OPEN-STATE SUBSTRUCTURE OF SINGLE CHLORIDE CHANNELS FROM TORPEDO ELECTROPLAX

Cl- channels from T. californica electroplax were inserted into planar phospholipid membranes, and single-channel currents were studied at high time-resolution. The open channel fluctuates rapidly between 3 substates, with conductances of 18.5, 9.4 and 0 picosiemens in 150 mM Cl-. Under various ionic conditions the 3 substates are always equally spaced in conductance; at various voltages leading to different probabilities of observing the 3 substates, the substate frequencies are always binomially distributed. The conducting unit of this Cl- channel is composed of 2 identical Cl- diffusion pathways, each with a voltage-dependent gate.