Analysis of a novel double-barreled anion channel from rat liver rough endoplasmic reticulum

Abstract

The presence of anionic channels in stripped rough endoplasmic reticulum membranes isolated from rat hepatocytes was investigated by fusing microsomes from these membranes to a planar lipid bilayer. Several types of anion-selective channels were observed including a voltage-gated Cl- channel, the activity of which appeared in bursts characterized by transitions among three distinct conductance levels of 0 pS (0 level), 160 pS (O1 level), and 320 pS (O2 level), respectively, in 450 mM (cis) 50 mM (trans) KCl conditions. A chi 2 analysis on current records where interburst silent periods were omitted showed that the relative probability of current levels 0 (baseline), O1, and O2 followed a binomial statistic. However, measurements of the conditional probabilities W(level 0 at tau/level O2 at 0) and W(level O2 at tau/level 0 at 0) provided clear evidence of direct transitions between the current levels 0 and O2 without any detectable transitions to the intermediate level O1. It was concluded on the basis of these results that the observed channel was controlled by at least two distinct gating processes, namely 1) a voltage-dependent activation mechanism in which the entire system behaves as two independent monomeric channels of 160 pS with each channel characterized by a simple Open-Closed kinetic, and 2) a slow voltage-dependent process that accounts for both the appearance of silent periods between bursts of channel activity and the transitions between the current levels 0 and O2. Finally, an analysis of the relative probability for the system to be in levels 0, O1, and O2 showed that our results are more compatible with a model in which all the states resulting from the superposition of the two independent monomeric channels have access at different rates to a common inactivated state than with a model where a simple Open-Closed main gate either occludes or exposes simultaneously two independent 160-pS monomers.