Voltage-dependent cationic channel of Escherichia coli

A fraction highly enriched with inner membranes of E. coli was fused with liposomes, using the dehydration-rehydration technique, to produce giant liposomes amenable to patch-clamp recordings. Among the several channels present in this type of preparation, one was further characterized. The channel has a conductance of some 200 pS (in 0.1 M KCl) and is weakly selective for cations (PK/PCl = 4). The channel stays open at negative and low positive membrane potentials and shows an increasing probability of closure with increasing voltage. High positive membrane potentials favor transitions to a long-lived inactivated state, following slow kinetics. Voltage-dependent rapid flickerings of the same amplitude, between open state and other short-lived closed states, are superposed on these kinetics. The channel is presumed to be localized in the inner membrane, but its characteristics are also compatible with those of porins of the outer membrane. However, the major porins OmpF and OmpC, purified and reconstituted into giant liposomes, exhibited a marked different behavior.