Binding of a Telomestatin Derivative Changes the Mechanical Anisotropy of a Human Telomeric G-Quadruplex

DNA origami nano-mechanics

Invention of DNA origami has transformed the fabrication and application of biological nanomaterials. In this review, we discuss DNA origami nanoassemblies according to their four fundamental mechanical properties in response to external forces: elasticity, pliability, plasticity and stability. While elasticity and pliability refer to reversible changes in structures and associated properties, plasticity shows irreversible variation in topologies. The irreversible property is also inherent in the disintegration of DNA nanoassemblies, which is manifested by its mechanical stability. Disparate DNA origami devices in the past decade have exploited the mechanical regimes of pliability, elasticity, and plasticity, among which plasticity has shown its dominating potential in biomechanical and physiochemical applications. On the other hand, the mechanical stability of the DNA origami has been used to understand the mechanics of the assembly and disassembly of DNA nano-devices. At the end of this review, we discuss the challenges and future development of DNA origami nanoassemblies, again, from these fundamental mechanical perspectives.

Characterization of G-Quadruplexes Folding/Unfolding Dynamics and Interactions with Proteins from Single-Molecule Force Spectroscopy

G-quadruplexes (G4s) are stable secondary nucleic acid structures that play crucial roles in many fundamental biological processes. The folding/unfolding dynamics of G4 structures are associated with the replication and transcription regulation functions of G4s. However, many DNA G4 sequences can adopt a variety of topologies and have complex folding/unfolding dynamics. Determining the dynamics of G4s and their regulation by proteins remains challenging due to the coexistence of multiple structures in a heterogeneous sample. Here, in this mini-review, we introduce the application of single-molecule force-spectroscopy methods, such as magnetic tweezers, optical tweezers, and atomic force microscopy, to characterize the polymorphism and folding/unfolding dynamics of G4s. We also briefly introduce recent studies using single-molecule force spectroscopy to study the molecular mechanisms of G4-interacting proteins.

G4 Sensing Pyridyl-Thiazole Polyamide Represses c-KIT Expression in Leukemia Cells

Specific sensing and functional tuning of nucleic acid secondary structures remain less explored to date. Herein, we report a thiazole polyamide TPW that binds specifically to c-KIT1 G-quadruplex (G4) with sub-micromolar affinity and ∼1 : 1 stoichiometry and represses c-KIT proto-oncogene expression. TPW shows up to 10-fold increase in fluorescence upon binding with c-KIT1 G4, but shows weak or no quantifiable binding to other G4s and ds26 DNA. TPW can increase the number of G4-specific antibody (BG4) foci and mark G4 structures in cancer cells. Cell-based assays reveal that TPW can efficiently repress c-KIT expression in leukemia cells via a G4-dependent process. Thus, the polyamide can serve as a promising probe for G-quadruplex recognition with the ability to specifically alter c-KIT oncogene expression.

Molecular mechanisms underlying the extreme mechanical anisotropy of the flaviviral exoribonuclease-resistant RNAs (xrRNAs)

Mechanical anisotropy is an essential property for many biomolecules to assume their structures, functions and applications, however, the mechanisms for their direction-dependent mechanical responses remain elusive. Herein, by using a single-molecule nanopore sensing technique, we explore the mechanisms of directional mechanical stability of the xrRNA1 RNA from ZIKA virus (ZIKV), which forms a complex ring-like architecture. We reveal extreme mechanical anisotropy in ZIKV xrRNA1 which highly depends on Mg²⁺ and the key tertiary interactions. The absence of Mg²⁺ and disruption of the key tertiary interactions strongly affect the structural integrity and attenuate mechanical anisotropy. The significance of ring structures in RNA mechanical anisotropy is further supported by steered molecular dynamics simulations in combination with force distribution analysis. We anticipate the ring structures can be used as key elements to build RNA-based nanostructures with controllable mechanical anisotropy for biomaterial and biomedical applications.

RNA Nanoparticles as Rubber for Compelling Vessel Extravasation to Enhance Tumor Targeting and for Fast Renal Excretion to Reduce Toxicity

Rubber is a fascinating material in both industry and daily life. The development of elastomeric material in nanotechnology is imperative due to its economic and technological potential. By virtue of their distinctive physicochemical properties, nucleic acids have been extensively explored in material science. The Phi29 DNA packaging motor contains a 3WJ with three angles of 97°, 125°, and 138°. Here, the rubber-like property of RNA architectures was investigated using optical tweezers and in vivo imaging technologies. The 3WJ 97° interior angle was contracted or stretched to 60°, 90°, and 108° at will to build elegant RNA triangles, squares, pentagons, cubes, tetrahedrons, dendrimers, and prisms. RNA nanoarchitecture was stretchable and shrinkable by optical tweezer with multiple extension and relaxation repeats like a rubber. Comparing to gold and iron nanoparticles with the same size, RNA nanoparticles display stronger cancer-targeting outcomes, while less accumulation in healthy organs. Generally, the upper limit of renal excretion is 5.5 nm; however, the 5, 10, and 20 nm RNA nanoparticles passed the renal filtration and resumed their original structure identified in urine. These findings solve two previous mysteries: (1) Why RNA nanoparticles have an unusually high tumor targeting efficiency since their rubber or amoeba-like deformation property enables them to squeeze out of the leaky vasculature to improve the EPR effect; and (2) why RNA nanoparticles remain non-toxic since they can be rapidly cleared from the body via renal excretion into urine with little accumulation in the body. Considering its controllable shape and size plus its rubber-like property, RNA holds great promises for industrial and biomedical applications especially in cancer therapeutics delivery.

