Control of subunit stoichiometry in single-chain MspA nanopores
Transmembrane protein channels enable fast and highly sensitive electrical detection of single molecules. Nanopore sequencing of DNA was achieved using an engineered Mycobacterium smegmatis porin A (MspA) in combination with a motor enzyme. Due to its favorable channel geometry, the octameric MspA pore exhibits the highest current level as compared to other pore proteins. To date, MspA is the only protein nanopore with a published record of DNA sequencing. While widely used in commercial devices, nanopore sequencing of DNA suffers from significant base-calling errors due to stochastic events of the complex DNA-motor-pore combination and the contribution of up to five nucleotides to the signal at each position. Asymmetric mutations within subunits of the channel protein offer an enormous potential to improve nucleotide resolution and sequencing accuracy. However, random subunit assembly does not allow control of the channel composition of MspA and other oligomeric protein pores. In this study, we showed that it is feasible to convert octameric MspA into a single-chain pore by connecting eight subunits using peptide linkers. We constructed single-chain MspA trimers, pentamers, hexamers and heptamers to demonstrate that it is feasible to alter the subunit stoichiometry and the MspA pore diameter. All single-chain MspA proteins formed functional channels in lipid bilayer experiments. Importantly, we demonstrated that single-chain MspA discriminated all four nucleotides identical to MspA produced from monomers. Thus, single-chain MspA constitutes a new milestone in its development and adaptation as a biosensor for DNA sequencing and many other applications. STATEMENT OF SIGNFICANCENanopore sequencing of DNA is a fast and cheap technology that uniquely delivers multi-kilobase reads. It is currently used world-wide in many applications such as genome sequencing, epigenetics, and surveillance of viral and bacterial pathogens and has started to revolutionize human lives in medicine, agriculture and environmental studies. However, the high base-calling error rates prevent nanopore DNA sequencing from reaching its full potential. In this study, we converted octameric MspA into a single-chain pore enabling asymmetric mutations to fine-tune the pore geometry and chemistry and address the shortcomings of nanopores. Thus, single-chain MspA constitutes a new milestone in its development and adaptation as a biosensor for DNA sequencing and many other applications.