Our Science – Le Grice Website
Stuart F.J. Le Grice, Ph.D.
Research Focus: Protein/Nucleic Acid Interactions Controlling Retroviral Replication
The primary research objective of the RT Biochemistry Section is dissecting proteins, nucleic acids and their nucleoprotein complexes as they relate to replication of RNA viruses, retroviruses, and LTR-containing retrotransposons. Projects in the laboratory use a combination of biochemical, biophysical, structural, biological, and computational strategies to better understand processes of reverse transcription, RNA export, and genome packaging. Single-molecule spectroscopy has shown HIV-1 reverse transcriptase (RT) to be a highly dynamic enzyme, capable of sliding and changing orientation on its nucleic acid substrate, while at the same time making the novel observation that nonnucleoside RT inhibitors can influence enzyme orientation. We have recently generated high-resolution structures for lentiviral (HIV-1) and gammaretroviral (XMRV) RTs in the presence of a non-polypurine tract (PPT) RNA/DNA hybrid, demonstrating a catalytically competent conformation with respect to nucleic acid in the RNase H active site. These structures provide a platform for both structure/function studies and drug development, where we focus on developing allosteric inhibitors that bind adjacent to the RNase H active site.
Understanding RNA structure and function has taken advantage of a novel chemoenzymatic probing method (SHAPE) that can be used both in vitro and in vivo. Modifications of this technique (ai-SHAPE) allow us to investigate long-range tertiary interactions (i.e., kissing-loop interactions and pseudoknots) that control both genome replication and transport of unspliced RNAs. SHAPE studies are combined with computational methods designed to develop improved algorithms for predicting RNA tertiary structure. This work involves collaborative interactions with several investigators in both the intramural and extramural research communities.
Finally, we are investigating the potential of N-terminal Cu++/Ni++ motifs (ATCUNS) as metallotherapeutics. This tripeptide motif can be appended to the N-terminus of peptides and proteins and induced to release reactive oxygen species, thereby designating them chemical 'nucleases' and 'proteases' capable of irreversibly inactivating their target biomolecule. In addition to their therapeutic potential, ATCUN-derived metallopeptides are also under investigation as through-space cleavage reagents to provide information on RNA tertiary structure.
Research Highlights 2012-2014
Chung, S., Miller, J.T., Johnson, B.C., Hughes, S.H., and Le Grice, S.F.J. (2012) Mutagenesis of human immunodeficiency virus reverse transcriptase p51 subunit defines residues contributing to vinylogous urea inhibition of ribonuclease H activity. J. Biol. Chem. 287: 4066-4075.
The vinylogous urea, NSC727447, allosterically inhibits ribonuclease H (RNase H) activity of human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) by interacting with the thumb subdomain of the non-catalytic p51 subunit. Proximity of the p51 thumb to the p66 RNase H domain implied inhibitor-mediated alterations to active site geometry, while mass spectrometry suggested a contribution from alpha-helix I residues Cys280 and Lys281. To further characterize the inhibitor binding site, Chung et al. combined scanning mutagenesis between p51 residues Lys275 and Thr286, (comprising alpha-helix I and portions of the neighboring connecting loops) with a limited vertical scan of Cys280. An important role for Cys280 and Thr286 was suggested by the observation that these reconstituted, selectively mutated p66/p51 heterodimers were significantly resistant to inhibition, as were additional Cys280 mutants. In several instances, alanine substitutions led to significantly increased inhibitor sensitivity. In contrast, mutant enzymes retained equivalent sensitivity to an active site alpha-hydroxytropolone-derived RNase H inhibitor. p66/p51 heterodimers containing short p51 C-terminal truncations also displayed increased sensitivity to vinylogous urea inhibition. Cumulatively, these data suggest a contribution from the p51 thumb subdomain to nucleic acid binding that is compromised following inhibitor binding.
[Link to 'Platinum Publications Highlight' in NCI-Frederick Poster: Novel Binding Site Targeting HIV-1 RNase H May Lead to Future Anti-HIV Drug Design]
Zhou, D., Chung, S., Miller, M., Le Grice, S.F.J., and Wlodawer, A. (2012) Crystal structures of the reverse transcriptase-associated ribonuclease H domain of xenotropic murine leukemia-virus related virus. J. Struct. Biol. 177: 638-645.
