unclear. Here we report distinct orientational dynamics of reverse transcriptase observed on different substrates with a single-molecule assay. The enzyme adopted opposite binding orientations on duplexes containing DNA or RNA primers, directing its DNA synthesis or RNA hydrolysis activity, respectively. On duplexes containing the unique polypurine RNA primers for plus-strand DNA synthesis, the enzyme can rapidly switch between the two orientations. The switching kinetics were regulated by cognate nucleotides and non-nucleoside reverse transcriptase inhibitors, a major class of anti-HIV drugs. These results indicate that the activities of reverse transcriptase are determined by its binding orientation on substrates.

HuR interacts with human immunodeficiency virus type 1 reverse transcriptase, and modulates reverse transcription in infected cells

Reverse transcription of the genetic material of human immunodeficiency virus type 1 (HIV-1) is a critical step in the replication cycle of this virus. This process, catalyzed by reverse transcriptase (RT), is well characterized at the biochemical level. However, in infected cells, reverse transcription occurs in a multiprotein complex - the reverse transcription complex (RTC) - consisting of viral genomic RNA associated with viral proteins (including RT) and, presumably, as yet uncharacterized cellular proteins. Very little is known about the cellular proteins interacting with the RTC, and with reverse transcriptase in particular. We report here that HIV-1 reverse transcription is affected by the levels of a nucleocytoplasmic shuttling protein - the RNA-binding protein HuR. A direct protein-protein interaction between RT and HuR was observed in a yeast two-hybrid screen and confirmed in vitro by homogenous time-resolved fluorescence (HTRF). We mapped the domain interacting with HuR to the RNAse H domain of RT, and the binding domain for RT to the C-terminus of HuR, partially overlapping the third RRM RNA-binding domain of HuR. HuR silencing with specific siRNAs greatly impaired early and late steps of reverse transcription, significantly inhibiting HIV-1 infection. Moreover, by mutagenesis and immunoprecipitation studies, we could not detect the binding of HuR to the viral RNA. These results suggest that HuR may be involved in and may modulate the reverse transcription reaction of HIV-1, by an as yet unknown mechanism involving a protein-protein interaction with HIV-1 RT.

A Mg2+-induced conformational switch rendering a competent DNA polymerase catalytic complex

The structural and dynamical changes occurring before nucleotide addition were studied using molecular dynamics (MD) simulations of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) complexes containing one or two Mg2+ ions in the presence of dNTP. Our models revealed that the formation of a catalytically competent DNA polymerase complex required subtle rearrangements at the catalytic site A, which occurred only when an Mg2+ ion was bound. This model has been validated using pre-steady-state kinetics to show that free Mg2+ is necessary to obtain a catalytically competent polymerase. Kinetic studies carried out with Be2+ as a cofactor permitted the functional discrimination between metal sites A and B. At low concentrations, Be2+ increased the catalytic efficiency of the polymerase, while at higher concentrations, it competed with Mg2+ for binding to site A, and inhibited DNA polymerization. In agreement with experimental data, MD simulations revealed that the catalytic attack distance between the 3-OH of the primer and the phosphorus in complexes containing Be2+ instead of Mg2+ at site A was above 4.5 A. Our findings provide a detailed description of the mechanism of DNA polymerization and should be helpful to understand the molecular basis of DNA replication fidelity.

Toward chaperone-assisted crystallography: protein engineering enhancement of crystal packing and X-ray phasing capabilities of a camelid single-domain antibody (VHH) scaffold

A crystallization chaperone is an auxiliary protein that binds to a target of interest, enhances and modulates crystal packing, and provides high-quality phasing information. We critically evaluated the effectiveness of a camelid single-domain antibody (V(H)H) as a crystallization chaperone. By using a yeast surface display system for V(H)H, we successfully introduced additional Met residues in the core of the V(H)H scaffold. We identified a set of SeMet-labeled V(H)H variants that collectively produced six new crystal forms as the complex with the model antigen, RNase A. The crystals exhibited monoclinic, orthorhombic, triclinic, and tetragonal symmetry and have one or two complexes in the asymmetric unit, some of which diffracted to an atomic resolution. The phasing power of the Met-enriched V(H)H chaperone allowed for auto-building the entire complex using single-anomalous dispersion technique (SAD) without the need for introducing SeMet into the target protein. We show that phases produced by combining SAD and V(H)H model-based phases are accurate enough to easily solve structures of the size reported here, eliminating the need to collect multiple wavelength multiple-anomalous dispersion (MAD) data. Together with the presence of high-throughput selection systems (e.g., phage display libraries) for V(H)H, the enhanced V(H)H domain described here will be an excellent scaffold for producing effective crystallization chaperones.

