1.D.48 The Pore-forming Syringomycin E (Syringomycin E) Family
Antifungal lipodepsipeptide syringomycin E (SRE) forms two major conductive states in lipid bilayers: 'small' and 'large'. Large SRE channels are clusters of several small ones, demonstrating synchronous opening and closure. Ostroumova et al. 2005 reported that the large SRE channels consist of 3-8 simultaneously gating small channels. An increase in the absolute value of the transmembrane potential (from 50 to 200 mV) decreases the number of synchronously gated channels in the clusters. Voltage-dependence of channel synchronization is influenced by the ionic strength of the bathing solution. They proposed a mechanism for the voltage-dependent cluster behavior that involves a voltage-induced reorientation of lipid dipoles associated with the channel pores (Ostroumova et al. 2005). A sharp decrease in the lifetime of single ion pores induced by the antifungal lipopeptide syringomycin E, after addition of benzylamines or the black pepper alkaloid, piperine, was mainly due to a reduction in the dipole potential (Efimova et al. 2020).
The lipodepsipeptide, syringomycin E (SR-E or SRE), interacts with two mercury-supported biomimetic membranes, which consist of a self-assembled phospholipid monolayer (SAM) and a tethered bilayer lipid membrane (tBLM), separated from the mercury surface by a hydrophilic tetraethyleneoxy (TEO) spacer that acts as an ionic reservoir. SR-E interacts more rapidly and effectively with a SAM of dioleoylphosphatidylserine (DOPS) than with one of dioleoylphosphatidylcholine (DOPC). The proximal lipid monolayer of the tBLM has no polar head region, being linked to the TEO spacer via an ether bond, while the distal monolayer consists of either a DOPC or a DOPS leaflet. Ion flow into or out of the spacer through the lipid bilayer moiety of the tBLM was monitored by potential step chronocoulometry and cyclic voltammetry (Becucci et al. 2015). With the distal monolayer bathed by aqueous 0.1M KCl and 0.8muM SR-E, an ion flow in two stages was monitored with DOPC at pH3 and 5.4 and with DOPS at pH3, while a single stage was observed with DOPS at pH5.4. This behavior was compared with that already described in conventional bilayer lipid membranes. The sigmoidal shape of the chronocoulometric charge transients points to an aggregation of SR-E monomers forming an ion channel via a mechanism of nucleation and growth. The ion flow was mainly determined by potassium ions, and was inhibited by calcium ions. The contribution to the transmembrane potential from the distal leaflet depended more on the nature of the lipid than that of the ion channel (Becucci et al. 2015).
The effects of the amide-linked (lidocaine (LDC), mepivacaine (MPV), prilocaine (PLC)) and ester-bound local anesthetics (benzocaine (BZC), procaine (PRC), and tetracaine (TTC)) on the pore-forming activity of the antifungal lipopeptide syringomycin E (SRE) in lipid bilayers have been studied (Zakharova et al. 2018). Independently of electrolyte concentration in the membrane bathing solution, the observed changes in conductance of SRE channels agreed with the altered membrane dipole potential under the action of ester-bound local anesthetics. Effects of aminoamides in diluted and concentrated solutions were completely different. At 0.1 M KCl (pH 7.4) the effects of amide-linked anesthetics were in accordance with changes in the membrane surface potential, while at 2 M KCl aminoamides blocked ion passage through the SRE channels, leading to sharp reductions in pore conductance at negative voltages and 100-fold decreases in the channel lifetimes. The effects were not influenced by the membrane lipid composition. The interaction cooperativity implied the existence of specific binding sites for amide-bound anesthetics in SRE channels (Zakharova et al. 2018).