The ribonuclease H (RNase H) domain of retroviral reverse transcriptase (RT) plays a critical role in the life cycle by degrading the RNA strands of DNA/RNA hybrids. In addition, RNase H activity is required to precisely remove the RNA primers from nascent (-) and (+) strand DNA. Zhou et al. report here crystal structures for several variants of the RNase H domain of xenotropic murine leukemia virus-related virus (XMRV) RT, namely (i) the previously identified construct from which helix C was deleted, (ii) the intact domain, and (iii) the intact domain complexed with an active site alpha-hydroxytropolone inhibitor. Enzymatic assays showed that the intact RNase H domain retained catalytic activity, whereas the variant lacking helix C was only marginally active, corroborating the importance of this helix for enzymatic activity. Modeling of the enzyme-substrate complex elucidated the essential role of helix C in binding a DNA/RNA hybrid and its likely mode of recognition. The crystal structure of the RNase H complexed with beta-thujaplicinol clearly showed that coordination by two divalent cations mediated recognition of the inhibitor.
Kessl, J.J., Jena, N., Koh, Y., Taskent-Sezgin, H., Slaughter, A., Feng, L., de Silva, S., Wu, L., Le Grice, S.F.J., Engelman, A., Fuchs, J.R., and Kvaratskhelia, M. (2012) Multimode, cooperative mechanism of action of allosteric HIV-1 integrase inhibitors. J. Biol. Chem. 287: 16801-16811.
HIV-1 integrase (IN) interacts with viral DNA and its key cellular cofactor LEDGF to effectively integrate viral DNA into a host cell chromosome. These interactions are crucial for HIV-1 replication and present attractive targets for antiviral therapy. 2-(quinolin-3-yl) acetic acid derivatives have been reported to selectively inhibit the IN-LEDGF interaction in vitro and impair HIV-1 replication in infected cells. Kessl et al. show here that this class of compounds impairs both IN-LEDGF binding and LEDGF-independent IN catalytic activities with similar IC50 values, defining them as bona fide allosteric inhibitors of IN function. Furthermore, this study shows show that 2-(quinolin-3-yl) acetic acid derivatives block formation of the stable synaptic complex between IN and viral DNA by allosterically stabilizing an inactive multimeric form of IN. In addition, these compounds inhibit LEDGF binding to the stable synaptic complex. This multimode mechanism of action concordantly results in cooperative inhibition of the concerted integration of viral DNA ends in vitro and HIV-1 replication in cell culture. Such findings, coupled with the fact that high cooperativity of antiviral inhibitors correlates with their increased instantaneous inhibitory potential, argue strongly that improved 2-(quinolin-3-yl) acetic acid derivatives could exhibit desirable clinical properties.
Purzycka, K.J., Legiewicz, M., Matsuda, E., Eizentstat, L.D., Lusvarghi, S., Saha, A., Le Grice, S.F.J., and Garfinkel, D.J. (2013) Exploring Ty1 retrotransposon RNA structure within virus-like particles. Nucleic Acids Res. 41: 463-473.
Ty1, a long terminal repeat (LTR) retrotransposon of Saccharomyces, is structurally and functionally related to retroviruses. However, a differentiating aspect between these retroelements is the diversity of the replication strategies used by LTR-retrotransposons. To understand the structural organization of cis-acting elements present on Ty1 genomic RNA from the GAG region that control reverse transcription, Purzcyka et al. applied chemoenzymatic probing (SHAPE) to RNA/tRNA complexes assembled in vitro and to the RNA in virus-like particles (VLPs). By comparing different RNA states, this analysis provides a comprehensive structure of the primer-binding site (pbs), a novel pseudoknot adjacent to the pbs, three regions containing palindromic sequences that may be involved in RNA dimerization or packaging, and candidate protein interaction sites. This study additionally determined the impact of a novel form of transposon control based on Ty1 antisense transcripts that associate with VLPs. The results of this work support the idea that antisense RNAs inhibit retrotransposition by targeting Ty1 protein function rather than annealing with the RNA genome.