Reduced dNTP binding affinity of 3TC-resistant M184I HIV-1 reverse transcriptase variants responsible for viral infection failure in macrophage

We characterized HIV-1 reverse transcriptase (RT) variants either with or without the (-)-2',3'-deoxy-3'-thiacytidine-resistant M184I mutation isolated from a single HIV-1 infected patient. First, unlike variants with wild-type M184, M184I RT variants displayed significantly reduced DNA polymerase activity at low dNTP concentrations, which is indicative of reduced dNTP binding affinity. Second, the M184I variant displayed a approximately 10- to 13-fold reduction in dNTP binding affinity, compared with the Met-184 variant. However, the k(pol) values of these two RTs were similar. Third, unlike HIV-1 vectors with wild-type RT, the HIV-1 vector harboring M184I RT failed to transduce cell types containing low dNTP concentrations, such as human macrophage, likely due to the reduced DNA polymerization activity of the M184I RT under low cellular dNTP concentration conditions. Finally, we compared the binary complex structures of wild-type and M184I RTs. The Ile mutation at position 184 with a longer and more rigid beta-branched side chain, which was previously known to alter the RT-template interaction, also appears to deform the shape of the dNTP binding pocket. This can restrict ground state dNTP binding and lead to inefficient DNA synthesis particularly at low dNTP concentrations, ultimately contributing to viral replication failure in macrophage and instability in vivo of the M184I mutation.

Murine leukemia virus reverse transcriptase: structural comparison with HIV-1 reverse transcriptase

Recent X-ray crystal structure determinations of Moloney murine leukemia virus reverse transcriptase (MoMLV RT) have allowed for more accurate structure/function comparisons to HIV-1 RT than were formerly possible. Previous biochemical studies of MoMLV RT in conjunction with knowledge of sequence homologies to HIV-1 RT and overall fold similarities to RTs in general, provided a foundation upon which to build. In addition, numerous crystal structures of the MoMLV RT fingers/palm subdomain had also shed light on one of the critical functions of the enzyme, specifically polymerization. Now in the advent of new structural information, more intricate examination of MoMLV RT in its entirety can be realized, and thus the comparisons with HIV-1 RT may be more critically elucidated. Here, we will review the similarities and differences between MoMLV RT and HIV-1 RT via structural analysis, and propose working models for the MoMLV RT based upon that information.

Mechanisms of resistance to nucleoside analogue inhibitors of HIV-1 reverse transcriptase

Human immunodeficiency virus (HIV) reverse transcriptase (RT) inhibitors can be classified into nucleoside and nonnucleoside RT inhibitors. Nucleoside RT inhibitors are converted to active triphosphate analogues and incorporated into the DNA in RT-catalyzed reactions. They act as chain terminators blocking DNA synthesis, since they lack the 3'-OH group required for the phosphodiester bond formation. Unfortunately, available therapies do not completely suppress viral replication, and the emergence of drug-resistant HIV variants is facilitated by the high adaptation capacity of the virus. Mutations in the RT-coding region selected during treatment with nucleoside analogues confer resistance through different mechanisms: (i) altering discrimination between nucleoside RT inhibitors and natural substrates (dNTPs) (e.g. Q151M, M184V, etc.), or (ii) increasing the RT's phosphorolytic activity (e.g. M41L, T215Y and other thymidine analogue resistance mutations), which in the presence of a pyrophosphate donor (usually ATP) allow the removal of chain-terminating inhibitors from the 3' end of the primer. Both mechanisms are implicated in multi-drug resistance. The excision reaction can be modulated by mutations conferring resistance to nucleoside or nonnucleoside RT inhibitors, and by amino acid substitutions that interfere with the proper binding of the template-primer, including mutations that affect RNase H activity. New developments in the field should contribute towards improving the efficacy of current therapies.

