RNA Dimerization Initiation Site Research Articles

Native Electrospray Ionization Mass Spectrometry of RNA-Ligand Complexes.

Native electrospray ionization mass spectrometry (native ESI-MS) is a powerful tool to investigate non-covalent biomolecular interactions. It has been widely used to study protein complexes, but only few examples are described for the analysis of complexes involving RNA-RNA interactions. Here, we provide a detailed protocol for native ESI-MS analysis of RNA complexes. As an example, we present the analysis of the HIV-1 genomic RNA dimerization initiation site (DIS) extended duplex dimer bound to the aminoglycoside antibiotic lividomycin.

Structural basis of genomic RNA (gRNA) dimerization and packaging determinants of mouse mammary tumor virus (MMTV).

BackgroundOne of the hallmarks of retroviral life cycle is the efficient and specific packaging of two copies of retroviral gRNA in the form of a non-covalent RNA dimer by the assembling virions. It is becoming increasingly clear that the process of dimerization is closely linked with gRNA packaging, and in some retroviruses, the latter depends on the former. Earlier mutational analysis of the 5’ end of the MMTV genome indicated that MMTV gRNA packaging determinants comprise sequences both within the 5’ untranslated region (5’ UTR) and the beginning of gag.ResultsThe RNA secondary structure of MMTV gRNA packaging sequences was elucidated employing selective 2’hydroxyl acylation analyzed by primer extension (SHAPE). SHAPE analyses revealed the presence of a U5/Gag long-range interaction (U5/Gag LRI), not predicted by minimum free-energy structure predictions that potentially stabilizes the global structure of this region. Structure conservation along with base-pair covariations between different strains of MMTV further supported the SHAPE-validated model. The 5’ region of the MMTV gRNA contains multiple palindromic (pal) sequences that could initiate intermolecular interaction during RNA dimerization. In vitro RNA dimerization, SHAPE analysis, and structure prediction approaches on a series of pal mutants revealed that MMTV RNA utilizes a palindromic point of contact to initiate intermolecular interactions between two gRNAs, leading to dimerization. This contact point resides within pal II (5’ CGGCCG 3’) at the 5’ UTR and contains a canonical “GC” dyad and therefore likely constitutes the MMTV RNA dimerization initiation site (DIS). Further analyses of these pal mutants employing in vivo genetic approaches indicate that pal II, as well as pal sequences located in the primer binding site (PBS) are both required for efficient MMTV gRNA packaging.ConclusionsEmploying structural prediction, biochemical, and genetic approaches, we show that pal II functions as a primary point of contact between two MMTV RNAs, leading to gRNA dimerization and its subsequent encapsidation into the assembling virus particles. The results presented here enhance our understanding of the MMTV gRNA dimerization and packaging processes and the role of structural motifs with respect to RNA-RNA and possibly RNA-protein interactions that might be taking place during MMTV life cycle.Electronic supplementary materialThe online version of this article (doi:10.1186/s12977-014-0096-6) contains supplementary material, which is available to authorized users.

SHAPE analysis of the 5′ end of the Mason-Pfizer monkey virus (MPMV) genomic RNA reveals structural elements required for genome dimerization

Earlier genetic and structural prediction analyses revealed that the packaging determinants of Mason Pfizer monkey virus (MPMV) include two discontinuous core regions at the 5' end of its genomic RNA. RNA secondary structure predictions suggested that these packaging determinants fold into several stem-loops (SLs). To experimentally validate this structural model, we employed selective 2' hydroxyl acylation analyzed by primer extension (SHAPE), which examines the flexibility of the RNA backbone at each nucleotide position. Our SHAPE data validated several predicted structural motifs, including U5/Gag long-range interactions (LRIs), a stretch of single-stranded purine (ssPurine)-rich region, and a distinctive G-C-rich palindromic (pal) SL. Minimum free-energy structure predictions, phylogenetic, and in silico modeling analyses of different MPMV strains revealed that the U5 and gag sequences involved in the LRIs differ minimally within strains and maintain a very high degree of complementarity. Since the pal SL forms a helix loop containing a canonical "GC" dyad, it may act as a RNA dimerization initiation site (DIS), enabling the virus to package two copies of its genome. Analyses of wild-type and pal mutant RNAs revealed that disruption of pal sequence strongly affected RNA dimerization. However, when in vitro transcribed trans-complementary pal mutants were incubated together showed RNA dimerization was restored authenticating that the pal loop (5'-CGGCCG-3') functions as DIS.

