On the different sources of cooperativity in pH titrating sites of a membrane protein channel

Vesicle-Encapsulated Chemosensing Ensembles Allow Monitoring of Transmembrane Uptake Coupled with Enzymatic Reactions

Compartmentalized models with coupled catalytic networks are considered as "protocells" in the context of research related to the origin of life. To model the kinetics of a simple cellular uptake-metabolism process, we use a compartmentalized protocell system that combines liposome-encapsulated intravesicular reporter pairs with co-encapsulated enzymes to monitor the membrane transport of a substrate (analyte uptake) and its subsequent enzymatic reaction inside the vesicles (metabolism to the product). The intravesicular chemosensing ensembles consist of the macrocycles cucurbit[7]uril or p-sulfonatocalix[4]arene and matching fluorescent dyes to set up suitable reporter pairs. When these macrocycle/dye reporter pairs are co-encapsulated with enzymes (trypsin, protein kinase A, or butyrylcholinesterase), it is possible to monitor first the transport of different substrates (polylysine, protamine, H-LRRWSLG-OH, or butyrylcholine) through added pores (outer membrane proteins F and C), with synthetic carriers (amphiphilic calixarenes), or by direct permeation (only for butyrylcholine). The subsequent enzymatic conversions of the substrates after they have entered the corresponding protocells can be monitored as consecutive reactions. The new type of in vitro assays can be applied to different enzymes and analytes, affording a comprehensive chemosensing system of high chemical complexity.

Electrophysiological dissection of the ion channel activity of the Pseudomonas aeruginosa ionophore protein toxin Tse5

We present an in-depth electrophysiological analysis of Tse5, a pore-forming toxin (PFT) delivered by the type VI secretion system (T6SS) of Pseudomonas aeruginosa. The T6SS is a sophisticated bacterial secretion system that injects toxic effector proteins into competing bacteria or host cells, providing a competitive advantage by disabling other microbes and modulating their environment. Our findings highlight the dependency of Tse5 insertion on membrane charge and electrolyte concentration, suggesting an in vivo effect from the periplasmic space. Conductance and selectivity experiments reveal a predominant and reproducible pore architecture of Tse5, characterized by a weak cation selectivity without chemical specificity. pH titration experiments suggest a proteolipidic pore structure influenced by both protein and lipid charges, a hypothesis further supported by experiments involving engineered mutants of Tse5 with altered glycine zippers. These results significantly advance our understanding of Tse5's molecular mechanism of toxicity, paving the way for potential applications in biosensing and macromolecular delivery.

Gating of β-Barrel Protein Pores, Porins, and Channels: An Old Problem with New Facets

β barrels are ubiquitous proteins in the outer membranes of mitochondria, chloroplasts, and Gram-negative bacteria. These transmembrane proteins (TMPs) execute a wide variety of tasks. For example, they can serve as transporters, receptors, membrane-bound enzymes, as well as adhesion, structural, and signaling elements. In addition, multimeric β barrels are common structural scaffolds among many pore-forming toxins. Significant progress has been made in understanding the functional, structural, biochemical, and biophysical features of these robust and versatile proteins. One frequently encountered fundamental trait of all β barrels is their voltage-dependent gating. This process consists of reversible or permanent conformational transitions between a large-conductance, highly permeable open state and a low-conductance, solute-restrictive closed state. Several intrinsic molecular mechanisms and environmental factors modulate this universal property of β barrels. This review article outlines the typical signatures of voltage-dependent gating. Moreover, we discuss recent developments leading to a better qualitative understanding of the closure dynamics of these TMPs.

Effect of endosomal acidification on small ion transport through the anthrax toxin PA63 channel

Tight regulation of pH is critical for the structure and function of cells and organelles. The pH environment changes dramatically along the endocytic pathway, an internalization transport process that is 'hijacked' by many intracellularly active bacterial exotoxins, including the anthrax toxin. Here, we investigate the role of pH on single-channel properties of the anthrax toxin protective antigen (PA₆₃ ). Using conductance and current noise analysis, blocker binding, ion selectivity, and poly(ethylene glycol) partitioning measurements, we show that the channel exists in two different open states ('maximum' and 'main') at pH ≥ 5.5, while only a maximum conductance state is detected at pH < 5.5. We describe two substantially distinct patterns of PA₆₃ conductance dependence on KCl concentration uncovered at pH 6.5 and 4.5.