Probes of the conduction process of a voltage-gated Cl- channel from Torpedo electroplax

The open-channel conductance properties of a voltage-gated Cl- channel derived from Torpedo californica electroplax and incorporated into planar bilayers were studied by several approaches. In neutral bilayers the channel conductance saturates with Cl- activity according to a rectangular hyperbolic relation with a half-saturation activity of 75 mM and a maximum conductance of 32 pmho. The observation of identical behavior in charged membranes implies that ions permeating the channel do not sense the surface potential of the bulk membrane. The Cl-:Br- permeability ratio, measured under biionic conditions, is independent of salt concentration. SCN- ion reversibly blocks the channel. The voltage dependence of the block implies the existence of two separate blocking sites within the channel: one accessible from the cis side only (the side to which vesicles are added) and the other accessible from the trans side only. The block at each site is competitive with Cl-. The results are consistent with a single-ion Eyring model of the conduction process in which the ion must traverse three kinetic barriers as it permeates the channel and in which the channel can accommodate at most one ion at a time.