Chapter 3 Voltage‐Dependent Ion Channels Induced by Cyclic Lipodepsipeptides in Planar Lipid Bilayers: Structure, Properties, and Resemblance to Native Channels

Abstract

Abstract The syringomycins (SRs) are cyclic lipodepsipeptides produced by the phytopathogenic bacterium Pseudomonas syringae pv. syringae . Discovered and described during the 1980s and 1990s, the effects of the SRs on plant cells and inhibition of fungal and yeast growth are due to specific interactions with membrane lipids of target cells. Using one‐side addition of SRs to a system consisting of a bilayer lipid membrane and two aqueous compartments, researchers have shown that the SRs form anion selective channels that possess unique and highly reproducible voltage sensitive properties. Moreover, SRs form two types of ion channels, namely “small” and “large,” that differ several‐fold in their conductance. These initial findings generated interest in the SRs to (1) consider development of the SRs as antifungal agents, and (2) use SR channels as mechanistic models for native ion channels. Follow‐up kinetic studies were conducted on the opening/closure of ion channels formed by the most abundant form of the SRs, syringomycin E (SRE). It was established that SRE's channel activities and properties depend on several factors, including the surface charge and composition of membranes, the ionic strength of bathing solutions, and the concentration of membrane modifying agents. Estimations of the inner radius of SRE channels and their anion selectivity showed that the large channels are clusters of synchronously functioning small ones and both clustered and nonclustered single channels have similar radii. Determinations of the gating charge of the SRE channels showed that they are asymmetrical lipid pores stabilized by lipopeptide molecules. The involvement of membrane lipids in the channel structure provides explanations for the dependencies of SRE channel properties on the earlier‐mentioned factors, and a model is proposed that features the influence of a dipolar membrane potential on the gating charge of SRE channels. More recent studies have focused on the effects of actin on SRE channel‐forming activity. Activity is induced if SRE and actin species are added to opposite sides of the lipid membrane but absent when added on the same side. The observed actin influence is explained as the consequence of changing the bilayer structure with one‐side cooperative sorption of actin molecules. As actin is a major component of the cytoskeleton, the sensitivity of cells (fungi, yeasts, and human erythrocytes) to SRs may be modulated by intracellular actin. Other recent studies have been devoted to the properties of the large SRE channel clusters. It was established that the number of small channels synchronously working in such clusters depends on the transmembrane potential, whereas the lifetimes of the small and large channels are not dependent on this parameter. The dependence on electrical potential is altered with addition of dipole‐modifying agents. Overall, the resemblance of the properties of the SRE channels with those of natural ion channels suggests their value as models for investigating ion channel structure, regulation, and function.