Chamanian, M., Purzycka, K.J., Wille, P.T., Ha, J.S., McDonald, D., Gao, Y., Le Grice, S.F.J., and Arts, E.J. (2013) A cis-acting element in retroviral genomic RNA links Gag-Pol ribosomal frameshifting to selective viral RNA encapsidation. Cell Host Microbe 13: 181-192.
During retroviral RNA encapsidation, two full-length genomic RNAs are selectively incorporated into assembling virions. Genome packaging involves a cis-acting packaging element (Y) within the 5’ untranslated region (UTR) of the unspliced genome. However, the mechanism(s) that selects and limits genomic RNAs for packaging remains uncertain. Using a dual complementation system involving bipartite HIV-1 genomic RNA, Chamanian et al. observed that packaging was additionally dependent on a cis-acting RNA element, designated the genomic RNA packaging enhancer (GRPE), found within the Gag p1-p6 domain and overlapping the Gag-Pol ribosomal frameshift signal. Deleting or disrupting the two conserved GRPE stem-loops diminished genome packaging and infectivity >50-fold, while deleting gag sequences between Y and the GRPE had no effect. Downregulating the translation termination factor eRF1 produces defective virus particles containing approximately 20 times more genomic RNA. Thus, only the HIV-1 RNAs employed for Gag-Pol translation may be specifically selected for encapsidation.
[Link to Cell Host & Microbe Commentary feature related to this article: Durney, M.A., and D'Souza, V.M. (2013) HIV-1: Packaging a shifty genome? Cell Host Microbe 13: 123-125.]
Lapkouski, M., Tian, L., Miller, J.T., Le Grice, S.F.J., and Yang, W. (2013) Complexes of HIV-1 RT, NNRTI and RNA/DNA hybrid reveal a structure compatible with RNA degradation. Nat. Struct. Mol. Biol. 20: 230-236.
Although a large number of structures of HIV-1 reverse transcriptase (RT) have been determined, only one contains an RNA/DNA hybrid. In this publication, Lapkouski et al. report three novel structures of HIV-1 RT complexed with a nonnucleotide RT inhibitor (NNRTI) and an RNA/DNA hybrid. In the presence of an NNRTI, these RNA/DNA structures differ from all prior nucleic acid-RT structures including the polypurine tract (PPT)-containing RNA/DNA hybrid. The enzyme structure also differs from all previous RT-DNA complexes. Thus, the hybrid has ready access to the RNase H active site. These observations collectively indicate that an RT-nucleic acid complex may adopt alternate structural states, namely one competent for DNA synthesis and the other for RNA degradation. RT mutations that confer drug resistance but are distant from the inhibitor-binding sites often map to the unique RT-hybrid interface that undergoes conformational changes between two catalytic states.
Huang, Q., Purzycka, K.J., Lusvarghi, S., Li, D., Le Grice, S.F.J., and Boeke, J.D. (2013) Retrotransposon Ty1 RNA contains a 5'-terminal long-range pseudoknot required for efficient reverse transcription. RNA 19: 320-332.
The RNA genome of the Saccharomyces cerevisiae LTR-retrotransposon Ty1 has the potential to fold into a variety of distinct structures, mutation of which has been shown to affect retrotransposition frequency. Huang et al. show in this communication that one potential functional structure is located at the 5' end of the genome and can assume a pseudoknot conformation. Chemoenzymatic probing of wild-type and mutant mini-Ty1 RNAs via SHAPE supports the existence of such a structure, while molecular genetic analyses show that mutations disrupting pseudoknot formation interfere with retrotransposition, indicating that it provides a critical biological function. These defects are enhanced at higher temperatures. When these mutants are combined with compensatory changes, retrotransposition is restored, consistent with pseudoknot architecture. Analyses of mutants suggest a defect in Ty1 reverse transcription. Collectively, data in this study allow modeling of a three-dimensional structure for this novel critical cis-acting signal of the Ty1 genome.
Nowak, E., Potrzebowski, W., Konarev, P.V., Rausch, J.W., Bona, M.K., Svergun, D.I., Bujnicki, J.M., Le Grice, S.F.J., and Nowotny, M. (2013) Structural analysis of monomeric retroviral reverse transcriptase in complex with an RNA/DNA hybrid. Nucleic Acids Res. 41: 3874-3887.
