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Mol. Cell. Biol., Oct 1997, 5727-5738, Vol 17, No. 10
E Blachly-Dyson, J Song, WJ Wolfgang, M Colombini and M Forte
The permeability of the outer mitochondrial membrane to most metabolites is
believed to be based in an outer membrane, channel- forming protein known
as VDAC (voltage-dependent anion channel). Although multiple isoforms of
VDAC have been identified in multicellular organisms, the yeast
Saccharomyces cerevisiae has been thought to contain a single VDAC gene,
designated POR1. However, cells missing the POR1 gene (delta por1) were
able to grow on yeast media containing a nonfermentable carbon source
(glycerol) but not on such media at elevated temperature (37 degrees C). If
VDAC normally provides the pathway for metabolites to pass through the
outer membrane, some other protein(s) must be able to partially substitute
for that function. To identify proteins that could functionally substitute
for POR1, we have screened a yeast genomic library for genes which, when
overexpressed, can correct the growth defect of delta por1 yeast grown on
glycerol at 37 degrees C. This screen identified a second yeast VDAC gene,
POR2, encoding a protein (YVDAC2) with 49% amino acid sequence identity to
the previously identified yeast VDAC protein (YVDAC1). YVDAC2 can
functionally complement defects present in delta por1 strains only when it
is overexpressed. Deletion of the POR2 gene alone had no detectable
phenotype, while yeasts with deletions of both the POR1 and POR2 genes were
viable and able to grow on glycerol at 30 degrees C, albeit more slowly
than delta por1 single mutants. Like delta por1 single mutants, they could
not grow on glycerol at 37 degrees C. Subcellular fractionation studies
with antibodies which distinguish YVDAC1 and YVDAC2 indicate that YVDAC2 is
normally present in the outer mitochondrial membrane. However, no YVDAC2
channels were detected electrophysiologically in reconstituted systems.
Therefore, mitochondrial membranes made from wild-type cells, delta por1
cells, delta por1 delta por2 cells, and delta por1 cells overexpressing
YVDAC2 were incorporated into liposomes and the permeability of resulting
liposomes to nonelectrolytes of different sizes was determined. The results
indicate that YVDAC2 does not confer any additional permeability to these
liposomes, suggesting that it may not normally form a channel. In contrast,
when the VDAC gene from Drosophila melanogaster was expressed in delta por1
yeast cells, VDAC-like channels could be detected in the mitochondria by
both bilayer and liposome techniques, yet the cells failed to grow on
glycerol at 37 degrees C. Thus, channel-forming activity does not seem to
be either necessary or sufficient to restore growth on nonfermentable
carbon sources, indicating that VDAC mediates cellular functions that do
not depend on the ability to form channels.
Copyright © 1997, American Society for Microbiology
Multicopy suppressors of phenotypes resulting from the absence of yeast VDAC encode a VDAC-like protein
Vollum Institute, Oregon Health Sciences University, Portland 97201, USA.
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