Matrix protein from Escherichia coli outer membranes forms voltage-controlled channels in lipid bilayers.

Matrix protein from Escherichia coli was integrated into planar lipid bilayers. The incorporated protein generates aqueous channels across these membranes. Channels are induced irreversibly by voltage, and their number is proportional to the protein content of the membrane and stays constant over hours. They are uniform in size, with a diameter of about 1 nm and a single-channel conductance of 0.14 nS in 0.1 M NaCl. In addition to ionic conductance, the channels allow free diffusion of small, uncharged molecules. Channels assume either an open or a closed state. Membrane potentials shift this two-state equilibrium distribution in favor of closed channels, an observation that explains both negative resistance and inactivation at high potentials. Channels are not randomly distributed in the membrane but interact cooperatively within aggregates. The smallest entity inducible consists of three channels.