Structure-Guided Discovery of a Novel Aminoglycoside Conjugate Targeting HIV-1 RNA Viral Genome

The dimerization initiation site (DIS) of the HIV-1 genomic RNA is a conserved stem-loop that promotes viral genome dimerization by forming a loop-loop complex. The DIS constitutes a potentially interesting target because it is crucial for several key steps of the viral replication. In this work we describe the synthesis of a rationally designed aminoglycoside conjugate that binds the HIV-1 DIS viral RNA with high specificity, as shown by an extensive in vitro binding characterization. We propose a three-dimensional model of the drug-RNA interaction that perfectly fits with binding data. Our results show the feasibility of targeting the HIV DIS viral RNA dimer and open the way to the rationale design of a new class of antiviral drugs. In addition, due to similarities between the HIV-1 DIS RNA and the bacterial aminoacyl decoding site (A site) RNA, we show that this novel aminoglycoside conjugate also binds the bacterial A site with a similar affinity as natural aminoglycoside antibiotics.

Sequence and structure requirements for specific recognition of HIV-1 TAR and DIS RNA by the HIV-1 Vif protein

The HIV-1 Vif protein plays an essential role in the regulation of the infectivity of HIV-1 virion and in vivo pathogenesis. Vif neutralizes the human DNA-editing enzyme APOBEC3 protein, an antiretroviral cellular factor from the innate immune system, allowing the virus to escape the host defence system. It was shown that Vif is packaged into viral particles through specific interactions with the viral genomic RNA. Conserved and structured sequences from the 5'-noncoding region, such as the Tat-responsive element (TAR) or the genomic RNA dimerization initiation site (DIS), are primary binding sites for Vif. In the present study we used isothermal titration calorimetry to investigate sequence and structure determinants important for Vif binding to short viral RNA corresponding to TAR and DIS stem-loops. We showed that Vif specifically binds TAR and DIS in the low nanomolar range. In addition, Vif primarily binds the TAR UCU bulge, but not the apical loop. Determinants for Vif binding to the DIS loop-loop complex are likely more complex and involve the self-complementary loop together with the upper part of the stem. These results suggest that Tat-TAR inhibitors or DIS small molecule binders might be also effective to disturb Vif-TAR and Vif-DIS binding in order to reduce Vif packaging into virions.

Cation-dependent cleavage of the duplex form of the subtype-B HIV-1 RNA dimerization initiation site.

The crystal structure of subtype-B HIV-1 genomic RNA Dimerization Initiation Site duplex revealed chain cleavage at a specific position resulting in 3′-phosphate and 5′-hydroxyl termini. A crystallographic analysis showed that Ba2+, Mn2+, Co2+ and Zn2+ bind specifically on a guanine base close to the cleaved position. The crystal structures also point to a necessary conformational change to induce an ‘in-line’ geometry at the cleavage site. In solution, divalent cations increased the rate of cleavage with pH/pKa compensation, indicating that a cation-bound hydroxide anion is responsible for the cleavage. We propose a ‘Trojan horse’ mechanism, possibly of general interest, wherein a doubly charged cation hosted near the cleavage site as a ‘harmless’ species is further transformed in situ into an ‘aggressive’ species carrying a hydroxide anion.

Binding of Aminoglycoside Antibiotics to the Duplex Form of the HIV‐1 Genomic RNA Dimerization Initiation Site

Targeting HIV RNA: High-resolution X-ray structures of the HIV-1 genomic RNA dimerization initiation site (DIS) extended duplex bound to several aminoglycoside antibiotics have been obtained (see ribostamycin complex as an example). They show the feasibility of targeting viral RNA with small drugs, and open up the possibility of structure-based rational drug design aimed against viral replication.

