Characterization of the Channel Formed by the Mycobacterial Porin in Lipid Bilayer Membranes. Demonstration of Voltage Gating and of Negative Point Charges at the Channel Mouth

We studied the channel formed by the mycobacterial porin from the cell wall of Mycobacterium chelonae (Trias, J., Jarlier, V., and Benz, R. (1992) Science 258, 1479-1481) by reconstituting the mycobacterial porin and cell wall extracts in lipid bilayer membranes. The channel exhibited two different states in lipid bilayer membranes at 10 mV of applied voltage. One was characterized by a steplike appearance while the other showed a fast, voltage-dependent, flickering behavior between a closed and an open state. The channel was voltage-gated, and starting at 40 mV of applied voltage the mycobacterial porin channel switched to a closed configuration in an asymmetric fashion. The channel was cation-selective and had 2.5-point negative charges at both sides of the channel. Identical channels were observed when membranes were reconstituted with cell wall extracts, suggesting that there is only one porin species in the mycobacterial cell wall.