A key step in proliferation of retroviruses is the converting their RNA genome to double-stranded DNA via a process catalyzed by the multifunctional reverse transcriptase (RT). Dimeric and monomeric RTs have been described, the latter exemplified by enzyme from Moloney murine leukemia virus (Mo-MLV). However, structural information that describes the substrate-binding mechanism for a monomeric RT is lacking. Nowak et al. report here the first crystal structure of a complex between an RNA/DNA hybrid substrate and the single-subunit RT from xenotropic murine leukemia virus-related virus (XMRV), a close relative of Mo-MLV. A comparison with p66/p51 HIV-1 RT shows that substrate binding around the DNA polymerase active site is conserved but differs in the thumb and connection subdomains. Small-angle X-ray scattering was used to model full-length XMRV RT, demonstrating that its mobile RNase H domain becomes ordered in the presence of a substrate, highlighting a key difference between monomeric and dimeric RTs.
Sztuba-Solinska, J., Teramoto, T., Rausch, J.W., Shapiro, B.A., Padmanabhan, R., and Le Grice, S.F.J. (2013) Structural complexity of Dengue virus untranslated regions: cis-acting RNA motifs and pseudoknot interactions modulating functionality of the viral genome. Nucleic Acids Res. 41: 5075-5089.
The Dengue virus (DENV) genome contains multiple cis-acting elements required for translation and replication. Previous studies indicated that a 719-nt subgenomic minigenome (DENV-MINI) is an efficient template for translation and (-) strand RNA synthesis in vitro. In this study, Sztuba-Solinska et al. performed a detailed structural analysis of DENV-MINI RNA, combining chemical acylation techniques, Pb2+ ion-induced hydrolysis, and site-directed mutagenesis. This multidisciplinary study highlighted protein-independent 5'-3' terminal interactions involving cis-acting motifs that assume a 'panhandle' structure. Probing analyses identified tandem dumbbell structures (DBs) within the 3' terminus spaced by single-stranded regions, and internal loops and hairpins with embedded GNRA-like motifs. Analysis of conserved motifs and top loops (TLs) of these dumbbells, and their predicted interactions with downstream pseudoknot (PK) regions, predicted an H-type pseudoknot involving TL1 of the 5' DB and the complementary region, PK2. Since disrupting the TL1/PK2 interaction, via 'flipping' mutations of PK2, previously attenuated DENV replication, this pseudoknot may participate in regulation of RNA synthesis. Computer modeling implied that this motif might function as an autonomous structural/regulatory element. In addition, these studies targeting elements of the 3' DB and its complementary region PK1 indicated that communication between 5'-3' terminal regions strongly depends on structure and sequence composition of the 5' cyclization region.
Lusvarghi, S., Sztuba-Solinska, J., Purzycka, K.J., Pauly, G.T., Rausch, J.W., and Le Grice, S.F.J. (2013) The HIV-2 Rev-response element: Determining secondary structure and defining folding intermediates. Nucleic Acids Res. 41: 6637-6649.
Interaction between the human immunodeficiency virus (HIV) Rev protein and the RNA motifs known as Rev response elements (RREs) is required for transport of unspliced and partially spliced HIV-1 and HIV-2 RNAs from the nucleus to the cytoplasm during the later stages of virus replication. A more detailed understanding of these nucleoprotein complexes and the host factors with which they interact should accelerate the development of new antiviral drugs targeting cis-acting RNA regulatory signals. In this communication, we identified the secondary structures of the HIV-2 RRE and two RNA-folding precursors by combining chemical probing with a novel mathematical approach for determining the secondary structures of RNA conformers present in a mixture. A complementary chemical probing technique was also used to support these secondary structure models, confirm that the RRE2 RNA undergoes a folding transition, and obtain information about the relative positioning of RRE2 substructures in three dimensions. Our analysis collectively suggests that the HIV-2 RRE undergoes two conformational transitions before assuming the energetically most favorable conformer. Three-dimensional models for the HIV-2 RRE and folding intermediates are also presented, wherein the Rev-binding stem-loops (IIB and I) are located coaxially in the former, which is in agreement with previous models for HIV-1 Rev-RRE binding.