Binding of Aminoglycoside Antibiotics to the Duplex Form of the HIV‐1 Genomic RNA Dimerization Initiation Site

Binding of Aminoglycoside Antibiotics to the Duplex Form of the HIV‐1 Genomic RNA Dimerization Initiation Site

Conformational transitions of flanking purines in HIV‐1 RNA dimerization initiation site kissing complexes studied by CHARMM explicit solvent molecular dynamics

Dimerization of HIV-1 genomic RNA is initiated by kissing loop interactions at the Dimerization Initiation Site (DIS). Dynamics of purines that flank the 5' ends of the loop-loop helix in HIV-1 DIS kissing complex were explored using explicit solvent molecular dynamics (MD) simulations with the CHARMM force field. Multiple MD simulations (200 ns in total) of X-ray structures for HIV-1 DIS Subtypes A, B, and F revealed conformational variability of flanking purines. In particular, the flanking purines, which in the starting X-ray structures are bulged-out and stack in pairs, formed a consecutive stack of four bulged-out adenines at the beginning of several simulations. This conformation is seen in the crystal structure of DIS Subtype F with no interference from crystal packing, and was frequently reported in our preceding MD studies performed with the AMBER force field. However, as CHARMM simulations progressed, the four continuously stacked adenines showed conformational transitions from the bulged-out into the bulged-in geometries. Although such an arrangement has not been seen in any X-ray structure, it has been suggested by a recent NMR investigation. In CHARMM simulations, in the longer time scale, the flanking purines display the tendency to move to bulged-in conformations. This is in contrast with the AMBER simulations, which indicate a modest prevalence for bulged-out flanking base positions in line with the X-ray data. The simulations also suggest that the intermolecular stacking between purines from the opposite hairpins can additionally stabilize the kissing complex.

Primary T-lymphocytes rescue the replication of HIV-1 DIS RNA mutants in part by facilitating reverse transcription

The dimerization initiation site (DIS) stem-loop within the HIV-1 RNA genome is vital for the production of infectious virions in T-cell lines but not in primary cells. In comparison to peripheral blood mononuclear cells (PBMCs), which can support the replication of both wild type and HIV-1 DIS RNA mutants, we have found that DIS RNA mutants are up to 100 000-fold less infectious than wild-type HIV-1 in T-cell lines. We have also found that the cell-type-dependent replication of HIV-1 DIS RNA mutants is largely producer cell-dependent, with mutants displaying a greater defect in viral cDNA synthesis when viruses were not derived from PBMCs. While many examples exist of host–pathogen interplays that are mediated via proteins, analogous examples which rely on nucleic acid triggers are limited. Our data provide evidence to illustrate that primary T-lymphocytes rescue, in part, the replication of HIV-1 DIS RNA mutants through mediating the reverse transcription process in a cell-type-dependent manner. Our data also suggest the presence of a host cell factor that acts within the virus producer cells. In addition to providing an example of an RNA-mediated cell-type-dependent block to viral replication, our data also provides evidence which help to resolve the dilemma of how HIV-1 genomes with mismatched DIS sequences can recombine to generate chimeric viral RNA genomes.

Aminoglycoside binding to the HIV-1 RNA dimerization initiation site: thermodynamics and effect on the kissing-loop to duplex conversion

