A VOLTAGE-DEPENDENT CHLORIDE CONDUCTANCE CHANNEL FROM zyxwvu TORPEDO ELECTROPLAX MEMBRANE zy * zy Christopher Miller and Michael M. White Graduate Department of Biochemistry Brandeis University Waltham, Massachusetts 02254 During the past few years, we have been studying the possibility of incor- porating ion-conductance channels from animal cell membranes into artificial bilayers, as a first step towards characterizing these channels in a biochemically defined system. The strategy for this work is to bring about the fusion of native membrane vesicles with a planar phospholipid bilayer, a system upon which electrical conductance may be measured at a controllable applied voltage. Using this approach, we have documented the existence of a voltage-gated K+-selective channel from rabbit sarcoplasmic reticulum zyx (SR) zy .1-4 In attempting to extend the technique to other and better-understood membrane systems, we recently showed that “microsac” vesicles of the electric organ of Torpedo californica, an electric ray whose electroplax membrane carries a high concen- tration of acetylcholine receptor, can be made to induce a large increase in conductance of planar bilayer~.~ Quite surprisingly, this microsac-induced con- ductance had no properties to be expected of the acetylcholine-gated channel, but rather displayed anion selectivity and voltage-dependence, along with clear single-channel fluctuation behavior. In this paper, we extend the initial results presented on this channel. The major new finding here is that the channel is exquisitely selective to chloride ion and is actually blocked by certain other anions. In addition, we demon- strate the asymmetric inhibition of the channel by those anion-transport work- horses, SITS and DIDS. While we can only speculate upon the physiological role of the system, we consider that the results demonstrate that the C1- channel is present in the electroplax of Torpedo and that it must be taken into account in any model of the operation of this cell. Fusion of Microsacs with Planar Bilayers The experimental setup, shown in FIGURE 1, consists of a phospholipid bilayer separating two aqueous chambers (labelled cis and trans) containing an appropriate electrolyte (usually 0.1 M KCI). The voltage across the bilayer may be adjusted at will, and the resulting current flow is measured by a simple feedback circuit.’ Voltage is always defined as zero on the trans side of the bilayer, and microsac vesicles are always added to the cis side, to final protein concentrations in the range 1-50 pgm/ml. Before addition of vesicles, the bilayers have very low conductance, in the range of 0.5-2 X lo-* mho/cm2. * This research was supported by NSF Grant #BNS76-23212 and NIH predoctoral Training Grant g00212-20 (to M.M.W.). C.M. is the recipient of NIH R.C.D.A. No. KO4 AM 00354-01. M.M.W. is the recipient of a Brandeis University Klein Fellowship. 534 0077-8923/80/0341-0534 $01.75/0 @) 1980, zyxw NYAS