Engineering a rigid protein tunnel for biomolecular detection.

One intimidating challenge in protein nanopore-based technologies is designing robust protein scaffolds that remain functionally intact under a broad spectrum of detection conditions. Here, we show that an extensively engineered bacterial ferric hydroxamate uptake component A (FhuA), a ß-barrel membrane protein, functions as a robust protein tunnel for the sampling of biomolecular events. The key implementation in this work was the coupling of direct genetic engineering with a refolding approach to produce an unusually stable protein nanopore. More importantly, this nanostructure maintained its stability under many experimental circumstances, some of which, including low ion concentration and highly acidic aqueous phase, are normally employed to gate, destabilize, or unfold ß-barrel membrane proteins. To demonstrate these advantageous traits, we show that the engineered FhuA-based protein nanopore functioned as a sensing element for examining the proteolytic activity of an enzyme at highly acidic pH and for determining the kinetics of protein-DNA aptamer interactions at physiological salt concentration.

Redesign of a plugged beta-barrel membrane protein.

The redesign of biological nanopores is focused on bacterial outer membrane proteins and pore-forming toxins, because their robust ß-barrel structure makes them the best choice for developing stochastic biosensing elements. Using membrane protein engineering and single-channel electrical recordings, we explored the ferric hydroxamate uptake component A (FhuA), a monomeric 22-stranded ß-barrel protein from the outer membrane of Escherichia coli. FhuA has a luminal cross-section of 3.1 × 4.4 nm and is filled by a globular N-terminal cork domain. Various redesigned FhuA proteins were investigated, including single, double, and multiple deletions of the large extracellular loops and the cork domain. We identified four large extracellular loops that partially occlude the lumen when the cork domain is removed. The newly engineered protein, FhuAΔC/Δ4L, was the result of a removal of almost one-third of the total number of amino acids of the wild-type FhuA (WT-FhuA) protein. This extensive protein engineering encompassed the entire cork domain and four extracellular loops. Remarkably, FhuAΔC/Δ4L forms a functional open pore in planar lipid bilayers, with a measured unitary conductance of ∼4.8 nanosiemens, which is much greater than the values recorded previously with other engineered FhuA protein channels. There are numerous advantages and prospects of using such an engineered outer membrane protein not only in fundamental studies of membrane protein folding and design, and the mechanisms of ion conductance and gating, but also in more applicative areas of stochastic single-molecule sensing of proteins and nucleic acids.