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Vol. 16, Issue 9, 4163-4171, September 2005

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Yeast Aconitase in Two Locations and Two Metabolic Pathways: Seeing Small Amounts Is Believing

Department of Molecular Biology, Hebrew University Medical School, Jerusalem 91120, Israel

Submitted November 26, 2004; Revised May 31, 2005; Accepted June 9, 2005
Monitoring Editor: Randy Schekman

The distribution of identical enzymatic activities between different subcellular compartments is a fundamental process of living cells. At present, the Saccharomyces cerevisiae aconitase enzyme has been detected only in mitochondria, where it functions in the tricarboxylic acid (TCA) cycle and is considered a mitochondrial matrix marker. We developed two strategies for physical and functional detection of aconitase in the yeast cytosol: 1) we fused the peptide of the -galactosidase enzyme to aconitase and observed complementation in the cytosol; and 2) we created an ACO1-URA3 hybrid gene, which allowed isolation of strains in which the hybrid protein is exclusively targeted to mitochondria. These strains display a specific phenotype consistent with glyoxylate shunt elimination. Together, our data indicate that yeast aconitase isoenzymes distribute between two distinct subcellular compartments and participate in two separate metabolic pathways; the glyoxylate shunt in the cytosol and the TCA cycle in mitochondria. We maintain that such dual distribution phenomena have a wider occurrence than recorded currently, the reason being that in certain cases there is a small fraction of one of the isoenzymes, in one of the locations, making its detection very difficult. We term this phenomenon of highly uneven isoenzyme distribution "eclipsed distribution."

Address correspondence to: Ophry Pines (ophry{at}md.huji.ac.il).

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