Mutation of K234 and K236 in the Voltage-Dependent Anion Channel 1 Impairs Its Insertion into the Mitochondrial Outer Membrane

Abstract

Previous in vitro studies indicated that mutation of bothK234 and K236 to arginine, glutamine, or glutamic acid impaired the abilityof the voltage-dependent anion channel (VDAC1) to insert into the outermembrane of the mitochondria (Smith et al. 1995). These same mutantswere expressed in a strain of Saccharomyces cerevisiae with adisruption in the VDAC1 gene. The mutant VDAC1 forms were found in themitochondria suggesting that they were correctly sorted to the outermembrane. However, only very small amounts of the mutants were inserted intothe mitochondrial membranes. Mitochondria isolated from the strains expressingthe mutants were capable of catalyzing the translocation of both wild-typeVDAC1 and pre-alcohol dehydrogenase III indicating that the translocationapparatus was functional. These results confirm the previously drawnconclusion that K234 and K236 are part of a membrane insertion motif. Thefailure of the mutant VDAC1 forms to insert did not cause VDAC1 precursors toaccumulate in the soluble cell cytoplasm or in the microsomal fraction. Theapparent lack of a “precursor pool” suggested that apost-transcriptional control mechanism might limit the amounts of VDAC1precursors in the cell. Such a control mechanism is consistent with theobservation that the amount of VDAC1 was very similar after epichromosomal(gene in a 2u plasmid controlled by a Gal1 promoter) and chromosomalexpression (endogenous gene controlled by the endogenous promoter).