RNase H activity: structure, specificity, and function in reverse transcription

This review compares the well-studied RNase H activities of human immunodeficiency virus, type 1 (HIV-1) and Moloney murine leukemia virus (MoMLV) reverse transcriptases. The RNase H domains of HIV-1 and MoMLV are structurally very similar, with functions assigned to conserved subregions like the RNase H primer grip and the connection subdomain, as well as to distinct features like the C-helix and loop in MoMLV RNase H. Like cellular RNases H, catalysis by the retroviral enzymes appears to involve a two-metal ion mechanism. Unlike cellular RNases H, the retroviral RNases H display three different modes of cleavage: internal, DNA 3' end-directed, and RNA 5' end-directed. All three modes of cleavage appear to have roles in reverse transcription. Nucleotide sequence is an important determinant of cleavage specificity with both enzymes exhibiting a preference for specific nucleotides at discrete positions flanking an internal cleavage site as well as during tRNA primer removal and plus-strand primer generation. RNA 5' end-directed and DNA 3' end-directed cleavages show similar sequence preferences at the positions closest to a cleavage site. A model for how RNase H selects cleavage sites is presented that incorporates both sequence preferences and the concept of a defined window for allowable cleavage from a recessed end. Finally, the RNase H activity of HIV-1 is considered as a target for anti-virals as well as a participant in drug resistance.

Mechanistic insights into the role of Val75 of HIV-1 reverse transcriptase in misinsertion and mispair extension fidelity of DNA synthesis

The side chain of Val75 stabilizes the fingers subdomain of the human immunodeficiency virus type 1 reverse transcriptase (RT), while its peptide backbone interacts with the single-stranded DNA template (at nucleotide +1) and with the peptide backbone of Gln151. Specific DNA polymerase activities of mutant RTs bearing amino acid substitutions at position 75 (i.e., V75A, V75F, V75I, V75L, V75M, V75S and V75T) were relatively high. Primer extension experiments carried out in the absence of one deoxyribonucleoside-triphosphate suggested that mutations did not affect the accuracy of the RT, except for V75A, V75F, V75I, and to a lesser extent V75T. The fidelity of RTs bearing mutations V75F and V75I increased 1.8- and 3-fold, respectively, as measured by the M13 lacZ alpha forward mutation assay, while V75A showed 1.4-fold decreased accuracy. Steady- and pre-steady-state kinetics demonstrated that the increased fidelity of V75I and V75F was related to their decreased ability to extend mismatched template-primers, while misincorporation efficiencies were not significantly affected by mutations. The increased mispair extension fidelity of mutant V75I RT could be attributed to the nucleotide affinity loss, observed in reactions with mismatched template-primers. Altogether, these data suggest that Val75 interactions with the 5' template overhang are important determinants of fidelity.

Comparison of the thermal stabilities of reverse transcriptases from avian myeloblastosis virus and Moloney murine leukaemia virus

Reverse transcriptases (RTs) from avian myeloblastosis virus (AMV) and Moloney murine leukaemia virus (MMLV) have been most extensively used as a tool for conversion of RNA to DNA. In this study, we compared the thermal stabilities of AMV RT and MMLV RT by observing their irreversible thermal inactivation. The temperatures reducing initial activity by 50% in 10-min incubation, T(50), of AMV RT were 47 degrees C without the template-primer (T/P), poly(rA)-p(dT)(12-18), and 52 degrees C with the T/P (28 microM). T(50) of MMLV RT were 44 degrees C without the T/P and 47 degrees C with the T/P (28 microM). Unexpectedly, AMV RT was considerably activated when incubated with the T/P at 45 and 48 degrees C. Such activation was not observed in MMLV RT. These results suggest that AMV RT and MMLV RT are different in the following: (i) The intrinsic thermal stability of AMV RT is higher than that of MMLV RT; (ii) AMV RT is activated by thermal treatment with the T/P at 45-48 degrees C; and (iii) AMV RT is stabilized by the T/P more potently than MMLV RT. Thermodynamic analysis indicates that thermal inactivation of AMV RT and MMLV RT is due to the large entropy change of activation for thermal inactivation.