Chung, S., Miller, J.T., Lapkouski, M., Tian, L., Yang, W., and Le Grice, S.F.J. (2013) Examining the role of the HIV-1 reverse transcriptase p51 subunit in positioning and hydrolysis of RNA/DNA hybrids. J. Biol. Chem. 288: 16177-16184.
Recent crystallographic analysis of p66/p51 human immunodeficiency virus (HIV) type 1 reverse transcriptase (RT) complexed with a non-polypurine tract RNA/DNA hybrid has highlighted novel and important contacts between structural elements at the C-terminus of the non-catalytic p51 subunit and the nucleic acid duplex in the vicinity of the ribonuclease H (RNase H) active site. In particular, a short peptide spanning residues Phe416-Pro421 was demonstrated to interact with the DNA strand, cross the minor groove of the helix, and finally form Van der Waals contacts with the RNA strand adjacent to the scissile phosphate. At the base of the adjoining alpha-helix M', p51 residue Tyr427 forms a hydrogen bond with Asn348, the latter of which, when mutated to Ile, has been implicated in resistance to both nucleoside and nonnucleoside RT inhibitors. Based on this recently reported structural data, Chung et al. analyzed the contribution from p51 C-terminal elements by evaluating selectively mutated p66/p51 heterodimers carrying (i) truncations that encroach on alpha-M, (ii) alterations that interrupt the Asn348:Tyr427 interaction, and (iii) alanine substitutions throughout the region Tyr416-Pro421. Collectively, this strategy supports the notion that the p51 C-terminus makes an important contribution toward hybrid binding and orienting the RNA strand for catalysis at the RNase H active site.
Masaoka, T., Chung, S., Caboni, P., Rausch, J., Wilson, J.A., Taskent-Sezgin, H., Beutler, J.A., Tocco, G., and Le Grice, S. (2013) Exploiting drug-resistant enzymes as tools to identify thienopyrimidinone inhibitors of human immunodeficiency virus reverse transcriptase-associated ribonuclease H. J. Med. Chem. 56: 5436-5445.
The thienopyrimidinone 5,6-dimethyl-2-(4-nitrophenyl)thieno[2,3-d]pyrimidin-4(3H)-one (DNTP) has been proposed to occupy the interface between the p66 ribonuclease H (RNase H) domain and p51 thumb subdomain of human immunodeficiency virus reverse transcriptase (HIV RT), thereby inducing a conformational change incompatible with catalysis. In this communication, Masaoka et al. report the synthesis, activity, and antiviral properties of 39 novel thienopyrimidinones bearing substitutions on the thiophene or the C2 position of the pyrimidinone ring. Exploiting a panel of selectively mutated HIV-1 RT mutants allowed identification of four groups of molecules, based on their resistance and sensitivity profiles. Among these, compounds with a 3',4'-dihydroxyphenyl (catechol) substitution displayed activity against both wild-type and drug-resistant RT variants at submicromolar concentrations and, importantly, inhibited HIV-1 replication in cells. Differential scanning fluorimetry indicated that these compounds, in contrast to alpha-hydroxytropolone-derived RNase H inhibitors (such as the natural product manicol), destabilized the RT heterodimer, in some instances lowering the Tm by almost 5oC. Collectively, these data provide an important structural platform for the continued development of thienopyrimidinone-based RNase H inhibitors and highlight the value of genetically engineered HIV-1 RT variants with altered inhibitor sensitivity profiles for secondary screening.
Le Grice, S.F.J., and Goette, M., eds. (2013) Human Immunodeficiency Virus Reverse Transcriptase: A Bench-to-Bedside Success, Springer Publishing, New York.
HIV-1 RT arguably ranks among one of the most extensively studied retroviral enzymes. Heterologous expression and purification of recombinant enzyme in the early 1980s, approval of the first nucleoside analogue RT inhibitor (NRTI) in 1987, discovery of resistance to RT inhibitors, and approval of the first nonnucleoside analogue RT inhibitor (NNRTI) in 1996 and the various crystal structures of RT with or without bound substrate(s) and/or inhibitors represent only a few of the important milestones that describe a bench-to-bedside success in the continuing effort to combat HIV-1 infection and its consequences. NRTIs and NNRTIs remain important components in combination regimens to treat the infection. RT inhibitors also play important roles in recently validated strategies to prevent virus transmission. The relevance of HIV-1 RT as a drug target has simultaneously triggered interest in basic research studies aimed at providing a more detailed understanding of interactions between proteins, nucleic acids, and small-molecule ligands in general terms. In light of the ever-growing knowledge on its structure and function, HIV-1 RT serves as a valuable model system in efforts to develop novel experimental tools and to explain biochemical processes.
