Pep-1 is a cell-penetrating peptide that represents a powerful strategy for delivering large, hydrophilic therapeutic molecules into cells. Model membranes, such as lipid vesicles and planar bilayers, have been useful for investigating the direct translocation of cell-penetrating peptides. Here, we present a droplet interface bilayer-based approach to quantify pep-1-mediated β-galactosidase translocation. We found that β-galactosidase translocation is driven only by the negative transmembrane potential resulting from the asymmetric bilayers. The asymmetric droplet interface bilayer method may be generally applicable for high-throughput screening of the efficacy of cell-penetrating peptides.

We have previously shown that a biotin ligand tethered to the rim of an OmpG nanopore can be used to detect biotin-binding proteins. Here, we investigate the effect of the length of the polyethylene glycol tether on the nanopore’s sensitivity and selectivity. When the tether length was increased from 2 to 45 ethylene repeats, sensitivity decreased substantially for a neutral protein streptavidin and slightly for a positively charged protein (avidin). In addition, we found that two distinct avidin binding conformations were possible when using a long tether. These conformations were sensitive to the salt concentration and applied voltage. Finally, a longer tether resulted in reduced sensitivity due to slower association for a monoclonal antibiotin antibody. Our results highlight the importance of electrostatic, electroosmotic, and electrophoretic forces on nanopore binding kinetics and sensor readout.Keywords: antibody; biotin; nanopore; OmpG; sensing; streptavidin

Outer membrane protein G is a monomeric β-barrel porin that has seven flexible loops on its extracellular side. Conformational changes of these labile loops induce gating spikes in current recordings that we exploited as the prime sensing element for protein detection. The gating characteristics, open probability, frequency, and current decrease, provide rich information for analyte identification. Here, we show that two antibiotin antibodies each induced a distinct gating pattern, which allowed them to be readily detected and simultaneously discriminated by a single OmpG nanopore in the presence of fetal bovine serum. Our results demonstrate the feasibility of directly profiling proteins in real-world samples with minimal or no sample pretreatment.

The flexible loops decorating the entrance of OmpG nanopore move dynamically during ionic current recording. The gating caused by these flexible loops changes when a target protein is bound. The gating is characterized by parameters including frequency, duration, and open-pore current, and these features combine to reveal the identity of a specific analyte protein. Here, we show that OmpG nanopore equipped with a biotin ligand can distinguish glycosylated and deglycosylated isoforms of avidin by their differences in surface charge. Our studies demonstrate that the direct interaction between the nanopore and analyte surface, induced by the electrostatic attraction between the two molecules, is essential for protein isoform detection. Our technique is remarkably sensitive to the analyte surface, which may provide a useful tool for glycoprotein profiling.

Oligomeric protein nanopores with rigid structures have been engineered for the purpose of sensing a wide range of analytes including small molecules and biological species such as proteins and DNA. We chose a monomeric β-barrel porin, OmpG, as the platform from which to derive the nanopore sensor. OmpG is decorated with seven flexible loops that move dynamically to create a distinct gating pattern when ionic current passes through the pore. Biotin was chemically tethered to the most flexible one of these loops. The gating characteristic of the loop’s movement in and out of the porin was substantially altered by analyte protein binding. The gating characteristics of the pore with bound targets were remarkably sensitive to molecular identity, even providing the ability to distinguish between homologues within an antibody mixture. A total of five gating parameters were analyzed for each analyte to create a unique fingerprint for each biotin-binding protein. Our exploitation of gating noise as a molecular identifier may allow more sophisticated sensor design, while OmpG’s monomeric structure greatly simplifies nanopore production.Keywords: nanopore; OmpG; protein sensor; single-molecule detection;