Mouse VDAC isoforms expressed in yeast: Channel properties and their rolesin mitochondrial outer membrane permeability

The channel-forming protein called VDAC forms the major pathway in the mito chondrial outer membrane and controls metabolite flux across that membrane. The different VDAC isoforms of a species may play different roles in the r egulation of mitochondrial functions. The mouse has three VDAC isoforms (VD AC1, VDAC2 and VDAC3). These proteins and different versions of VDAC3 were expressed in yeast cells (S. cerevisiae) missing the major yeast VDAC gene and studied using different approaches. When reconstituted into liposomes, each isoform induced a permeability in the liposomes with a similar molecul ar weight cutoff (between 3,400 and 6,800 daltons based on permeability to polyethylene glycol). In contrast, electrophysiological studies on purified proteins showed very different channel properties. VDAC1 is the prototypic version whose properties are highly conserved among other species. VDAC2 a lso has normal gating activity but may exist in 2 forms, one with a lower c onductance and selectivity. VDAC3 can also form channels in planar phosphol ipid membranes. It does not insert readily into membranes and generally doe s not gate well even at high membrane potentials (up to 80 mV). Isolated mi tochondria exhibit large differences in their outer membrane permeability t o NADH depending on which of the mouse VDAC proteins was expressed. These d ifferences in permeability could not simply be attributed to different amou nts of each protein present in the isolated mitochondria. The roles of thes e different VDAC proteins are discussed.