Owing to a striking, and most likely fortuitous, structural and sequence similarity with the bacterial 16 S ribosomal A site, the RNA kissing-loop complex formed by the HIV-1 genomic RNA dimerization initiation site (DIS) specifically binds 4,5-disubstituted 2-deoxystreptamine (2-DOS) aminoglycoside antibiotics. We used chemical probing, molecular modeling, isothermal titration calorimetry (ITC) and UV melting to investigate aminoglycoside binding to the DIS loop–loop complex. We showed that apramycin, an aminoglycoside containing a bicyclic moiety, also binds the DIS, but in a different way than 4,5-disubstituted 2-DOS aminoglycosides. The determination of thermodynamic parameters for various aminoglycosides revealed the role of the different rings in the drug–RNA interaction. Surprisingly, we found that the affinity of lividomycin and neomycin for the DIS (Kd ∼ 30 nM) is significantly higher than that obtained in the same experimental conditions for their natural target, the bacterial A site (Kd ∼ 1.6 µM). In good agreement with their respective affinity, aminoglycoside increase the melting temperature of the loop–loop interaction and also block the conversion from kissing-loop complex to extended duplex. Taken together, our data might be useful for selecting new molecules with improved specificity and affinity toward the HIV-1 DIS RNA.

HIV controls the selective packaging of genomic, spliced viral and cellular RNAs into virions through different mechanisms

In addition to genomic RNA, HIV-1 particles package cellular and spliced viral RNAs. In order to determine the encapsidation mechanisms of these RNAs, we determined the packaging efficiencies and specificities of genomic RNA, singly and fully spliced HIV mRNAs and different host RNAs species: 7SL RNA, U6 snRNA and GAPDH mRNA using RT-QPCR. Except GAPDH mRNA, all RNAs are selectively encapsidated. Singly spliced RNAs, harboring the Rev-responsible element, and fully spliced viral RNAs, which do not contain this motif, are enriched in virions to similar levels, even though they are exported from the nucleus by different routes. Deletions of key motifs (SL1 and/or SL3) of the packaging signal of genomic RNA indicate that HIV and host RNAs are encapsidated through independent mechanisms, while genomic and spliced viral RNA compete for the same trans-acting factor due to the presence of the 5′ common exon containing the TAR, poly(A) and U5-PBS hairpins. Surprisingly, the RNA dimerization initiation site (DIS/SL1) appears to be the main packaging determinant of genomic RNA, but is not involved in packaging of spliced viral RNAs, suggesting a functional interaction with intronic sequences. Active and selective packaging of host and spliced viral RNAs provide new potential functions to these RNAs in the early stages of the virus life cycle.

A structure-based approach for targeting the HIV-1 genomic RNA dimerization initiation site

Dimerization of the genomic RNA is an important step of the HIV-1 replication cycle. The Dimerization Initiation Site (DIS) promotes dimerization of the viral genome by forming a loop–loop complex between two DIS hairpins. Crystal structures of the DIS loop–loop complex revealed an unexpected and strong similitude with the bacterial 16S ribosomal aminoacyl-tRNA site (A site), which is the target of aminoglycoside antibiotics. As a consequence of these structural and sequence similarities, the HIV-1 DIS also binds some aminoglycosides, not only in vitro, but also ex vivo, in lymphoid cells and in viral particles. Crystal structures of the DIS loop–loop in complex with several aminoglycoside antibiotics provide a detailed-view of the DIS/drug interaction and reveal some hints about possible modifications to increase the drug affinity and/or specificity.

Fluorescent HIV-1 Dimerization Initiation Site: Design, Properties, and Use for Ligand Discovery

