AANT: the Amino Acid-Nucleotide Interaction Database

Research progress on prediction of RNA-protein binding sites in the past five years

Accurately predicting RNA-protein binding sites is essential to gain a deeper comprehension of the protein-RNA interactions and their regulatory mechanisms, which are fundamental in gene expression and regulation. However, conventional biological approaches to detect these sites are often costly and time-consuming. In contrast, computational methods for predicting RNA protein binding sites are both cost-effective and expeditious. This review synthesizes already existing computational methods, summarizing commonly used databases for predicting RNA protein binding sites. In addition, applications and innovations of computational methods using traditional machine learning and deep learning for RNA protein binding site prediction during 2018-2023 are presented. These methods cover a wide range of aspects such as effective database utilization, feature selection and encoding, innovative classification algorithms, and evaluation strategies. Exploring the limitations of existing computational methods, this paper delves into the potential directions for future development. DeepRKE, RDense, and DeepDW all employ convolutional neural networks and long and short-term memory networks to construct prediction models, yet their algorithm design and feature encoding differ, resulting in diverse prediction performances.

Exploring the functionality and conservation of Alba proteins in Trypanosoma cruzi: A focus on biological diversity and RNA binding ability

Trypanosoma cruzi is the causative agent of Chagas disease, as well as a trypanosomatid parasite with a complex biological cycle that requires precise mechanisms for regulating gene expression. In Trypanosomatidae, gene regulation occurs mainly at the mRNA level through the recognition of cis elements by RNA-binding proteins (RBPs). Alba family members are ubiquitous DNA/RNA-binding proteins with representatives in trypanosomatid parasites functionally related to gene expression regulation. Although T. cruzi possesses two groups of Alba proteins (Alba1/2 and Alba30/40), their functional role remains poorly understood. Thus, herein, a characterization of T. cruzi Alba (TcAlba) proteins was undertaken. Physicochemical, structural, and phylogenetic analysis of TcAlba showed features compatible with RBPs, such as hydrophilicity, RBP domains/motifs, and evolutionary conservation of the Alba-domain, mainly regarding other trypanosomatid Alba. However, in silico RNA interaction analysis of T. cruzi Alba proteins showed that TcAlba30/40 proteins, but not TcAlba1/2, would directly interact with the assayed RNA molecules, suggesting that these two groups of TcAlba proteins have different targets. Given the marked differences existing between both T. cruzi Alba groups (TcAlba1/2 and TcAlba30/40), regarding sequence divergence, RNA binding potential, and life-cycle expression patterns, we suggest that they would be involved in different biological processes.

Structure-based virtual screening of unbiased and RNA-focused libraries to identify new ligands for the HCV IRES model system

Targeting RNA including viral RNAs with small molecules is an emerging field. The hepatitis C virus internal ribosome entry site (HCV IRES) is a potential target for translation inhibitor development to raise drug resistance mutation preparedness. Using RNA-focused and unbiased molecule libraries, a structure-based virtual screening (VS) by molecular docking and pharmacophore analysis was performed against the HCV IRES subdomain IIa. VS hits were validated by a microscale thermophoresis (MST) binding assay and a Förster resonance energy transfer (FRET) assay elucidating ligand-induced conformational changes. Ten hit molecules were identified with potencies in the high to medium micromolar range proving the suitability of structure-based virtual screenings against RNA-targets. Hit compounds from a 2-guanidino-quinazoline series, like the strongest binder, compound 8b with an EC50 of 61 μM, show low molecular weight, moderate lipophilicity and reduced basicity compared to previously reported IRES ligands. Therefore, it can be considered as a potential starting point for further optimization by chemical derivatization.

The Inhibition of Gag-Pol Expression by the Restriction Factor Shiftless Is Dispensable for the Restriction of HIV-1 Infection

The interferon-induced host cell protein Shiftless (SFL) inhibits -1 programmed ribosomal frameshifting (-1PRF) required for the expression of HIV-1 Gal-Pol and the formation of infectious HIV-1 particles. However, the specific regions in SFL required for antiviral activity and the mechanism by which SFL inhibits -1PRF remain unclear. Employing alanine scanning mutagenesis, we found that basic amino acids in the predicted zinc ribbon motif of SFL are essential for the suppression of Gag-Pol expression but dispensable for anti-HIV-1 activity. We have shown that SFL inhibits the expression of the murine leukemia virus (MLV) Gag-Pol polyprotein and the formation of infectious MLV particles, although Gag-Pol expression of MLV is independent of -1PRF but requires readthrough of a stop codon. These findings indicate that SFL might inhibit HIV-1 infection by more than one mechanism and that SFL might target programmed translational readthrough as well as -1PRF signals, both of which are regulated by mRNA secondary structure elements.

Peptide conjugates with polyaromatic hydrocarbons can benefit the activity of catalytic RNAs

Early stages of life likely employed catalytic RNAs (ribozymes) in many functions that are today filled by proteins. However, the earliest life forms must have emerged from heterogenous chemical mixtures, which included amino acids, short peptides, and many other compounds. Here we explored whether the presence of short peptides can help the emergence of catalytic RNAs. To do this, we conducted an in vitro selection for catalytic RNAs from randomized sequence in the presence of ten different peptides with a prebiotically plausible length of eight amino acids. This in vitro selection generated dozens of ribozymes, one of them with ∼900-fold higher activity in the presence of one specific peptide. Unexpectedly, the beneficial peptide had retained its N-terminal Fmoc protection group, and this group was required to benefit ribozyme activity. The same, or higher benefit resulted from peptide conjugates with prebiotically plausible polyaromatic hydrocarbons (PAHs) such as fluorene and naphthalene. This shows that PAH-peptide conjugates can act as potent cofactors to enhance ribozyme activity. The results are discussed in the context of the origin of life.

