Benjamin I. Tickman

Subangstrom single-molecule measurements of motor proteins using a nanopore

Techniques for measuring the motion of single motor proteins, such as FRET and optical tweezers, are limited to a resolution of ∼300 pm. We use ion current modulation through the protein nanopore MspA to observe translocation of helicase Hel308 on DNA with up to ∼40 pm sensitivity. This approach should be applicable to any protein that translocates on DNA or RNA, including helicases, polymerases, recombinases and DNA repair enzymes.

Direct Detection of Unnatural DNA Nucleotides dNaM and d5SICS using the MspA Nanopore

Malyshev et al. showed that the four-letter genetic code within a living organism could be expanded to include the unnatural DNA bases dNaM and d5SICS. However, verification and detection of these unnatural bases in DNA requires new sequencing techniques. Here we provide proof of concept detection of dNaM and d5SICS in DNA oligomers via nanopore sequencing using the nanopore MspA. We find that both phi29 DNA polymerase and Hel308 helicase are capable of controlling the motion of DNA containing dNaM and d5SICS through the pore and that single reads are sufficient to detect the presence and location of dNaM and d5SICS within single molecules.

Revealing dynamics of helicase translocation on single-stranded DNA using high-resolution nanopore tweezers

Significance DNA helicases are enzymes that use energy from ATP hydrolysis to move along nucleic acid tracks and unwind double-stranded DNA. Helicases are involved in every aspect of DNA metabolism and are therefore vital to maintaining genomic integrity. Using the single-molecule technique single-molecule picometer-resolution nanopore tweezers (SPRNT), which measures the position of DNA through the biological membrane protein MspA as an enzyme moves along the DNA, we monitored the kinetics of the helicase Hel308 at 1,000 times better temporal resolution than was previously possible. We derived a detailed mechanism for how ATP hydrolysis coordinates the motion of Hel308 along single-stranded DNA that can likely be applied to other structurally similar helicases and showed that the DNA sequence in Hel308 affects its kinetics. Enzymes that operate on DNA or RNA perform the core functions of replication and expression in all of biology. To gain high-resolution access to the detailed mechanistic behavior of these enzymes, we developed single-molecule picometer-resolution nanopore tweezers (SPRNT), a single-molecule technique in which the motion of polynucleotides through an enzyme is measured by a nanopore. SPRNT reveals two mechanical substates of the ATP hydrolysis cycle of the superfamily 2 helicase Hel308 during translocation on single-stranded DNA (ssDNA). By analyzing these substates at millisecond resolution, we derive a detailed kinetic model for Hel308 translocation along ssDNA that sheds light on how superfamily 1 and 2 helicases turn ATP hydrolysis into motion along DNA. Surprisingly, we find that the DNA sequence within Hel308 affects the kinetics of helicase translocation.

Investigating asymmetric salt profiles for nanopore DNA sequencing with biological porin MspA

Nanopore DNA sequencing is a promising single-molecule analysis technology. This technique relies on a DNA motor enzyme to control movement of DNA precisely through a nanopore. Specific experimental buffer conditions are required based on the preferred operating conditions of the DNA motor enzyme. While many DNA motor enzymes typically operate in salt concentrations under 100 mM, salt concentration simultaneously affects signal and noise magnitude as well as DNA capture rate in nanopore sequencing, limiting standard experimental conditions to salt concentrations greater than ~100 mM in order to maintain adequate resolution and experimental throughput. We evaluated the signal contribution from ions on both sides of the membrane (cis and trans) by varying cis and trans [KCl] independently during phi29 DNA Polymerase-controlled translocation of DNA through the biological porin MspA. Our studies reveal that during DNA translocation, the negatively charged DNA increases cation selectivity through MspA with the majority of current produced by the flow of K+ ions from trans to cis. Varying trans [K+] has dramatic effects on the signal magnitude, whereas changing cis [Cl-] produces only small effects. Good signal-to-noise can be maintained with cis [Cl-] as small as 20 mM, if the concentration of KCl on the trans side is kept high. These results demonstrate the potential of using salt-sensitive motor enzymes (helicases, polymerases, recombinases) in nanopore systems and offer a guide for selecting buffer conditions in future experiments to simultaneously optimize signal, throughput, and enzyme activity.

Direct Single Molecule Measurement of ATP Hydrolysis Substates in Hel308 DNA Helicase using Nanopore Tweezers

Single-molecule picometer resolution nanopore tweezers (SPRNT) is a single molecule tool for studying enzymes that move on nucleic acids (NA). SPRNT measures NA position relative to the enzyme at sub-Angstrom spatial resolution on millisecond timescales, while simultaneously providing the NA sequence near the enzyme. We use this method to resolve two substates of the ATP hydrolysis cycle of a Hel308 DNA Helicase. By examining the dwell times of each state, we derive a kinetic model of Hel308s translocation along single-stranded DNA, and find that the models rate constants depend on the DNA sequence within the enzyme. This is, to our knowledge, the first evidence of sequence-dependent translocase behavior in a helicase system.

Continuously Scanning DNA with Nanopore MspA

Nanopore sequencing is a promising next-generation technology that is being enabled by the biological nanopore MspA. In this sequencing technique, current is driven through the ∼1 nm constriction of MspA. Single stranded DNA molecules are first drawn into the pore by an applied voltage. Next, a polymerase or helicase moves the DNA by discrete steps as it interacts with the pore. As the DNA moves through the pore, different nucleobases modulate the pores conductance to varying extents, allowing for extraction of sequence information from the current trace. In this experiment, we gain more complete information by using a variable applied voltage to stretch the DNA within the pore. Changing the voltage continuously repositions the DNA in the constriction. Coupling this voltage-induced movement with the enzyme-induced motion we reconstruct a profile characteristic of the DNA as it is pulled continuously through the constriction. This profile provides a tool for improving the de novo sequencing accuracy of the nanopore technique.