The HIV-1 Dimerization Initiation Site (DIS) is an intriguing, yet underutilized, viral RNA target for potential antiretroviral therapy. To study the recognition features of this target and to provide a quantitative, rapid, and real-time tool for the discovery of new binders, a fluorescence-based assay has been constructed. It relies on strategic incorporation of 2-aminopurine, an isosteric fluorescent adenosine analogue, into short hairpin RNA constructs. These oligomers self-associate to form a kissing loop that thermally rearranges into a more stable extended duplex, thereby mimicking the association and structural features of the native RNA sequence. We demonstrate the ability of two fluorescent DIS constructs, DIS272(2AP) and DIS273(2AP), to report the binding of known DIS binders via changes in their emission intensity. Binding of aminoglycosides such as paromomycin to DIS272(2AP) results in significant fluorescence enhancement, while ligand binding to DIS273(2AP) results in fluorescence quenching. These observations are rationalized by comparison to the sequence-analogous bacterial A-site, where the relative emission of the fluorescent probe is dependent on the placement of the flexible purine residues inside or outside the helical domain. Analysis of binding isotherms generated using DIS272(2AP) yields submicromolar EC50 values for paromomycin (0.5 +/- 0.2 microM) and neomycin B (0.6 +/- 0.2 microM). Other neomycin-family aminoglycosides are less potent binders with neamine, the core pharmacophore, displaying the lowest affinity of 21 +/- 1 microM. Screening of additional aminoglycosides and their derivatives led to the discovery of new, previously unreported, aminoglycoside binders of the HIV DIS RNA, among them butirosin A (5.5 +/- 0.6 microM) and apramycin (7.6 +/- 1.0 microM). A conformationally constrained neomycin B analogue displays a rather high affinity to the DIS (1.9 +/- 0.2 microM). Among a series of nucleobase aminoglycoside conjugates, only the uracil derivatives display a measurable affinity using this assay with EC50 values in the 2 microM range. In addition, similarity between the solution behavior of HIV-1 DIS and the bacterial decoding A-site has been observed, particularly with respect to the intra- and extra-helical residence of the conformationally flexible A residues within the bulge. Taken together, the observations reported here shed light on the solution behavior of this important RNA target and are likely to facilitate the design of new DIS selective ligands as potential antiretroviral agents.

A new NMR solution structure of the SL1 HIV-1Lai loop-loop dimer

Dimerization of genomic RNA is directly related with the event of encapsidation and maturation of the virion. The initiating sequence of the dimerization is a short autocomplementary region in the hairpin loop SL1. We describe here a new solution structure of the RNA dimerization initiation site (DIS) of HIV-1Lai. NMR pulsed field-gradient spin-echo techniques and multidimensional heteronuclear NMR spectroscopy indicate that this structure is formed by two hairpins linked by six Watson–Crick GC base pairs. Hinges between the stems and the loops are stabilized by intra and intermolecular interactions involving the A8, A9 and A16 adenines. The coaxial alignment of the three A-type helices present in the structure is supported by previous crystallography analysis but the A8 and A9 adenines are found in a bulged in position. These data suggest the existence of an equilibrium between bulged in and bulged out conformations in solution.

Polymorphism of Bulged-out Residues in HIV-1 RNA DIS Kissing Complex and Structure Comparison with Solution Studies

All retroviruses encapsidate their genome as a dimer of homologous single-stranded RNAs. The dimerization initiation site (DIS) of human immunodeficiency virus type 1 (HIV-1) is located in the 5'-untranslated region of the viral genome and consists of a hairpin with a 6 nt self-complementary loop sequence. Genomic RNA dimerization, a crucial step for virion infectivity, is promoted by the formation of a loop-loop complex (or kissing complex) between two DIS hairpins. Crystal structures for the subtypes A, B and F of the HIV-1 DIS kissing complex have now been solved at 2.3 A, 1.9 A and 1.6 A, respectively. They revealed a polymorphism of bulged-out residues showing clearly that their conformation is not a mere consequence of crystal packing. They also provide more insights into ion binding, hydration, and RNA conformation and flexibility. In particular, we observed the binding of spermine to the loop-loop helix, which displaced a magnesium cation important for subtype A DIS dimerization. The excellent agreement between X-ray structures and the results of chemical probing and interference data on larger viral RNA fragments shows that the crystal structures are relevant for the DIS kissing complex present in solution and in viral particles. Accordingly, these structures will be helpful for designing new drugs derived from aminoglycoside antibiotics and targeted against the RNA dimerization step of the viral life-cycle.