Web tools support predicting protein-nucleic acid complexes stability with affinity changes

Numerous biological processes, such as transcription, replication, and translation, rely on protein-nucleic acid interactions (PNIs). Demonstrating the binding stability of protein-nucleic acid complexes is vital to deciphering the code for PNIs. Numerous web-based tools have been developed to attach importance to protein-nucleic acid stability, facilitating the prediction of PNIs characteristics rapidly. However, the data and tools are dispersed and lack comprehensive integration to understand the stability of PNIs better. In this review, we first summarize existing databases for evaluating the stability of protein-nucleic acid binding. Then, we compare and evaluate the pros and cons of web tools for forecasting the interaction energies of protein-nucleic acid complexes. Finally, we discuss the application of combining models and capabilities of PNIs. We may hope these web-based tools will facilitate the discovery of recognition mechanisms for protein-nucleic acid binding stability. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Efficient mapping of RNA-binding residues in RNA-binding proteins using local sequence features of binding site residues in protein-RNA complexes

Protein-RNA interactions play vital roles in plethora of biological processes such as regulation of gene expression, protein synthesis, mRNA processing and biogenesis. Identification of RNA-binding residues (RBRs) in proteins is essential to understand RNA-mediated protein functioning, to perform site-directed mutagenesis and to develop novel targeted drug therapies. Moreover, the extensive gap between sequence and structural data restricts the identification of binding sites in unsolved structures. However, efficient use of computational methods demanding only sequence to identify binding residues can bridge this huge sequence-structure gap. In this study, we have extensively studied protein-RNA interface in known RNA-binding proteins (RBPs). We find that the interface is highly enriched in basic and polar residues with Gly being the most common interface neighbor. We investigated several amino acid features and developed a method to predict putative RBRs from amino acid sequence. We have implemented balanced random forest (BRF) classifier with local residue features of protein sequences for prediction. With 5-fold cross-validations, the sequence pattern derived dipeptide composition based BRF model (DCP-BRF) resulted in an accuracy of 87.9%, specificity of 88.8%, sensitivity of 82.2%, Mathew's correlation coefficient of 0.60 and AUC of 0.93, performing better than few existing methods. We further validated our prediction model on known human RBPs through RBR prediction and could map ~54% of them. Further, knowledge of binding site preferences obtained from computational predictions combined with experimental validations of potential RNA binding sites can enhance our understanding of protein-RNA interactions. This may serve to accelerate investigations on functional roles of many novel RBPs.

c-di-GMP regulates activity of the PlzA RNA chaperone from the Lyme disease spirochete

PlzA is a c-di-GMP-binding protein crucial for adaptation of the Lyme disease spirochete Borrelia (Borreliella) burgdorferi during its enzootic life cycle. Unliganded apo-PlzA is important for vertebrate infection, while liganded holo-PlzA is important for survival in the tick; however, the biological function of PlzA has remained enigmatic. Here, we report that PlzA has RNA chaperone activity that is inhibited by c-di-GMP binding. Holo- and apo-PlzA bind RNA and accelerate RNA annealing, while only apo-PlzA can strand displace and unwind double-stranded RNA. Guided by the crystal structure of PlzA, we identified several key aromatic amino acids protruding from the N- and C-terminal domains that are required for RNA-binding and unwinding activity. Our findings illuminate c-di-GMP as a switch controlling the RNA chaperone activity of PlzA, and we propose that complex RNA-mediated modulatory mechanisms allow PlzA to regulate gene expression during both the vector and host phases of the B. burgdorferi life cycle.

Ned Seeman and the prediction of amino acid-basepair motifs mediating protein-nucleic acid recognition

Fifty years ago, the first atomic-resolution structure of a nucleic acid double helix, the mini-duplex (ApU)2, revealed details of basepair geometry, stacking, sugar conformation, and backbone torsion angles, thereby superseding earlier models based on x-ray fiber diffraction, including the original DNA double helix proposed by Watson and Crick. Just 3 years later, in 1976, Ned Seeman, John Rosenberg, and Alex Rich leapt from their structures of mini-duplexes and H-bonding motifs between bases in small-molecule structures and transfer RNA to predicting how proteins could sequence specifically recognize double helix nucleic acids. They proposed interactions between amino acid side chains and nucleobases mediated by two hydrogen bonds in the major or minor grooves. One of these, the arginine-guanine pair, emerged as the most favored amino acid-base interaction in experimental structures of protein-nucleic acid complexes determined since 1986. In this brief review we revisit the pioneering work by Seeman et al. and discuss the importance of the arginine-guanine pairing motif.

DNA and RNA Binding Proteins: From Motifs to Roles in Cancer

DNA and RNA binding proteins (DRBPs) are a broad class of molecules that regulate numerous cellular processes across all living organisms, creating intricate dynamic multilevel networks to control nucleotide metabolism and gene expression. These interactions are highly regulated, and dysregulation contributes to the development of a variety of diseases, including cancer. An increasing number of proteins with DNA and/or RNA binding activities have been identified in recent years, and it is important to understand how their activities are related to the molecular mechanisms of cancer. In addition, many of these proteins have overlapping functions, and it is therefore essential to analyze not only the loss of function of individual factors, but also to group abnormalities into specific types of activities in regard to particular cancer types. In this review, we summarize the classes of DNA-binding, RNA-binding, and DRBPs, drawing particular attention to the similarities and differences between these protein classes. We also perform a cross-search analysis of relevant protein databases, together with our own pipeline, to identify DRBPs involved in cancer. We discuss the most common DRBPs and how they are related to specific cancers, reviewing their biochemical, molecular biological, and cellular properties to highlight their functions and potential as targets for treatment.

Structure- and Interaction-Based Design of Anti-SARS-CoV-2 Aptamers

Aptamer selection against novel infections is a complicated and time-consuming approach. Synergy can be achieved by using computational methods together with experimental procedures. This study aims to develop a reliable methodology for a rational aptamer in silico et vitro design. The new approach combines multiple steps: (1) Molecular design, based on screening in a DNA aptamer library and directed mutagenesis to fit the protein tertiary structure; (2) 3D molecular modeling of the target; (3) Molecular docking of an aptamer with the protein; (4) Molecular dynamics (MD) simulations of the complexes; (5) Quantum-mechanical (QM) evaluation of the interactions between aptamer and target with further analysis; (6) Experimental verification at each cycle for structure and binding affinity by using small-angle X-ray scattering, cytometry, and fluorescence polarization. By using a new iterative design procedure, structure- and interaction-based drug design (SIBDD), a highly specific aptamer to the receptor-binding domain of the SARS-CoV-2 spike protein, was developed and validated. The SIBDD approach enhances speed of the high-affinity aptamers development from scratch, using a target protein structure. The method could be used to improve existing aptamers for stronger binding. This approach brings to an advanced level the development of novel affinity probes, functional nucleic acids. It offers a blueprint for the straightforward design of targeting molecules for new pathogen agents and emerging variants.

