Voltage Dependence and Modulation of Bacterial Channel MscL

Abstract

Channels carrying canonical voltage sensor domains with multiple charges respond to voltage steeply. Non-voltage-gated channels lacking specialized voltage sensors, nevertheless, can also be modulated by voltage acting on other domains. The reason is that a typical channel residing in the polarized membrane represents a non-uniform protein insert characterized by local parameters of dielectric permeability, dipole and charge whose distributions may change as the channel transitions between the states. Here we describe a strong voltage modulation of bacterial mechanosensitive channel MscL and especially its gain-of-function mutant (V23T), which is exerted through a non-specific dielectric mechanism by water present in the pore with some contribution from the dipole moment of the protein itself. Patch-clamp measurements obtained under linear pressure ramp stimulation at different voltages reveal that both versions of MscL have dipole (linear with voltage) and dielectric/capacitive (quadratic with voltage) components that contribute to the transition energy. In contrast to WT, which exhibits a larger dipole and a smaller dielectric contribution, V23T shows a dominating dielectric effect likely related to the more pronounced hydration of the mutant pore. The increasing aqueous volume of the pore and the flattening of the protein complex impart the capacitive/dielectric energy contribution that stabilizes the expanded and subconductive states of V23T MscL. To further explore the nature of the electric field effect on open probability, we have compared the V23T MscL behavior in electrically symmetric and asymmetric droplet interface bilayers where a large intrinsic electric bias was created by using lipids with different interfacial dipole potentials. The results confirm that V23T MscL senses the local intramembrane electric field in the membrane core between the interfacial layers, and thus the voltage imposed by external electrodes is sensed not directly, but rather in combination with the potential imparted by the lipids.