Channel-forming activity of syringopeptin 25 A in mercury-supported phospholipid monolayers and negatively charged bilayers

Interactions of the cationic lipodepsipeptide syringopeptin 25 A (SP25A) with mercury-supported dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylserine (DOPS) and dioeleoylphosphatidic acid (DOPA) self-assembled monolayers (SAMs) were investigated by AC voltammetry in 0.1 M KCl at pH 3, 5.4 and 6.8. SP25A targets and penetrates the DOPS SAM much more effectively than the other SAMs not only at pH 6.8, where the DOPS SAM is negatively charged, but also at pH 3, where it is positively charged just as SP25A. Similar investigations at tethered bilayer lipid membranes (tBLMs) consisting of a thiolipid called DPTL anchored to mercury, with a DOPS, DOPA or DOPC distal monolayer on top of it, showed that, at physiological transmembrane potentials, SP25A forms ion channels spanning the tBLM only if DOPS is the distal monolayer. The distinguishing chemical feature of the DOPS SAM is the ionic interaction between the protonated amino group of a DOPS molecule and the carboxylate group of an adjacent phospholipid molecule. Under the reasonable assumption that SP25A preferentially interacts with this ion pair, the selective lipodepsipeptide antimicrobial activity against Gram-positive bacteria may be tentatively explained by its affinity for similar protonated amino-carboxylate pairs, which are expected to be present in the peptide moieties of peptidoglycan strands

The GM1 Ganglioside Forms GM1-Rich Gel Phase Microdomains within Lipid Rafts

Abstract: Mercury-supported, self-assembled monolayers of the sole (SAMs) dioleoylphosphatidylcholine (DOPC)and of a raft -forming mixture of DOPC, cholesterol (Chol) and palmitoylsphingomyelin of (59:26:15) mol% composition(PSM) , were investigated by electrochemical impedance spectroscopy (EIS), both in the absence and in the presence of the monosialoganglioside GM1. The impedance spectra of these four SAMs were fitted by a series of parallel combinations of a resistance and a capacitance ( RC meshes) and displayed on plots of ω Z ′ against −ω Z ″, where Z ′ and Z ″ are the in -phase and quadrature components of the impedance and is the angular frequency. A comparison among these ωdifferent impedance spectra points to the formation of GM1-rich gel phase microdomains within the lipid rafts of the DOPC/Chol/PSM mixture, thanks to the unique molecular-level smooth support provided by mercury, whichallows EIS to detect the protruding gel phase microdomains by averaging them over a macroscopically large area

Can proton pumping by SERCA enhance the regulatory role of phospholamban and sarcolipin?

AbstractThe effect of the incorporation of phosphorylated phospholamban (pPLN) and sarcolipin (SLN) in mercury-supported self-assembled lipid monolayers and in lipid bilayers tethered to mercury via a hydrophilic spacer was investigated by voltammetric techniques and electrochemical impedance spectroscopy. It was shown that pPLN and SLN do not permeabilize lipid bilayers toward ions at physiological pH. However, they exert a permeabilizing action toward inorganic monovalent cations such as K+ and Tl+, but not toward divalent cations such as Ca2+ and Cd2+, following a small decrease in pH. This behavior can be associated with their regulatory action on the Ca-ATPase of the sarcoplasmic reticulum (SERCA). SERCA pumps two Ca2+ ions from the cytosol to the lumen of the sarcoplasmic reticulum (SR) and two protons in the opposite direction, causing a transient decrease of pH in the immediate vicinity of its cytoplasmic domain. This decrease is expected to activate the liberated pPLN molecules and SLN to make the SR membrane leakier toward K+ and Na+ and the SLN ion channel to translocate small inorganic anions, such as Cl−. The effect of pPLN and SLN, which becomes synergic when they are both present in the SR membrane, is expected to favor a rapid equilibration of ions on both sides of the membrane