Initial Screening of Poly(ethylene glycol) Amino Ligands for Affinity Purification of Plasmid DNA in Aqueous Two-Phase Systems

Gene therapy and DNA vaccination are among the most expected biotechnological and medical advances for the coming years. However, the lack of cost-effective large-scale production and purification of pharmaceutical-grade plasmid DNA (pDNA) still hampers their wide application. Downstream processing, which is mainly chromatography-based, of pDNA remains the key manufacturing step. Despite its high resolution, the scaling-up of chromatography is usually difficult and presents low capacity, resulting in low yields. Alternative methods that are based on aqueous two-phase systems (ATPSs) have been studied. Although higher yields may be obtained, its selectivity is often low. In this work, modified polymers based on poly(ethylene glycol) (PEG) derivatisation with amino groups (PEG–amine) or conjugation with positively charged amino acids (PEG–lysine, PEG–arginine, and PEG–histidine) were studied to increase the selectivity of PEG–dextran systems towards the partition of a model plasmid. A two-step strategy was employed to obtain suitable pure formulations of pDNA. In the first step, a PEG–dextran system with the addition of the affinity ligand was used with the recovery of the pDNA in the PEG-rich phase. Then, the pDNA was re-extracted to an ammonium-sulphate-rich phase in the second step. After removing the salt, this method yielded a purified preparation of pDNA without RNA and protein contamination.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

Recognition of nucleolin through interaction with RNA G-quadruplex

The development of novel biomarkers for early-stage diagnosis of prostate cancer (PCa) has attracted the attention of researchers in the last decade. Nucleolin (NCL) has emerged as a possible biomarker of PCa due to its high expression levels in the surface of PCa cells and affinity towards parallel G4s since it contains four RNA-binding domains (RBDs). Herein, we developed a novel strategy based on a microfluidic platform for the detection of NCL in biological samples, such as human plasma. The RNA G4 (rG4) sequence found in human precursor microRNA 92b (pre-miR-92b) was used as a molecular recognition probe since it forms a single dominant parallel rG4 conformation in the presence of 0.1 mM K+ as confirmed by NMR spectroscopy. The additional stability of the rG4 structure was provided by the acridine orange derivative ligand C8, which stabilizes the pre-miR-92b rG4 structure, as denoted by an increase in more than 30 °C of its melting temperature. FRET-melting assay revealed a remarkable synergistic effect of NCL RBD1,2 and C8 on the stabilization of the pre-miR-92b rG4. The binding of pre-miR-92b to NCL RBD1,2 was determined by in silico studies, which revealed a binding pocket formed by a 12-residue linker between RBD1 and RBD2. Both, pre-miR-92b rG4 and pre-miR-92b rG4/C8 complex demonstrated high affinity towards NCL RBD1,2, as proved by fluorimetric titrations (KD range between 10-12 and 10-9 M). The stability and nuclease resistance of pre-miR-92b rG4 and pre-miR-92b rG4/C8 complex were evaluated as molecular recognition probes to capture and detect NCL. Finally, the microfluidic platform detects NCL in complex biological samples, such as human plasma. Overall, this work demonstrates the usefulness of the microfluidic platform based on the pre-miR-92b to detect NCL and the possibility to be used as a valuable biomedical tool in PCa diagnosis.

Computational methods for predicting hotspots at protein-RNA interfaces

Protein-RNA interactions play essential roles in many critical biological events. A comprehensive understanding of the mechanisms underlying these interactions is helpful when studying cellular activities and therapeutic applications. Hotspots are a small portion of residues contributing much toward protein-RNA binding affinity. In pharmaceutical research, the hotspot residues are seen as the best option for designing small molecules to target proteins of therapeutic interest. With the accumulation of experimental data about protein-RNA interactions, computational methods have been produced for hotspot prediction on a large scale. In this review, we first present an overview of the existing databases for protein-RNA binding data. Furthermore, we outline the most adopted computational methods for hotspots prediction in protein-RNA interactions. Finally, we discuss the applications of hotspot prediction. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Methods > RNA Analyses In Vitro and In Silico.

The bridge helix of Cas12a imparts selectivity in cis-DNA cleavage and regulates trans-DNA cleavage

Cas12a is an RNA-guided DNA endonuclease of the type V-A CRISPR-Cas system that has evolved convergently with the type II Cas9 protein. We previously showed that proline substitutions in the bridge helix (BH) impart target DNA cleavage selectivity in Streptococcus pyogenes (Spy) Cas9. Here, we examined a BH variant of Cas12a from Francisella novicida (FnoCas12aKD2P ) to test mechanistic conservation. Our results show that for RNA-guided DNA cleavage (cis-activity), FnoCas12aKD2P accumulates nicked products while cleaving supercoiled DNA substrates with mismatches, with certain mismatch positions being more detrimental for linearization. FnoCas12aKD2P also possess reduced trans-single-stranded DNA cleavage activity. These results implicate the BH in substrate selectivity in both cis- and trans-cleavages and show its conserved role in target discrimination among Cas nucleases.

Minicircle Biopharmaceuticals–An Overview of Purification Strategies

Minicircles are non-viral delivery vectors with promising features for biopharmaceutical applications. These vectors are plasmid-derived circular DNA molecules that are obtained in vivo in Escherichia coli by the intramolecular recombination of a parental plasmid, which generates a minicircle containing the eukaryotic therapeutic cassette of interest and a miniplasmid containing the prokaryotic backbone. The production process results thus in a complex mixture, which hinders the isolation of minicircle molecules from other DNA molecules. Several strategies have been proposed over the years to meet the challenge of purifying and obtaining high quality minicircles in compliance with the regulatory guidelines for therapeutic use. In minicircle purification, the characteristics of the strain and parental plasmid used have a high impact and strongly affect the purification strategy that can be applied. This review summarizes the different methods developed so far, focusing not only on the purification method itself but also on its dependence on the upstream production strategy used.

