Effect of Gating Modifier Toxins on Membrane Thickness: Implications for Toxin Effect on Gramicidin and Mechanosensitive Channels

Abstract

Various gating modifier toxins partition into membranes and interfere with the gating mechanisms of biological ion channels. For example, GsMTx4 potentiates gramicidin and several bacterial mechanosensitive channels whose gating kinetics are sensitive to mechanical properties of the membrane, whereas binding of HpTx2 shifts the voltage-activity curve of the voltage-gated potassium channel Kv4.2 to the right. The detailed process by which the toxin partitions into membranes has been difficult to probe using molecular dynamics due to the limited time scale accessible. Here we develop a protocol that allows the spontaneous assembly of a polypeptide toxin into membranes in atomistic molecular dynamics simulations of tens of nanoseconds. The protocol is applied to GsMTx4 and HpTx2. Both toxins, released in water at the start of the simulation, spontaneously bind into the lipid bilayer within 50 ns, with their hydrophobic patch penetrated into the bilayer beyond the phosphate groups of the lipids. It is found that the bilayer is about 2 Å thinner upon the binding of a GsMTx4 monomer. Such a thinning effect of GsMTx4 on membranes may explain its potentiation effect on gramicidin and mechanosensitive channels.

Effects of Lys to Glu mutations in GsMTx4 on membrane binding, peptide orientation, and self-association propensity, as analyzed by molecular dynamics simulations

“…For WT peptide, the CG and AT PMFs yielded similar PMF depths, which were also consistent with the results of the CHARMM36-based analysis by Chen and Chung, who reported a PMF depth of −26 kT [10]. Binding strengths for these peptides were also consistent with our AT and CG analyses, suggesting that these force fields are reliable, at least for this application.…”

“…We preliminarily replicated the findings of Chen and Chung related to membrane thinning with binding of multiple peptides to the membrane [10] (our unpublished results). Using AT (Berger and OPLS-AA force fields), when two WT peptides were placed at normal binding positions at the center of a bilayer of 256 (125/131) DPPC (dipalmitoylphosphatidylcholine) molecules, thinning was prominent in the close vicinity of GsMTx4.…”

“…Using the CHARMM36 force field, Chen and Chung reported thickening of POPC bilayer in the proximity of GsMTx4 and thinning in more distal parts of the POPC bilayer [10]. However, our similar analyses using the CHARMM36 force field uncovered slow undulatory motions of the membrane and vertical drifts of the peptides, but did not show the membrane thickening reported by the latter paper.…”

“…GsMTx4 has been subjected to several computational analyses [9,10], but the interactions with membranes remain poorly understood. We expected that peptide binding would produce local deformation and local changes in membrane thickness and curvature [8,10], but these effects have not been systematically evaluated.…”

“…We expected that peptide binding would produce local deformation and local changes in membrane thickness and curvature [8,10], but these effects have not been systematically evaluated. One notable feature of GsMTx4 is that it has high net positive charge (+5 e ).…”