Functional Differences among Wheat Voltage-dependent Anion Channel (VDAC) Isoforms Expressed in Yeast: INDICATION FOR THE PRESENCE OF A NOVEL VDAC-MODULATING PROTEIN?

Abstract

VDAC is a voltage-gated anion channel located in the mitochondrial outer membrane, presumably participating in controlling aerobic metabolism. Three distinct wheat vdac cDNAs were expressed in a vdac-minus yeast strain and successfully complemented its defective phenotype. The growth curves of these transformants were different. The wheat channel isoforms were functionally characterized following purification from yeast mitochondria and reconstitution into soybean phospholipid planar membranes. All three isoforms yielded voltage-dependent anion channels with electrophysiological parameters comparable to known VDACs. Isoform-related functional features (specific conductance levels, kinetics, and gating behaviors) are reported for the first time in VDACs. The presence (or absence) of protease inhibitors during the purification procedure, and the use of Pronase on reconstituted channels, strongly suggest that some of the unique wheat VDAC properties are due to co-purification of a yeast channel-modulating protein. Its effects, different from the reported functional interactions of the channel with hexo- or creatine kinases, could not be mimicked by the protein termed VDAC modulator, indicating the presence of a novel VDAC modulator. In addition to strengthening VDAC presumed role in metabolism, the functional diversity of the channels (as shown here in two different systems) implies a highly dynamic outer membrane permeability. Our results are consistent with VDAC functioning as a heteromer including one pore protein and other modulating subunits.