How RNA-Binding Proteins Interact with RNA: Molecules and Mechanisms

RNA-binding proteins (RBPs) comprise a large class of over 2,000 proteins that interact with transcripts in all manner of RNA-driven processes. The structures and mechanisms that RBPs use to bind and regulate RNA are incredibly diverse. In this review, we take a look at the components of protein-RNA interaction, from the molecular level to multi-component interaction. We first summarize what is known about protein-RNA molecular interactions based on analyses of solved structures. We additionally describe software currently available for predicting protein-RNA interaction and other resources useful for the study of RBPs. We then review the structure and function of seventeen known RNA-binding domains and analyze the hydrogen bonds adopted by protein-RNA structures on a domain-by-domain basis. We conclude with a summary of the higher-level mechanisms that regulate protein-RNA interactions.

A solid-state NMR tool box for the investigation of ATP-fueled protein engines

Motor proteins are involved in a variety of cellular processes. Their main purpose is to convert the chemical energy released during adenosine triphosphate (ATP) hydrolysis into mechanical work. In this review, solid-state Nuclear Magnetic Resonance (NMR) approaches are discussed allowing studies of structures, conformational events and dynamic features of motor proteins during a variety of enzymatic reactions. Solid-state NMR benefits from straightforward sample preparation based on sedimentation of the proteins directly into the Magic-Angle Spinning (MAS) rotor. Protein resonance assignment is the crucial and often time-limiting step in interpreting the wealth of information encoded in the NMR spectra. Herein, potentials, challenges and limitations in resonance assignment for large motor proteins are presented, focussing on both biochemical and spectroscopic approaches. This work highlights NMR tools available to study the action of the motor domain and its coupling to functional processes, as well as to identify protein-nucleotide interactions during events such as DNA replication. Arrested protein states of reaction coordinates such as ATP hydrolysis can be trapped for NMR studies by using stable, non-hydrolysable ATP analogues that mimic the physiological relevant states as accurately as possible. Recent advances in solid-state NMR techniques ranging from Dynamic Nuclear Polarization (DNP), ³¹P-based heteronuclear correlation experiments, ¹H-detected spectra at fast MAS frequencies >100 kHz to paramagnetic NMR are summarized and their applications to the bacterial DnaB helicase from Helicobacter pylori are discussed.

Effect of Chromatographic Conditions on Supercoiled Plasmid DNA Stability and Bioactivity

The dysfunction of the tumor suppressor gene TP53 has been associated with the pathogenesis of the majority of the cases of cancer reported to date, leading the cell to acquire different features known as the cancer hallmarks. In normal situations, the protein p53 protects the cells against tumorigenesis. By detecting metabolic stress or DNA damage in response to stress, p53 can lead the cell to senescence, autophagy, cell cycle arrest, DNA repair, and apoptosis. Thus, in the case of p53 mutations, it is reasonable to assume that the reestablishment of its function, may restrain the proliferation of cancer cells. The concept of cancer gene therapy can be based on this assumption, and suitable biotechnological approaches must be explored to assure the preparation of gene-based biopharmaceuticals. Although numerous procedures have already been established to purify supercoiled plasmid DNA (sc pDNA), the therapeutic application is highly dependent on the biopharmaceutical’s activity, which can be affected by the chromatographic conditions used. Thus, the present work aims at comparing quality and in vitro activity of the supercoiled (sc) isoform of the p53 encoding plasmid purified by three different amino acids-based chromatographic strategies, involving histidine–agarose, arginine–macroporous, and histidine–monolith supports. The B-DNA topology was maintained in all purified pDNA samples, but their bioactivity, related to the induction of protein p53 expression and apoptosis in cancer cells, was higher with arginine–macroporous support, followed by histidine–monolith and histidine–agarose. Despite the purity degree of 92% and recovery yield of 43% obtained with arginine–macroporous, the sc pDNA sample led to a higher expression level of the therapeutic p53 protein (58%) and, consequently, induced a slightly higher apoptotic effect (27%) compared with sc pDNA samples obtained with histidine–monolithic support (26%) and histidine–agarose support (24%). This behavior can be related to the mild chromatographic conditions used with arginine–macroporous support, which includes the use of low salt concentrations, at neutral pH and lower temperatures, when compared to the high ionic strength of ammonium sulfate and acidic pH used with histidine-based supports. These results can contribute to field of biopharmaceutical preparation, emphasizing the need to control several experimental conditions while adapting and selecting the methodologies that enable the use of milder conditions as this can have a significant impact on pDNA stability and biological activity.

DNA-protein interaction: identification, prediction and data analysis

Life in living organisms is dependent on specific and purposeful interaction between other molecules. Such purposeful interactions make the various processes inside the cells and the bodies of living organisms possible. DNA-protein interactions, among all the types of interactions between different molecules, are of considerable importance. Currently, with the development of numerous experimental techniques, diverse methods are convenient for recognition and investigating such interactions. While the traditional experimental techniques to identify DNA-protein complexes are time-consuming and are unsuitable for genome-scale studies, the current high throughput approaches are more efficient in determining such interaction at a large-scale, but they are clearly too costly to be practice for daily applications. Hence, according to the availability of much information related to different biological sequences and clearing different dimensions of conditions in which such interactions are formed, with the developments related to the computer, mathematics, and statistics motivate scientists to develop bioinformatics tools for prediction the interaction site(s). Until now, there has been much progress in this field. In this review, the factors and conditions governing the interaction and the laboratory techniques for examining such interactions are addressed. In addition, developed bioinformatics tools are introduced and compared for this reason and, in the end, several suggestions are offered for the promotion of such tools in prediction with much more precision.

The conformational changes coupling ATP hydrolysis and translocation in a bacterial DnaB helicase

DnaB helicases are motor proteins that couple ATP-hydrolysis to the loading of the protein onto DNA at the replication fork and to translocation along DNA to separate double-stranded DNA into single strands during replication. Using a network of conformational states, arrested by nucleotide mimics, we herein characterize the reaction coordinates for ATP hydrolysis, DNA loading and DNA translocation using solid-state NMR spectroscopy. AMP-PCP is used as pre-hydrolytic, ADP:AlF4- as transition state, and ADP as post-hydrolytic ATP mimic. 31P and 13C NMR spectra reveal conformational and dynamic responses to ATP hydrolysis and the resulting DNA loading and translocation with single amino-acid resolution. This allows us to identify residues guiding the DNA translocation process and to explain the high binding affinities for DNA observed for ADP:AlF4-, which turns out to be optimally preconfigured to bind DNA.

