Voltage-dependent anion channel transports calcium ions through biomimetic membranes

The mitochondrial outer membrane channel (VDAC), a central player in mitochondria and cell death, was reconstituted in polymer-supported phospholipid bilayers. Highly purified VDAC was first reconstituted in vesicles; channel properties and NADH-ferricyanide reductase activity were ascertained before deposition onto solid substrates. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol)-N-hydroxysuccinimide mixed vesicles containing VDAC were linked onto amine-grafted surfaces (glass and gold) and disrupted to form a VDAC-containing polymer-tethered planar bilayer. Surface plasmon spectroscopy, fluorescence microscopy, and atomic force microscopy measurements ascertained the membrane thickness, fluidity, and continuity. VDAC reconstituted in bilayers efficiently transported calcium ions and was modulable by two channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and l-glutamate. The novel setup may allow the study of the assembly of a polyprotein complex centered on VDAC and its role in mitochondrial biology, calcium fluxes, and apoptosis.

Differential mechanisms for calcium-dependent protein/membrane association as evidenced from SPR-binding studies on supported biomimetic membranes

In this work, two different types of supported biomimetic membranes were designed to study the membrane binding properties of two different proteins that both interact with cellular membranes in a calcium-dependent manner. The first one, neurocalcin, is a member of a subfamilly of EF-hand calcium-binding proteins that exhibit a calcium-myristoyl switch. The second protein is a bacterial toxin, the adenylate cyclase produced by Bordetella pertussis, the causative agent of whooping cough. The biomimetic membranes constructed in this study were either hybrid bilayer membranes or polymer-tethered membranes. Hemimembrane formation was obtained in two steps: a monolayer of 1-octadecanethiol or octadecyltrichlorosilane was self-assembled on top of the gold or glass surface, respectively, and then the egg-phosphatidyl choline (PC) vesicle fused on the hydrophobic alkyl layer. Polymer-tethered membranes on solid support were obtained using N-hydroxysuccinimide (NHS)-terminated-poly(ethyleneglycol) (PEG)-phospholipids as anchoring molecules. Egg-PC/1,2-distearoyl-sn-glycero-3-phospho-ethanolamine-poly(ethyleneglycol)-N-hydroxy-succinimide (DSPE-PEG-NHS) mixture liposomes were injected on the top of an amine grafted surface (cysteamine-coated gold or silanized glass); vesicles were linked to the surface and disrupted, leading to the formation of a bilayer. The biomimetic membrane constructions were followed by surface plasmon spectroscopy, while membrane fluidity and continuity were observed by fluorescence microscopy. Protein/membrane binding properties were determined by resonance surface plasmon measurements. The tethered bilayer, designed here, is very versatile as it can be adapted easily to different types of support. The results demonstrate the potentialities of such polymer-tethered artificial membranes for the study of proteins that insert into biological membranes such as toxins and/or integral membrane proteins.

Ca2+-myristoyl switch and membrane binding of chemically acylated neurocalcins

Neurocalcin is a member of a novel family of neuronal calcium sensors that belongs to the superfamily of EF-hand Ca(2+)-binding proteins. Neurocalcin is myristoylated on its N-terminus in vivo and can associate with biological membranes in a calcium and myristoyl-dependent manner. This process known as "Ca(2+)-myristoyl switch" has been best described for the photoreceptor specific protein, recoverin, as well as for several other neuronal calcium sensors. Here, we used reversed micelles to chemically acylate nonmyristoylated neurocalcin at its N-terminus with fatty acids of different lengths (from C12 to C16). This approach allowed us to prepare neurocalcin derivatives in which a single fatty acid is selectively linked to the N-terminal glycine of the polypeptide chain through an amide bond. The membrane binding properties of the monoacylated neurocalcins were then examined by cosedimentation with phospholipid vesicles and direct binding to lipid monolayers by surface plasmon resonance spectroscopy (Biacore). Our results show that neurocalcins monoacylated with lauric, myristic, or palmitic acid were able to associate with membrane in a calcium-dependent manner. This indicates that the Ca(2+)-myristoyl switch can function with different lipid moieties and is not strictly restricted to myristate. The ability to modify at will the fatty acid linked to the N-terminal glycine should be useful to analyze the contribution of the fatty acid moiety to the biological function of this family of neuronal calcium sensors.