CHARACTERIZATION OF ION CHANNELS FROM ACETABULARIA PLASMA-MEMBRANE INPLANAR LIPID BILAYERS

Plasma membrane from Acetabularia acetabulum was prepared by aqueous-p olymer two-phase partitioning and incorporated into planar 1-palmitoyl -2-oleoyl phosphatidylethanolamine bilayers by stirring in the presenc e of a (cis : trans) 325 : 100 mM KCl gradient. Under these conditions five distinct K+-selective channels were observed which had unitary c hord-conductances (determined between 30 mV either side of the reversa l potential) and frequencies of incorporation (in parentheses) of 1.60 0 pS (26%), 485 pS (21%), 259 pS (53%), 140 pS (37%) and 27 pS (37%). Two Cl--selective channels were also observed, which had unitary chord -conductances of 8 and 48 pS and were present in 21 and 16% of bilayer s, respectively. The voltage dependencies of channel open probability (P0), open-state time constant (tau0) and closed-state time constant ( tau(c)) were determined for the 259, 140 and 27 pS K+ channels. The P0 of all three channels increased with increasingly positive membrane p otentials. Thus, since these channels were oriented with their extrace llular face adjacent to the cis chamber, which was grounded, all would exhibit outward rectification in vivo. Changes in P0 were effected by modulation of tau(c) in all channels, which shortened as membrane pot entials became more positive, and also of tau0 in the 140 and 27pS cha nnels, which increased as membrane potentials became more positive. Ex tracellular (cis) KCl concentration (and/or the KCl gradient across th e bilayer) affected the P0 of all three K+ channels, shifting the P0/m embrane potential relationship in the direction of the change in the p otassium reversal potential. In all channels this was achieved largely by changes in tau(c).