Amino Acid Stabilization of Nucleic Acid Secondary Structure: Kinetic Insights from Single-Molecule Studies

Amino acid and nucleic acid interactions are central in biology and may have played a role in the evolutionary development of protein-based life from an early "RNA Universe." To explore the possible role of single amino acids in promoting nucleic acid folding, single-molecule Förster resonance energy transfer experiments have been implemented with a DNA hairpin construct (7 nucleotide double strand with a 40A loop) as a simple model for secondary structure formation. Exposure to positively charged amino acids (arginine and lysine) is found to clearly stabilize the secondary structure. Kinetically, each amino acid promotes folding by generating a large increase in the folding rate with little change in the unfolding rate. From analysis as a function of temperature, arginine and lysine are found to significantly increase the overall exothermicity of folding while imposing only a small entropic penalty on the folding process. Detailed investigations into the kinetics and thermodynamics of this amino acid-induced folding stability reveal arginine and lysine to interact with nucleic acids in a manner reminiscent of monovalent cations. Specifically, these observations are interpreted in the context of an ion atmosphere surrounding the nucleic acid, in which amino acid salts stabilize folding qualitatively like small monovalent cations but also exhibit differences because of the composition of their side chains.

RNA-protein interactions in an unstructured context

Despite their importance, our understanding of noncovalent RNA-protein interactions is incomplete. This especially concerns the binding between RNA and unstructured protein regions, a widespread class of such interactions. Here, we review the recent experimental and computational work on RNA-protein interactions in an unstructured context with a particular focus on how such interactions may be shaped by the intrinsic interaction affinities between individual nucleobases and protein side chains. Specifically, we articulate the claim that the universal genetic code reflects the binding specificity between nucleobases and protein side chains and that, in turn, the code may be seen as the Rosetta stone for understanding RNA-protein interactions in general.

Interrogation of Base Pairing of the Spiroiminodihydantoin Diastereomers Using the α-Hemolysin Latch

Spiroiminodihydantoin (Sp) is a hyperoxidized form of guanine (G) resulting from oxidation by reactive oxygen species. The lesion is highly mutagenic, and the stereocenter renders the two isomers with distinct behaviors in chemical, spectroscopic, enzymatic, and computational studies. In this work, the α-hemolysin (αHL) latch sensing zone was employed to investigate the base pairing properties of the Sp diastereomers embedded in a double-stranded DNA. Duplexes containing (S)-Sp consistently gave deeper current blockage, and a baseline resolution of ∼0.8 pA was achieved between (S)-Sp:G and (R)-Sp:G base pairs. Ion fluxes were generally more hindered when Sp was placed opposite pyrimidines. Analysis of the current noise of blockade events further provided dynamics information about the Sp-containing base pairs. In general, base pairs comprised of (S)-Sp generated current fluctuations larger than those of their (R)-Sp counterparts, suggesting enhanced base pairing dynamics. The current noise was also substantially affected by the identity of the base opposite Sp, increasing in the following order: A < G < T < C. This report provides information about the dynamic structure of Sp in the DNA duplex and therefore has implications for the enzymatic repair of the Sp diastereomers.

Noncovalent Interactions in Specific Recognition Motifs of Protein-DNA Complexes

In view of the importance of protein-DNA interactions in biological processes, we extracted from the Protein Data Bank several one-to-one complexes of amino acids with nucleotides that matched certain geometric and energetic specificity criteria and investigated them using quantum chemistry methods. The CCSD(T)/CBS interaction energies were used as a benchmark to compare the performance of the MP2.5, MP2-F12, DFT-D3, and PM6-D3H4 methods. All methods yielded good agreement with the reference values, with declining accuracy from MP2.5 to PM6-D3H4. Regardless of the site of interaction, the minima found after full optimization in implicit solvent with high dielectric constant were close to the structures experimentally detected in protein-DNA complexes. According to DFT-SAPT analysis, the nature of noncovalent interactions strongly depends on the type of amino acid. The negatively charged sugar-phosphate backbone of DNA heavily influences the strength of interactions and must be included in the computational model, especially in the case of interactions with charged amino acids.

Amino Acid Specific Effects on RNA Tertiary Interactions: Single-Molecule Kinetic and Thermodynamic Studies

In light of the current models for an early RNA-based universe, the potential influence of simple amino acids on tertiary folding of ribozymal RNA into biochemically competent structures is speculated to be of significant evolutionary importance. In the present work, the folding-unfolding kinetics of a ubiquitous tertiary interaction motif, the GAAA tetraloop-tetraloop receptor (TL-TLR), is investigated by single-molecule fluorescence resonance energy transfer spectroscopy in the presence of natural amino acids both with (e.g., lysine, arginine) and without (e.g., glycine) protonated side chain residues. By way of control, we also investigate the effects of a special amino acid (e.g., proline) and amino acid mimetic (e.g., betaine) that contain secondary or quaternary amine groups rather than a primary amine group. This combination permits systematic study of amino acid induced (or amino acid like) RNA folding dynamics as a function of side chain complexity, pK_a, charge state, and amine group content. Most importantly, each of the naturally occurring amino acids is found to destabilize the TL-TLR tertiary folding equilibrium, the kinetic origin of which is dominated by a decrease in the folding rate constant (k_dock), also affected by a strongly amino acid selective increase in the unfolding rate constant (k_undock). To further elucidate the underlying thermodynamics, single-molecule equilibrium constants (K_eq) for TL-TLR folding have been probed as a function of temperature, which reveal an amino acid dependent decrease in both overall exothermicity (ΔΔH° > 0) and entropic cost (-TΔΔS° < 0) for the overall folding process. Temperature-dependent studies on the folding/unfolding kinetic rate constants reveal analogous amino acid specific changes in both enthalpy (ΔΔH^⧧) and entropy (ΔΔS^⧧) for accessing the transition state barrier. The maximum destabilization of the TL-TLR tertiary interaction is observed for arginine, which is consistent with early studies of arginine and guanidine ion-inhibited self-splicing kinetics for the full Tetrahymena ribozyme [ Yarus , M. ; Christian , E. L. Nature 1989 , 342 , 349 - 350 ; Yarus , M. Science 1988 , 240 , 1751 - 1758 ].

