In nanopore analytics, individual molecules pass through a single nanopore giving rise to detectable temporary blockades in ionic pore current. Reflecting its simplicity, nanopore analytics has gained popularity and can be conducted with natural protein as well as man-made polymeric and inorganic pores. The spectrum of detectable analytes ranges from nucleic acids, peptides, proteins, and biomolecular complexes to organic polymers and small molecules. Apart from being an analytical tool, nanopores have developed into a general platform technology to investigate the biophysics, physicochemistry, and chemistry of individual molecules (critical review, 310 references).

The creation of synthetic devices that mimic functionality of biological systems is a task of fundamental importance for the future development of bio- and nanotechnology and also an ultimate test of our understanding of the biological systems. Among a plethora of bio-inspired devices, designed nanopores and nanochannels with an embedded functionality are of particular interest because of their potential applications in nanofluidic electronics, biosensing, separation, synthetic biology, and single-molecule manipulation. In this respect, nanopores with built-in stimulus-responsive properties are of special benefit. A transmembrane potential is a particularly useful stimulus as it is non-invasive, tunable, and can act over a short time scale. This critical review considers engineered solid-state and protein nanopores with voltage-responsive properties. The engineered systems show nonlinear current–voltage curves, and/or voltage-dependent switching between discrete conductance states (141 references).

CsgG and CgsE form an encaging translocon for selective, iterative diffusion of curli subunits across the non-energized bacterial outer membrane. This manuscript reports the atomic structure of the bacterial amyloid transporter CsgG, an outer-membrane lipoprotein that forms the translocation channel for curli (amyloid fibre) subunits that once secreted, polymerize into cross β-fibres that mediate biofilm formation. Curli fibres constitute the major protein component of the extracellular matrix in biofilms formed by Bacteroidetes and Proteobacteria. The CsgG structure reveals a 36-stranded β-barrel that forms a channel spanning the membrane bilayer and suggests a potential mechanism for guiding substrates through the secretion pore via a channel constriction. Curli are functional amyloid fibres that constitute the major protein component of the extracellular matrix in pellicle biofilms formed by Bacteroidetes and Proteobacteria (predominantly of the α and γ classes)1,2,3. They provide a fitness advantage in pathogenic strains and induce a strong pro-inflammatory response during bacteraemia1,4,5. Curli formation requires a dedicated protein secretion machinery comprising the outer membrane lipoprotein CsgG and two soluble accessory proteins, CsgE and CsgF6,7. Here we report the X-ray structure of Escherichia coli CsgG in a non-lipidated, soluble form as well as in its native membrane-extracted conformation. CsgG forms an oligomeric transport complex composed of nine anticodon-binding-domain-like units that give rise to a 36-stranded β-barrel that traverses the bilayer and is connected to a cage-like vestibule in the periplasm. The transmembrane and periplasmic domains are separated by a 0.9-nm channel constriction composed of three stacked concentric phenylalanine, asparagine and tyrosine rings that may guide the extended polypeptide substrate through the secretion pore. The specificity factor CsgE forms a nonameric adaptor that binds and closes off the periplasmic face of the secretion channel, creating a 24,000 Å3 pre-constriction chamber. Our structural, functional and electrophysiological analyses imply that CsgG is an ungated, non-selective protein secretion channel that is expected to employ a diffusion-based, entropy-driven transport mechanism.