This book provides an overview of important aspects in past and current HIV-1 RT research, with emphasis on mechanistic aspects and translation of knowledge into drug discovery and development. The first section includes chapters emphasizing coordination of the RT-associated DNA polymerase and RNase H activities. The second covers mechanisms of action and future perspectives associated with NRTIs and NNRTIs, while the third section includes chapters focusing on novel strategies to target the RT enzyme. The final chapters are intended to discuss mechanisms involved in HIV variability, development of drug resistance and the use of NNRTIs as microbicides. We hope these contributions will stimulate interest and encourage research aimed at the continued development of novel RT inhibitors.
Kenyon, J.C., Prestwood, L.J., Le Grice, S.F.J., and Lever, A.M.L. (2013) In-gel probing of individual RNA conformers within a mixed population reveals a dimerization structural switch in the HIV-1 leader. Nucleic Acids Res. 41 (18): e174.
Accurate and unambiguous determination of RNA structures in vitro can be complicated by the presence of interchanging conformers or multimerization of some of these molecules. Until now, probing a single structure of conformationally flexible RNA molecules has traditionally required either the introduction of stabilizing mutations or adjustment buffer conditions or RNA concentration. In this communication, Kenyon et al. developed an 'in-gel' SHAPE (selective 2'OH acylation analyzed by primer extension) approach, where a mixed structural population of RNA molecules is initially separated by nondenaturing polyacrylamide gel electrophoresis, after which conformers are individually probed within the gel matrix. Validation of the technique using a well-characterized RNA stem-loop structure, the HIV-1 TAR (transactivation response element), showed that authentic structure was maintained and that the method was both accurate and highly reproducible. To further demonstrate the utility of in-gel SHAPE, monomeric and dimeric species of the HIV-1 packaging signal RNA were fractionated and individually probed. Extensive differences in acylation sensitivity between monomer and dimer could be observed, supporting a recently proposed structural switch model of RNA genomic dimerization and packaging and demonstrating the discriminatory power of in-gel SHAPE.
Fang, X., Wang, J., O'Carroll, I.P., Mitchell, M., Zuo, X., Wang, Y., Yu, P., Liu, Y., Rausch, J.W., Dyba, M., Kjems, J., Schwieters, C.D., Seifert, S., Winans, R.E., Watts, N.R., Stahl, S.J., Wingfield, P.T., Byrd, R.A., Le Grice, S.F.J., Rein, A., and Wang, X. (2013) An unusual topological structure of the HIV-1 Rev response element. Cell 155: 594-605.
Retrovirus replication requires nucleocytoplasmic export of intron-containing unspliced and singly spliced RNAs, which ultimately serve as mRNAs for the gag, gag-pol, and env gene products and the genome of progeny virions. HIV-1 fulfills this requirement via the accessory protein Rev, which binds to the Rev response element (RRE) element within the env-coding region of the viral RNA. The Rev-RRE complex then engages host proteins to form a host export complex that facilitates translocation through the nuclear pore complex. The secondary structure of the HIV-1 RRE includes a series of stems (I, II, III/IV, and V), which are arranged around a central 4-way junction, with stem-loop II comprising a proximal stem (IIA) and two distal stem-loops (IIB and IIC) around a 3-way junction. Initial occupancy of the high-affinity Rev-binding site in stem-loop IIB is a prerequisite to oligomerization, in which up to 12 Rev molecules may bind to a single RRE ultimately resulting in Rev-mediated nuclear export of RRE-containing RNAs. In this communication, Fang et al. describe the structure of the HIV-1 RRE (RRE-1), derived by small-angle X-ray scattering, together with a model for its interaction with the viral Rev protein. The structure shares many features in common with the HIV-2 RRE recently elucidated by ensemble-SHAPE by our group.