Large-Scale Quantitative Assessment of Binding Preferences in Protein-Nucleic Acid Complexes

The growing number of high-quality experimental (X-ray, NMR) structures of protein–DNA complexes has sufficient enough information to assess whether universal rules governing the DNA sequence recognition process apply. While previous studies have investigated the relative abundance of various modes of amino acid–base contacts (van der Waals contacts, hydrogen bonds), relatively little is known about the energetics of these noncovalent interactions. In the present study, we have performed the first large-scale quantitative assessment of binding preferences in protein–DNA complexes by calculating the interaction energies in all 80 possible amino acid–DNA base combinations. We found that several mutual amino acid–base orientations featuring bidentate hydrogen bonds capable of unambiguous one-to-one recognition correspond to unique minima in the potential energy space of the amino acid–base pairs. A clustering algorithm revealed that these contacts form a spatially well-defined group offering relatively little conformational freedom. Various molecular mechanics force field and DFT-D ab initio calculations were performed, yielding similar results.

NIMS: a database on nucleobase compounds and their interactions in macromolecular structures

The intense exploration of nucleotide-binding protein structures has created a whirlwind in the field of structural biology and bioinformatics. This has led to the conception and birth of NIMS. This database is a collection of detailed data on the nucleobases, nucleosides and nucleotides, along with their analogues as well as the protein structures to which they bind. Interaction details such as the interacting residues and all associated values have been made available. As a pioneering step, the diffraction precision index for protein structures, the atomic uncertainty for each atom, and the computed errors on the interatomic distances and angles are available in the database. Apart from the above, provision has been made to visualize the three-dimensional structures of both ligands and protein–ligand structures and their interactions inJmolas well asJSmol. One of the salient features of NIMS is that it has been interfaced with a user-friendly and query-based efficient search engine. It was conceived and developed with the aim of serving a significant section of researchers working in the area of protein and nucleobase complexes. NIMS is freely available online at http://iris.physics.iisc.ernet.in/nims and it is hoped that it will prove to be an invaluable asset.

Interaction preferences between nucleobase mimetics and amino acids in aqueous solutions

Despite the paramount importance of protein-nucleic acid interactions in different cellular processes, our understanding of such interactions at the atomistic level remains incomplete. We have used molecular dynamics (MD) simulations and 15 μs of sampling time to study the behavior of amino acids and amino-acid sidechain analogs in aqueous solutions of different mimetics of naturally occurring nucleobases, including dimethylpyridine (DMP) and unsubstituted purine and pyrimidine rings. By using structural and energetic analysis, we have derived preference scales for the interaction of amino acids and their sidechain analogs with different nucleobase mimetics and have exhaustively compared them with each other. A close correspondence with a standard hydrophobicity measure in the case of the pyrimidine mimetic DMP and purines suggests that the hydrophobic effect is the main defining factor behind such interactions. We analyze our findings in the context of the origin of the genetic code and the recently proposed cognate mRNA-protein complementarity hypothesis. Most importantly, we show that unsubstituted purine and pyrimidine rings alone cannot differentiate between predominantly purine- and pyrimidine-coded amino acids, suggesting that for such specificity to exist, it must primarily reside in ring substituents.

5-Thiocyanato-2'-deoxyuridine as a possible radiosensitizer: electron-induced formation of uracil-C5-thiyl radical and its dimerization

In this work, we have synthesized 5-thiocyanato-2'-deoxyuridine (SCNdU) along with the C6-deuterated nucleobase 5-thiocyanatouracil (6-D-SCNU) and studied their reactions with radiation-produced electrons. ESR spectra in γ-irradiated nitrogen-saturated frozen homogeneous solutions (7.5 M LiCl in H2O or D2O) of these compounds show that electron-induced S-CN bond cleavage occurs to form a thiyl radical (dU-5-S˙ or 6-D-U-5-S˙) and CN(-)via the initial π-anion radical (SCNdU˙(-)) intermediate in which the excess electron is on the uracil base. HPLC and LC-MS/MS studies of γ-irradiated N2-saturated aqueous solutions of SCNdU in the presence of sodium formate as a OH-radical scavenger at ambient temperature show the formation of the dU-5S-5S-dU dimer in preference to dU by about 10 to 1 ratio. This shows that both possible routes of electron-induced bond cleavage (dUC5-SCN and S-CN) in SCNdU˙(-) and dU-5-S˙ formation are preferred for the production of the σ-type uracilyl radical (dU˙) by 10 fold. DFT/M06-2x/6-31++G(d,p) calculations employing the polarizable continuum model (PCM) for aqueous solutions show that dU-5-S˙ and CN(-) formation was thermodynamically favored by over 15 kcal mol(-1) (ΔG) compared to dU˙ and SCN(-) production. The activation barriers for C5-S and S-CN bond cleavage in SCNdU˙(-) amount to 8.7 and 4.0 kcal mol(-1), respectively, favoring dU-5-S˙ and CN(-) formation. These results support the experimental observation of S-CN bond cleavage by electron addition to SCNdU that results in the formation of dU-5-S˙ and the subsequent dU-5S-5S-dU dimer. This establishes SCNdU as a potential radiosensitizer that could cause intra- and inter-strand crosslinking as well as DNA-protein crosslinking via S-S dimer formation.

