Cytoplasmic Inorganic Pyrophosphatase

We have cloned a gene encoding a mitochondrial inorganic pyrophosphatase (PPase) in the yeast Saccharomyces cerevisiae by low stringency hybridization to PPA1, the yeast gene for cytoplasmic PPase. The new gene, PPA2, is located on chromosome 13 and encodes a protein whose sequence is 49% identical to the cytoplasmic enzyme. The protein differs from cytoplasmic PPase in that it has a leader sequence enriched in basic and hydroxylated residues, which is typically found in mitochondrial proteins. Yeast cells overproducing PPA2 had a 47-fold increase in mitochondrial PPase activity. This activity was further stimulated 3-fold by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone, which suggests that PPA2 is part of an energy-linked enzyme. Using gene disruptions, we found that PPA1 is required for cell growth. In contrast, cells disrupted for PPA2 are viable, but unable to grow on respiratory carbon sources. Fluorescence microscopy revealed that these cells have lost their mitochondrial DNA. We conclude that the mitochondrial PPase encoded by PPA2 is essential for mitochondrial function and maintenance of the mitochondrial genome.