Mutations in the U5 region adjacent to the primer binding site affect tRNA cleavage by human immunodeficiency virus type 1 reverse transcriptase in vivo

In retroviruses, the first nucleotide added to the tRNA primer defines the end of the U5 region in the right long terminal repeat, and the subsequent removal of this tRNA primer by RNase H exactly defines the U5 end of the linear double-stranded DNA. In most retroviruses, the entire tRNA is removed by RNase H cleavage at the RNA/DNA junction. However, the RNase H domain of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase cleaves the tRNA 1 nucleotide from the RNA/DNA junction at the U5/primer binding site (PBS) junction, which leaves an rA residue at the U5 terminus. We made sequence changes at the end of the U5 region adjacent to the PBS in HIV-1 to determine whether such changes affect the specificity of tRNA primer cleavage by RNase H. In some of the mutants, RNase H usually removed the entire tRNA, showing that the cleavage specificity was shifted by 1 nucleotide. This result suggests that the tRNA cleavage specificity of the HIV-1 RNase domain H depends on sequences in U5.

Probing nonnucleoside inhibitor-induced active-site distortion in HIV-1 reverse transcriptase by transient kinetic analyses

Nonnucleoside reverse transcriptase inhibitors (NNRTI) are a group of structurally diverse compounds that bind to a single site in HIV-1 reverse transcriptase (RT), termed the NNRTI-binding pocket (NNRTI-BP). NNRTI binding to RT induces conformational changes in the enzyme that affect key elements of the polymerase active site and also the association between the two protein subunits. To determine which conformational changes contribute to the mechanism of inhibition of HIV-1 reverse transcription, we used transient kinetic analyses to probe the catalytic events that occur directly at the enzyme's polymerase active site when the NNRTI-BP was occupied by nevirapine, efavirenz, or delavirdine. Our results demonstrate that all NNRTI-RT-template/primer (NNRTI-RT-T/P) complexes displayed a metal-dependent increase in dNTP binding affinity (K(d) ) and a metal-independent decrease in the maximum rate of dNTP incorporation (k (pol)). The magnitude of the decrease in k (pol) was dependent on the NNRTI used in the assay: Efavirenz caused the largest decrease followed by delavirdine and then nevirapine. Analyses that were designed to probe direct effects on phosphodiester bond formation suggested that the NNRTI mediate their effects on the chemistry step of the DNA polymerization reaction via an indirect manner. Because each of the NNRTI analyzed in this study exerted largely similar phenotypic effects on single nucleotide addition reactions, whereas each of them are known to exert differential effects on RT dimerization, we conclude that the NNRTI effects on subunit association do not directly contribute to the kinetic mechanism of inhibition of DNA polymerization.

The excision mechanism in reverse transcriptase: pyrophosphate leaving and fingers opening are uncoupled events with the analogues AZT and d4T

We conducted molecular dynamics simulations of complexes of HIV-1 reverse transcriptase (RT) with the substrate and the antiretrovirals AZT and d4T for which resistance emerges via the excision mechanism. It is currently believed that excision results from the inability of AZT to translocate to the P site because of the steric hindrances imposed by the azide group. However, such explanation is far from satisfactory as d4T does not have such steric hindrances and still suffers from excision. Such contradiction motivated us for the present study. The results point to a new explanation for excision. RT preferably excises these inhibitors over the substrate as a consequence of a different pattern of hydrogen bridges they establish with the N site after incorporation. In the complexes with normal nucleotides, the fingers residues K65 and R72 establish hydrogen bonds mainly with the leaving PPi. With the inhibitors, those same residues establish hydrogen bonds primarily with the substituted nucleotides. Consequently, pyrophosphate is eliminated before the opening of the fingers domain, which allows ATP binding, with subsequent excision and development of drug resistance.

