Nanopore-Based Single-Molecule Investigation of DNA Sequences with Potential to Form i-Motif Structures

Nanopore-Based Single-Molecule Investigation of Cation Effect on the i-Motif Structure

The i-motif, a secondary structure of a four-helix formed by cytosine-rich DNA (i-DNA) through C-C+ base pairing, is prevalent in human telomeres and promoters. This structure creates steric hindrance, thereby inhibiting both gene expression and protein coding. The conformation of i-DNA is intricately linked to the intracellular ionic environment. Hence, investigating its conformation under various ion conditions holds significant importance. In this study, we explored the impact of cations on the i-motif structure at the single-molecule level using the α-hemolysin (α-HL) nanochannel. Our findings reveal that the ability of i-DNA to fold into the i-motif structure follows the order Cs+ > Na+ > K+ > Li+ for monovalent cations. Furthermore, we observed the interconversion of single-stranded DNA (ss-DNA) and the i-motif structure at high and low concentrations of Mg2+ and Ba2+ electrolyte solutions. This study not only has the potential to extend the application of i-motif-based sensors in complex solution environments but also provides a new idea for the detection of metal ions.

i-Motif DNA: identification, formation, and cellular functions

An i-motif (iM) is a four-stranded (quadruplex) DNA structure that folds from cytosine (C)-rich sequences. iMs can fold under many different conditions in vitro, which paves the way for their formation in living cells. iMs are thought to play key roles in various DNA transactions, notably in the regulation of genome stability, gene transcription, mRNA translation, DNA replication, telomere and centromere functions, and human diseases. We summarize the different techniques used to assess the folding of iMs in vitro and provide an overview of the internal and external factors that affect their formation and stability in vivo. We describe the possible biological relevance of iMs and propose directions towards their use as target in biology.

Nanopore single-molecule biosensor in protein denaturation analysis

Unclear issues in protein studies include but not limited to the stability and denaturation mechanism in the presence of denaturants. Herein, we report a dynamic monitoring approach based on nanopore single-molecule biosensor, which can detect the protein's folding and unfolding transitions by recording a nanopore ionic current. When gradually increasing the concentration of denaturant guanidine hydrochloride (GdmCl), sensitive responses were observed with lysozyme unfolding. The emergence of the featured biphasic-pulse demonstrated the existence of a stable intermediate. It was the first time to experimentally confirm the dynamic equilibrium between the intermediate and the native states at single molecule level, therefore consolidating the standpoint of lysozyme denaturation process following the three-state model. Additionally, we got more insights into the conformation about the intermediate as globular-like structure, larger gyration radius, and enhanced positive charge density. We considered that the manner of denaturant toward lysozyme adopts the "direct" model based on stronger electrostatic and van der Waals forces. Nanopore biosensor exhibited excellent sensitivity with a low detection concentration of 280 pM and reproducibility in analysing the folding intermediate of lysozyme.

Label-free Sensing of Main Protease Activity of SARS-CoV-2 with an Aerolysin Nanopore

The main protease (M pro ), which is highly conserved and plays a critical role in the replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a natural biomarker for SARS-CoV-2. Accurate assessment of the M pro activity is crucial for the detection of SARS-CoV-2. Herein, we report a nanopore-based sensing strategy that uses an enzyme-catalyzed cleavage reaction of a peptide substrate to measure the M pro  activity. The peptide was specifically cleaved by the M pro , thereby releasing the output products that, when translocated through aerolysin, quantitatively produced the signature current events. The proposed method exhibited high sensitivity, allowing the detection of M pro concentrations as low as 1 nM without the use of any signal amplification techniques. This simple, convenient, and label-free nanopore assay may expand the diagnostic tools for viruses.