The supply of substrates to the respiratory chain as well as of other metabolites (e.g. ATP) into inner compartments of mitochondria is crucial to preprotein import into these organelles. Transport of the compounds across the outer mitochondrial membrane is enabled by mitochondrial porin, also known as the voltage-dependent an- ion-selective channel (VDAC). Our previous studies led to the conclusion that the transport of metabolites through the outer membrane of the yeast Saccharomyces cerevisiae mitochondria missing VDAC (now termed YVD AC 1) is considerably restricted. Therefore we expected that depletion of YVDAC1 should also hamper protein import into the mutant mitochondria. We report here that YVD AC 1-depleted mitochondria are able to import a fusion protein termed pSu9-DHFR in the amount comparable to that of wild type mitochondria, although over a considerably longer time. The rate of import of the fusion protein into YVD AC 1-depleted mitochondria is dis-tinctly lower than into wild type mitochondria probably due to restricted ATP access to the intermembrane space and is additionally influenced by the way the supporting respiratory substrates are transported through the outer membrane. In the presence of ethanol, diffusing freely through lipid membranes, YVDAC1-depleted mitochon- dria are able to import the fusion protein at a higher rate than in the presence of external NADH which is, like ATP, transported through the outer membrane by facilitated diffusion. It has been shown that transport of external NADH across the outer membrane of YVDAC1-depleted mitochondria is supported by the protein import machin- ery, i.e. the TOM complex (Kmita & Budziñska, 2000, Biochim. Biophys. Acta 1509, 86.94.). Since theTOMcomplex might also contribute to the permeability of themem-brane to ATP, it seems possible that external NADH and ATP as well as the imported preprotein could compete with one another for the passage through the outer mem- brane in YVDAC1-depleted mitochondria.
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