Molecular recognition of oligonucleotides and plasmid DNA by l-methionine

The present study explores the effect of oligonucleotide composition on the mechanism of retention to l-methionine agarose support by chromatography and saturation transfer difference (STD)-nuclear magnetic resonance (NMR) techniques. All chromatographic experiments were performed using 1.5 M (NH4 )2 SO4 . The binding profiles obtained by chromatography show that oligonucleotides with thymine had the highest retention time. In general, the larger homo-oligonucleotides are more retained to the l-methionine agarose support. Moreover, the study with hetero-oligonucleotides confirms that the presence of guanine reduces the retention on the l-methionine chromatographic support. These results are in accord with STD-NMR experiments, which show that the strongest signals were observed for the methyl group of thymine, and no STD signals were observed for the guanosine protons. Finally, the retention behaviour of linear plasmid DNA (pDNA) with different sizes and base composition (2.7-kbp pUC19, 6.05-kbp pVAX1-LacZ, 7.4-kbp pVAX1-LacZgag and 14-kbp pcDNA-based plasmid) was also evaluated by chromatography. The results indicate that the underlying mechanism of retention involves not only hydrophobic interactions but also other elementary interactions responsible for the biorecognition of pDNA molecules by l-methionine ligands.

Absolute binding-free energies between standard RNA/DNA nucleobases and amino-acid sidechain analogs in different environments

Despite the great importance of nucleic acid-protein interactions in the cell, our understanding of their physico-chemical basis remains incomplete. In order to address this challenge, we have for the first time determined potentials of mean force and the associated absolute binding free energies between all standard RNA/DNA nucleobases and amino-acid sidechain analogs in high- and low-dielectric environments using molecular dynamics simulations and umbrella sampling. A comparison against a limited set of available experimental values for analogous systems attests to the quality of the computational approach and the force field used. Overall, our analysis provides a microscopic picture behind nucleobase/sidechain interaction preferences and creates a unified framework for understanding and sculpting nucleic acid-protein interactions in different contexts. Here, we use this framework to demonstrate a strong relationship between nucleobase density profiles of mRNAs and nucleobase affinity profiles of their cognate proteins and critically analyze a recent hypothesis that the two may be capable of direct, complementary interactions.

Looked at life from both sides now

As the molecular top–down causality emerging through comparative genomics is combined with the bottom–up dynamic chemical networks of biochemistry, the molecular symbiotic relationships driving growth of the tree of life becomes strikingly apparent. These symbioses can be mutualistic or parasitic across many levels, but most foundational is the complex and intricate mutualism of nucleic acids and proteins known as the central dogma of biological information flow. This unification of digital and analog molecular information within a common chemical network enables processing of the vast amounts of information necessary for cellular life. Here we consider the molecular information pathways of these dynamic biopolymer networks from the perspective of their evolution and use that perspective to inform and constrain pathways for the construction of mutualistic polymers.

DBBP: database of binding pairs in protein-nucleic acid interactions

Background

Interaction of proteins with other molecules plays an important role in many biological activities. As many structures of protein-DNA complexes and protein-RNA complexes have been determined in the past years, several databases have been constructed to provide structure data of the complexes. However, the information on the binding sites between proteins and nucleic acids is not readily available from the structure data since the data consists mostly of the three-dimensional coordinates of the atoms in the complexes.

Results

We analyzed the huge amount of structure data for the hydrogen bonding interactions between proteins and nucleic acids and developed a database called DBBP (DataBase of Binding Pairs in protein-nucleic acid interactions, http://bclab.inha.ac.kr/dbbp). DBBP contains 44,955 hydrogen bonds (H-bonds) of protein-DNA interactions and 77,947 H-bonds of protein-RNA interactions.

Conclusions

Analysis of the huge amount of structure data of protein-nucleic acid complexes is labor-intensive, yet provides useful information for studying protein-nucleic acid interactions. DBBP provides the detailed information of hydrogen-bonding interactions between proteins and nucleic acids at various levels from the atomic level to the residue level. The binding information can be used as a valuable resource for developing a computational method aiming at predicting new binding sites in proteins or nucleic acids.

Computational analysis of amino acids and their sidechain analogs in crowded solutions of RNA nucleobases with implications for the mRNA-protein complementarity hypothesis

Many critical processes in the cell involve direct binding between RNAs and proteins, making it imperative to fully understand the physicochemical principles behind such interactions at the atomistic level. Here, we use molecular dynamics simulations and 15 μs of sampling to study the behavior of amino acids and amino acid sidechain analogs in high-concentration aqueous solutions of standard RNA nucleobases. Structural and energetic analysis of simulated systems allows us to derive interaction propensity scales for different amino acid/nucleobase combinations. The derived scales closely match and greatly extend the available experimental data, providing a comprehensive foundation for studying RNA-protein interactions in different contexts. By using these scales, we demonstrate a statistically significant connection between nucleobase composition of human mRNA coding sequences and nucleobase interaction propensities of their cognate protein sequences. For example, pyrimidine density profiles of mRNAs match uracil-propensity profiles of their cognate proteins with a median Pearson correlation coefficient of R = -0.70. Our results provide support for the recently proposed hypotheses that mRNAs and their cognate proteins may be physicochemically complementary to each other and bind, especially if unstructured, with the complementarity level being negatively influenced by mRNA adenine content. Finally, we utilize the derived scales to refine the complementarity hypothesis and closely examine its physicochemical underpinnings.

Quantitative analysis of the interaction between l-methionine derivative and oligonucleotides

This study explores the use of l-methionine derivative as a potential affinity ligand for nucleic acids purification. The l-methionine derivative is synthesized by activation of the carboxylic acid group with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide follow by immobilization on amine sensor surface, previously activated and treated with ethylenediamine. Their affinity towards oligonucleotides has been determined by surface plasmon resonance biosensor. The highest affinity is found for cytosine and thymine, followed by adenine, whereas the lowest affinity is found for guanine. For hetero-oligonucleotides the affinity order is CCCTTT > CCCAAA ≈ AAATTT > GGGTTT, showing that nucleotides with cytosine have the highest affinity, and the presence of guanine reduces the affinity, corroborating with the results obtained with homo-oligonucleotides.

Analogue encoding of physicochemical properties of proteins in their cognate messenger RNAs

Being related by the genetic code, mRNAs and their cognate proteins exhibit mutually interdependent compositions, which implies the possibility of a direct connection between their general physicochemical properties. Here we probe the general potential of the cell to encode information about proteins in the average characteristics of their cognate mRNAs and decode it in a ribosome-independent manner. We show that average protein hydrophobicity, calculated from either sequences or 3D structures, can be encoded in an analogue fashion by many different average mRNA sequence properties with the only constraint being that pyrimidine and purine bases be clearly distinguishable on average. Moreover, average characteristics of mRNA sequences enable discrimination between cytosolic and membrane proteins even in the absence of topogenic signal-based mechanisms. Our results suggest that protein and mRNA localization may be partly determined by basic physicochemical rationales and interdependencies between the two biomolecules.

mRNA transport contributes to the proper localization of its cognate proteins. Here the authors report a correlation between the physicochemical properties of mRNAs and their cognate proteins, suggesting that these properties of mRNAs can predict the subcellular localization of their cognate proteins.

