Multiplex Single-Molecule Kinetics of Nanopore-Coupled Polymerases

Absorption-Based Rapid Acquisition of Single-Molecule Kinetics from Unstable Enzymes in Microdroplets

Single-molecule catalysis reflects the heterogeneity of each molecule, providing a unique insight into the complex catalytic mechanism through the statistics of stochastic individuals. However, the present study methods for single-molecule catalysis are either complicated or have low throughput, limiting their rapid acquisition of single-molecule reaction kinetics with statistical significance. Here, a label-free imaging method is developed for the study of single-molecule catalysis in microdroplets with high throughput based on the absorption of the reaction molecules. A wide distribution of the catalytic reaction rate constant value of 238-2026 molecules s-1 is observed from 68 single enzymes. Interestingly, an exponential decayed distribution of the enzyme activity can be clearly observed due to the rapid denaturation of the enzymes. The denaturation mechanism of the Horse Radish Peroxidase (HRP) enzyme is clarified. It is revealed that the denaturation of each enzyme goes through a gradual decay rather than a truncated turn-off process from a single molecule point of view. This absorption-based method can be applied to most of the catalytic reactions with high throughput, which offers an indispensable route for the rapid statistical analysis of various single-molecule catalytic reactions, making it particularly suitable for the acquisition of catalytic kinetics from highly unstable enzymes.

Tuning single-molecule ClyA nanopore tweezers for real-time tracking of the conformational dynamics of West Nile viral NS2B/NS3 protease

The flaviviral NS2B/NS3 protease is a conserved enzyme required for flavivirus replication. Its highly dynamic conformation poses major challenges but also offers opportunities for antiviral inhibition. Here, we established a nanopore tweezers-based platform to monitor NS2B/NS3 conformational dynamics in real-time. Molecular simulations coupled with electrophysiology revealed that the protease could be captured in the middle of the ClyA nanopore lumen, stabilized mainly by dynamic electrostatic interactions. We designed a new Salmonella typhi ClyA nanopore with enhanced nanopore/protease interaction that can resolve the open and closed states at the single-molecule level for the first time. We demonstrated that the tailored ClyA could track the conformational transitions of the West Nile NS2B/NS3 protease and unravel the conformational energy landscape of various protease constructs through population and kinetic analysis. The new ClyA-protease platform paves a way to high-throughput screening strategies for discovering new allosteric inhibitors that target the NS2B and NS3 interface.

Unraveling the salt tolerance of Phi29 DNA polymerase using compartmentalized self-replication and microfluidics platform

In Phi29-α-hemolysin (α-HL) nanopore sequencing systems, a strong electrochemical signal is dependent on a high concentration of salt. However, high salt concentrations adversely affect polymerase activity. Sequencing by synthesis (SBS) requires the use of phi29 polymerase without exonuclease activity to prevent the degradation of modified nucleotide tags; however, the lack of exonuclease activity also affects polymerase processivity. This study aimed to optimize phi29 polymerase for improved salt tolerance and processivity while maintaining its lack of exonuclease activity to meet the requirements of nanopore sequencing. Using salt tolerance compartmentalized self-replication (stCSR) and a microfluidic platform, we obtained 11 mutant sites with enhanced salt tolerance attributes. Sequencing and biochemical analyses revealed that the substitution of conserved amino acids such as G197D, Y369E, T372N, and I378R plays a critical role in maintaining the processivity of exonuclease-deficient phi29 polymerase under high salt conditions. Furthermore, Y369E and T372N have been identified as important determinants of DNA polymerase binding affinity. This study provides insights into optimizing polymerase processability under high-salt conditions for real-time polymerase nanopore sequencing, paving the way for improved performance and applications in nanopore sequencing technologies.

An Engineered OmpG Nanopore with Displayed Peptide Motifs for Single-Molecule Multiplex Protein Detection

Molecular detection via nanopore, achieved by monitoring changes in ionic current arising from analyte interaction with the sensor pore, is a promising technology for multiplex sensing development. Outer Membrane Protein G (OmpG), a monomeric porin possessing seven functionalizable loops, has been reported as an effective sensing platform for selective protein detection. Using flow cytometry to screen unfavorable constructs, we identified two OmpG nanopores with unique peptide motifs displayed in either loop 3 or 6, which also exhibited distinct analyte signals in single-channel current recordings. We exploited these motif-displaying loops concurrently to facilitate single-molecule multiplex protein detection in a mixture. We additionally report a strategy to increase sensor sensitivity via avidity motif display. These sensing schemes may be expanded to more sophisticated designs utilizing additional loops to increase multiplicity and sensitivity.

Profiling single-molecule reaction kinetics under nanopore confinement

The study of a single-molecule reaction under nanoconfinement is beneficial for understanding the reactive intermediates and reaction pathways. However, the kinetics model of the single-molecule reaction under confinement remains elusive. Herein we engineered an aerolysin nanopore reactor to elaborate the single-molecule reaction kinetics under nanoconfinement. By identifying the bond-forming and non-bond-forming events directly, a four-state kinetics model is proposed for the first time. Our results demonstrated that the single-molecule reaction kinetics inside a nanopore depends on the frequency of individual reactants captured and the fraction of effective collision inside the nanopore confined space. This insight will guide the design of confined nanopore reactors for resolving the single-molecule chemistry, and shed light on the mechanistic understanding of dynamic covalent chemistry inside confined systems such as supramolecular cages, coordination cages, and micelles.

A four-state kinetics model is proposed to reveal the kinetics of a single-molecule reaction under nanopore confinement.

Machine Learning Assisted Simultaneous Structural Profiling of Differently Charged Proteins in a Mycobacterium smegmatis Porin A (MspA) Electroosmotic Trap

The nanopore is emerging as a means of single-molecule protein sensing. However, proteins demonstrate different charge properties, which complicates the design of a sensor that can achieve simultaneous sensing of differently charged proteins. In this work, we introduce an asymmetric electrolyte buffer combined with the Mycobacterium smegmatis porin A (MspA) nanopore to form an electroosmotic flow (EOF) trap. Apo- and holo-myoglobin, which differ in only a single heme, can be fully distinguished by this method. Direct discrimination of lysozyme, apo/holo-myoglobin, and the ACTR/NCBD protein complex, which are basic, neutral, and acidic proteins, respectively, was simultaneously achieved by the MspA EOF trap. To automate event classification, multiple event features were extracted to build a machine learning model, with which a 99.9% accuracy is achieved. The demonstrated method was also applied to identify single molecules of α-lactalbumin and β-lactoglobulin directly from whey protein powder. This protein-sensing strategy is useful in direct recognition of a protein from a mixture, suggesting its prospective use in rapid and sensitive detection of biomarkers or real-time protein structural analysis.

Real-Time and Label-Free Measurement of Deubiquitinase Activity with a MspA Nanopore

Covalently attaching ubiquitin (Ub) to cellular proteins as a post-translational modification can result in altered function of modified proteins. Enzymes regulating Ub as a post-translational modification, such as ligases and deubiquitinases, are challenging to characterize in part due to the low throughput of in-vitro assays. Single-molecule nanopore based assays have the advantage of detecting proteins with high specificity and resolution, and in a label-free, real-time fashion. Here we demonstrate the use of a MspA nanopore for discriminating and quantifying Ub proteins. We further applied the MspA pore to measure the Ub-chain disassembly activity of UCH37, a proteasome associated deubiquitinase. The implementation of this MspA system into nanopore arrays could enable high throughput characterizations of unknown deubiquitinases as well as drug screening against disease related enzymes.