Asymmetry of syringomycin E channel studied by polymer partitioning

Abstract

To probe the size of the ion channel formed by Pseudomonas syringae lipodepsipeptide syringomycin E, we use the partial blockage of ion current by penetrating poly(ethylene glycol)s. Earlier experiments with symmetric application of these polymers yielded a radius estimate of approximately 1 nm. Now, motivated by the asymmetric non-ohmic current-voltage curves reported for this channel, we explore its structural asymmetry. We gauge this asymmetry by studying the channel conductance after one-sided addition of differently sized poly(ethylene glycol)s. We find that small polymers added to the cis-side of the membrane (the side of lipodepsipeptide addition) reduce channel conductance much less than do the same polymers added to the trans-side. We interpret our results to suggest that the water-filled pore of the channel is conical with cis- and trans-radii differing by a factor of 2-3 and that the smaller cis-radius is in the 0.25-0.35 nm range. In symmetric, two-sided addition, polymers entering the pore from the larger opening dominate blockage.

Figure 1

The effect of cis- and trans-side addition of PEG200 and PEG4600 (15 % w/w) on the current through a single SRE channel in PS/PE bilayer, bathed by 1 M NaCl, pH 6, at V = −200 mV. The decrease of the channel conductance in the presence of PEG is related to its partitioning into the channel pore: PEG4600 is strongly excluded, while PEG200 penetrates the channel pore, displaces the ions, and increases solution viscosity. However, permeating PEG200 affects SRE-channel conductance asymmetrically.

Figure 2

The conductance-voltage characteristics of SRE-channel in the presence of PEG200 and PEG4600 added from the different sides of the membrane. PEG200 reduces the channel conductance to a larger extent when applied from the trans-side.

Figure 3

The relative changes in SRE-channel conductance as functions of the PEG molecular weight. Hydrodynamic radii of the polymer [30] are denoted on the top axis. Open circles and solid squares correspond to the cis- and trans-side side application of polymer of varying molecular weight, respectively (the impermeant PEG4600 was on the opposite side). The dotted line at 0.6 corresponds to the ratio of bulk solution conductivities with and without polymers. Panels A, B, and C show changes of the conductance ratio at V = +200 mV, 0 mV, and −200 mV, respectively. The solid lines in all three panels present the best-fit predictions based on the scaling law (Eqs. (1), (2), (4), and (5)) for the given pair of cis and trans-side channel radii; the dashed lines are the best fits using α = 3 (see text) and χ as an adjustable parameter.

Figure 4

The comparison of the partition coefficient profiles along the axis of a cone-shaped channel, which are given by Eq. (3) (panel A), and Eq. (4), (panel B). The permeating polymer is applied from the side of the larger channel opening, which corresponds to the trans-side in the case of the SRE channel.