Hepatitis C virus translation inhibitors targeting the internal ribosomal entry site

The internal ribosome entry site (IRES) in the 5' untranslated region (UTR) of the hepatitis C virus (HCV) genome initiates translation of the viral polyprotein precursor. The unique structure and high sequence conservation of the 5' UTR render the IRES RNA a potential target for the development of selective viral translation inhibitors. Here, we provide an overview of approaches to block HCV IRES function by nucleic acid, peptide, and small molecule ligands. Emphasis will be given to the IRES subdomain IIa, which currently is the most advanced target for small molecule inhibitors of HCV translation. The subdomain IIa behaves as an RNA conformational switch. Selective ligands act as translation inhibitors by locking the conformation of the RNA switch. We review synthetic procedures for inhibitors as well as structural and functional studies of the subdomain IIa target and its ligand complexes.

Purification of human papillomavirus 16 E6/E7 plasmid deoxyribonucleic acid-based vaccine using an arginine modified monolithic support

The development of efficient plasmid DNA (pDNA) purification processes has fostered therapeutic applications like gene therapy and DNA vaccination. In fact, monolithic supports have emerged as interesting approaches to purify pDNA due to their excellent mass transfer properties and high binding capacity for large biomolecules. The present study describes a method that combines the high selectivity of arginine affinity ligands with the versatility of monoliths to efficiently purify the supercoiled (sc) plasmid HPV-16 E6/E7. Quality control tests indicated that the level of impurities (proteins, endotoxins, gDNA and RNA) in the final plasmid sample was in accordance with the guidelines proposed by regulatory agencies. Breakthrough experiments were designed to compare the dynamic binding capacity of pDNA in the conventional arginine-agarose matrix with the modified monolithic support. The arginine monolith capacity was substantially higher than the conventional arginine-agarose matrix at 10% of breakthrough under the flow rate and pDNA concentration used. Overall, given that the pDNA final product complies with regulatory specifications, this combined support can be the key to obtain an adequate non-viral vaccine against a HPV infection.

The DNA-binding domain of BenM reveals the structural basis for the recognition of a T-N11-A sequence motif by LysR-type transcriptional regulators

LysR-type transcriptional regulators (LTTRs) play critical roles in metabolism and constitute the largest family of bacterial regulators. To understand protein–DNA interactions, atomic structures of the DNA-binding domain and linker-helix regions of a prototypical LTTR, BenM, were determined by X-ray crystallography. BenM structures with and without bound DNA reveal a set of highly conserved amino acids that interact directly with DNA bases. At the N-terminal end of the recognition helix (α3) of a winged-helix–turn–helix DNA-binding motif, several residues create hydrophobic pockets (Pro30, Pro31 and Ser33). These pockets interact with the methyl groups of two thymines in the DNA-recognition motif and its complementary strand, T-N11-A. This motif usually includes some dyad symmetry, as exemplified by a sequence that binds two subunits of a BenM tetramer (ATAC-N7-GTAT). Gln29 forms hydrogen bonds to adenine in the first position of the recognition half-site (ATAC). Another hydrophobic pocket defined by Ala28, Pro30 and Pro31 interacts with the methyl group of thymine, complementary to the base at the third position of the half-site. Arg34 interacts with the complementary base of the 3′ position. Arg53, in the wing, provides AT-tract recognition in the minor groove. For DNA recognition, LTTRs use highly conserved interactions between amino acids and nucleotide bases as well as numerous less-conserved secondary interactions.

Evidence of direct complementary interactions between messenger RNAs and their cognate proteins

Recently, the ability to interact with messenger RNA (mRNA) has been reported for a number of known RNA-binding proteins, but surprisingly also for different proteins without recognizable RNA binding domains including several transcription factors and metabolic enzymes. Moreover, direct binding to cognate mRNAs has been detected for multiple proteins, thus creating a strong impetus to search for functional significance and basic physico-chemical principles behind such interactions. Here, we derive interaction preferences between amino acids and RNA bases by analyzing binding interfaces in the known 3D structures of protein-RNA complexes. By applying this tool to human proteome, we reveal statistically significant matching between the composition of mRNA sequences and base-binding preferences of protein sequences they code for. For example, purine density profiles of mRNA sequences mirror guanine affinity profiles of cognate protein sequences with quantitative accuracy (median Pearson correlation coefficient R = -0.80 across the entire human proteome). Notably, statistically significant anti-matching is seen only in the case of adenine. Our results provide strong evidence for the stereo-chemical foundation of the genetic code and suggest that mRNAs and cognate proteins may in general be directly complementary to each other and associate, especially if unstructured.

Computational approaches for predicting the binding sites and understanding the recognition mechanism of protein-DNA complexes

Protein-DNA recognition plays an important role in the regulation of gene expression. Understanding the influence of specific residues for protein-DNA interactions and the recognition mechanism of protein-DNA complexes is a challenging task in molecular and computational biology. Several computational approaches have been put forward to tackle these problems from different perspectives: (i) development of databases for the interactions between protein and DNA and binding specificity of protein-DNA complexes, (ii) structural analysis of protein-DNA complexes, (iii) discriminating DNA-binding proteins from amino acid sequence, (iv) prediction of DNA-binding sites and protein-DNA binding specificity using sequence and/or structural information, and (v) understanding the recognition mechanism of protein-DNA complexes. In this review, we focus on all these issues and extensively discuss the advancements on the development of comprehensive bioinformatics databases for protein-DNA interactions, efficient tools for identifying the binding sites, and plausible mechanisms for understanding the recognition of protein-DNA complexes. Further, the available online resources for understanding protein-DNA interactions are collectively listed, which will serve as ready-to-use information for the research community.