Cooperative Heteroligand Interaction with G-Quadruplexes Shows Evidence of Allosteric Binding

Although allosteric binding of small molecules is commonplace in protein structures, it is rather rare in DNA species such as G-quadruplexes. By using CD melting, here, we found binding of the small-molecule ligands PDS and L2H2-6OTD to the telomeric DNA G-quadruplex was cooperative. Mass spectrometry indicated a 1:1:1 ratio in the ternary binding complex of the telomeric G-quadruplex, PDS, and L2H2-6OTD. Compared to the binding of each individual ligand to the G-quadruplex, single-molecule mechanical unfolding assays revealed a significantly decreased dissociation constant when one ligand is evaluated in the presence of another. This demonstrates that cooperative binding of PDS and L2H2-6OTD to the G-quadruplex is allosteric, which is also supported by the mass spectra data that indicated the ejection of coordinated sodium ions upon binding of the heteroligands to the G-quadruplex. The unprecedented observation of the allosteric ligand binding to higher-ordered structures of DNA may help to design more effective ligands to target non-B DNA species involved in many critical cellular processes.

A poly(thymine)-melamine duplex for the assembly of DNA nanomaterials

The diversity of DNA duplex structures is limited by a binary pair of hydrogen-bonded motifs. Here we show that poly(thymine) self-associates into antiparallel, right-handed duplexes in the presence of melamine, a small molecule that presents a triplicate set of the hydrogen-bonding face of adenine. X-ray crystallography shows that in the complex two poly(thymine) strands wrap around a helical column of melamine, which hydrogen bonds to thymine residues on two of its three faces. The mechanical strength of the thymine-melamine-thymine triplet surpasses that of adenine-thymine base pairs, which enables a sensitive detection of melamine at 3 pM. The poly(thymine)-melamine duplex is orthogonal to native DNA base pairing and can undergo strand displacement without the need for overhangs. Its incorporation into two-dimensional grids and hybrid DNA-small-molecule polymers highlights the poly(thymine)-melamine duplex as an additional tool for DNA nanotechnology.

Structural Features of Nucleoprotein CST/Shelterin Complex Involved in the Telomere Maintenance and Its Association with Disease Mutations

Telomere comprises the ends of eukaryotic linear chromosomes and is composed of G-rich (TTAGGG) tandem repeats which play an important role in maintaining genome stability, premature aging and onsets of many diseases. Majority of the telomere are replicated by conventional DNA replication, and only the last bit of the lagging strand is synthesized by telomerase (a reverse transcriptase). In addition to replication, telomere maintenance is principally carried out by two key complexes known as shelterin (TRF1, TRF2, TIN2, RAP1, POT1, and TPP1) and CST (CDC13/CTC1, STN1, and TEN1). Shelterin protects the telomere from DNA damage response (DDR) and regulates telomere length by telomerase; while, CST govern the extension of telomere by telomerase and C strand fill-in synthesis. We have investigated both structural and biochemical features of shelterin and CST complexes to get a clear understanding of their importance in the telomere maintenance. Further, we have analyzed ~115 clinically important mutations in both of the complexes. Association of such mutations with specific cellular fault unveils the importance of shelterin and CST complexes in the maintenance of genome stability. A possibility of targeting shelterin and CST by small molecule inhibitors is further investigated towards the therapeutic management of associated diseases. Overall, this review provides a possible direction to understand the mechanisms of telomere borne diseases, and their therapeutic intervention.

Submolecular dissection reveals strong and specific binding of polyamide-pyridostatin conjugates to human telomere interface

To modulate biological functions, G-quadruplexes in genome are often non-specifically targeted by small molecules. Here, specificity is increased by targeting both G-quadruplex and its flanking duplex DNA in a naturally occurring dsDNA-ssDNA telomere interface using polyamide (PA) and pyridostatin (PDS) conjugates (PA-PDS). We innovated a single-molecule assay in which dissociation constant (Kd) of the conjugate can be separately evaluated from the binding of either PA or PDS. We found Kd of 0.8 nM for PA-PDS, which is much lower than PDS (Kd ∼ 450 nM) or PA (Kd ∼ 35 nM). Functional assays further indicated that the PA-PDS conjugate stopped the replication of a DNA polymerase more efficiently than PA or PDS. Our results not only established a new method to dissect multivalent binding into actions of individual monovalent components, they also demonstrated a strong and specific G-quadruplex targeting strategy by conjugating highly specific duplex-binding molecules with potent quadruplex ligands.