Determination of ion permeability by fluorescence quenching

Membrane fusion studied by colloidal probes

Membrane-coated colloidal probes combine the benefits of solid-supported membranes with a more complex three-dimensional geometry. This combination makes them a powerful model system that enables the visualization of dynamic biological processes with high throughput and minimal reliance on fluorescent labels. Here, we want to review recent applications of colloidal probes for the study of membrane fusion. After discussing the advantages and disadvantages of some classical vesicle-based fusion assays, we introduce an assay using optical detection of fusion between membrane-coated glass microspheres in a quasi two-dimensional assembly. Then, we discuss free energy considerations of membrane fusion between supported bilayers, and show how colloidal probes can be combined with atomic force microscopy or optical tweezers to access the fusion process with even greater detail.

Probing and Tuning the Permeability of Polymersomes

Polymersomes are a class of synthetic vesicles composed of a polymer membrane surrounding an aqueous inner cavity. In addition to their overall size, the thickness and composition of polymersome membranes determine the range of potential applications in which they can be employed. While synthetic polymer chemists have made great strides in controlling polymersome membrane parameters, measurement of their permeability to various analytes including gases, ions, organic molecules, and macromolecules remains a significant challenge. In this Outlook, we compare the general methods that have been developed to quantify polymersome membrane permeability, focusing in particular on their capability to accurately measure analyte flux. In addition, we briefly highlight strategies to control membrane permeability. Based on these learnings, we propose a set of criteria for designing future methods of quantifying membrane permeability such that the passage of a variety of molecules into and out of their lumens can be better understood.

Stopped-Flow Fluorometric Ion Flux Assay for Ligand-Gated Ion Channel Studies

Quantitative investigations into functional properties of purified ion channel proteins using standard electrophysiological methods are challenging, in particular for the determination of average ion channel behavior following rapid changes in experimental conditions (e.g., ligand concentration). Here, we describe a method for determining the functional activity of liposome-reconstituted K⁺ channels using a stopped-flow fluorometric ion flux assay. Channel activity is quantified by measuring the rate of fluorescence decrease of a liposome-encapsulated fluorophore, specifically quenched by thallium ions entering the liposomes via open channels. This method is well suited for studying the lipid bilayer dependence of channel activity, the activation and desensitization kinetics of ligand-dependent K⁺ channels, and channel modulation by channel agonists, blockers, or other antagonists.

Regulation of ion channel function by the host lipid bilayer examined by a stopped-flow spectrofluorometric assay

To examine the function of ligand-gated ion channels in a defined membrane environment, we developed a robust sequential-mixing fluorescence-based stopped-flow assay. Channel activity is determined using a channel-permeable quencher (e.g., thallium, Tl(+)) of a water-soluble fluorophore (8-aminonaphthalene-1,3,6-trisulfonic acid) encapsulated in large unilamellar vesicles in which the channel of interest has been reconstituted, which allows for rapid solution changes. To validate the method, we explored the activation of wild-type KcsA channel, as well as it's noninactivating (E71A) KcsA mutant, by extravesicular protons (H(+)). For both channel types, the day-to-day variability in the reconstitution yield (as judged from the time course of fluorescence quenching) is <10%. The activation curve for E71A KcsA is similar to that obtained previously using single-channel electrophysiology, and the activation curves for wild-type and E71A KcsA are indistinguishable, indicating that channel activation and inactivation are separate processes. We then investigated the regulation of KcsA activation by changes in lipid bilayer composition. Increasing the acyl chain length (from C18:1 to C22:1 in diacylphosphatidylcholine), but not the mole fraction of POPG (>0.25) in the bilayer-forming phospholipid mixture, alters KcsA H(+) gating. The bilayer-thickness-dependent shift in the activation curve is suggestive of a decrease in an apparent H(+) affinity and cooperativity. The control over bilayer environment and time resolution makes this method a powerful assay for exploring ligand activation and inactivation of ion channels, and how channel gating varies with changes in the channels' lipid bilayer environment or other regulatory processes.

Direct integration of cell-free-synthesized connexin-43 into liposomes and hemichannel formation

Proteoliposomes were directly prepared by synthesizing membrane proteins with the use of minimal protein synthesis factors isolated from Escherichia coli (the PURE system) in the presence of liposomes. Connexin-43 (Cx43), which is a water-insoluble integral membrane protein that forms a hexameric complex in membranes, was cotranslationally integrated with an essentially uniform orientation in liposomes. The addition of liposomes following protein expression (post-translational presence of liposomes) did not lead to the integration of Cx43 into the liposome membranes. The amount of integrated Cx43 increased as the liposome concentration increased. The presence of liposomes did not influence the total amount of synthesized Cx43. The Cx43 integrated into the liposome membranes formed open membrane pores. These results indicate that the liposomes act in a chaperone-like manner by preventing Cx43 from aggregating in solution, because of integration into the bilayer, and also by functionalization of the integrated Cx43 in the membrane. This is the first report that cell-free-synthesized water-insoluble membrane protein is directly integrated with a uniform orientation as a functional oligomer into liposome membranes. This simple proteoliposome preparation procedure should be a valuable approach for structural and functional studies of membrane proteins.