Nowak, E., Miller, J.T., Bona, M.K., Studnicka, J., Szczepanowski, R.H., Jurkowski, J., Le Grice, S.F.J., and Nowotny, M. (2014) Ty3 reverse transcriptase complexed with an RNA-DNA hybrid shows structural and functional asymmetry. Nat. Struct. Mol. Biol. 21: 389-396.
Retrotransposons represent a class of mobile genetic elements that replicate by converting their single-stranded RNA genome into double-stranded DNA through the combined DNA polymerase and ribonuclease H (RNase H) activities of the element-encoded reverse transcriptase (RT). Although a wealth of structural and biochemical information is available for lentiviral and gammaretroviral RTs, equivalent studies on counterpart enzymes of long terminal repeat (LTR)-containing retrotransposons, from which they are evolutionarily derived, are lacking. In this study, Nowak et al. report the first crystal structure of a complex of RT from the Saccharomyces cerevisiae LTR-retrotransposon Ty3 in the presence of its polypurine tract-containing RNA/DNA hybrid. In contrast to its retroviral counterparts, Ty3 RT adopts an asymmetric homodimeric architecture, whose assembly is substrate dependent. More strikingly, and in contrast to data from other dimeric RTs, this structure, in combination with biochemical analysis via phenotypic mixing, suggests that the RNase H and DNA polymerase activities are contributed by individual subunits of the Ty3 RT homodimer.
[Link to Center for Cancer Research (CCR) 'In the Journals' feature related to this article: Novel Structure of Ty3 Reverse Transcriptase, April 2014.]
Costi, R., Metifiot, M., Chung, S., Cuzzucoli Crucitti, G., Maddali, K., Pescatori, L., Messore, A., Madia, V.N., Pupo, G., Scipione, L., Tortorella, S., Di Leva, F.S., Cosconati, S., Marinelli, L., Novellino, E., Le Grice, S.F.J., Corona, A., Pommier, Y., Marchand, C., and Di Santo, R. (2014) Basic quinolinonyl diketo acid derivatives as inhibitors of HIV integrase and their activity against RNase H function of reverse transcriptase. J. Med. Chem. 57: 3223-3234.
Based on the similarities of the IN and RNase H catalytic sites and reports on dual activities of a number of compounds against IN and RNase H, Costi et al. tested newly designed quinolonyl DKAs against both HIV-1 enzymes. Among these derivatives, three compounds inhibited HIV-1 IN with IC50 values below 100 nM for strand transfer and showed a two-order of magnitude selectivity over 3' processing. These strand transfer selective inhibitors also inhibited HIV-1 RNase H with low micromolar potencies. Molecular modeling studies based on both the HIV- 1 IN and RNase H catalytic core domains provided new structural insights for the future development of these compounds as dual HIV-1 IN and RNase H inhibitors.
Sztuba-Solinska, J., Shenoy, S.R., Gareiss, P., Krumpe, L., Le Grice, S.F.J., O'Keefe, B.R., and Schneekloth, J., Jr. (2014) Identification of biologically active, HIV TAR RNA-binding small molecules using small molecule microarrays. J. Am. Chem. Soc. 136: 8402-8410.
Identifying small molecules that selectively bind to structured RNA motifs remains an important challenge in developing potent and specific therapeutics. Most strategies to uncover RNA-binding molecules have identified highly charged compounds or aminoglycosides that commonly have modest selectivity. In this communication, Sztuba-Solinska et al. report a strategy to screen a large unbiased library of drug-like small molecules in a microarray format against an RNA target. This strategy identified a novel chemotype that selectively targets the HIV transactivation response (TAR) RNA hairpin in a manner not dependent on cationic charge. The candidate thienopyridine binds to and stabilizes the TAR hairpin with a Kd of 2.4 uM. Structure activity relationships demonstrate that this compound achieves activity through hydrophobic substituents on a heterocyclic core, rather than cationic groups typically required. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis was performed on a 365-nucleotide sequence derived from the 5' UTR of the HIV-1 genome to determine global structural changes in the presence of the molecule. The interaction of this compound could be mapped to the TAR hairpin without broadly disrupting any other structured elements of the 5' UTR. Cell-based assays indicated that this novel small molecule inhibits HIV-induced cytopathicity in the micromolar range, while cytotoxicity was not observed at concentrations of 1 mM.
This page was last updated on 6/13/2014.