Insights in peptide diffusion channels from the bacterial amyloid secretor CsgG

Molecular Insights into Distinct Detection Properties of α-Hemolysin, MspA, CsgG, and Aerolysin Nanopore Sensors

Protein nanopores have been widely used as single-molecule sensors for the detection and characterization of biological polymers such as DNA, RNA, and polypeptides. A variety of protein nanopores with various geometries have been exploited for this purpose, which usually exhibit distinct sensing capabilities, but the underlying molecular mechanism remains elusive. Here, we systematically characterize the molecular transport properties of four widely studied protein nanopores, α-hemolysin, MspA, CsgG, and aerolysin, by extensive molecular dynamics simulations. It is found that a sudden drop in electrostatic potentials occurs at the sole constriction in MspA and CsgG nanopores in contrast to the gradual potential change inside α-hemolysin and aerolysin pores, indicating the crucial role of pore geometry in ionic and molecular transport. We further demonstrate that these protein nanopores exhibit open-pore currents and ssDNA-induced current blockades both in the order MspA > α-hemolysin > CsgG > aerolysin, but an equivalent blockade percentage around 80%. In addition, the substitution of key amino acids at the pore constriction, especially by charged ones, provides an efficient way to modulate the pore electrostatic potential and ionic current. This work sheds new light on the search for high-performance nanopores, engineering of protein nanopores, and design of bioinspired solid-state nanopores.

Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway

In 1989, Normark and coworkers reported on fibrous surface structures called curli on strains of Escherichia coli that were suspected of causing bovine mastitis. Subsequent work by many groups has revealed an elegant and highly regulated curli biogenesis pathway also referred to as the type VIII secretion system. Curli biogenesis is governed by two divergently transcribed operons, csgBAC and csgDEFG. The csgBAC operon encodes the structural subunits of curli, CsgA and CsgB, along with a chaperone-like protein, CsgC. The csgDEFG operon encodes the accessory proteins required for efficient transcription, secretion, and assembly of the curli fiber. CsgA and CsgB are secreted as largely unstructured proteins and transition to β-rich structures that aggregate into regular fibers at the cell surface. Since both of these proteins have been shown to be amyloidogenic in nature, the correct spatiotemporal synthesis of the curli fiber is of paramount importance for proper functioning and viability. Gram-negative bacteria have evolved an elegant machinery for the safe handling, secretion, and extracellular assembly of these amyloidogenic proteins.

Single-Molecule Sensing Using Nanopores in Two-Dimensional Transition Metal Carbide (MXene) Membranes

Label-free nanopore technology for sequencing biopolymers such as DNA and RNA could potentially replace existing methods if improvements in cost, speed, and accuracy are achieved. Solid-state nanopores have been developed over the past two decades as physically and chemically versatile sensors that mimic biological channels, through which transport and sequencing of biomolecules have already been demonstrated. Of particular interest is the use of two-dimensional (2D) materials as nanopore substrates, since these can in theory provide the highest resolution readout (<1 nm of a biopolymer segment) and opportunities for electronic multiplexed readout through their interesting electronic properties. In this work, we report on nanopores comprising atomically thin flakes of 2D transition metal carbides called MXenes. We demonstrate a high-yield (60%), contamination-free, and alignment-free transfer method that involves their self-assembly at a liquid-liquid interface to large-scale (mm-sized) films composed of sheets, followed by nanopore fabrication using focused electron beams. Our work demonstrates the feasibility of MXenes, a class of hydrophilic 2D materials with over 20 compositions known to date, as nanopore membranes for DNA translocation and single-molecule sensing applications.

A new class of hybrid secretion system is employed in Pseudomonas amyloid biogenesis

Gram-negative bacteria possess specialised biogenesis machineries that facilitate the export of amyloid subunits for construction of a biofilm matrix. The secretion of bacterial functional amyloid requires a bespoke outer-membrane protein channel through which unfolded amyloid substrates are translocated. Here, we combine X-ray crystallography, native mass spectrometry, single-channel electrical recording, molecular simulations and circular dichroism measurements to provide high-resolution structural insight into the functional amyloid transporter from Pseudomonas, FapF. FapF forms a trimer of gated β-barrel channels in which opening is regulated by a helical plug connected to an extended coil-coiled platform spanning the bacterial periplasm. Although FapF represents a unique type of secretion system, it shares mechanistic features with a diverse range of peptide translocation systems. Our findings highlight alternative strategies for handling and export of amyloid protein sequences.

Gram-negative bacteria assemble biofilms from amyloid fibres, which translocate across the outer membrane as unfolded amyloid precursors through a secretion system. Here, the authors characterise the structural details of the amyloid transporter FapF in Pseudomonas.

Outer membrane lipoprotein biogenesis: Lol is not the end

Bacterial lipoproteins are lipid-anchored proteins that contain acyl groups covalently attached to the N-terminal cysteine residue of the mature protein. Lipoproteins are synthesized in precursor form with an N-terminal signal sequence (SS) that targets translocation across the cytoplasmic or inner membrane (IM). Lipid modification and SS processing take place at the periplasmic face of the IM. Outer membrane (OM) lipoproteins take the localization of lipoproteins (Lol) export pathway, which ends with the insertion of the N-terminal lipid moiety into the inner leaflet of the OM. For many lipoproteins, the biogenesis pathway ends here. We provide examples of lipoproteins that adopt complex topologies in the OM that include transmembrane and surface-exposed domains. Biogenesis of such lipoproteins requires additional steps beyond the Lol pathway. In at least one case, lipoprotein sequences reach the cell surface by being threaded through the lumen of a beta-barrel protein in an assembly reaction that requires the heteropentomeric Bam complex. The inability to predict surface exposure reinforces the importance of experimental verification of lipoprotein topology and we will discuss some of the methods used to study OM protein topology.

Bacterial amyloid formation: structural insights into curli biogensis

Curli are functional amyloid fibers assembled by many Gram-negative bacteria as part of an extracellular matrix that encapsulates the bacteria within a biofilm. A multicomponent secretion system ensures the safe transport of the aggregation-prone curli subunits across the periplasm and outer membrane, and coordinates subunit self-assembly into surface-attached fibers. To avoid the build-up of potentially toxic intracellular protein aggregates, the timing and location of the interactions of the different curli proteins are of paramount importance. Here we review the structural and molecular biology of curli biogenesis, with a focus on the recent breakthroughs in our understanding of subunit chaperoning and secretion. The mechanistic insight into the curli assembly pathway will provide tools for new biotechnological applications and inform the design of targeted inhibitors of amyloid polymerization and biofilm formation.