Insertions in the β3–β4 loop of reverse transcriptase of human immunodeficiency virus type 1 and their mechanism of action, influence on drug susceptibility and viral replication capacity

Introduction of antiretroviral therapy combining protease and reverse transcriptase (RT) inhibitors has dramatically improved the quality of life and survival of patients infected with the human immunodeficiency virus (HIV). However, effective long-term therapy of HIV-infection has been severely hampered by the development of drug resistance. Resistance to antiretroviral drugs is generally conferred by specific amino acid substitutions in the target gene of the drug. Yet, occasionally gene insertions are being observed. The most commonly observed insertion is seen during substrate analogue RT inhibitor therapy and is selected in the beta3-beta4 loop of the RT enzyme. This flexible loop is located in the fingers subdomain of the enzyme and plays an important role in substrate binding. The acquisition of drug resistance related mutations or insertions might come at a price, which is reduced performance of the enzyme resulting in a diminished replication capacity of the virus. Various types of insertions have been described, and, in this review, we have summarized these data and discussed the mechanism of action of the RT inserts and their impact on both drug susceptibility and replication capacity.

Stepping statistics of single HIV-1 reverse transcriptase molecules during DNA polymerization

DNA polymerases are protein machines that processively incorporate complimentary nucleotides into a growing double-stranded DNA (ds-DNA). Single-base nucleotide incorporation rates have been determined by stalling and restarting various polymerases, but intrinsic processive rates have been difficult to obtain, particularly for polymerases with low processivity, such as the human immunodeficiency virus type 1 reverse transcriptase (HIV RT) polymerase. Here we find, using a new fluorescence-based single-molecule polymerization assay, that the intrinsic processive DNA-dependent polymerization of HIV RT is approximately Poissionian (i.e. each nucleotide is added sequentially) with a rate of about 100 bases per second at 21 degrees C. From the same experiments, based on the stepping statistics of polymerization, we also estimate the rates for HIV RT early termination and final release of completely replicated primer-template DNA. In addition, by measuring the rate of polymerization as a function of temperature, we have estimated the activation energy for processive nucleotide incorporation.

HIV-1 protease dimer interface mutations that compensate for viral reverse transcriptase instability in infectious virions

Mature enzymes encoded within the human immunodeficiency virus type 1 (HIV-1) genome (protease (PR), reverse transcriptase (RT) and integrase (IN)) derive from proteolytic processing of a large polyprotein (Gag-Pol). Gag-Pol processing is catalyzed by the viral PR, which is active as a homodimer. The HIV-1 RT functions as a heterodimer (p66/p51) composed of subunits of 560 and 440 amino acid residues, respectively. Both subunits have identical amino acid sequence, but p51 lacks 120 residues that are removed by the HIV-1 PR during viral maturation. While p66 is the catalytic subunit, p51 has a primarily structural role. Amino acid substitutions affecting the stability of p66/p51 (i.e. F130W) have a deleterious effect on viral fitness. Previously, we showed that the effects of F130W are mediated by p51 and can be compensated by mutation T58S. While studying the dynamics of emergence of the compensatory mutation, we observed that mutations in the viral PR-coding region were selected in HIV clones containing the RT substitution F130W, before the imposition of T58S/F130W mutations. The PR mutations identified (G94S and T96S) improved the replication capacity of the F130W mutant virus. By using a trans-complementation assay, we demonstrate that the loss of p66/p51 heterodimer stability caused by Trp130 can be attributed to an increased susceptibility of RT to viral PR degradation. Recombinant HIV-1 PRs bearing mutations G94S or T96S showed decreased dimer stability and reduced catalytic efficiency. These results were consistent with crystallographic data showing the location of both residues in the PR dimerization interface.

