Genetic Demonstration That the Plasma Membrane Maxianion Channel and Voltage-dependent Anion Channels Are Unrelated Proteins

Tatiana Sheiko,Defeng Deng,Hongtao Liu,Ravshan Z Sabirov,William J Craigen,Yasunobu Okada

doi:10.1074/jbc.m509482200

Abstract

The maxianion channel is widely expressed in many cell types, where it fulfills a general physiological function as an ATP-conductive gate for cell-to-cell purinergic signaling. Establishing the molecular identity of this channel is crucial to understanding the mechanisms of regulated ATP release. A mitochondrial porin (voltage-dependent anion channel (VDAC)) located in the plasma membrane has long been considered as the molecule underlying the maxianion channel activity, based upon similarities in the biophysical properties of these two channels and the purported presence of VDAC protein in the plasma membrane. We have deleted each of the three genes encoding the VDAC isoforms individually and collectively and demonstrate that maxianion channel (approximately 400 picosiemens) activity in VDAC-deficient mouse fibroblasts is unaltered. The channel activity is similar in VDAC1/VDAC3-double-deficient cells and in double-deficient cells with the VDAC2 protein depleted by RNA interference. VDAC deletion slightly down-regulated, but never abolished, the swelling-induced ATP release. The lack of correlation between VDAC protein expression and maxianion channel activity strongly argues against the long held hypothesis of plasmalemmal VDAC being the maxianion channel.

Highlights

The maxianion channel has been observed in a wide variety of cell types and exhibits roughly uniform behavior [9], suggesting that it has a general physiological function
Single Maxianion Channels in Wild-type Mouse Fibroblasts—In the cell-attached configuration, no single channel events were observed from wild-type mouse embryonic fibroblasts (WT-MEFs)
In a separate set of experiments, we determined the ionic selectivity of WT-MEF maxianion channels in conditions close to those commonly used for mitochondrial voltage-dependent anion channel (VDAC) proteins reconstituted in lipid bilayers

Summary

Introduction

The maxianion channel has been observed in a wide variety of cell types and exhibits roughly uniform behavior [9], suggesting that it has a general physiological function. The biophysical properties of the WT-MEF maxianion channel, such as the single channel conductance, voltage-dependent inactivation, ATP permeability, and sensitivity to Gd3ϩ and arachidonate, are very similar to those observed previously in C127 cells [5, 6], kidney macula densa cells [7], and cardiomyocytes [8].

Results

Conclusion