The role of structural elements in dimerization of RNA fragments in avian retroviruses

The slipped loop structure, earlier identified as an unusual DNA structure, was found to be a possible element of the RNA folding. In order to experimentally test this suggestion, model oligoribonucleotides capable of forming the SLS were synthesized. Treatment of the oligoribonucleotides with nuclease S1 and RNases specific for single- and double-stranded RNA demonstrated the steric possibility of SLS formation. To determine the possible functional role of SLS-RNA, various naturally occurring RNAs were screened in silico. Among the most interesting findings were dimerization initiation sites of avian retroviral genomic RNAs. Analysis of RNA from 31 viruses showed that formation of the intermolecular SLS during RNA dimerization is theoretically possible, competing with the formation of an alternative hairpin structure. Identification of the secondary structure of selected RNA dimers employing nuclease digestion techniques as well as covariance analysis of the retroviral RNA dimerization initiation site sequences were used to show that the alternative conformation (loop-loop interaction of two hairpins) is the most preferred. Alternative structures and conformational transitions in RNA dimerization mechanisms in avian retroviruses are discussed.

HIV-1 RNA Dimerization Initiation Site Is Structurally Similar to the Ribosomal A Site and Binds Aminoglycoside Antibiotics

Human immunodeficiency virus (HIV) genomic RNA is packaged into virions as a dimer. The first step of dimerization is the formation of a kissing-loop complex at the so-called dimerization initiation site (DIS). We found an unexpected and fortuitous resemblance between the HIV-1 DIS kissing-loop complex and the eubacterial 16 S ribosomal aminoacyl-tRNA site (A site), which is the target of aminoglycoside antibiotics. Similarities exist not only at the primary and secondary structure level but also at the tertiary structure level, as revealed by comparison of the respective DIS and A site crystal structures. Gel shift, inhibition of lead-induced cleavage, and footprinting experiments showed that paromomycin and neomycin specifically bind to the kissing-loop complex formed by the DIS, with an affinity and a geometry similar to that observed for the A site. Modeling of the aminoglycoside-DIS complex allowed us to identify antibiotic modifications likely to increase the affinity and/or the specificity for the DIS. This could be a starting point for designing antiviral drugs against HIV-1 RNA dimerization.

Crystal structures of coaxially stacked kissing complexes of the HIV-1 RNA dimerization initiation site.

We describe the crystal structures of the RNA dimerization initiation sites (DIS) of HIV-1 subtypes A and B. Both molecules adopt a hairpin conformation, with loop sequences consisting of 272-AGGUGCACA-280 and 272-AAGCGCGCA-280, respectively. This includes a six-base self-complementary stretch (underlined) that allows homodimerization through the formation of a loop-loop, or 'kissing-loop', complex. The DISs for the two sequences have identical conformations, and the two interacting hairpins show a perfect coaxial alignment. The conserved purines, A272 and R273, are stacked in a bulged-out conformation and symmetrically join the upward and downward strands of each hairpin by crossing the helix major groove. There is good agreement between these structures and previous results from chemical probing in solution, as well as with differences in magnesium dependence for dimerization. The overall shape of the kissing-loop complex is very similar to that of the previously determined subtype A DIS duplex form. Unexpectedly, the purine R273 is the only base seen at a different position and is responsible for the difference in topology between the two forms. We propose that the transition from kissing-loop duplex could occur by a recombination mechanism based on symmetrical chain cleavage at R273 of each hairpin and subsequent cross-religation, rather than by base-pair melting and subsequent reannealing.

Conformational change of dimerization initiation site of HIV-1 genomic RNA by NCp7 or heat treatment.

Dimerization initiation site (DIS) of the human immunodeficiency virus type 1 (HIV-1) genome has been identified as a primary sequence that can form a stem-loop structure with a self-complementary sequence in the loop and a bulge in the stem. A DIS RNA fragment spontaneously forms a kissing dimer and is converted into an extended-duplex dimer by supplement of nucleocapsid protein NCp7. This two-step dimerization reaction can be also executed by a heat treatment instead of the binding proteins. However, it has not identified whether mechanisms of the conformational conversion by two different treatments are identical or not. In the present study, we used a series of DIS RNA oligonucleotides and the conformations of two extended-duplex dimers produced by the two different treatments were compared by the analysis of NMR spectra in the imino proton region. It was found that the effects of the two kinds of treatments are quite similar and the conformations of the two extended-duplex dimers are identical. These findings suggest that the conversion mechanisms DIS RNA by NCp7 and heat treatments are similar.