DISCOVERY OF AN ESCHERICHIA COLI CHANNEL WITH HIGH VOLTAGE DEPENDENCE AND COOPERATIVITY.

Abstract

Channels are essential for controlling the permeability of cellular membranes. The regulation of channel gating therefore plays an important role physiologically. Voltage-gating is one of the regulations that cells utilize wherein the change in transmembrane potential cause conformational changes in channels. Here a novel bacterial channel from Escherichia coli with remarkable voltage-gating properties is reported.

When the channel-forming protein was reconstituted into a planar phospholipid membrane, two different types of channel activities were observed. Type A is weakly cation-selective, with a single channel conductance about 1.5 nS (in 1M KCl solution), corresponding to a pore size of 0.9 nm. High positive voltages cause step-wise closures. Type B is voltage-independent, with much larger and noisier conductance. When LaCl3 was added, Type B channels first showed a decrease in conductance, and the residual conductance became voltage-gated, indistinguishable from Type A channels.

Under triangular voltage waves, more interesting voltage-gating behaviors were revealed. The single conducting unit seems to be composed of three channels, each with the identical 1.5 nS conductance (namely channel (1), channel (2), and channel (3)). Based on the voltages at which they close/reopen, and the sequence of their closure/reopening, a model was proposed as follows. All three channels are proposed to be molecularly identical but, channel (1) and channel (3) have the same orientation, which is opposite to that of channel (2). Altogether these three channels form the conducting unit in a linear array. The voltage sensor domain of each channel is proposed to take the form of a dipole moment. The interaction between dipole moments of the channels, each with an opposite orientation with its neighbor(s), leads to the impressively high cooperativity between channels. Although the physiological roles of these channels are not clear yet, the remarkably steep voltage dependence (n~14) rivals that of the channels in excitable membranes.