Molecular mechanisms of HIV-1 resistance to nucleoside and nucleotide reverse transcriptase inhibitors

The nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs) were the first drugs used to treat HIV-1 infection and they remain integral components of nearly all antiretroviral regimens. However, the long-term efficacy of combination therapies that contain NRTIs is limited by the selection of drug-resistant variants of HIV-1. In general, NRTI therapy selects for viruses that have mutations in reverse transcriptase (RT). These mutations can be broadly categorized into two groups depending on their phenotypic mechanism of resistance. Mutations such as K65R, K70E, L74V, Q151M and M184V allow RT to discriminate against the NRTI triphosphate by increasing the enzyme’s selectivity for incorporation of the natural deoxynucleotide triphosphate substrate. By comparison, the thymidine analog mutations – such as M41L, D67N, K70R, L210W, T215F/Y and K219Q – augment the ability of HIV-1 RT to excise a chain-terminating NRTI monophosphate from a prematurely terminated DNA chain. A comprehensive knowledge of resistance mechanisms, cross-resistance patterns and interplay between mutations – as described in this review – can help optimize antiretroviral treatment strategies and possibly aid in the design of NRTIs that are active against drug-resistant HIV-1.

A new methodology for the simulation of flexible protein–ligand interactions

A methodology has been developed for the simulation of induced fit between a ligand and its target protein. It utilizes constrained molecular dynamics where atoms determined to be immobile from difference distance matrix studies are fixed. Application of this methodology to HIV-1 reverse transcriptase (RT) as the example target protein has demonstrated its robustness. Short simulation times are sufficient to achieve good refinement of docking poses resulting from exchange of structurally dissimilar inhibitors between crystal structures.

Sequential structures provide insights into the fidelity of RNA replication

RNA virus replication is an error-prone event caused by the low fidelity of viral RNA-dependent RNA polymerases. Replication fidelity can be decreased further by the use of mutagenic ribonucleoside analogs to a point where viral genetic information can no longer be maintained. For foot-and-mouth disease virus, the antiviral analogs ribavirin and 5-fluorouracil have been shown to be mutagenic, contributing to virus extinction through lethal mutagenesis. Here, we report the x-ray structure of four elongation complexes of foot-and-mouth disease virus polymerase 3D obtained in presence of natural substrates, ATP and UTP, or mutagenic nucleotides, ribavirin triphosphate and 5-fluorouridine triphosphate with different RNAs as template-primer molecules. The ability of these complexes to synthesize RNA in crystals allowed us to capture different successive replication events and to define the critical amino acids involved in (i) the recognition and positioning of the incoming nucleotide or analog; (ii) the positioning of the acceptor base of the template strand; and (iii) the positioning of the 3'-OH group of the primer nucleotide during RNA replication. The structures identify key interactions involved in viral RNA replication and provide insights into the molecular basis of the low fidelity of viral RNA polymerases.

Expression and characterization of a novel reverse transcriptase of the LTR retrotransposon Tf1

The LTR retrotransposon of Schizosacharomyces pombe, Tf1, has several distinctive properties that can be related to the unique properties of its reverse transcriptase (RT). Consequently, we expressed, purified and studied the recombinant Tf1 RT. This monomeric protein possesses all activities typical to RTs: DNA and RNA-dependent DNA polymerase as well as an inherent ribonuclease H. The DNA polymerase activity shows preference to Mn(+)(2) or Mg(+)(2), depending on the substrate used, whereas the ribonuclease H strongly prefers Mn(+)(2). The most outstanding feature of Tf1 RT is its capacity to add non-templated nucleotides to the 3'-ends of the nascent DNA. This is mainly apparent in the presence of Mn(+)(2), as is the noticeable low fidelity of DNA synthesis. In all, Tf1 RT has a marked infidelity in synthesizing DNA at template ends, a phenomenon that can explain, as discussed herein, some of the features of Tf1 replication in the host cells.

