Membrane transport

A range of mechanisms have evolved for communicating information across cell membranes, but designing synthetic analogues is far from trivial. A collection of articles in this issue discuss different methods of passing chemical information across lipid bilayers using artificial systems.

Life has evolved elaborate means of communicating essential chemical information across cell membranes. Inspired by biology, two new artificial mechanisms have now been developed that use synthetic messenger molecules to relay chemical signals into or across lipid membranes.

The assembly of transmembrane barrels formed from short synthetic peptides has not been previously demonstrated. Now, a transmembrane pore has been fabricated via the self-assembly of peptides. The 35-amino-acid α-helical peptides are based on the C-terminal D4 domain of the Escherichia coli polysaccharide transporter Wza.

The transmission of chemical information across lipid bilayer membranes is crucial in biological systems. Now, an artificial chemical system able to both transduce and amplify chemical signals across a membrane has been developed. The system works by exploiting the controlled translocation of a synthetic molecule that is embedded within a vesicle membrane.

Genetic circuits are important for synthetic biology, biochemistry and bioengineering. Now, the encapsulation of genetic circuits into liposomes has been shown to enable a more modular design, the selective isolation of reactions from the environment and from each other, and the hierarchical assembly of reaction products.

A dynamic foldamer scaffold has now been ligated to a water-compatible, metal-centred binding site and a conformationally responsive fluorophore to form a receptor mimic that inserts into the membrane of artificial vesicles. Binding of specific carboxylate ligands induces a global conformational change that depends on the structure of the ligand, and can be detected via fluorescence.

Synthetic anion transporters that replace the activity of defective anion channels have been proposed as treatments for cystic fibrosis; however, it remains uncertain whether such molecules are fundamentally toxic. A series of bis- and tris-(thio)ureas capable of transporting anions have now been tested in cells expressing halide-sensitive yellow fluorescent protein. One bis-urea compound proved especially effective while showing almost no toxicity.

Anion transporters that disrupt cellular ion homeostasis could represent a new approach for generating therapeutic lead compounds. Now, two pyridine diamide-strapped calix[4]pyrroles have been shown to induce coupled chloride anion and sodium cation transport in liposomal models and cells. These compounds promote cell death by increasing intracellular chloride and sodium ion concentrations.

Amiloride is a widely used diuretic that blocks epithelial sodium channels (ENaCs); however, the functional role of the different ENaC isoforms is still poorly understood and no pharmacological tools exist to differentiate between them. Now, photoswitchable amilorides that enable the optical control of ENaCs, and can distinguish between different ENaC isoforms have been developed.

Light-driven proton pumps are used in biology to create a proton gradient that can be subsequently converted into chemical energy. Here, an artificial light-harvesting system based on a membrane doped with a spiropyran is described. Irradiation with UV light generates a proton flux across the membrane and results in the generation of an electrical current.

Capsular polysaccharides (CPS) enclose many pathogenic strains of Escherichia coli, protecting the bacteria from the host. Here, an extracellular blocker of Wza, a pore-forming protein that transports CPS to the cell surface, has been discovered by single-channel electrical recording. Treatment with the blocker exposes the bacterial cell surface and thereby facilitates killing by the human immune system.

The transport of bicarbonate anions across cell membranes by proteins is an important biological process, and if not regulated properly it can lead to cystic fibrosis and heart disease. Now, it has been shown that ‘small’ molecules can also promote efficient bicarbonate transport across lipid membranes and could be used to probe this process.