The nucleoside analogs 4'C-methyl thymidine and 4'C-ethyl thymidine block DNA synthesis by wild-type HIV-1 RT and excision proficient NRTI resistant RT variants

HIV-1 can become resistant to nucleoside analogs by developing an enhanced ability to excise the analogs after they have been incorporated. Excision requires that the analog be located at the 3' terminus of the primer. We have describe nucleoside analogs that do not block DNA synthesis at the point of incorporation, but only after additional dNTPs have been added to the DNA. These nucleoside analogs are called "delayed chain terminators" and are relatively effective inhibitors of drug-resistant HIV-1 reverse transcriptases (RTs) that are excision proficient. However, the first delayed chain terminator that we characterized was poorly phosphorylated in cultured cells. We have examined other nucleoside analogs to determine whether these compounds also act as delayed chain terminators, but were more efficiently converted to the triphosphate form by cellular kinases. These analogs contain substitutions on the deoxyribose sugar ring at the 4' carbon (4'C-methyl dT and 4'C-ethyl dT). Unlike true delayed chain terminators, which terminate DNA synthesis in a spatial sense (DNA synthesis is halted only after additional dNTPs have been incorporated after the analog), 4'C-methyl dTTP causes a pause in DNA synthesis at the point of incorporation. However, HIV-1 RT can eventually extend the primer blocked by the 4' C-Me dTMP analog. 4'C-methyl dTTP blocks DNA synthesis in a temporal sense, rather than in a spatial sense. A primer blocked by 4'C-ethyl dTMP is not extended by HIV-1 RT, and this compound acts like a conventional chain terminator, despite the presence of a 3'-OH group. These compounds effectively block the replication of an HIV-1-based vector that replicates using wild-type HIV-1 RT, but only in the presence of herpes simplex virus thymidine kinase (HSV TK). These compounds are effective against many NRTI drug-resistant RT variants; however, the M184V mutant is relatively resistant.

Mutations in human immunodeficiency virus type 1 RNase H primer grip enhance 3'-azido-3'-deoxythymidine resistance

We recently observed that mutations in the human immunodeficiency type 1 (HIV-1) reverse transcriptase (RT) connection domain significantly increase 3'-azido-3'-deoxythymidine (AZT) resistance up to 536 times over wild-type (WT) RT in the presence of thymidine analog resistance mutations (TAMs). These mutations also decreased RT template switching, suggesting that they altered the balance between nucleotide excision and template RNA degradation, which in turn increased AZT resistance. Several residues in the HIV-1 connection domain contact the primer strand and form an RNase H primer grip structure that helps to position the primer-template at the RNase H and polymerase active sites. To test the hypothesis that connection domain mutations enhanced AZT resistance by influencing the RNase H primer grip, we determined the effects of alanine substitutions in RNase H primer grip residues on nucleoside RT inhibitor resistance in the context of a WT, TAM-containing, or K65R-containing polymerase domain. Ten of the 11 RNase H primer grip mutations increased AZT resistance 20 to 243 times above WT levels in the context of a TAM-containing polymerase domain. Furthermore, all mutations in the RNase H primer grip decreased template switching, suggesting that they reduced RNase H activity. These results demonstrate that mutations in the RNase H primer grip region can significantly enhance AZT resistance and support the hypothesis that mutations in the connection and RNase H domains can increase resistance by altering the RNase H primer grip region, changing interactions between RT and the template-primer complex and/or shifting the balance between the polymerase and RNase H activities.

Stable complexes formed by HIV-1 reverse transcriptase at distinct positions on the primer-template controlled by binding deoxynucleoside triphosphates or foscarnet

Binding of the next complementary dNTP by the binary complex containing HIV-1 reverse transcriptase (RT) and primer-template induces conformational changes that have been implicated in catalytic function of RT. We have used DNase I footprinting, gel electrophoretic mobility shift, and exonuclease protection assays to characterize the interactions between HIV-1 RT and chain-terminated primer-template in the absence and presence of various ligands. Distinguishable stable complexes were formed in the presence of foscarnet (an analog of pyrophosphate), the dNTP complementary to the first (+1) templating nucleotide or the dNTP complementary to the second (+2) templating nucleotide. The position of HIV-1 RT on the primer-template in each of these complexes is different. RT is located upstream in the foscarnet complex, relative to the +1 complex, and downstream in the +2 complex. These results suggest that HIV-1 RT can translocate along the primer-template in the absence of phosphodiester bond formation. The ability to form a specific foscarnet complex might explain the inhibitory properties of this compound. The ability to recognize the second templating nucleotide has implications for nucleotide misincorporation.